多孔材料：制备·应用·表征=Porous materials：processing and applications：英文(txt+pdf+epub+mobi电子书下载)

作者：(中)P. S. Liu刘培生,(中)G. F. Chen陈国锋

出版社：清华大学出版社

格式: AZW3, DOCX, EPUB, MOBI, PDF, TXT

多孔材料：制备·应用·表征=Porous materials：processing and applications：英文试读：

内容简介

多孔材料具有优秀的物理和力学性能，特别是在功能结构一体化方面展示出优异的综合性能。本书系统介绍了此类材料的概念、制备、应用和表征等基本知识以及近年来的相关研究进展。全书共分10章：在第1章对多孔材料作了整体性的概述，第2章至第8章依次论述了多孔金属、多孔陶瓷、泡沫塑料三大类多孔材料的各种制备方法和不同用途，第9章和第10章分别介绍多孔材料的几个基本参量的表征，包括孔隙因素和基本物理性能。本书可供材料领域的科研人员、工程技术人员参考，也可作为高等院校材料类和相关专业（如物理、化学、生物、医学、机械、冶金、建筑等专业）的教材。

ABOUT THE AUTHORS

Dr. P. S. Liu is professor at the College of Nuclear Science and Technology, Beijing Normal University (BNU), Beijing, China. He graduated from the Chinese Academy of Science in 1998 and received his Ph.D. in materials science and engineering. He has served as the committeeman of the academic committee of the Key Laboratory of Beam Technology and Material Modification of Ministry of Education of China. Additionally, he was the first director of the Material Physics division and then the vice director of the Nuclear Physics Research Institute at BNU from 2004 to 2008. Investigating porous materials and high-temperature coatings for many years, he has published extensively in the area of materials science and engineering as the lead author, including about 60 SCI papers, more than 50 EI articles, and six academic books. In addition, he has authorized nine Chinese invention patents as the principal originator.

Dr. G. F. Chen, Ph.D. from the Institute of Metal Research, Chinese Academy of Sciences; Alexander von Humboldt Fellow in the Max-Planck-Institute for Metals Research in Stuttgart; research scientist at the National Physical Laboratory in London; research specialist at Cleveland State University in Cleveland, Ohio, professor at the Northwestern Polytechnical University in Xi’an, China, and now an expert in corporate technology at Siemens Ltd., China, in Beijing. He has more than 20 years of experience in materials research and development, particularly with energy materials.

PREFACE

Porous materials are a class of functionalstructural materials with the optimal index of physical and mechanical properties, thanks to their porous structure. This book systematically introduces the basic concept behind these materials, as well as their major types, characteristics, applications, and main parameters. In addition, it presents various methods that can be used to process porous metals, porous ceramics, and polymer foams (foamed plastics) in accordance with their respective categories.

The concept of porous materials has been known for a number of years, but its radiation is far less successful than that of other materials. By the end of the 20th century, studies on porous materials have made a number of important discoveries. Based on this background, we spent a good deal of time and energy on collecting relevant literatures, combining with our own accumulated work experience, to write the Chinese version of the book, Introduction to Porous Materials, published in 2004 by Tsinghua University Press. This book focuses on production methods and applications of porous materials, considering that a classic work about porous materials, Cellular Solids: Structure and Properties, ” by L. J. Gibson and M. F. Ashby, has made a great contribution to expounding the structure and properties of porous solids. This is aimed at providing more information to scientific researchers and engineering or technical personnel who interact with porous materials (including the present authors themselves, of course). The formation and the publication of Introduction to Porous Materials were quite hasty at that time, with some immature viewpoints. In addition, at that time, there were only a few researchers in China studying porous materials. However, the results of our previous effort (including its reception) far exceeded our expectations, and that development further encouraged our future work. In turn, the publication of this book may play a part in promoting the development in China of the porous material field, as well as research in relevant or potential relevant fields. Because we have seen that research into porous materials has been growing rapidly in recent years in China, and the number of the scientific research institutes, universities, and companies involved in this area also have increased rapidly.

In order not to let down the readers, the publisher and the author jointly determined to revise the original book for a second edition, published in Chinese, to better meet the needs of the wider readership. In the second edition published in 2012, we corrected some errors and inappropriate content that appeared in the first edition, and we added relevant new content reflecting the research progress made over the previous decade. In addition, we readjusted the layout of the book to give it a scientific and reasonable arrangement; in particular, we devoted a lot of time to revising chapters 2, 3, and 4.

Learning of Elsevier's interest in the topic of this book and considering the international demand for it, we comprehensively rewrote and rearranged the book again for a third edition. In so doing, we expanded on the relevant contents with an emphasis on supplementing the information about the processing, applications, and characterization of porous materials.

In the process of writing this book, we referred to the relevant papers and works published in the last 40 years, and especially those from the last 20 years, and made good use of them. Here, we would like to express our heartfelt thanks to all the authors of these documents. However, we should note that due to space and time limitations, we had to leave out a good many worthy books, papers, and articles, and we regret this deeply. Certainly, we also should acknowledge the assistance of many of our colleagues in the field of porous materials, and our friends that have helped and supported us greatly. In the process of writing and publishing this book, P. Liu provided excellent assistance, and C. Y. Yang and Y. J. Guo worked hard to collate the references and draw the figures for this book. The combined effort of all these fine people have allowed this project to reach a suc-cessful conclusion.P.S. Liu, G.F. ChenOctober 2013

CHAPTER ONE General Introduction to Porous Materials

Porous materials widely exist around us and play a role in many aspects of our daily lives; among the fields they can be found in are energy management, vibration suppression, heat insulation, sound absorption, and fluid filtration. Highly porous solids have relatively high structural rigidity and low density of mass, so porous solids often serve as structural bodies in nature, including in wood and bones [1, 2]; but human beings use porous materials more functionally than structurally, and develop many structural and functional integrative applications that use these materials fully [3, 4]. This chapter will introduce the elementary concepts and features of this kind of material.

1.1 ELEMENTARY CONCEPTS FOR POROUS MATERIALS

Just as their name implies, porous materials contain many pores. Porous solids are made of a continuously solid phase that forms the basic porous frame and a fluid phase that forms the pores in the solid. The latter can consist of gas, when there is a gaseous medium in the pore, or of liquid, when there is a liquid medium in the pore.

In that case, can all materials with pores be referred to as porous? Perhaps surprisingly, the answer is “no.” For instance, holes and crannies that are the result of defects will lower a material's performance. This result is not what designers want, and so these materials cannot be termed porous. So-called porous materials must possess two essential characteristics: one is that the material contains a lot of pores, and the other is that the pores are designed specifically to achieve the expectant index of the material's performance. Thus, the pore of porous materials may be thought as a functional phase what designers and users hope to come forth within the material, and it supplies an optimizing action for the performance of the material.

1.2 MAIN GROUPS OF POROUS MATERIALS

The number of pores (i.e., porosity) will vary for different porous materials. Porous materials can be classified as low porosity, middle porosity, or high porosity based on the number of pores. Generally, porous materials with low and middle porosity have closed pores (Figure 1.1) which behave like a phase of impurity. For porous materials with high porosity (Figures 1.2–1.4), there are two different cases according to various morphologies of the pore and the continuous solid phase. In the first case, the continuous solid constructs a two-dimensional array of polygons; the pore is isolated in space, taking on polygonal columniations accordingly; and the cross-sectional shape of the pore is commonly triangle, quadrangle, or hexagon (Figure 1.2). This structure looks similar to the hexagonal cell of a honeycomb, and such two-dimensional porous materials are called honeycomb materials. Porous materials with directional pores [5], which are called lotus-type porous materials, have a similar structure as honeycomb materials, but the cross-sectional shape of the pores for these materials is circular or elliptic, and the pore often cannot run through it, resulting in less uniformity of distribution and a lower density of the array. In the second case, the continuous solid presents a three-dimensional reticulated structure (Figure 1.3), and such porous materials can be termed three-dimensional reticulated foamed materials. These materials have connective pores that are of a typical opencell structure. In the third case, the continuous solid shows the cell wall structure of pores of sphericity, elliptical sphericity, or polyhedron shape (Figure 1.4), and such three-dimensional porous materials can be called bubblelike foamed materials. Within these materials, the cell wall may separate many isolated closed pores or cells, forming a closed-cell, bubblelike foamed substance (Figure 1.4a). The cell wall may make up open-cell, bubblelike foamed material as well (Figure 1.4b). In the literature, three-dimensional, reticulated foamed materials are referred to as “open-cell foamed materials, ” closed-cell, bubblelike foamed materials are called “closed-cell foamed materials, ” and open-cell, bubblelike foamed materials are “half open-cell foamed materials.”Figure 1.1　Porous composite oxide ceramics, which is a low-porosity material, shown as a cross-sectional image.Figure 1.2　Two-dimensional honeycomb materials: (a) conductive honeycomb TiC ceramics with quasi-square pores [6]; (b) thermal storage of honeycomb ceramics with square pores (with dimensions of 100 mm×100 mm×100 mm, cell-wall thickness of 1 mm, and square-pore side length of 2.5 mm) [7].Figure 1.3　Three-dimensional reticulated foamed materials: (a) nickel foam; (b) iron foam.Figure 1.4　Bubblelike foamed materials: (a) a closed-cell bubblelike foamed material of aluminum foam [8]; (b) an open-cell bubblelike foamed material of iron foam.

Porous solids include two types of porous bodies (i.e., natural and artificial). Natural porous solids can be found universally [1], such as bones that support the bodies and limbs of animals and human beings (see Figure 1.5), plant leaves, wood, sponge, coral (Figure 1.6), pumice (Figure 1.7), and lava (Figure 1.8). Lava is a sort of natural porous material that can be used in construction or for creating artwork (Figure 1.9). It is not accurate to refer to the natural, porous solids of living animal bones and tree trunks as “natural porous materials.” However, when a tree is cut down to make materials used by human beings to make things like furniture, it becomes natural porous materials. The fluid phase contained in the pores of plant leaves and living tree trunks always consists of liquid (namely sap), while that within artificial porous materials is mostly gas. Artificial porous materials can be subclassified further into porous metals, porous ceramics, and polymer foams.Figure 1.5　Cross-sectional view of a reticulated porous bone of a whale.Figure 1.6　An optical photograph showing the porous morphology of coral.Figure 1.7　An image showing the porous morphology of pumice.Figure 1.8　Cross-sectional view of the porous morphology of lava.Figure 1.9　A vase made of lava.

1.3 POROUS METALS

Porous metals are a relatively new class of engineering materials that can serve functional and structural purposes [9–11]. They have undergone rapid development over the last thirty years. These lightweight materials not only have the typical characteristics of metals (weldability, electrical conductivity, and ductibility), but also possess other useful characteristics, such as low bulk density, great specific surface area, low thermal conductivity, good penetrability, energy management, mechanical damping, vibration suppression, sound absorption, noise attenuation, and electromagnetic shielding. Consequently, these materials have increasing applications, and have emerged as a focus of great attention in the international material field [12]. The next sections describe the main characteristics of these types of metals [11, 13–15].

1.3.1 Powder-Sintering Type

The powder-sintering type of porous metallic material is commonly made from metal or alloy powder with spherical or irregular shapes via molding and sintering. The porous bodies obtained in this manner will have various porosities, pore sizes, and pore-size distribution due to differences of the selected raw materials or technological systems. However, all of them have the characteristics of good penetrability, controllable pore sizes and levels of porosity, and great specific surface area, as well as endurance under high or low temperatures and resistance to heat fluctuation.

Powder-sintering porous metals were developed early, with pore size usually less than 0.3 mm and porosity mostly less than 30％. However, the production with porosity much higher than 30％ can be prepared by using special technological processes, e.g, the space-holder method. In the metallurgy and chemical engineering fields, high-temperature and high-pressure environments are frequent, and accordingly, filtration and separation materials are needed; during catalysis reactions, catalyzer materials with great specific surface area are needed to supply the reactive interface area; and many typesofoilsand working gases must be filtered strictly to guarantee that the aviation and hydraulic pressure systems work safely. The areas of aviation and rockets demand that porous materials with great heat endurance and heat fluctuation resistance and well-proportioned pore structures be used as the basic structural material for volatilization cooling. In general, porous polymer or ceramic bodies are difficult to adapt to these conditions, which require great strength, plasticity, and high temperature tolerance at the same time, but powder-sintering type porous metallic materials can do this well, and therefore scientists worked to develop them speedily.

The first patents mentioning powder-sintering porous components were approved as early as 1909, and patents dealing with the techniques to make powder-sintering filters were developed until the early 1930s. During World War II, powder-sintering porous materials underwent rapid development for military applications. Powder-sintering filters were applied to airplanes and tanks, porous nickel was adopted to make radar switches, porous iron was employed to make cannonball hoops instead of dense metallic copper, and iron filters were used as flame extinguisher. In the mid-twentieth century, porous materials with oxidation resistance were applied to the fireboxes and blades of jet engines for volatilization cooling to heighten the efficiency of engines. In response to developments in chemical engineering, metallurgy, atomic energy, aviation, and rocketry, many types of powdersintering porous materials with high penetrability and resistance to corrosion, high temperatures, and high pressure were created. Some more advanced porous materials were produced in the 1960s, including the corrosion-and heat-resistant porous materials of Hastelloy, Inconel, titanium, stainless steel, tungsten, tantalum, and other refractory metals and alloys. At present, powder-sintering porous materials of bronze, stainless steel, nickel, titanium, and aluminium alloys have been mass-produced and employed. Figure 1.10 shows a powder-sintering type of porous titanium alloy.Figure 1.10　SEM image of the porous TiNiFe alloy fabricated by powder sintering [16].

1.3.2 Fiber-Sintering Type

The fiber-sintering type of porous metal is an improvement over powdersintering porous metals for the above mentioned purpose. Porous materials made of metallic fiber may be superior to that of metallic powder in some ways. For example, filtration materials fabricated of metallic fiber will have a much greater degree of penetrability than those made of metallic powder with the same diameter as the metallic fiber. In addition, they have a higher mechanical strength, corrosion resistance, and thermal stability. These materials can reach a porosity of over 90％, with all through pores, good plasticity and impact toughness, and a high dust retention capacity. Known as secondgeneration porous metallic filtration materials, they may be used by many businesses under rigorous filtration conditions. Figure 1.11 shows a porous structure crafted by metallic fiber sintering.

1.3.3 Melt-Casting Type

The melt-casting type of porous metal is formed via cooling molten metals or alloys, which can include a very wide range of porosities and have diversely shaped pores with different casting manners. One example of this is aluminum foam produced by melt-foaming and infiltration-casting processes. Materials made from melt foaming are mostly closed-cell or half open-cell porous materials (Figure 1.12), and those made from infiltration casting commonly take the form of three-dimensional, reticulated, open-cell ones with high porosity.Figure 1.11　Micrograph of a porous material fabricated by metallic fiber sintering [11].Figure 1.12　An aluminum foam produced by melt foaming [17].

1.3.4 Metal-Deposition Type

The metal-deposition type of porous metal is created via depositing atomic metal on open-cell polymer foam, followed by eliminating polymers and sintering. The main features of such metals include connective pores, high porosity, and a three-dimensional, reticulated structure. This porous material, a new type of functionally and structurally integrative substance with excellent properties, is a very important class of porous metals. When used in certain settings, its merits include low density, high porosity, great specific surface area, good pore connectivity, and uniform structure, which is difficult to achieve for other types of porous metals. However, the feature also results in some limits to the strength of metal-deposition type porous metals. These materials first were manufactured and utilized in the 1970s, and then, during the 1980s, they were speedily developed for a wide variety of applications and demands. At present, these porous materials are produced on a large scale in many countries, with the products of nickel and copper foams typically made by the electrodeposition process. Such metal foams are shown in Figure 1.13.Figure 1.13　SEM images of nickel foam samples of various thicknesses made by the metal deposition process: (a) a thinner nickel layer; (b) a thicker nickel layer.

1.3.5 Directional-Solidification Type

The directional-solidification type of porous metal forms via dissolved gas in molten metal releasing in the course of directional cooling [5, 18], namely by GASAR. The resultant products have a very similar structure to plant lotus roots (Figure 1.14), so they are called lotus-type porous metals, porous metals with directional pores, or Gasarite.

1.3.6 Composite Type

Composite-type porous metals are porous metal composite materials. They can be obtained by compositing different metal species or metal species and nonmetal species to form a porous body. Examples of this type of metal include graphite-nickel composite porous material created by electroplating a nickel layer onto a graphite felt, and a composite of aluminum alloy and nickel foam made by pouring a melted aluminum alloy into a threedimensional, reticulated nickel foam. Such materials also can be fabricated by using porous metals as a core to form a metallic composite porous “sandwich”; for example, by putting together stainless steel fiber felt and wire netting or by integrating aluminum foam and metallic panels. Compositing makes the materials acquire the respective merits of these different ingredients and improved their properties; the result is a completely new synthetic material that better meets the demands placed on products made from this substance.Figure 1.14　A lotus-type porous metal formed by gas-metal eutectic directional solidification [18].

In addition, certain porous metallic materials are prepared by particular routes, some of which can be ascribed to those of the above mentioned types, and others can be those of new types.

1.4 POROUS CERAMICS

Porous ceramics, also known as cellular ceramics, began developing in the 1970s. They are comprised of a kind of heat-resistant porous material with many gaseous pores. Their pore size mostly ranges between the angstrom and millimeter levels, the porosity usually spans from 20％ to 95％, and the serving temperature varies from room temperature to 1, 600 C [19, 20].

1.4.1 Classifying Porous Ceramics

In general, porous ceramics may be divided into two main classes [20–22]: honeycomb ceramics (Figure 1.15) [23] and ceramic foam (Figure 1.16). The former has polygonal columnar pores that form a two-dimensional array (see Figure 1.2), and the latter has hollow polyhedron pores that form a three-dimensional array. Figure 1.16 shows two ceramic foams with different pore structures, both of which were made from compounded oxides.

There are two sorts of ceramic foam: the open-cell, reticulated ceramic foam (Figure 1.16a) and the closed-cell, bubblelike ceramic foam (Figure 1.16b). When the solid species constituting the foamed body is comprised only of pore struts, the connective pores will generate reticulated structures, resulting in open-cell ceramic foams. When pores are separated by solid cell walls, the closed-cell ceramic foam will be achieved. Such differences can be clearly seen by comparing the fluid penetrability of these two sorts of foamed bodies. The distinction between the two types depends on whether the pore is enveloped by solid cell walls or not [20–22]. In addition, there are half open-cell ceramic foams.Figure 1.15　An optical photograph showing two-dimensional honeycomb ceramic products [23].Figure 1.16　Three-dimensional ceramic foams: (a) an open-cell reticulated ceramic foam, (b) a closed-cell bubblelike ceramic foam.

Apparently, some ceramic foams have both open and closed pores.

These porous structures take on a relatively low level of bulk density and thermal conductivity, as well as varying levels of fluid penetrability which is high for the open-cell body. By properly matching the ceramic raw material to the preparation technique, porous ceramics may be created that have relatively high levels of mechanical strength, corrosion resistance, and stability under high temperatures that can satisfy the demands of severe conditions [21].

Porous ceramics also can be classified according to the size of their pores, as follows [24]:● Microporous material, for pore sizes of less than 2 nm● Mesoporous material, for pore sizes of 2–50 nm● Macroporous material, for pore sizes over 50 nm

This classification standard has not been adopted abroad because the rules about using porous materials vary widely from country to country.

In light of the differences among their materials, there are several types of porous ceramics: silicate; aluminosilicate; diatomite; carbon; corundum; silicon carbide; and ocordierite [25].

Ceramic foam is an important part of porous ceramics, and the open-cell type of ceramic foam, which is a new type of highly porous ceramics, has a three-dimensional, reticulated structure with connective pores, resulting in great specific surface area, high fluid contact efficiency, and a small loss of fluid pressure [26, 27]. In particular, these materials have many connective pores and capillary holes and have high specific surface energy on the inside, so they perform well in terms of filtration and adsorption under low fluid resistance loss conditions. They can be used in many fields, including metallurgy, chemical engineering, environment protection, energy, and biology, for such applications as metal melt filtration, high-temperature gas purification, and catalyst support [26]. Moreover, the porosity, density, fluid resistance loss, and penetrability of these materials can be modulated by various processing techniques, and the commonly used material species includes alumina and cordierite. Cordierite is used as a raw material with the primary purpose of improving the heat fluctuation resistance of products, and alumina is used to increase a material's strength and thermal stability. As the demand of thermal stability heightens for such products, porous silicon nitride and silicon carbide ceramics also have been developed [19].

The research on porous ceramics has been expansively attended, and lots of technological applications have become possible for these materials in practice. In some areas (such as energy and environmental protection), the applications of porous ceramics can have enormous economic and societal benefits [25].

1.4.2 Characteristics of Porous Ceramics

Porous ceramics have several common characteristics [25]:1. Good chemical stability. Choosing the appropriate material

species and techniques can make porous products suitable for

various corrosive conditions in which the products are expected to

function.2. Great specific strength and rigidity. The shape and size of pores

in porous ceramics will not change under gas pressure, liquid

pressure, and other stress loadings.3. Fine thermal stability. Porous products made of heat-resistant

ceramics can filtrate molten steel or high-temperature burning gas.

These excellent characteristics promise a great future for porous ceramics being used in a wide variety of applications, and make such materials adaptable in many areas, including chemical engineering, environment protection, energy source, metallurgy, and electronic industry. The specific cases for which porous ceramics are suitable depend on both the composition and structure of the products. At first, porous ceramics were used as filtration materials to filtrate bacteria belonging to the microorganism. Once the level of controlling the fine pores of porous ceramics was increased, the resulting products gradually became used in more and more applications, including separation, dispersion, and adsorption; and they are presently being used in many industrial areas, including the chemical engineering, metal smelting, petroleum, textile, pharmaceutical, and foodstuff machinery industries. Also, these porous ceramics have been used increasingly in sound-absorbing materials, sensitive components, artificial bones, and tooth root materials.

1.5 POLYMER FOAMS

Polymer foams, also called plastic foams, are porous plastics filled with bubblelike pores, but products with a reticulated structure also can be seen frequently in this category [28, 29]. These materials contain many pores filled with gas, so they may be regarded as polymer composites or composite plastics in which the gas is stuffed. In general, all the thermoset plastics, general plastics, engineering plastics, and heat-resistant plastics can be made into foamed plastics. Such porous bodies are one kind of plastic products that are used on a large scale, and assume an important role in the plastics industry [28].

The density of plastic foams is determined by the volume ratio of gaseous pores to solid polymer. This ratio is about 9:1 for low-density plastic foams and about 1.5:1 for high-density ones [30].

1.5.1 Classifying Polymer Foams

There are a variety of polymer foams. They are classified as follows [28, 29]:

1. Open-and closed-cell polymer foams can be defined based on the pore structure of the foamed body. Open-cell polymer foams have mutually connected pores, with gaseous and solid phases, which are each continuously distributed (Figure 1.17a) [31]. The penetrability of fluids through the porous body is related to both open-cell porosity and polymer characteristics. Closed-cell polymer foams have pores that are separate from one another, and the solid polymer phase presents a continuous distribution, but the gaseous phase occurs inside the individual isolated pores (Figure 1.17b [32]). Actually, both structures of pores exist simultaneously in real polymer foams; that is, open-cell polymer foams contain some closed-cell pores, and closed-cell polymer foams contain some open-cell pores. In general, open-cell structures make up approximately 90％–95％ in so-called open-cell polymer foams.Figure 1.17　Three-dimensional porous polymer foams: (a) an open-cell polyurethane (PU) foam [31]; (b) a closed-cell polyolefin foam [32].

2. Polymer foams can be divided into three categories based conversely on their density: low foaming, moderate foaming, and high foaming. Lowfoamed or high-density polymer foams have a density of 3more than 0.4 g/cm and a gas/solid expansion ratio (a ratio of the density of dense plastic to the apparent density of foamed plastic with the same polymer species) of less than 1.5. Moderate-foamed or 3middle-density foams have a density of 0.1–0.4 g/cm and an expansion ratio of 1.5–9.0. Highfoamed or low-density foams have a 3density of less than 0.1 g/cm and an expansion ratio of more than 9.0. Another way of classifying these materials is to label products with an expansion ratio of less than 4 or 5 as low-foamed polymer foams, and those with a ratio of more than 4 or 5 as high-foamed. On occasion, 3the density with the value of 0.4 g/cm is adopted to bound the high-or low-foamed porous plastics. Products that commonly use polymer foams, such as mattresses, cushions, and packaging liners, mostly are the high-foamed types; other products, like frothed plastic plates, pipes, and abnormal components, fall into the low-foamed category.

3. Polymer foams may be grouped into three types based on their rigidity: rigid, semi-rigid, and flexible. With rigid foams, the polymer takes a crystal form at room temperature or has a glass transition temperature higher than room temperature, and it is quite rigid at room temperature. With flexible polymers, the melting point of the polymeric crystal or the glass transition temperature of the amorphous polymer is lower than room temperature. Semi-rigid foams fall between these two types. Based on these criteria, phenol formaldehyde resin (PF), epoxy resin (ER), polystyrene (PS), polycarbonate (PC), rigid polyvinyl chloride (PVC), and numerous polyolefin foams are rigid polymers, and porous rubber, elastic polyurethane (PU), flexible polyvinyl chloride (PVC), and a part of polyolefin foams are flexible [29].

From the viewpoint of modulus, rigid foamed plastics are characterized by porous polymers, of which the elastic modulus is more than 700 MPa at a temperature of 23°C and relative humidity of 50％. With flexible foamed plastics, the elastic modulus is less than 70 MPa at the same temperature and relative humidity, and with semi-rigid foamed plastics, the elastic modulus is between 70 MPa and 700 MPa [28].

The resin species most frequently used to make foamed plastics are polystyrene (PS), polyurethane (PU), polyvinyl chloride (PVC), polyethylene (PE), and urea formaldehyde (UF). Other commonly used varieties include phenol formaldehyde resin (PF), epoxy resin (ER), organosilicon resin (OS), polyethylene formaldehyde, cellulose acetate, and polymethyl methacrylate (PMMA). In recent years, some material species have begun to be used to produce polymer foams, such as polypropylene (PP), polycarbonate (PC), polytetrafluoroethylene (PTFE), and polyamide (PA; i.e., nylon).

1.5.2 Characteristics of Polymer Foams

Although there are many kinds of polymer foams, all of them contain a lot of pores. Therefore, they have several common characteristics, including low density, low thermal conductivity, good thermal barrier effect, effective impact energy absorption, excellent sound insulation, and great specific strength [28, 29]. These characteristics are described in the next sections.Low Relative Density

There are lots of pores in polymer foams, and correspondingly, the density of porous products is only a small percentage of that of dense products. Additionally, the polymer itself is a class of low-density material species, so the products of polymer foams may have a very low density, which is the lowest of all the porous materials. (Note that polymers consist of light atoms, and the molecules inside are linked by a weak Van der Waals force, causing it to have a constitution without compactness, with low density and rigidity.)Excellent Performance of Heat Insulation

The thermal conductivity of foamed polymers is greatly reduced compared to the corresponding dense plastics due to the fact that porous products have so many pores, and the gas in these pores has a thermal conductivity with an order of magnitude less than that of dense solid plastics. Furthermore, the gaseous phase in pores is separate for closed-cell foamed bodies, which reduces the convection heat transfer of gas. As a result, the thermal barrier effect for polymer foams is improved.Good Impact Energy Absorption

Gas in the pores of polymer foams under impact loading will be compressed, resulting in hesitation. Such compression, springback, and hesitation will consume the energy from the impact load. Moreover, the foamed body also can terminate the impact load step by step with a small deceleration, so it will acquire an excellent damping ability.Excellent Sound Insulation

The sound insulation effect of polymer foams comes into play in the following two ways: (1) the porous body absorbs sound wave energy to terminate the reflection and transferal of the sound waves; (2) the porous body eliminates resonance and decreases noise. When the sound wave arrives at the cell wall of a pore in polymer foams, it will strike the pore and make the gas within it to be compressed. This causes hesitation, so the impact energy of the sound wave will dissipate. In addition, increasing the rigidity of the polymer foams can eliminate or decrease the resonance and noise caused by the sound wave hitting the pores.Great Specific Strength

Specific strength is the ratio of material strength to relative density. The mechanical strength of polymer foams will decrease when porosity increases, but the specific strength as a whole will be much higher than that of porous metals or porous ceramics with equivalent porosities.

Polymer foams that are made from hollow globular stuffing and resin matrix have a very great specific strength of compression, and they can be used for such applications as the elastic material on the hulls of ships serving in deep seawater [32]. Usually, the stuffing may employ hollow or porous granules of glass and ceramics, as well as thermoset plastics or thermoplastic resins. The tiny ball stuffing also may be used in fiber-reinforced plastics and enhances the toughness of fiber-reinforced resins.

Strengthening polymer foams advances the potential development in material sciences. The exploitation and application insufficiencies make the virtue not adequately utilized yet, but the reinforced thermoplastic materials have some advantages both in economy and in technology. In many cases when specific strength is demanded, these recent applications of reinforced plastics may come in handy. Also, using the reinforcement technique and the other materials can give some of the composite porous materials a number of outstanding properties which integrate the low density, low combustibility, low cost, and great specific strength.

Of course, all of the abovementioned porous metals, porous ceramics, and polymer foams can be incorporated with other materials to form excel-lent porous composites, whose combined properties can be well suited to more demanding purposes.

1.6 CONCLUSIONS

Making a dense material porous endows it with brand-new, very useful properties. These additional properties make porous materials suitable for many applications for which dense ones are not well suited. This enhances the degree of creativity that is possible usingporous materials andgreatlyopens up the range that these materials will be applied in engineering. There are many varieties of porous material, but all the types have some common characteristics, including low relative density, large specific surface area, high specific strength, small thermal conductivity, and good energy absorption compared to the dense version of the same materials. Low-density porous materials may be used to design lightweight rigid components, large portable structural frames, and various flotages. Low-thermal-conductivity products canbeapplied to simpleand convenient forms of heatinsulation, and the effect is just a little inferior to that of more expensive and difficult varieties. Lowrigidity foamed bodies serve as the perfect material for mechanical damping. For example, elastic foams are standard materials used to install machinery bases. In addition, the large compressive strain of these materials make them quite attractive for energy absorption applications, and there is a huge market for porous materials to protect articles. This book mainly discusses artificial porous materials, their production, application, and characteristics, as well as the results of relevant research on these substances in recent years.

REFERENCES

[1] Gibson LJ, Ashby MF. Cellular solids: structure and properties. Cambridge, UK: Cambridge University Press; 1997.

[2] Banhart J, Baumeister J, Weber M. Metal foams near commercialization. Met Powder Rep 1997; 4:38–41.

[3] Liu PS, Lang KM. Functional materials of porous metals made by P/M, electroplating, and some other techniques. J Mater Sci 2001; 36(21):5059–72.

[4] Liu PS, Yu B, Hu AM, Lang KM, Gu SR. Development in applications of porous metals. Trans Nonferrous Met Soc China 2001; 11:629–38.

[5] Nakajima H. Fabrication, properties, and applications of porous metals with directional pores. Proc Jpn Acad B Phys Bio Sci 2010; 86(9):884–99.

[6] Sun JS, Chen QH, Ye JF. Fabrication of AlO-TiC electric 23honeycomb ceramics by SHS. China Ceram 2008; 44(4):24–7.

[7] Ouyang DG, Jiang YH, Wang HQ, Luo W, Zhu SH, Li MH. Development of honeycomb ceramics thermal storage with low stress. Indus Furn 2009; 31(5):8–10.

[8] Liu H, Xie MZ, Li K, Wang DQ. Numerical simulation of production process of aluminum foam by air injecting and melt stirring. Chin J Process Eng 2007; 7(5):889–94.

[9] Zhu ZG. Metallic foam materials. Physics 1999; 28(2):84–8.

[10] Nakajima H, Hyun SK, Ohashi K, Ota K, Murakami K. Fabrication of porous copper by unidirectional solidification under hydrogen and its properties. Coll Surf 2001; 179:209–14.

[11] Zeng HM. General introduction to high technology and new materials. Beijing: China Science and Technology Press; 1993.

[12] Banhart J, Ashby MF, Fleck N. Metal foams and porous metal structures, In: Intl conf metal foams porous metal struct. Bremen: Verl MIT publ; 1999.

[13] Baoji Institute of Nonferrous Metal Research. Powder-metallurgical porous materials. Beijing: Metallurgical Industry Press; 1979.

[14] Tang HP, Zhang ZD. Developmental states of porous metal materials. Rare Metal Mat Eng 1997; 26(1):1–6.

[15] Liu PS, Bing Yu, Hu AM, Liang KM, Gu SR. Techniques for the preparation of porous metals. J Mater Sci Technol 2002; 18(4):299–305.

[16] Li YH, Qi GX, Li YH, Deng ZY, Wang CZ. Porous TiNiFe alloy fabricated by combustion synthesis and powder sintering. Rare Metal Mat Eng 2010; 39(S1):227–30.

[17] Li YX. Comparison of aluminum foams produced by melt forming and gas injection processes. Spec Cast Nonferr Alloys 2011; 31(12):1097–9.

[18] Li YX, Liu Y, Zhang HW. Research progress in GASAR and Gasarite. Spec Cast Nonferr Alloys 2004; 1:9–11.

[19] Wang LX, Ning QJ, Yao ZC. Development of porous ceramics material. Bull Chin Ceram Soc 1998; 1(1):41–5.

[20] Scheffler M, Colombo P. Cellular ceramics. Weinheim: Wiley-VCH; 2005.

[21] Montanaro L, Jorand Y, Fantozzi G, Negro A. Ceramic foams by powder processing. J Eur Ceram Soc 1998; 18:1339–50.

[22] Hirschfeld DA, Li TK, Liu DM. Processing of porous oxide ceramics. Key Eng Mat 1996; 115:65–80.

[23] Meng XQ, Li P. Porous ceramic materials. China Build Mat 2008; 10:92–4.

[24] Nettleship I. Applications of porous ceramics. Key Eng Mat 1996; 122–124:305–24.

[25] Zhu XL, Su XJ. Porous ceramics materials. China Ceram 2000; 36(4):36–9.

[26] Li JH, Lu AH, Song TB, Huang Y. Research on a new kind of environmental mineral materials: cordierite-matrix foam ceramic. Acta Min Sin 2001; 21(3):481–4.

[27] Ren XT, Zeng LK, Wang H. Investigation of the preparation technology of foam ceramics. Mat Sci Eng 2001; 19(1):102–3.

[28] Zhang YL, Li CD. Primary introduction to polymer foams. Hangzhou: Zhejiang Science and Technology Press; 2000.

[29] Wu XY, Xu JY. Polymer foams formation. Beijing: Chemical Industry Press; 2002.

[30] Qian ZP. Polymer foams. Beijing: China Petrochemical Press; 1998.

[31] Liu PS. Mechanical relation for porous metal foams under complex loads of triaxial tension and compression. Mater Des 2010; 31(4):2264–9.

[32] Liu H, Han CY, Dong LS. Research progress in structure-properties relationships of closed-cell polymer foams. Chin Poly Bull 2008; 3:29–42.

CHAPTER TWO Making Porous Metals

The making of porous metals has a long history. The first preparation of porous metals by the powder metallurgy process was reported at the beginning of the twentieth century. With the progress of technology and the emergence of new methods and processes, metals with porosity of 98％ or even more can be obtained today. However, metals prepared at the beginning of the twentieth century only had porosity as low as about 30％. Currently, a number of other porous metal preparation methods are available [1–5], such as sintering metal powders for the filter and melt foaming for the light porous aluminum. In practice, porous metals can be prepared by different processes, including powder metallurgy, melt foaming, electrical deposition, and infiltration. All these methods will be described in detail throughout this chapter.

2.1 POWDER METALLURGY

Porous metals were first prepared in the form of powder by sintering or other similar processes, and these metal powders maintain their solid state during the process. The sintered porous metals have either an isolated closed structure with low porosity or a connected open structure with high porosity. The framework is constructed by more or less individual spherical particles through connection of the sintered necks of particles. Sintering metal powders is the earliest approach to making porous metals, and it also has been the general production method used in the powder metallurgy industry.

Powder metallurgy is a process through which porous metals, composites, and other materials can be prepared by mixing powders, molding, and sintering [6, 7]. Porous products created by powder metallurgy were first mentioned in a patent in 1909, and similar patents concerning the preparation of porous filters by powder metallurgy were released in the late 1920s and early 1930s. The pore ratio, radius, and distribution of the porous materials prepared by powder metallurgy can be controlled effectively. For instance, there are near-dense materials, with porosity of less than 1–2％; semi-dense materials, with porosity of around 10％; porous materials, with porosity of＞15％; and more porous materials with porosity as high as 98％. Spherical powders are widely used to make porous materials through the typical powder metallurgy process, which has the advantages of easy control of the pore radius and good permeation. Accordingly, for the preparation of porous materials with high pore radius and permeation requirements, irregular shaped powders other than spheric powders shall be removed. However, for the preparation of porous materials with powders of a nonspheric shape, pore-forming agents like ammonium acid carbonate, urea, and methyl cellulose shall be used in order to increase porosity and permeation.

2.1.1 Preparation of Metal Powders

In general, preparing metal powders means to transform metals, alloys, or compounds that are in a solid, liquid, or gaseous state into powder. Metals and alloys in the solid state can be made into powders by mechanical crushing, electrochemical corrosion, and reduction of metal oxides or chloride. For metals and alloys in the liquid state, atomization, permutation reduction, and electrolytic methods can be applied. The condensation of gaseous metals, thermal dissociation of gaseous metal carbonyl compounds, and gas phase reduction of halide can be used to change gaseous metals to powder. The general methods are summarized in Table 2.1, the most widely used of which are atomization, reduction, mechanical pulverization, and vapor phase.

The general methods for the preparation of spheric powders are atomization, the carbonyl method, and gas deposition. For nonspheric powder preparation, in addition to the alloy ingot crushing and ball milling processes, nonspheric metal powder mixing followed by alloying and crushing processes can be used. The refractory metals and alloys are not easy to make into spheric powders, and the spheroidizing treatment can be applied if necessary.

The following are brief discussions of atomization, mechanical crushing, reduction, vapor phase, and liquid phase methods [6, 7].Atomization

Atomization, also called the spraying method, is a process in which molten metals are broken into small drops of liquid by high-speed fluids (gas as air or inert gas; liquid as water) or fluids with centrifugal force, and then solidified into powder. The schematic diagram for the spraying process is shown in Figure 2.1 [7]. Pb, Sn, Al, Zn, Cu, Ni, Fe metal powders, Cu-Zn, Cu-Sn, alloyed steels, and stainless steels (Figure 2.2 [6]), and bronze and Ni spherical powders can be made by the spraying process.Table 2.1　Preparation Methods for Metal Powders [6, 7]Table 2.1　Preparation Methods for Metal Powders [6, 7]—cont'dMechanical Crushing

Mechanical crushing is not just an independent powder preparation process; it also is a supplementary procedure in some other powder preparation processes. It uses mechanical forces like crushing (pulverizing, rolling, and jawing), striking (with hammer or similar tools), grinding (with ball and rod), and then breaking the large blocks and particles into powder. The pulverizer, double-roller, and jaw crusher can make large particles, and then a further fine-down process is required to make the powders into porous metal. Much finer powders can be produced by hammer mills, rod mills, normal ball mills (Figure 2.3), vibration ball mills, or stirring ball mills [7]. In the ball milling process, the balls are generally made of corundum, with great hardness and strength, and it takes place in air or in water, alcohol, gasoline, or acetone liquid.Figure 2.1　Schematic diagram of the molten metal atomization process.Figure 2.2　Stainless steel spheric powder created by gas atomization (× 300).Figure 2.3　Materials in a ball miller at different rotation speeds: (a) low speed; (b) appropriate speed; (c) critical speed.Reduction

Reduction is a widely used method to generate powder by reducing metal oxides or chlorides. As the reducing agent, solid carbon can be used to prepare Fe and W powders. H, H+N, or both are 22used to produce W, Mo, Fe, Cu, Co, and Ni powders. Transformed natural gas (H or CO) can be used for the preparation of Fe powders. 2And Na, Ca, and Mg metals are used for the preparation of rare metal powders like Ta, Nb, Ti, Zr, Th, and U.Vapor Phase Deposition

The following methods can be used to prepare the metal powders:

1. Metal vapor condensation: This method is used with alloys with low melting points and high vapor pressures to produce Zn and Cd powder.

2. Thermal decomposition of carbonyl: In this process, metal powders can be created by decomposing a metal's carbonyl compounds.

3. Gas reduction: This method includes the gaseous H reduction and the gaseous metal thermal reduction. In fact, it also can be part of the second method, because thermal decomposition of carbonyl is one important way of obtaining the raw powders (like Ni, Fe, and Co) to prepare porous metals, particularly for microporous filter/separation products. These transition metals can react with CO to form metal carbonyl compounds [like Me(CO)] that are either in the liquid state n(which tend to evaporate), or in the solid state (which are easy to sublimate). For instance, Ni(CO) is a colorless liquid with melting point 4of 43°C, Fe(CO) is an amber liquid with melting point of 103°C, and 5Co(CO), Cr(CO), W(CO), and Mo(CO) are all crystals of easy 28666sublimation. Also, these carbonyl compounds have the tendency to decompose into metal powders and CO. The reaction of carbonyl compounds isMe+nCO→Me(CO) (2-1)n

For instance, nickel carbonyl can be formed byNi+4CO→Ni (CO) (2-2)4

The decomposition of carbonyl compound isMe(CO)→Me+nCO (2-3)n

and nickel carbonyl can decompose intoNi (CO)→Ni+4CO (2-4)4

This decomposition is an endothermic reaction. In the decomposition temperature range, the higher the temperature is and the higher the decomposition rates are, the more crystal nuclei form and the finer the particles will be. The gas released from the thermal decomposition is toxic, in that CO can be absorbed by the cuprammonium solutions and then purified for recycling.Liquid Phase Deposition

Liquid phase deposition, like metal replacement, gas reduction in solution, and thermal reduction in molten salts, can be performed in different ways. Metal replacement is a process in which one metal takes the place of another in a water solution. And thermodynamically, only metals with higher negative potentials can replace metals with higher positive potentials, and the reaction is2+2+(Me)+Me→Me+Me). 　(2-5)1212

For instance,2+2+Cu+Zn→Cu+Zn.　(2-6)

In this way, Cu, Pb, Sn, Ag, and Au powders can be prepared.

CO, SO, HS, and H can be used as the reductant in solution in 222the gas reduction method, in which H is more popularly used. The 2reaction isn++Me+(1/2)nH→Me+nH.　(2-7)2

For example,Ni (NH) SO+H→Ni+(NH)SO+(n－2)NH.　(2-8)3n424243

In this way, Cu, Ni, Co, and Ni-Co powders can be prepared.

Sedimentation in molten salts achieves a thermal reduction of the metals. For example, Zr powders can be reduced and broken down after cooling through mixing ZrCl4 and KCl and adding Mg and increasing the temperature to 750°C, and then they are treated with water and HCl.Spherization of Powder

At times, the further spherization of nonspheric powders is necessary to make porous materials. Spherization processes include vertical spherization, plasma spherization, and inert liner spherization. In the vertical spherization process, metal particles are heated to temperatures higher than their melting points, melted in a furnace, and then allowed to fall freely due to surface tension. The molten drops are spherized and form spheric powders after cooling.

In plasma spherization, nonspheric powders are melted in the plasma beam and then sprayed into the water trough to form spheric particles. Generally, N was used to transport the powder. Due to the 2much higher plasma arc temperature, it is more practical to use this process to produce metal powders with higher melting points.

For the inert liner spherization process, metal or alloy powders are mixed with an inert filler (like AlO powder) and then heated in a 23nonoxidative temperature until they melt. The spheric particles form due to surface tension and then are separated from the inert filler after cooling. Various fillers shall be applied to the different metal powders. For instance, Cu uses carbon black as the filler, whereas Fr and Ni will take MgO and AlO as fillers.23

The properties of porous metals produced by powder metallurgy are related to the size and shape of the particles to a high degree. The grading of particle sizes can be realized by vibrating screens, compressed gas flows, powder sedimentation rate interaction, and gas discharge enrichment separation due to specific surface charges. The spheric and nonspheric powders can be separated by the discharge on the different specific surface areas (higher specific surface areas with the nonspheric particles), as well as the centrifugal separation process due to the different friction forces (smaller friction forces for the spheric particles). More details will not be given here; sources with further information are given in the “References” section at the end of this chapter.

2.1.2 Molding of the Porous Body

There are three types of molding of the porous body [6]:1. Pressure molding: The powders can be molded under pressure,

and they are deformed into green bodies under high pressure by

pressing, extruding, and rolling.2. Non-pressure molding: The powders can be molded without

pressure via various methods, like powder slurry pouring and

loose sintering.3. Other molding: This category includes methods like spraying,

vacuum deposition, and other forming processes.

The selection of which molding method to employ depends on the shape and size of the final product and the property of raw materials. Mold pressing can be applied to small parts with simple shapes to be produced in large numbers. Extrusion molding is more suitable for the continuous production of tubes, bars, and rods with uniform pore ratios. Isostatic pressing can be used to obtain the green body of a uniform structure with binders as additives, and then the green body can be subjected to machining and finally made into complex-shaped and large products. The rolling of powders can make porous plates and belts continuously, and the various products can be formed with further rolling, welding, and clipping. Slurry pouring is used for molding when the raw materials are metal fiber, finer spheric, and nonspheric powders. Complex-shaped, large products with uniform pore ratios can be produced by slurry pouring, and loose sintering is used to mold spheric powders.

The sintering process is the key procedure of making porous products, and it should be strictly controlled. The green body with a pore-forming agent shall be heated slowly during the sintering process to avoid cracking from the volatilization of the agent. If the sintered porous parts need further machining, metals, alloys, plastics, or resins are immersed into the porous body for ease of processing and achieving precise control of the size. The metals and alloys shall have low melting points and are insoluble and not chemically reactive with the porous body. The immersed metals, alloys, plastics, or resins shall be removed during heating to avoid blocking the pores. For the economical preparation of porous parts with corrosion resistance, porous irons or low-carbon steel is prepared and then subjected to Sn, Cu, Ni, and Cr plating or the vapor-phase Cr plating process. Alternatively, they can be made using the obtained plated powders.Press Molding

Porous tubes and sheets can be made by press molding, and the main equipment required for this process is the pressing machine. During molding, the additives have the following features: (1) the proper viscosity to achieve the required strength; (2) lubrication for the demolding process; (3) sufficient pore-forming ability to meet the requirements of the pore ratio; (4) no harmful residues are left after sintering; (5) being in liquid or solid state with a low melting point, or with the ability for solvents to mix with powders; (6) lack of reaction with the powders and lack of damage to the facility's components in the heated atmosphere. Depending on their roles in the molding, the additives can be lubricants, binders, pore-forming agents, or plasticizers. The lubricants in general use are oil, glycerin, petroleum jelly, stearate, sulfate, oxide, and graphite. The binders are resin, amylum, and polythene alcohol. The pore-forming agents include ammonium acid carbonate, ammonium carbonate, sodium carbonate, organic fibers, granule of carbon, naphthaline, urea, fiber, plastic, and sawdust (like TiH, ZrH, CaCO, NaCO, and KCO). The 2232323plasticizers are olefin and beeswax. The additives are selected based on the characteristics of the powders and the pressing requirements, and they normally are dissolved in an organic solution (such as gasoline, benzene, acetone, alcohol, or carbon tetrachloride) and then mixed with the powders.Isostatic Pressing

There can be both cold and hot isostatic pressings. For cold pressing, water or oil is used as the pressure medium, while Ar gas is used as the pressure medium for hot pressing. Usually, the preparation of porous materials is conducted by cold isostatic pressing. The fluid medium is pressed into a sealed steel container with high-pressure resistance by using a high-pressure pump. The resulting high pressure will be applied to the powders in the elastic mold isotropically at the same time. The friction from powder/powder, powders/mold wall is small enough and then the green body with uniform density will be produced. Molds used in isostatic pressing shall meet the following requirements: (1) the original geometrical shape must be maintained in loading powders, with high strength and certain elasticity; (2) there must be high abrasion resistance and easy machinability; and (3) there must be no reaction with the powders physically and chemically. Natural and synthetic rubbers are generally used as mold material, and they are now gradually substituted by the plastic due to the problem of deformation and wrinkling after contacting with the mineral oil for the rubber. Thermal plastic soft resin is one of the important materials for mold application; its softness and hardness can be adjusted by the composition and content of the plasticizer. A typical recipe used to make molds in China is as follows: 100 portions of PVC resin (in weight), 100 di-octylphthalate (or dibutyl phthalate), 3–5 tribasic lead sulfate, and 0.3 stearic acid [8]. The loaded sealed mold is sheathed by the porous metallic tube and then put in the highpressure container. Next, the pressure is increased slowly to avoid creating internal soft parts in the green body. The applied pressure cannot be reduced quickly, or else the green body will crack due to the expansion of compressed gas in the green product.Rolling of Powders

Green bodies can be prepared when the metal powder continuously passes between a pair of rollers in contrarotation and undergo pressure from them. The final porous sheet products will be obtained after the rolled green body completes the pre-sintering and sintering, rolling, and heat-treatment processes. Essentially, the metal powders with rolling capability are loaded in a specially designed funnel to the required height and then are fed into the rollers continuously due to the action of external and internal friction between powder and roller and powder and powder. Three zones can be seen during the movement of the powders (Figure 2.4) [7]:● Zone I: A free zone from the gravity effect on the powders● Zone II: A feeding zone from the interaction of powder and rollers● Zone III: A rolling zone at which a green belt with a certain density

and strength is produced from the loose individual powders

Zone III covers the feeding of powder into the roller and the production of the green belt from the rollers. Usually, the porous belt is produced by cold rolling, as demonstrated in Figure 2.5 [6]. During rolling, the rolling speed and feeding speed must be compatible to prevent damage to the final product.Figure 2.4　Schematic diagram of the powder-rolling process: Ⅰ—Free zone; Ⅱ—feeding zone; Ⅲ—rolling zone.Figure 2.5　Porous belt-rolling process.

The rolling properties for these powders include the plasticity, moldability, and fluidity, which have great effect on the rolling process. The density and thickness of the final green belt product decrease with the low fluidity of the powders, increase with the high apparent density of the powders, and significantly increase with the height and the applied force on the powders. However, as the apparent density of powders increases, the bending property of the final green belt product will be reduced. This is because the mechanical engagement strength between the powders will decrease if the size of the powders decrease and the roundness of the powders increase for the high apparent density of the powders. With the fixed feeding speed and roller gap, an increased rolling speed will reduce the green body's density and thickness. Moreover, if a low-viscosity gas like H is applied during 2rolling, the density and thickness of the green body increase. Sintering of the green body can be performed in a protective atmosphere like H, 2in an inert atmosphere like Ar or He, or in a vacuum.lastification Extruding

Extrusion molding (also known as plastification extruding) is a process that the stack of powders or the green body in die is pushed out to assume another form of green body or other final product under pressure. The cold extruding process is applied to mixtures of metal powders and organic binders, and extruding is performed at low temperatures (40–200°C) to form the green body. The processes include material preparation, preprocessing, extruding, cutting, and reforming. The porous products can be obtained after drying, pre-sintering, and sintering of the extruded green body. It is an effective way to produce a long porous tube with a small diameter.

The pretreatment of a mixture under pressure involves making full contact between the plasticizer and particle surfaces, to remove the gas inclusion and finally to ensure uniform density. Plasticizers have a large effect on a material's properties. Therefore, certain requirements are needed for them, including that there should not be any reaction with porous materials during sintering, and that they should be removable, sticky, and have great pore-forming ability. The plasticizers in common use are olefin, amylum, and polythene alcohol. Powders will be subjected to pressure from the side wall, friction from either the powder and the wall or the extrusion shaft and the wall, in addition to the normal compression from the extrusion shaft. The key factors affecting the properties of green body extrusion are the types of powders, the particle shape and size, the plasticizer type and content, the precision of the mold, the pressure from extrusion, the extrusion speed, and the preheating temperatures.

The selection of the preheating temperature depends on the optimal plastic used for the plasticizer at the selected temperature. The extrusion speed can be determined experimentally, and it is closely related to the particle size, shape, extrusion ratio, fluidity, extrusion force, and plasticizer. Higher extrusion speeds may cause the green body to crack.Slurry Pouring

The slurry pouring process were better suited to porous products with complex shapes. It requires simple facilities with low cost, needs a long production cycle, and has low productivity. The powders or fibers are first prepared as slurry (suspension), poured into a plaster mold, dehydrated for some time, and finally dried and sintered to obtain the porous product [7]. The slurrypouring process is illustrated in Figure 2.6.

The slurry is composed of the metal powders and a liquid solution of water mixed with additives. These additives are binder, dispersant (stabilizer), degasifier, and titrant. The presence of a binder contributes to the viscosity of slurry as well as the binding of the powders after drying. The binder shall not react to the powders and plaster, and less residue shall be left after sintering. The binders commonly used in this process are alginic acid sodium and poluthene alcohol. Dispersants can prevent the agglomeration of the particles and help form the stable suspension to improve the wetting of powders with liquid and to control the sedimentation speed. A certain amount of ammonium hydroxide, hydrochloric acid, ferric chloride, and sodium silicate mixed in water can be a perfect dispersant. Alginic acid sodium is another common dispersant. The titrant is used to control the pH value and the viscosity of slurry. Caustic soda, ammonia water, hydrochloric acid, and ferric chloride can be used as titrants. Degasifiers can remove the absorbed gas on the powder surface, and n-caprylic alcohol is widely used for that purpose. In addition, the gas can be removed in a time-controlled method, in which the stirred slurry is put aside for some time and the gas escapes due to the air density difference. The vacuum treatment is also a good way to remove the gas in slurry.Figure 2.6　Diagram of slurry pouring: (a) Plaster mold; (b) pouring of slurry; (c) dehydrating of slurry; (d) molding.

The sedimentation speed of powders in the slurry, its liquid/solid ratio, the dehydrating rate of plaster, and the viscosity, pH value, and stability of slurry are all factors that influence the quality of the poured product. The liquid/solid ratio of slurry is the weight ratio for the water versus metal powder, and it determines the viscosity of slurry and the sedimentation speed. The smaller the liquid/solid ratio is, the higher the viscosity is and the slower the sedimentation speed is. However, if the liquid/solid ratio is too small, the slurry is more difficult to pour. If the pH value of the slurry falls below a certain value, good fluidity will be obtained and the agglomeration of particles is prevented. The lower sedimentation speed of slurry is well suited to the preparation of parts with complex shapes and small cross-section areas.

The metal powders and foaming agents can be mixed to form slurry and then poured into the mold [8]. The metal powders can be Ni, Fe, Al, Cu, brass, and stainless steel; and the foaming agents can be hydrochloric acid, hydrafil, and orthophosphate.

2.1.3 Sintering of the Porous Body

The purpose of sintering is to control the microstructure and property of a product. Technically, it can be regarded as a heat treatment—that is, the semifinished product is heated to the temperature below the melting point of its main materials for a period of time and then cooled down to room temperature. After sintering, the agglomeration of particles will change to the agglomeration of crystals. At last, materials or products with the desired physical and mechanical properties are obtained. Sintering is different from the solid reaction since sintering may have some chemical reaction or even have no chemical reaction at all. Many types of migration processes exist. The following migration phases during sintering of a pressed green body can be described in terms of the pore change in the porous materials: the initial combination between particles (adhesion and linkage of unsaturated bonds on the particle surface), the growth of a sintered neck, and the shrinking and coarsing of pores. Based on the appearance of the liquid-and sintered-phase compositions, the process can be divided into single-phase sintering, multiphase solid sintering, and multiphase liquid sintering (sintering at temperatures above the low melting point of the elements).

Single-phase sintering can be regarded as a solid-state reaction that is determined by the change in the system energy state. Multiphase sintering is influenced by the alloy's thermodynamics. Both sintering processes exhibit the free energy reduction of the system as the driving force, including (1) the reduction of the total surface area and the total free energy of the surface led by the increased powder reaction area (sintering neck) and the flat powder surface; (2) the reduction of the total volume and total surface area of the pores in the sintered body; and (3) the elimination of lattice distortion in the powder. The grain boundaries might move through recrystallization or polycrystallization, and the number of grain boundaries will decrease. The total surface area of the pores tends to decrease due to the cylindrication of the metal frame or spheroidization of the pores, regardless of the change of the total pores. The closed pores stop shrinking when the inner pressure exceeds the surface tension.

At the early stage of sintering, the required activation energy is low because van der Waals forces exist among the powders and no obvious atom displacements are required. Other migration processes such as diffusion, evaporization and agglomeration, and flowing can occur only at high enough temperatures or under high enough external forces because the required activation energy is high.

In general, the sintering temperature is the highest one that will be maintained during the sintering process.Migration Mechanism During Sintering

The migration mechanism of sintering involves several elements, as follows:

1. Viscous flow: According to this model, the sintering process includes two stages: increase of the contact surface area between the adjacent particles, and gradual reduction of the size of the closed pores formed. Atoms and interstices in the crystal preferably will move along the direction of the surface tension, and the migration volume is in proportion to the surface tension.

2. Evaporation and agglomeration: Inside the powder, the vapor pressure at the convex area is large, while it is small in the concave area. The atoms evaporate from the convex surface and agglomerate again at the concave surface, such as the sintering neck, due to the pressure difference. The vapor pressure at the convex and concave surface has the following relationship with the radius of curvature (Kelvin equation):

where p is the vapor pressure at the convex and concave surface; p is vapor pressure at the flat surface; g is the surface tension; r and 01r are the two principal radii of curvature surfaces (positive at convex, 2negative at concave, infinite at flat); r is the solid density; M is the molecular weight; and R is the Mol gas constant.

3. Volume diffusion: With high density of the interstices at the contact surface of the particles, the atoms migrate toward the contact surface by exchanging positions with the interstices to make the sintering neck grows. At the specified temperature, the interstices density is in proportion to the surface tension.

4. Surface diffusion: The migration of the atoms on the particle surface will expand the contact surface, and the concave surface will be flattened. Essentially, the sintering of powders is a thermodynamic phenomenon due to the extremely high surface area and high surface energy. At low or mid-level sintering temperatures, the surface diffusion dominates, while at higher temperatures, the volume diffusion is preeminent. The smoothness and roundness of the closed pore will be promoted due to surface diffusion. The diffusion of atoms along the surface of particles or pores is mainly the vacancy mechanism, since its activation energy is much lower than that of the interstice and transposition of atoms. The vacancy will migrate from the concave to the convex area, while the atoms migrate to the concave area and the sintering neck due to the vacancy intensity and chemical potential differences on the surface with different curvatures.

5. Grain boundary diffusion: The grain boundary can “trap” the vacancy during vacancy migration. The activation energy for the grain boundary diffusion is only half that of volume diffusion, and it will be much less with decreasing temperatures. The pores close to the grain boundary always disappear or reduce in number, and the grain growth for the metals during sintering is accompanied by the movement of grain boundaries and pore disappearance. The grain boundary moves from the concave surface, with high energy, to the center of curvature, with low energy. The surplus surface energy at the grain boundary is also the driving force for the grain growth.

6. Plastic flow: A row of atoms will move or the crystal planes will slide with the generation of dislocations in the crystal caused by surface tension. The sintering is analogous to metal diffusional creep. High-temperature creep is a process of the continuous microdeformation for metals under constant low stress (driving force). The surface tension (driving force) will decrease during sintering, and then the sintering rate will slow accordingly.

7. Combined theory of sintering: In fact, the abovementioned mechanisms will play simultaneously or alternately in the same sintering process. The sintering of powders with high vapor pressure is conducted through the vaporation and agglomeration mechanisms. The surface and grain boundary diffusion mechanism is popular for sintering at lower temperatures or for sintering of ultrafine powders. For isothermal sintering, surface diffusion contributes only to the formation and growth of the sintering neck and pore spheroidization, not to the shrinkage of the sintering body. The grain boundary diffusion always accompanies volume diffusion and helps the densification of the sintering body. At much higher temperatures, the volume diffusion is predominant for most metal and compound crystal powders. The distinct shrinkage of the sintered body is the result of volume diffusion.

There are many sintering mechanisms, and the driving forces always come from surface tension. The main barrier for the grain growth and movement of the grain boundary in sintering are the presence of pores, and other barriers include the secondary phases and grain boundary groove.Influential Factors in Sintering

The influential factors involved in sintering include the following:1. Metal powder type: The intial sintering temperature will increase

with the reduction of crystal lattice symmetry.2. Powder activity: The diffusion (grain boundary) is promoted with

much smaller and finer grains. The higher activity for the ball-

milled particles is due to the generation of crystal defects, reduced

particle size, and increased total surface area.3. Oxides on the powder surface: When a thin layer of oxides

(smaller than a certain thickness) is formed on the surface, it is

prone to sintering due to the quick reduction of the oxides to the

metals and the increased activity. In addition, the diffusion and

sintering will be hindered with the thicker layer of oxides or the

lack of reduction in the oxides.4. Additives: Diffusion and sintering can be accelerated if the

additives can form a solid solution with the powders to reduce the

sintering temperature due to activation by crystal lattice distortion.5. Sintering atmosphere: Vacuum sintering can be done with most

metals; however, it will cause more loss of metals due to the

volatilization and deformation of the final products. Some

additives may be introduced to activate the powders in the

sintering atmosphere. The physical effect of the atmosphere is

that the gas compositions and properties in the pores are different

and demonstrate different diffusivities and solubilities in solids in

different sintering atmospheres. The chemical effect of the

atmosphere refers to the chemical reaction between the gas and

the sintering matter. In a sintering process controlled by positive

ion diffusion, for instance, it is advantageous that it take place in

an oxygen atmosphere or under higher oxygen partial pressure;

this is because of the increased positive ion vacancy from the

excessive negative oxygen ions. It is favorable for sintering with

any contributions to the diffusion.Features of Porous Material Sintering

Porous materials require a certain porosity and strength. Therefore, powders with narrow size ranges and spherical or near-spherical shapes shall be used to prepare them, and pore-forming agents are usually added to the powders. No shrinkage is expected for the loosely compacted or premolded green body after sintering; that is, there are no changes of porosity or pore sizes after sintering. As indicated in the sintering model of porous materials by powders, the atoms at the contact area of powders will leave their crystal lattices and then diffuse to form the intial bonding at the temperature of 0.4 T m(i.e., the melting point of the metal powders). At a temperature of 0.5 T, the atoms on the free surface at the convex area will migrate mtoward the neighboring powders to form the sintering neck. The growth of the neck needs more atoms to migrate without affecting the porosity (that is, no shrinkage of the sintered body occurs). The connections of the pores continue to exist and the growth of the neck leads only to the smoothness of the pore channels. Finally, the pore channels will become stable with the growth of the neck and the progress of sintering. It is then known from this model that the ideal porous body with round channels can be obtained at low temperatures (about 0.5 T) and over a long period of sintering.mSintering Methods for Porous Materials

There are several methods of sintering porous materials:

1. Sintering of molded powders: In this commonly used sintering process, the mixture of metal powders and pore-forming agents is loaded in the preformed body and then heated in a reductive atmosphere. During sinterting, the organic materials decompose and the atoms diffuse and combine to form the porous metals. The metal powders can be Al, Mo, Mo alloy, W, W alloys, or mixtures of these materials.

2. Loose sintering: The powders are loaded into the mold for sintering without any other pressing (though shaking may be applied). They make contact with one another from the effect of capillarity and the surface tension during sintering. It is mostly used in the production of porous filter materials with more permeation and low purification, sound and thermal insulation porous materials, and sealing materials. The mold materials used for this process shall not react with the powders and have enough high-temperature strength and stiffness. The thermal expansion coefficient of the mold materials is also as close as that of the powder materials for sintering.

Bronze (Cu-Sn alloy) filters are usually produced by this route. It can also be used to make brass (Cu-Zn alloy) filters, as well as the Ni diaphragm used as the electrode of alkaline batteries and fuel cells, which has a porosity of 40％–60％. The higher porosity can be achieved with the addition of pore-forming agents. The filter of Fe, Ni, Cu, and their alloys. In some cases, porosity of 70％–90％ can be obtained if ammonium chloride and methyl cellulose are used as pore-forming agents [8].

3. Activated sintering: For the sintering of metal powders with high melting points, much higher temperatures and longer periods of time are needed. If the activator is used or the activating treatment is applied, the sintering temperature can be reduced and the sintering time can be shortened. Essentially, activated sintering can reduce the activation energy in the flow, diffusion, vaporization, and condensation processes thermodynamically and then increase the reaction rate.

Activated sintering can be conducted physically and chemically. In the physical method, the alternating magnetic field, high-energy particle radiation, static loading, ultrasound vibration, and periodical sintering around the allotropic transformation temperature are applied to consolidate the sintering. In the chemical way, the hydride, the reactive gas, the trace elements, preoxidation, and periodical oxidation and reduction are applied to consolidate the sintering. Sintering of porous materials is mostly prepared chemically and it is based on reduction and dissociation in the chemical reaction. The newly formed atoms of Ti and Zr will be dissociated from Ti and Zr hydrides in sintering. These Ti and Zr atoms are more effective in sintering than pure Ti and Zr. The most effective way is to introduce halide (HCl) vapor in the sintering atmosphere for the chemical method. However, it may cause corrosion to the products and the equipments. It is necessary to eliminate halides thoroughly with hydrogen after sintering. Moreover, it is also effective to add a small amount of alloying elements. For instance, the addition of less than 1％ of elements from iron family members (like Ni) or platinum family members to W and Mo powders (or fibers) will lower the sintering temperature considerably. However, it is not a good way to mix mechanically with Ni or Co as an activator since an activating layer cannot be formed on the metal particle surface. Therefore, these additives of the alloying elements shall be in the state of their chlorides or other substances and then introduced into the solution. After that, the metal powders are mixed into the solution and dried and a thin layer of oxide will form on the particle surface.

For the mechanism of activated sintering, it is generally believed that volume diffusion is predominant, but the grain boundary and surface dif-fusion also act in the process. Due to the different diffusion coefficients for the metal elements, the vacancy defects left in the surface area of particles may contribute to the migration of atoms.

In order to increase the permeation property for the iron-base filter, the Cu, Fe, and Ni chlorides and phosphates are usually used in activated and TiB2sintering, and the halides also can be added and dissociated to activate the sintering. Metal powders with low melting points like P, B, Ag, Cu, and Sn are used in the activated sintering of stainless steels in the industrial hydrogen atmosphere. For the activated sintering of TiC, WC, ZrB2, and TiB spheric powders, 5％ 2polyethylene resin in alcohol is used as the plasticizer and 3％ CoCl 3is used as the activator.

The sintering temperature of refractory metal fibers can reach 0.95 T, so activated sintering is more meaningful for the refactory mmetal.

4. Electic spark sintering: The spark discharge occurs among powders while applying a mid-or high-frequency alternating current (AC) and direct current (DC) to the powders, and then increasing to a high temperature. The spark discharge could last for 15 seconds, and the sintering process can be completed within a few minutes. This method can be used to make porous metals and refractory metals with high melting points.

5. Liquid phase sintering: The migration of atoms in the liquid phase is faster than that in the solid phase, if the components with low melting points melt, or form the eutectic phase with low melting points at the sintering temperature. The driving force of liquid phase sintering is surface tension in the liquid phase and interface tension in the solid-liquid phase. The typical application of this method for making porous materials is the Cu-Sn porous body.

Sintered bronze is the earliest porous material used for anti-friction purposes, and it contains 10％ Sn. Sometimes 1％–3％ graphite or less than 3％ Pb are added to it to enhance its anti-adhension and–friction characteristics [7]. The mixture of the powders or atomized pre-alloyed powders is first pressed and then sintered in the protective atmoshphere (reductive gas or solid carbon stuffing) at 800°C–850°C to form a product with 20％–30％ porosity. At the final stage of sintering, the Cu-Sn liquid phase disappears. Cu and Sn are mutually soluble to form a series of transional phases (electron compound) and the corresponding finite solid solutions, see Figure 2.7. For example, in the sintering process with an alloy with 10％ Sn, the Sn powders start to melt when the temperature increases to 232°C; furthermore, the melted alloy flows and fills in the interstices of the Cu green body. Cu will dissolve in the liquid Sn to form the η phase through a eutectic reaction (–60％ Sn). as temperature increases further, Cu continues dissolving until the ε phase (38％ Sn) forms at 415°C by the peritectic reaction, and the liquid phase increases correspondingly. Therefore, Cu keeps dissolving with increasing temperature until the remelting reaction temperature (640°C) is reached. The ε phase changes to the γ phase, and then the liquid phases reduce significantly. When the temperature reachs 755°C, the γ phase will change to the β phase by the peritectic reaction and the liquid phase will reappear. When the sintering temperature is higher than another peritectic reaction temperature (755°C), the β phase will decompose again and finally form the Cu-base α solid solution. From the critical temperature point in the phase diagram, the stable liquid phase can be formed only at temperatures above 850°C for alloy powders with 10％ Sn. The stable liquid phase also can be formed at lower temperatures with higher content of Sn (＞10％ Sn). The α and ε phases form the equi-librium structure after cooling. In fact, the phases at room temperature could be the unhomogenized α phase and a small amount of high-temperature δ phase when the mixture of powders are used without enough diffusion.Figure 2.7　The phase diagram of Cu-Sn.

Cu can be dissolved quickly in the Sn liquid phase. Specifically, Cu can reach its satuation state in the molten Sn when the size of the Cu powders is very small (＜15 μm). With increasing temperatures, the γ phase forms and then the liquid phase reduces or disappears. Before the disappearance of the liquid phase, the sintering progresses quickly and the density increases due to the dissolution of Cu. With the formation of the γ phase, sintering occurs in the solid phases. For the sintering that takes place above the peritectic reaction temperature (798°C), the homogenization of the α phase will be finished via a liquid phase and lead to expansion until the temperature reaches 820°C. Then it shrinks as the temperature increases further. The liquid phase diffuses into the α phase and disappears after the peritectic reaction (β→α+liquid), and the dissolved gas (H in Cu) in the liquid phase during 2the solidification process is expelled and then the pores are left in the alloy. Therefore, the heat preservation shall be conducted at the peritectic reaction temperature to facilitate the diffusion and solidify the liquid phase slowly. After that, sintering at temperatures above the peritectic point will not lead to expansion.

6. Slurry foaming sintering [2, 3]: Slurry is prepared by mixing the metal powder, foaming agent, and organics; after that, the finished slurry is heated and foamed, and finally the solid porous materials are obtained. This process is used to prepare Be, Ni, Fe, Cu, Al, stainless steel, and bronze porous materials.Sintering Process

1. Sintering temperature and holding time: The determination of the sintering temperature is related to the compositions, particle size, surface state, and the property required for the product. As mentioned previously, the ideal sintering temperature is 0.5 T and a long msintering period is needed. Considering the strength, hardness, toughness, ductility, porosity, and particularly mechanical strength, the sintering temperature shall be above 0.6–0.8 T. For powders that are mprone to oxide formation on the surface (e.g., chromia and titania on stainless steel powders), it can be reduced only in pure hydrogen at a high temperature.

For pure metal solid solutions, the sintering temperature is 2/3–3/4 T. For example, the sintering of Fe takes place at 1000°C–m1200°C, and for Cu at 700°C–900°C. For sintering of mixed metal powders, the sintering temperature is generally lower than that of the main composition, or a little above the low melting eutectic temperature according to the phase diagram. The finer the powders are, the more active the powder surface is, and then the lower the sintering temperature is. The sintering temperature and holding time varies with the different physical and mechanical requirements for the product.

The holding time during the sintering process depends on the temperature, the required porosity, and the pore shape [6]. With the required porosity, the holding time is short if sintering occurs at higher temperatures, and it is longer at low temperatures. In practice, the sintering temperature and holding time can be determined by experimentation. Genenrally, low sintering temperatures and short holding times are preferred for sintering in order to reduce the requirements and increase the productivity of the sintering facility.

It is less important for the calculation of shrinkage of porous materials than that of powder metallurgy alloys. Sintering is always controlled to minimize the amount of shrinkage as a porous body was produced. Sometimes pore-forming agents are used in sintering.

2. Sintering atmosphere: The proper sintering atmosphere (reductive, neutral, inert, vacuum, or air) needs to be in place to achieve the required physical and mechanical properties. No oxidation of the powders happens and the oxides in the mixtures must be reduced during sintering. Gas desorbtion, removal of impurities, reduction and dissociation of oxides, migration of gaseous metal, interaction of gas and sintering materials (formation of stable and unstable compunds), and surface diffusion will be influenced by the atmosphere.

The sintering atmosphere controls the chemical reaction between the powders and gas and the removal of decomposed lubricants. For example, it is used to prevent or reduce oxidation and decarbonization of the porous body; to remove the absorbed gas, surface oxides, and inclusions; and to maintain or change the effective compositions in the sintered body, such as the carbon control, nitriding, and preoxidation sintering of steels. Depending on their function, sintering atmospheres can be divided into five types:1. Oxidative atmosphere: Pure oxygen, air, and water steam2. Reductive atmosphere: Pure hydrogen, decomposed ammonia,

CO, and transformed gas (mixture of H and CO) from carbon 2

hydrides3. Inert or neutral atmosphere: N, Ar, He, and vacuum24. Carburization atmosphere: CO, CH, and other carbon hydrides45. Nitriding atmosphere: NH and N for sintering of stainless steels 32

and Cr-steels

The same atmosphere might be neutral, reductive, oxidative, carburizing, or neutral/decarburizing depending on the metal involved. For instance, CO and water steam is neutral to Cu, but oxidative and 2decarburizing to carbon steel; H is decarburizing to carbon steel, 2while CO and CH are carburizing; and N is neutral to most metals, 42but nitriding to Cr, V, Ti, and Ta. The most widely used gases in the sintering atmosphere are reductive or protective gases containing H 2and CO since they are reductive to most metals.

The sintering of porous materials is conducted most frequently in the reductive atmosphere or in vacuum to prevent the oxidation of metals and to purify the sintered body through removing the absorbed gas, oxides, and impurities. If chemical heat treatment and sintering are combined, the sintering atmosphere can realize the alloying, carburization, and nitrding processes simultaneously.

Different sintering atmospheres are needed for different materials. The noble metal powders and stable oxide coated powders can be sintered in air, while Cu, Fe, Co, Ni, W, and Mo metals/alloys must be sintered in a reductive atmosphere to reduce the surface oxides [7]. The most common reductive gases are transformed coal gas, decomposed ammonia, and H. Co, H, and transformed coal gas can 22be used to sinter Cu base alloys. No oxygen or water steam is allowed in the sintering atmosphere for the sintering of metals/alloys with high affinities to oxygen (e.g., Cr, Be, Mn, and stainless steel). The small presence of oxygen or steam can react with the sintered body to form oxides and then impede the sintering process and reduce the plasticity of the sintered body. The sintering of these metals and alloys must be con-ducted in a highly pure and protective atmosphere. Vacuum sintering is applied to metal products that are prone to absorbtion or dissolution of the gas in the atmosphere, such as Ta, Nb, Ti, and Zr. It also contributes to the vaporization and decomposition of the inpurities in Ti and Zr powders in a vacuum. In addition, the impurities of C and H will be vaporized in a vacuum and is helpful to the reduction of oxides at high temperatures. The Si, Al, Mg, and Ca impurities and their oxides can also be removed in a vacuum and their materials purified. However, it is also a concern that the vaporization loss of the metals during liquid-phase sintering in a vacuum will change the final compositions and micristructures of the sintered alloys; meanwhile, vaporization could impede the sintering process.

The metallic Ti can react with H, O, N, and C easily; therefore, Ti is usually sintered in a vacuum or in a highly pure Ar atmosphere. -3Under vacuum conditions, the degree of the vacuum shall be 10 –-410 mmHg and the sintering temperature shall be 800°C–1350°C. Ta and Nb can easily absorb H, O, N, and C and then become brittle. So -3-4they are sintered in vacuums of higher than 10 –10 mmHg.

3. Filler in sintering: A high content of lubricants or pore-forming agents is included in the pressed green body for the preparation of porous materials and a large amount of gases or evaporized materials will be released. Therefore, fillers are added to sustain the green body, reduce the release rate of the gases or evaporized materials, and absorb the fluid with a low melting point. Otherwise, collapsing, cracking, bubbling, and other defects will be generated during sintering [6]. Another function of fillers is to prevent the infiltration of air into the furnace to oxidize the products. Generally, burned AlO, MgO and 23graphite particles are used as fillers to cover the green body. In addition, the fillers can contribute to the uniform heating of the sintering body and prevent bonding of the sintering bodies.

The first criterion for the selection of filler materials is that there be no reaction between the fillers and the sintering body or the sintering boat. Second, there is no deformation at the sintering temperature and the fillers have a certain range of particle sizes. Graphite and charcoal can be used as fillers for the sintering of iron and copper products, while electically melted magnesia and alumina are used as the fillers for Ni, Monel alloy, and stainless steel products. The particle sizes of the fillers depend on the powder size for the sintering of porous products: filler sizes are a little larger than for powders so that the fillers cannot fill the gap of the powders, but they cannot be so large as to release the volatile materials in the green body slowly.

If the oxides of some elements in the sintered products are very difficult to reduce, some additional activators will be used to activate the sintering atmosphere. For instance, titanium hydride can be added to fillers for the sintering of stainless steels to reduce the oxides by the hydrogen atoms. Ammonia chloride in the fillers can consolidate the sintering of iron products.reparation of Materials with High Porosity

1. Addition of a pore-forming agent: This step connects the pores and increases porosity. There should be no water absorption, no decomposition at room temperature, and no chemical reaction with the metal powders. Pore-forming agents are decomposable when heated, and there is no harmful residue after vaporization in the base metals. They can be inorganic compounds, salts with a low melting point, such as camphor, urea, salvolatile, ammonium acid carbonate, and stearic acid [6]; and ammonia chloride and methyl cellulose [9]. Figure 2.8 shows an Al foam product with open cells prepared by the space-holder method [10].

2. Addition of pore-forming enhancers: In this step, the enhancers can be reduced by hydrogen or decomposed into metallic salts. Unmetallic compositions form an evaporated gas, which then creates the pores, while metallic compositions can form compounds with the base metals. If the melting point of the compounds is lower than that of the base metals, then the compounds will be melted, and the materials are strengthened by liquid phase sintering.Figure 2.8　The macrostructure (a) and a scanning electron microscope (SEM) micrograph (b) of Al foam prepared by the space-holder method [10].

3. Natural cellulose: Natural cellulose is dipped into a solution of one or more metallic salts that are thermally decomposable. Afterward, the dried cellulose is heated and burned in the reductive atmosphere, and the metallic salts can be decomposed into metals or alloys. The gases decomposed from the cellulose and salts can be used to form the pores, and in the end, materials of high porosity with interconnected pores can form.

The abovementioned metallic salts can be decomposed completely, and no stable oxides are formed. Under these conditions, the salts can be decomposed and foster the sintering of metals in a reductive atmosphere. It is applicable to Ni, Mo, Fe, Cu, and their alloys, as well as W, Mo, Au, and Ag noble metals. The natural cellulose can absorb the dipped solution, while synthetic fiber and high-molecular polymers cannot.

It is reported [11] that porous metals with a porosity of＞90％ have been prepared by mixing carbonyl fine Fe powders, Ti alloy coarse powders, and the double polyhydric alcohol-isocyanate with pore-forming agents. These porous bodies have a reticulated structure with pore size of 100–200 μm and are applicable to catalysis, biomaterials, and composite materials.Examples

Porous Al has been prepared successfully by powder metallurgy at the Fraunhofer Institute of Applied Materials (IFAM) in Bremen, Germany [12]. The base metal, alloy powders, or the mixtures of powders were mixed with the pore-forming agents, and then the semi-products of the powders were created by densification. During densification through uniaxial compression or extruding or rolling of powders, the pore-forming agents were buried in the base metals and no residual open pores appeared. Afterward, it was subjected to heat treatment at temperatures close to the melting point of the base metals. During this process, the uniformly distributed pore-forming agents in the dense base metals are decomposed. The released gases make the densified powders expand to form the porous materials. Before the formation of the pores, the preformed materials can be manufactured in flakes, rods, or other forms by rolling, forging, or extruding in order to improve fluidity during the pore-forming process. The density of the porous metals can be controlled by adjusting the content of pore-forming agents, temperatures, and heating rate. If the hydrides act as pore-forming agents, the content of hydrides is less than 1％ in most cases.

This process is widely used to prepare porous Al, Sn, Zn, brass, bronze, and Pb by using the proper pore-forming agents like pure Al, 2×××, and 6×××Al alloys, together with the proper processing parameters. The cast AlSi12 alloy is also widely used due to its low melting point and good poreforming ability. In principle, any kind of Al alloys can be used as the agents with the proper adjustment of processing parameters.

The porous metals can take any irregular form after sintering if the preformed product is put in the furnace without restriction. Therefore, in order to obtain the designed shape of porous metals, the preformed product is put into the hollow die for sintering. The core/shell structured sandwich plates can be prepared by sticking the sheet and porous metals together. If the pure metals are needed for sticking to occur, the metal sheet can be rolled onto the green body of the porous metals, the composite is deformed via a deep drawing process, and finally, heat treatment is applied to the drawn body.

Additionally, a dence structured body can be produced by putting the plate slides into the container; these plates are produced by extruding the mixtures of Al powders and hydride particles [13, 14]. A closed-cell porous core can be obtained when it is heated to the same temperature as the solid. This porous core is isotropical, and some large pores can be seen occasionally. It is easy to form the structure that is filled by the porous body, and the related products are the sandwiched panel and the tube with a foamed filler. It is particularly advantageous to the application of porous materials that are insensitive to mechanical properties.

For instance, a dense preformed product was prepared by the axial compression of the mixture of Al powders and TiH at a certain 2temperature, and then heated to release the gas to force the preformed product to expand into the Al foam [15]. Mixing, pressing, and foaming are the three important procedures in the foaming of powders. The sintering pressure is 130–150 MPa at the temperature of 400°C–450°C. Different structured Al foams can be prepared by adjusting the content of the pore-forming agent (about 1％) at a temperature range of 600°C–720°C for 3–15 min.

In another example, the shape memory alloy of TiNiFe is 50482prepared by powder metallurgy [16]. First, the commercial pure powders of TiH, Ni, and Fe, as well as the pore forming agent of 2NHHCO, were blended according to a certain ratio and made into 43green compacts, and then placed in the furnace in a vacuum. After the NHHCO powders have been decomposed at 200°C for 2 h and then 43the TiH powders have been dehydrogenating at 800°C for 1 h, the 2compacts were heated to sinter at 1, 000°C for 5 h. Figure 2.9 shows the sintered porous TiNiFe alloys.Figure 2.9　Sintered porous TiNiFe alloys after adding different amounts of NHHCO 43[16]: (a) 0％, (b) 12.5％, (c) 25％, and (d) 37.5％.Common Porous Filter Metallic Materials

There are two steps in the process of preparing porous filter materials by powder metallurgy [17]: the densification/packing step and the sintering step. If the binder is used in the molding, it should be removed before or during sintering. Today, most porous products are prepared by one of the following processes: (1) loose loading/gravity sintering (bronze); (2) axial/isostatic press densification and vacuum sintering (stainless steel, carbon steel, superalloy, Ti, and Al); (3) asymmetrical designed filter (i.e., the AS (asymmetrical) method).

Later, the developed asymmetrical filter is a powder/powder composite made of the supporting coarse metal powders and the thin active filter layer of the same alloy (＜200 μm). The plate-type supporting materials are prepared by axial pressing, while the filter tube is made by isostatic pressing. A thin metal film is used during the separation. The diffusional combination can be created between the supporting materials and the active filter during sintering. The next developed AS method is a process for the porous structural product and the porous coating preparation, and ultrafine metal powders can be used to do this.

The most widely used areas that employ sintered porous metal parts are process engineering and chemical engineering. The porous filter can undertake deep filtering by using the total volume of the pores due to the synthetic physical effects. This is the main difference between deep filtering and surface filtering, and it enhances porous products made by powder metallurgy. The involved physical effects are the reduction of the particle flow rate and the adherence particle to the pore wall. The decision of whether deep or surface filtering should be used depends on the particle distribution and the range of pore sizes in the fluid.

Powder metallurgical filters have the following characteristics:1. Shape stability: Self supporting can be realized via a high

pressure difference in the fluid.2. Good fatigue property: Higher impact and shock resistance are

demonstrated compared to other filters (such as those made of

paper, plastic, or ceramic).3. High-temperature and thermal-shock resistance: Bronze filters

can be used at a temperature of 400°C, and highly alloyed steels

can be used at 600°C, and filters made of a special alloy can

withstand temperatures of 950°C or even higher. Metallic filters

are better than organic filters in this respect. Under certain

conditions, thermal shock resistance is required, and this

characteristic is better for these filters than those made of ceramic.4. High reliability for separation during deep filtering: This

characteristic is favorable for these filters compared to fabric,

paper, and silk screen filters.5. Good back pressure flow: These filters can be cleaned extremely

well using high-pressure steam, chemicals, or burning.

With more stringent requirements imposed by recent environmental laws for recycling and environmentally friendly substitution for the trash, there are many advantages of powder metallurgical metal products with high porosity. It is more important for the selective competition of technical products.

2.2 FIBER SINTERING

In powder metallurgy, metal fibers are substituted, partly or totally, for metal powders, and then metal fiber porous materials can be prepared.

Metal fiber sintering is very similar to metal powder sintering, but it has some distinctive features.

2.2.1 Preparation of Metal Fibers

The methods used for preparation of metal fibers are cold drawing, spinning, cutting, and plating [2, 18, 19]. They are described in the next sections.Cold Drawing

In cold drawing, multiple drawings of a single wire obtain ultrafine fibers with the optimal cross-sectional shape (with a precise diameter) and the surface state (which is smooth). However, the productivity of this process is low and the cost of die is high, which are distinct disadvantages. With cluster drawing, where tens or even hundreds of wires are drawn simultaneously through the die, productivity can be improved significantly and the cost can be reduced as well. The metal wires are wrapped with copper and drawn several times with annealing, and then cut when a certain diameter is reached. Further drawing is performed with the bundled cut wires in the wrap until the required fiber diameter is reached. The wrapped material can be dissolved in acid (like nitric acid). Finally, the metal fibers are obtained.Spinning Method

In the spinning method, metal fibers can be prepared from liquid metal at low cost, but special equipment is needed. For example, the molten metals can flow out through small holes in the bottom of the container from mechanical force or gas pressure, and then they solidify in a proper atmosphere. The connection between the metal fiber crystals is weak after solidification, so the substance is so thermally brittle that the short fiber can be cut easily with only a little bit of force. Therefore, uniform metal short fibers can be produced with shear force during the dropping of the solidified metal fibers. The shear force can be applied by being struck with a metal plate with the metal fibers somewhat tilted. It is simple to manufacture and has a high production rate. In addition, there is no oil, water pollution, or residual stress.

There are three types of spinning methods:

1. Melt spinning: This method is widely used to prepare glass fibers and synthesized fibers. In addition, it is applied to the development of Al, Sn, and Pb long fibers with diameters of 25–250 μm and a low melting point. However, traditional melt spinning cannot be used on metals with high melting points due to the high surface tension on these liquid metals. The tension may lead to the breaking of the metal wires into balls; in such a case, long metal fibers cannot be produced. The following measures can be taken to overcome these problems: (a) stabilize the injection with indirect physical methods, (b) change the surface state of the liquid injection, and (c) accelerate the heat transfer of the injected metal to solidify the metal wire before breaking.

2. Pendant-drop melt-extraction: This process involves two parts: the heater and the quenching wheel. The metal wires are first put into the heater and melted. The molten liquid drops fall onto the quenching wheel, which has a high speed of rotation, and then they are spinned off centrifugally and solidified with a cooling rate of 105°C/s. The crosssectional shape of the metal fibers with small diameters (25–75 μm) is round, while the shape is a crescent for fibers with large diameters.

3. Glass-coated melt-spinning: In this process, liquid glass has a high viscosity and can be made easily into fibers as follows: A metal rod is inserted into a glass tube and then passed through a high-frequency induction coil and melted together. Then the molten metal 56covered with the glass is cooled at a speed of 10–10°C/s to form into long, round thread and then spinned onto the reel. After removing the outer layer of glass, the metal fiber with diameter of 1–100 μm can be produced with a fine-grained or an amorphous surface and thickness of 500–2, 000 μm. Due to the quenching effect, the thermal stress and deformation from the drawing are presented in the metal fibers; therefore, the fibers demonstrate great strength. For example, the tensile strength of IN865 stainless steel fiber with a diameter of 2 μm is 14, 500 MPa. Au, Ag, Ni, Co, Fe, Ti, V, Pt, Ir, Cu, Al, and intermetallic fibers can be produced by this method.

4. Free-flight melt-spinning: Here, a hole for the adjustment of the flow speed is made at the bottom of the liquid metal container. The liquid metal flows out under pressure, and a tough film forms on the fiber surface with the effect of the chemical active chilling agent, or the liquid fiber will be encouraged to solidify by the introduction of a magnetic field. A fiber of diameter of 25–1, 000 μm can be produced at 3a cooling rate of 1–10°C/s. Continuous long or short fibers can be produced by this method. Be, Al, B, stainless steel, and superalloy fibers are prepared by this method.

5. Melt-dropping: In this process, liquid metal flows out of a hole on the side of the container and onto a high-speed rotating metal drum, and then it is spinned off during solidification to form a metal thread. Al and Al alloys, steels, and bronze fibers are prepared by this method with a large crosssectional area. For example, the sectional area of the Al iron fiber is 0.2 mm×2 mm.Grinding

The metals are ground in a grinder containing abrasive material, and the metal fibers with the required diameters will be obtained by adjusting the feeding speed and the size of the abrasive materials. The size of the metal fibers is influenced by many factors. For example, the finer metal fibers can be produced with higher content of feeding, while the coarse metal fibers are made with coarse abrasive materials. Tough metal fibers of Ni and Ni alloy threads and NiFe alloys can be prepared by this method.Plated Metal Sintering

In the plated metal sintering process, organic fibers are coated with metals by chemcial plating, vacuum evaporation, and slurry dipping, or electric plating after conductive treatment of the fibers. Afterward, they are either sintered in a reductive atmosphere or burned in the air to remove the organics, and then the oxides are removed using a reduction treatment. Finally, hollow metal fibers are obtained.Other Methods

Fiber scraps can be produced by cutting solid metals by chatter machining, shaving, or slitting. These actions are simple to perform with short production cycles at low cost. However, it is difficult to obtain fibers with uniform section and smooth surfaces; therefore, these processes are used mainly to produce short metal fibers. Metal fibers also can be obtained by preparing the slurry of powders of metals or metal oxides with an organic binder, extruding the slurry into the fibers through a spinneret, removing the binder at high temperatures, sintering in a reductive atmosphere, or removing the binder directly in a reductive atmosphere. For example, Ni fibers can be produced by preparing cream of Ni(OH) powders and a binder, extruding, and 2sintering.

2.2.2 Preparation of Porous Bodies

The normal processes used to prepare porous materials by metal fiber sintering are threading, felting, and sintering [2, 3, 20]. The metal fibers with certain ranges of lengths, diameters, and length-to-diameter ratios are aligned to felt (also using suspension) and then sintered in a reductive atmosphere to obtain metal porous fibers. It is applicable to the preparation of Cu, Ni, and Ni-Cr alloys and stainless steel with a wide range of porosities.

Long or short fibers can be selected according to the following requirements: short fibers are used in metal molding, and long fibers are for knitting. Then porous metals are obtained after sintering. The fibers are combined in a three-dimensional (3-D) reticulated way to achieve a porosity of 98％. The strength of porous metals prepared from fibers is better than that from powders with the same porosity.

This kind of porous metal fiber has several advantages, including toughness, elasticity, and tension/compression resistance. All porous fibers are composed of a single fiber, except porous bodies prepared by sintering plated metals on organic felt. Short fibers can be distributed uniformly, while long fibers cannot. Porous bodies with long fibers have better mechanical strength. Therefore, this kind of porous material has disadvatnages as well, such as large pores and nonuniform pore size distribution. With this method, it is easy to prepare products with high porosity and interconnected pores.

Powder metallurgical porous stainless steels have properties of gas permeation, noise reduction, and corrosion resistance, and then are used widely in the aviation, chemical, and mechanical fields. If they are reinforced by stainless steel fibers, much better mechanical properties will result. A porous stainless steel with fiber reinforcement can be developed by adding stainless steel fiber of Φ0.15 mm× 5.00 mm to 0Cr18Ni9 with a particle size of 0.10–0.15 mm, molding in the polythene achohol solution, and sintering in a vacuum at 1160°C for 2 h with a heating rate of 10°C/min [21]. The sintering process is controlled by atom diffusion, and the diffusional coefficient is constant at the sintering temperature. The number of diffusional atoms increases by extending the sintering time and then the conditions are favorable to the growth of sintering neck and an increase in strength. Due to the exponential relationship between the diffusion coefficient and the temperature, neck growth is promoted by increasing the temperature to between 1, 070°C–1170°C, and the strength increases accordingly, as opposed to what happens when only the sintering time is extended. However, as the sintering temperature increases, the porosity and permeation coefficient are reduced. The porosity and permeation coefficient decrease somewhat by increasing the stainless steel fibers while keeping the sintering temperature and time constant. The reason for this is that stainless steel fibers are bigger than the particles and the permeation coefficient decreases with the reduced porosity by adding fibers to the stainless steel powders.Fig. 2.10　SEM image of a novel porous metal fiber–sintered sheet with a 3-D reticulated structure [22, 23].

Several studies [22, 23] have fabricated a novel porous metal fiber–sintered sheet with a 3-D reticulated structure (Figure 2.10) by using the solid-state sintering method on copper fibers. They found that the stress-strain plots of the uniaxial compressive test showed no obvious yield stage in the uniaxial compressive process. Additionally, the results showed that the obtained porous body with higher porosity exhibited greater strain under the given level of compressive stress, therefore producing less effective stiffness [22].

2.2.3 Electrode Plate with Porous Metal Fibers

The porous metal fiber electrode widely used in the battery industry is the nickel base plate, which is prepared by the following methods [24, 25]:1. The fiber felt is uniformly mixed by the metal fibers with a certain

diameter-to-length ratio, and then sintered to obtain the porous

body in a reductive atmospohere. The fibers are produced by

drawing, cutting, and sintering the metal-plated organic fiber.2. The porous fiber body is obtained through the reductive sintering

of metal-plated synthesized fibers (like polypropylene fibers) after

the thermal decomposition of the organics, or direct thermal

decomposition of the metal-plated fibers.3. The base plate of metal fibers is produced continuously by the

thermal decomposition of the metal carbonyl compounds. A high-

quality base plate can be obtained by this method, but at high cost

and with size limitations.4. Using the conventional facility and technique, like using the nickel

slurry of Ni fiber, Ni powders, a binder, and pore-forming agents,

the Ni fiber porous base plate can be produced by dipping it in the

slurry and then drying and sintering it.

The physical and mechanical properties of the porous metal plate are influenced by the optimal combination of porosity, pore size distribution, and structure. The mechanical strength, capacity of the filler, content of the active agents, and electric conductivity shall be considered together. The porosity shall be increased with the goal of achieving the required strength and conductivity. With a certain porosity, the pore size shall be determined by considering the effect of both ohm resistance and concentration polarization impedance on the electrode properties and tensile and compressive performace. The availability of active agents will decrease, and the concentration polarization will increase with reduced pore size, increased pore numbers, and decreased ohm resistance. In all, the uniform pore sizes, appropriate pore size distribution, regular structure, great strength, and good ductility are the basis for the high porosity and large capacity of the base plate.

The sintered composite substance (Ni.C.E.) was prepared by pressure sintering Ni-plated graphite fibers with high elasticity in the early 1980s, and it also had greater resistance than the sintered powders. The results in China have shown that the tensile strength of composite electrodes is much lower than that of sintered powders, and the pore shape is also less regular than that of sintered powders. However, this substrate material has a weightspecific capacity that is much higher than for sintered material and result in lower consumption of Ni.

In the late 1980s, the porous body was developed by sintering interlaced metal (i.e., stainless steel) and carbon fibers. The electrode capacity is closely related to sintering conditions, like temperature and time, which affect the connection points and connection state. The flexible materials can be used as the electrode base plate in the normal battery and fuel cells.

The porous Ni plate was prepared by sintering ultrathin Ni fibers (diameter of 2–10 μm, length of 1 mm) overlaid onto the Ni-plated perforated steel belt or the other base net at 1, 000°C–1, 200°C in the H atmosphere. An integrated effect was achieved with improved 2flexibility, specific surface area, and strength of the base plate materials.

The size of the fiber-type base plate is not stable when loading the active agents due to the large pore size and the low mechanical strength of these materials. The application of Ni fiber and powders to the sintered porous body can overcome these problems.

2.3 METALLIC MELT FOAMING

2.3.1 Preparation of Porous Bodies

The gas-releasing, pore-forming agent is introduced into the metallic melt with adjusted viscosity, and then it is decomposed thermally. The released gas from decomposition expands and drives the foaming of the melt, and finally, the metal foam is produced after cooling [26, 27]. Al, Al alloy, Pb, Sn, and Zn with low melting points can be prepared by this method, and the common pore-forming agents are TiH, ZrH, CaH, MgH, and ErH metal hydride powders [8, 28, 29]. 22222TiH, ZrH, and CaH are used to produce Al foam, while MgH and 2222ErH are used for Zn and Pb foams [8, 9, 29]. TiH will release H 222when heated to above 400°C [28]. Once making contact with the molten metal, the pore-forming agents will decompose quickly. Therefore, the gas-releasing powders should be distributed uniformly in a very short time.

The introduction of ultrafine ceramic powders or alloying elements to form the stabilized particles increases the viscosity of the molten metal. The foaming of Al, Mg, Zn, and their alloys can be realized in this way.

Metal foam is one important part of porous metals and it has a long history. Al foam was developed in 1948 by the evaporization of Hg in the molten Al (U.S. patent 2434775), and further developed in 1956 (U.S. patent 2751289) [29]. In 1960s, Ethyl Inc. in Richmond, VA, became the research and development (R&D) center of Al foam. Up to now, many technical patents have been released concerning the production of Al foam in the United States, Japan, the United Kingdom, Germany, China, and Canada, most of which are related to melt foaming.

Figure 2.11 illustrated the technical process for the small-scale commercial production of metal foams by the melt foaming method. Metallic Ca is added to the Al melt at 680°C and stirred for several minutes. Due to the formation of CaO, CaAlO, and even AlCa, the 244liquid Al became five times thicker [5]. In the actual production of the foam, 1.5–3 wt％ of Ca is usually added. TiH (1.6 wt％) was added as 2the pore-forming agent when the required viscosity was reached, and H was released in the hot, viscid liquid. The melt will expand slowly, 2eventually filling the container. The foaming of melt takes place at a constant pressure. The liquid foam will transform into solid when the temperature is below the melting point, and then the solid foam is taken from the mold after further treatment. The foaming time is about 15 min for the batch production in a large furnace. With careful regulation of the processing parameters, foam with a uniform structure can be obtained, such as Alporas Al foam. ZrH also can be used as 2the pore-forming agent with the recommended content of 0.5％–0.6％ (wt％) and foaming temperature of 670°C–750°C.Figure 2.11　Technical process of melt foaming during the production of metal foam [5].

There will be problems when metal hydrides (like MgH) are 2added to the Al melt: a eutectic alloy (Al-Mg) with a low melting point will be formed, and then the pore-forming agent will be combined with the eutectic alloy and do not decompose (the system temperature is lower than the foaming temperature of the pore-forming agent), and foaming can happen only with the pure Al. The pore-forming agent is added to the liquid metal at temperatures that are above the soildius but below the decomposition temperature of the vesicant. The metal solidifies in the designed mold after stirring. Only when the composite is heated above the decomposition temperature of the pore-forming agent can foaming truly begin. The released gas generates the bubble and increases the volume.

The general requirements for the pore-forming agents are minimal decomposition before the mixing of agents and melt, complete decomposition afterward, and enough gas released before solidification [15]. Currently, TiH or ZrH is used as the pore-forming 22agent, and sometimes eruption is used because its gas-releasing temperature is lower than TiH, less gas is generated, and the cost is 2lower.

2.3.2 Technical Problems and Solutions

Melt foaming is applicable to most industrial mass production of metal foams due to its simple process and low cost [30]. The Al foam bulks in the market are also produced by this process. The selection of proper metal poreforming agents is one of the technical difficulties of this method, however, and the basic requirement is quick foaming around the metal's melting point.

Melt foaming can be used to prepare closed-cell metal foam, but it is hard to control the pore size. Therefore, it is difficult to obtain uniform porous materials. There are a few possible solutions, including (1) high-speed stirring of the pore-forming agent particles and then uniform distribution of the particles in the molten metal; and (2) preventing the escape of gas and the coalescence and growth of bubbles with increased viscosity of molten metal [5]. The other problem is the short interval between the addition of the pore-forming agent and the foam formation, which complicates the process of cast operation. The solutions to this are to thicken the cast layer in order to maintain the foamed metal temperature and to lengthen the flowing time or to apply the continuous casting process.

In this process, the quick foaming of the pore-forming agent makes it hard to distribute it uniformly in the melt. An oxide-wrapped, pore-forming agent was invented at the Institute of Solid State Physics, Chinese Academy of Sciences in Beijing, and it can delay foaming so that uniform foaming of the agent can be realized [30].

Viscosity shall be controlled carefully to make sure that the pore structure has a uniform size and shape in the melt foaming process. The melt viscosity can be controlled by adjusting the temperature, and also the great temperature difference for the alloys between solidus and liquidus. In addition, a tackifier (which can be gas, liquid, or solid) can be used [5, 8, 30] and it works in a more practical way in this process. Tackifiers can be added in several ways, including melt oxidation, the addition of alloying elements, and the dispersion of nonmetal particles [8, 15]. Melt oxidation is a process that air, oxygen, or water steam is blown into the molten metal and then stirred; the oxides will formed a short time afterward. This method is highly efficient and can achieve great viscosity. Solid oxidant particles are also used in the melt, like MnO, with a particle size of 20 μm in the Al 2melt and the formation of AlO. The AlO particles will form the 2323nucleus of the foam, and then the foamed body with uniform pore size, distribution, and shape will be obtained [5, 8]. The most commonly used method is the addition of alloying elements like Ca to form fine solid particles in the melt and then increase the viscosity. It is simpler than the melt oxidation process [15].

Tackifiers can be nonmetal Si polymers, alumina powders, SiC, Al scum, N, Ar, and other metals [5, 8]. With the addition of a tackifier, 2the viscosity of the melt increases, the foam wall thickens, the foam size decreases, and then the uniformity of the density improves. More uniform foam is obtained with the degassing treatment if a gas tackifier is used. However, the addition of a tackifier may pollute the master materials to some degree, as well as increasing the cost. In a Chinese patent (96117125.1), a special tackifier was disclosed with advantages like no pollution, no cost, and ease of performance [31]. Good results were demonstrated in Zn foam production by using cyclic foaming [8]. Numerous kinds of tackifiers are used in the cyclic foaming process, and the broken foam will be foamed again during the stirring process or to form intermediate products.

2.3.3 Case Studies on Porous Aluminum Preparation

The foaming will be triggered when the hydride powders are mixed into the Al melt [27]. The addition of Ca will help attain higher viscosity in the temperature range of solidus and liquidus. The growth of pores in the liquid can be controlled by the overpressurized H. Large bulk products with uniform interspaces and the isotropical property can be prepared by this method, and uniformity results from the high viscosity of melt and the overpressurized H in the foaming process.

The technical process for the preparation of Al foam is shown in Figure 2.12 [20]. Based on the application, the Al alloys are first selected, and the foaming conditions vary depending on the different Al alloys. Tackifying is the most important step in the preparation of Al foam. The tackifying methods include particle dispersion (dispersion of nonmetal particles in the liquid metal), alloying (addition of alloying elements) and oxidation of liquid metal (dispersion of formed oxides in the liquid metals), in which the addition of active Ca to the liquid Al and a short period of stirring can tackify the Al melt effectively. The optimal viscosity for liquid Al is about 8.6 mPa·s; overtackifying may cause the bubbles to escape from the liquid, while undertackifying may lead to nonuniform distribution and the irregular shape of pores with low porosity.

The following procedure is used to add and mix the pore-forming agent. The pore-forming agents for the Al foam are TiH, ZrH, NHCl, 224NHI, (NH)SO, BaCl, Bi(SO), CaCO, CaH, CaSO, and 44242243324NaNO. The decomposition of the pore-forming agents shall be a little 3higher than the melting point of the metals, and CaH is the best pore-2forming agent for the Al alloys. The pore-forming agents shall take the form of particles, and they are stirred during the addition to the liquid metal for even dispersion. Delay release technology is used to extend the stirring time and then mix in the pore-forming agent more evenly. After these procedures, the Al liquid is ready for foaming upon standing. The foaming temperature and time shall be decided by the decomposition termperature and rate of the pore-forming agent. Higher foaming termperatures lead to irregular pore shapes, while lower temperatures are not favorable to the growth of the pores. At certain foaming temperatures, a too-long or too-short foaming time may lead to pores having smaller diameters or irregular pore shapes. Therefore, the pore structure can be regulated by the foaming temperature and time. Finally, the required foam will be obtained after quick cooling of the designed foaming state. The cooling methods can be air cooling, wind blowing and oil or water cooling. Different foam states can be obtained using various cooling methods. Due to the shrinkage of the solidified metal, the porosity is generally lower than that of the liquid metal. Al foam has the following features: lightness, noncombustion, stiffness, sound absorbility, and dampening. Therefore, it is one kind of noncombustible, nontoxic, and light material that is useful in construction. Open-cell Al foam is a good sound-absorbing material.Figure 2.12　The technical process of the preparation of Al foam with melt foaming.

In one study [32], the preparation of Al foam with the industrial pure Al and the additive was reported, in which TiH was mainly used 2as the poreforming agent. The processes involved are as follows:1. Melting and stirring the pure Al and alloys to control the viscosity

of the melt2. Addition of pore-forming agents and the even dispersion in the

melt, and the decomposition of TiH into Ti and H, which form 22

bubbles in the liquid Al3. Maintaining the temperature to control the formation of pores and

growth

The pores obtained are mostly of the equiaxial closed-cell type, as shown in Figure 2.13. The addition of more pore-forming agent (＞3％) may cause the formation of large pores in Al foam, while less of the agent may lead to low porosity, along with the the formation ot foam with irregular structures.

2.4 GAS INJECTION INTO THE METALLIC MELT

Precise control of the foaming termperature range and processing time is needed for the melt foaming method, while the gas injection method, introduced in this section, is easier to implement and also has a low cost [8, 33]. The metal foams used in gas injection have a broad range of pore sizes and very high porosity (up to＞90％). Gas from the outside is injected into the bottom of the molten metal, which produces bubbles, and the used gases can be air, steam, oxygen, CO, and inert 2gas. The key technical points behind this procedure are the proper viscosity for the melt and the large temperature range for the foaming. The formed foams must be stable and cannot break during the process [8]. The mixture, composed of metal and solid stabilizer particles, is heated to the temperature above the liquidus of the metal, and then gas is injected to create the closed-cell bubbles. After cooling the temperature below the solidus, the metal foam with a large number of closed cells is obtained [33]. The stabilizing materials can be alumina, Ti, ZrO, SiC, and silcon nitride, and the metal gases that can be 2injected are Al, steel, Zn, Pb, Ni, Mg, Cu, and their alloys. The particle size and the volume ratio for the stabilizing materials shall be selected carefully. The small size of particles causes the problem of mixing, and the high-volume ratio leads to low stability of the foam, while a low-volume ratio leads to excess viscosity. The pore size can be regulated through the gas flow rate.Figure 2.13　The cross section of the Al foam with melt foaming [32]: (a) pore size of 2.44 mm, porosity of 85.2％; (b) pore size of 2.50 mm, porosity of 70.7％; (c) pore size of 2.48 mm, porosity of 57.0％. The content of the pore-forming agent and the time of maintaining temperature decrease at the same foaming temperatures from (a) to (c).

The gas injection method for Al and Al alloy foaming was developed at HYDRO in Norway and Cymat Technologies in Toronto, Canada [5]. SiC, AlO, and MgO particles are used to raise the 23viscosity of the melt, as shown in Figure 2.14. In the first step, the Al melt, like cast AlSi10Mg (A359) or precision cast 1060, 3003, 6016, 6061 alloys with these particles, shall be prepared, and the wetting of the melt to the particles and the uniform distribution of the particles shall be resolved. The second step is the injection of gas into the melt through a specially designed rotating propeller or vibrational muzzle, which creates the foams. The function of the propeller or nozzle is to generate smaller, uniformly distributed bubbles in the melt. With the presence of these finer bubbles, high-quality foam is produced. During the drainage of the melt, the thick mixture of bubble and melt will float above the melt and then become liquid metal foam. Due to the presence of ceramic particles, this kind of liquid metal foam is quite stable and can be drawn from the liquid, cooled, and solidified. Before the complete solidification occurs, the semi-solid foam can be flattened by rollers or a belt. In principle, the length of the foam belt can be as long as needed, however, the width depends on the allowed width for a liquid metal container with normal thickness of about 10 cm. Figure 2.15 shows two metal foam products.Figure 2.14　Schematic illustration of the manufacture of metal foam by gas injection: (a) mode 1 [5]; (b) mode 2 [34].Figure 2.15　Metal foam prepared by gas injection: (a) two foam plates with different densities and pore sizes [5]; (b) an Al foam column [34].

The volume fraction of particles is about 10％–20％, with an average size of 5–20 μm [5, 33]. The particle size and content are selected based on past experience. The particles on the pore wall play a key role in the stabilization process. First, the particles increase the surface viscosity and then delay the exhaust. Second, the particles are partly wetted by the melt, and the contacting angle must be within a specified range to ensure the stability of the bubble/particle interface and the energy reduction of particles at the interface compared to the total energy of bubble and particles. Very good (with a much lower contacting angle) or poor (with a large contact angle) wetting do not achieve stabilization effectively.

The Al foam prepared by this approach has the porosity of 80％–398％, the density of 0.069–0.54 g/cm with an average pore size of 3–25 mm and wall thickness of 50–85 μm [5]. The average pore size has an inverse relationship with wall thickness and density, and it is influenced by the gas flow, propeller speed, vibration frequency, and other parameters [5, 35]. The density, pore size, and pore extension gradients are present in the foamed panel, which is the result of gravity-induced exhaust [35]. Additionally, the shear force of the conveyer belt will cause diagonal distorted pores in the final product and has a notable effect on its mechanical properties. This effect can be improved by the vertical drawing of the foam.

The advantages of this method are the capability of large volume production of foamed materials, as well as low density. Therefore, metal-based composite foams are less expensive than porous metals. One disadvantage, however, is the existence of open pores after the final cutting of the foam. In addition, the presence of particles for the reinforcement will cause the metal-based composite foams to be brittle, which is the unexpected side effect of the foaming process.

This process also can be conducted with zero gravity [20]. The problems of bubbles floating and increased viscosity, the need of foaming agents cannot be encountered in zero gravity. The foam can be produced with the injection of Ar into the liquid metal.

The foam can take one of two shapes: sphere or polyhedron [36]. Polyhedrical pores are usually formed during the gas injection process, while spheric pores are formed in the early stage of gas generation within the melt and a mixture of spheres and polyhedrons are presented in the late stage. The ratio of spheres to polyhedrons is related to the foaming time. Spheric pores may change to polyhedrical pores in some conditions. In order to obtain metal foam with high porosity, the key processing parameters in addition to the size and dispersion of foaming agents are composition, cooling time, and cooling rate. The wall effect of the container has an influence on the stability of polyhedrical foam, whereas it exerts no influence on the stability of spheric foam. The wall effect on the formation and stability of the polyhedrical foams reflects the formation rate of the dispersed bubbles and the free surface generated when the foam breaks. The breaking rate of bubbles in the foam layer is related to the free surface and the height of the foam layer, since the drainage rates of the isolated bubbles vary with the height of the foam layers.

2.5 INFILTRATION CASTING

In infiltration casting, inorganic/organic particles or low-density hollow balls are piled up in the mold or the preformed porous product is put in the mold, and then the molten metal is infiltrated in the interspaces created therein. The porous metals are obtained after removing the the pileups or the preformed body [1, 5, 8, 9], and their removal is realized by the dissolution in solvent or by a heat treatment process. Inorganic particles with heat resistance and solubility can be used for this kind of pileup, like NaCl particles. Inorganic materials like puffing clay particles, fired clay balls, sand balls, glass ball foams, and alumina hollow balls can also be used as the pileups, and the porous composites will be obtained by using them. If the infiltration and solidification of the melt is fast enough, the polymer balls can be accepted, and positive or negative pressure may be applied.

The porous Al, Mg, Zn, Pb, Sn, and cast iron can be prepared in the form of sponges by this method [1, 5]. The parts with the designed shape can be manufactured by using the mold of the defined geometry. However, with the presence of surface tension on the liquid metal (which is particular to liquid aluminum), quick infiltration of the molten metal into the interspaces cannot be attained. The wetting problems may result in unfilled interspaces as well. In order to prevent this phenomenon, a vacuum state in the interspaces may be produced to have a negative pressure, or pressure is applied to the melt to facilitate the infiltration of the molten metal. Moreover, in order to prevent the early solidification of melt, the preheating of pileups or the overheated melt may be used, particularly under the conditions that the pileups have a higher specific heat capacity or the infiltration pressure is low.Figure 2.16　Schematic diagram for infiltration casting.

Infiltration casting can be pressurized by the solid press head (Figure 2.16), gas, differential pressure, and vacuum suction casting [15]. The quality of the metal foams prepared by differential pressure and vacuum suction is high, the products have good mechanical properties due to the long infiltration distance of metal liquid and the dense metal frame.

The salt particles can be washed away with water, the sands are removed during the thermal decomposition of the binder, and the polymer balls are eliminated by the pyrogenation reaction. A higher pileup density can be achieved with vibration. A sandwich plate can be obtained by the infiltration of the melt in the preformed part inserted between two metal sheets and by the formation of the metallurgical combination due to shell surface melting. The presintered products are put into the mold and the porous core will be produced together with the casting of the out-shell structure.Figure 2.17 Porous Al prepared by infiltration casting: (a) open-cell Al foam with density 3of 1.1 g/cm [5]; (b) porous Al with porosity of 76.0％ and 84.3％ [37].

One of the advantages of using preformed parts is the precise control of the pore size distribution. The distribution will be derived from the size of the filler particles. Figure 2.17 shows the morphology of porous Al prepared by infiltration casting, in which the fillers have been removed totally.

Water-soluble sodium chloride (NaCl) particles are generally used to prepare the preformed mold due to the consideration of the source, cost, and dissolution from water [38]. NaCl is a type of white crystal 3with a density of 2.16 g/cm, melting point of 804°C, and boiling point of 1, 413°C, and it is water soluble as well. Before application, it must be pretreated to remove the crystal water; otherwise it will explode and crumble when heated to a certain temperature. Hence, the crumbling NaCl blocks the interspaces in the preformed mold and makes the mold change shape. Therefore, the dehydrated NaCl particles are kept in the desiccator before using. Potassium phosphate (KHPO) is 243another kind of salt with water solubility, a density of 2.564 g/cm, and a melting point of 1, 340°C. As with NaCl, the pretreatment is applied to KHPO, but at a much higher temperature. Therefore, the melting 24point of the alloys for the infiltration with KHPO is also higher than 24that of NaCl. The combustible particles (like charcoal scraps with the strength of cast iron) shall be used in the protective atmosphere. The preformed particles are not easy to deform and combust with high-speed infiltration. The particles can be removed via the high-temperature treatment after infiltration and solidification, and then the open-cell metal foams can be obtained. Additionally, in order to remove the preform with subsequent treatments, the processing must be continuous [15].

If insoluble particles are used to prepare the preformed mold instead of water-soluble particles, the particles are sealed in the solidified metals and the hollow pores will become isolated [20]. If the liquid metal is die cast in 3-D reticulated ceramics with a hollow framework, the pores in the metalceramic porous composites are mainly composed of the original hollow parts in the ceramic frameworks.

The preparation of porous Al and Al alloys by infiltration casting has been reported [31, 38, 39]. The 3-D reticulated porous Al alloys with different pore sizes and porosity can be prepared by the pressure adding cast process [39]. During this process, molten metals are poured into a preformed mold and pressure is applied to force the liquid metal to infiltrate the interspaces, and a metal-preformed particle composite is obtained after the metal solidifies. Al-12％Si alloy foam with maximum dimensions of Ф100 mm×100 mm, pore size of 0.5–1.6 mm, and porosity of 60％–80％ is obtained. The preformed part is prepared by mixing the composite salt particles with a binder and water, pressing into the graphite mold and sintering. Figure 2.18 shows the pressure adding infiltration casting facility. The preformed part is put at the bottom of the metal cylinder and heated to 450°C by resistance heating. The Al-12％Si alloy is melted, refined, deslagged, and finally poured into the metal mold. After that, the punch head is pressed quickly to force the liquid metal into the interspaces and then kept at a constant pressure for 15 min. After the metal has solidified, the metalpreformed cast part composite is obtained. At last, the porous metals are formed after the salt particle has dissolved in the preformed part.Figure 2.18　Schematic diagram of pressure-adding infiltration casting. 1—oil cylinder; 2—pillar; 3—upper table; 4—upper template; 5—punch; 6—metal mold; 7—prefab; 8—porous baseboard; 9—bottom table; 10—holding furnace; 11—manual handle; 12—hydraulic pressure gauge; 13—temperature controller; 14—melting furnace.

The preformed particles are shaped like triangles with limited contact areas, so the pores have more edges and corners. With further baking, the edges and corners of the pores disappear and the contact areas increase. These conditions are favorable to the preparation of porous Al with a smooth pore surface and interconnected pores. The melted binders gather and flow to the contact area of the particles, creating surface tension and leading to a smooth connection. Moreover, the binders react with the salt particles to form compounds with low melting points, further increasing the contacting area. It is reported that the content of the binder needs to be in the range of 7％–10％ to achieve this effect.

It is advantageous to prepare the open-cell Al foam by high-pressure infiltration rather than low-pressure infiltration, even when preforming flammable particles that cannot deform and burn during infiltration. The processing of Al foam can be simplified by vacuum infiltration rather than high-pressure infiltration.

2.6 METAL DEPOSITION

Porous metals can be prepared with gaseous metals or metal com-pounds in a metal ion solution [5]. A solid, preformed structure is needed to determine the geometrical shape of the porous materials. For example, a polyurethane porous plastic is used to prefabricate the substrate.

2.6.1 Vapor Deposition

In vacuum vapor deposition, materials in a vacuum is heated by the electron beam, electric arc, and resistance heating, and then they are evaporized and deposited onto the cold porous substrate. The metal vapor finally solidifies and covers the surface of the polymer porous base to form a metal film. The thickness of the film depends on the vapor density and deposition time [5, 40].

The vacuum-plated film is quite thin, particularly when the synthesized resin substrate is melted in a vacuum. Thin film (with a thickness of only 0.1–1.0 μm) can be deposited due to the heating of the substrate from the radiation of the molten metals in the vacuum deposition process. A 30°C cooling medium can be introduced, and then the temperature in a vacuum can be reduced. The temperature of the organic net belt or the porous material belt can be brought below 50°C by cooling the transit roller. Therefore, a thick film can be deposited on any kind of substrate, with any kinds of metal. The obtained pores are not prone to deformation in the porous metals [40]. The porous metals can be obtained by the thermal decomposition of the porous substrate in the H reductive atmosphere and the sintering 2treatment. The base can be made of synthesized resins, such as polyester, polypropylene, and poluurethane, and natural organic materials like natural fabric and cellulose. For the preparation of the porous composite body, inorganic materials like glass, ceramic, carbon, and mineral can be used. Cu, Ni, Zn, Sn, Pd, Pb, Co, Al, Mo, Ti, Fe, SUS304, SUS430, 30Cr, and Bs metals can be deposited. After the vacuum deposition occurs, Cu-Sn, Cu-Ni, Ni-Cr, Fe-Zn, Mo-Pb, and Ti-Pd composite film can also be deposited. The organic base can be removed in the H reductive atmosphere and then followed by 2sintering. Meanwhile, the strength and ductility of the porous metal can be improved. The sintering temperature is 300°C–1, 200°C, and sometimes the sintering process can be omitted depending on the application.Ambient Vapor Deposition

Another way to prepare porous metals is to evaporize the metal vapor in an inert atmosphere. The vapor coagulates to form the porous structure, like the physical vapor deposition after resistance evaporization [41]. In this way, the metal evaporizes slowly in the inert 24atmosphere (10–10 Pa) and the evaporized metal atoms collide with the inert gas molecules, scatter, and lose their kinetic energy. This fact is demonstrated by the temperature decrease seen in the metal vapors. The metal atoms will coagulate into clusters before arriving at the base, and the “metal smoke” is observed for these clusters. The reducedtemperature clusters are deposited on the base by the carrier of the inert gas from the effect of gravity. The metal smoke particles pile up loosely due to the difficulty in migration and diffusion from the low temperature, and then a porous metal foam forms.

This preparation method gives the metal foam a high porosity and a submicro structure for the effective restriction of the superheated electrons in the experiment of “effectively restrain inertial confinement fusion (ICF) laser.” In this experiment, the superheated electrons were transformed from laser energy via the radiation of a high-intensity laser on the target. The electrons heated the target materials to reduce the ablated compression quality, affecting the conversion rate of energy. Therefore, the low-density metal foams with large numbers of atoms are accepted as the target to reduce the preheating depth effectively and increase the conversion efficiency. It has an extremely low density (only about 1％ of the solid metal, reaching 0.5％ at its lowest point). It is composed of a large number of sub-micro metal particles and pores that is similar to the foam.

Flat targets of Au and Pb foams with relative density of 1％–10％ are prepared in the United Kingdom by this method. In the United States, the target of Au foam was also prepared, and then it was deposited on the organic micro balls and applied to the ICF experiment successfully.

The working principle of the machine for the preparation of Au, Cu, and Al foams at the Chinese Academy of Engineering Physics in Mianyang is shown in Figure 2.19. The main body for the facility is composed of a JK-300 high-vacuum unit with an evaporization and coagulation system in a glass cover, together with other parts like the 500A/10 V output transformer, the 5 kW booster (which controls the current), the mutual inductance, and the ammeter. The pilot gas system consists of the stainless steel gas tube, barometer, flowmeter, and micro-adjustable valve (which regulates the gas flow). The inert gas pressure in the vacuum chamber is measured by the digital -15vacuometer with a range of 10–10 Pa.Figure 2.19　Principle of the facility for sub-micro metal foam preparation with evaporization and coagulation.

The influencial factors for the formation of metal foams include the metal types, heating power, inert gas pressure/flow, evaporization source, heater type, distance between heating source and substrate, and substrate materials. The heating power, inert gas pressure, and flow are the most important of these. Sub-micro, low-density metal foams can be prepared only with the matching parameters.

The metal foam structure is formed by the nonequilibrium solidification of the metal vapors. The dendrite will be formed via nonequilibrium solidification with the abrupt thermal gradient. The growth of dentrites with radial symmetry can form a fluffy structure in the shape of a snowflower. Much fluffier structured foams are formed from the direct transformation of gas to solid, together with the higher thermal gradient, concentration gradient, and much lower crystallization rate.

The Al foam is considered stable since there is no presence of alumina or absorbed oxygen in the Al foam by the spectrum and electron microscopy examination. The Al foam can be formed only with 24an inert gas (Ar) pressure of 10–10 Pa. There is an analogously inverse relationship between the foam density and the pressure in the 23range of 10–10 Pa.

The cooling rate of the metal vapor depends on the inert gas pressure when a certain heating power is fixed. The porosity increases with the flow rate due to the increased thermal gradient around the vapor source, and the metal foam of lower density can be obtained. 4While increasing the inert gas pressure above 10 Pa, the Al metal smoke overflows from the sealed area of the bell cover and then leads to increased density and decreased film thickness. No “metal smoke” 2will be formed when the inert gas pressure is lower than 10 Pa.

The electrical resistivity and the optical absorption coefficient can be increased more for the metal foams prepared by this method than for solid metals. Therefore, the absorbability and chemical activity increase and then are widely used as gas-sensitive materials, temperature-sensitive materials, molecule sifter, catalyst carriers, wave absorption materials, and electron emission materials.

2.6.2 Electrodeposition

The metals are plated electrically on the open-cell polymer foam substrate from the metal ions in the electrolyte, and porous metals are finally obtained by removing the polymer [5]. Therefore, similar to the investment casting process, there is no real foaming of the metals.

Currently, the large scale of metals with high porosity can be prepared by using this process with features like high porosity (80％–99％), uniform pore distribution, and interconnection of the pores. It takes the open structure as the base, and usually 3-D reticulated organic foams are used. For example, organic foams could be polyurethane (including polyether and polyester series), polyester, vinyl polymer (such as polypropylene or polythene), vinyl and styrene polymers and polyamide, and fiberfelt materials. The major processes include pretreatment of the base, electric conduction treatment, electric plating, and reductive sintering (see Figure 2.20).

The pretreatment is performed with the alkali (acid) solution to remove the oil, roughen the surface, eliminate the closed pores, and then clean the surface before the electric deposition. The electric conduction treatment is conducted on the organic foam substrate. If the substrate is electrically conductive, this procedure is omitted. The electric conduction can be treated by evaporization plating (resistance heating), ion plating (arc ion plating), sputtering (magnetron sputtering), chemical plating (Cu, Ni, Co, Pd, Sn), conductive gluing (graphite colloid, carbonlolloid), conductive resin coating (polypyrrole, polythiophene), metal powders (Cu, Ag), and slurry coating [42–44]. The mostly used treatments are chemical plating and coating with conductive glue. The flat micro-carbon particles used in the glue can be overlaid on the synthesized resin frameworks to ensure that the plane maintains contact between each particle and then forms a conductive layer.Figure 2.20　The processes of using electrodeposition for making metal foams [5].

The plated metal with little defects on this overlaid surface is smooth and has a uniform thickness. Therefore, the porous metals of the 3-D reticulated structure can be obtained with high tensile strength and bending strength. If chemical plating is used, the oil removal, roughening, sensitizing, activating and reduction processes are performed. More detailed information shall be referred to related documents about plastic electroplating technology.

There are some special advantages for the conductive treatment of the 3-D reticulated substrate by painting the metal micro-powders with a binder. The resistance is small, and a large current can be applied in the plating process. Moreover, the plated layer cannot be burned out in the sintering process, while it composes the main part of the porous metal frames. The presence of this layer not only reduces the plating thickness but also saves plating time. The metal micro-powders can be Ni, Cu, Ag, Al, Au, Fe, Zn, Sn, P, Cr, Pb, or mixtures of these. The activation is applied to the metals that are easily oxidized to form nonconductive oxide film. The metals with intrinsic high resistance need to be substituted to reduce resistance. Another treatment that can be used instead of activation and substitution is to mix the metal powders with more conductive, softer powders like Au, Ag, or Cu, in the ball miller and then increase the conductivity of the metal powders.

A conductive macromolecule layer can be formed on the surface of the porous polymer by the chemical oxidation polymerization process, and then it is electroplated. The monomer for the chemical oxidation polymerization can be pyrrole, thiophene, and furan pentacyclic compounds and their derivatives. The inorganic acids or metal compounds can be used as the oxidants in the chemical oxidation polymerization. The inorganic acids are hydrochloric, sulfuric, and nitric acids, and the metal compounds are chlo-rides, sulphates, and nitrates. If there is no special restricition for the solvent (i.e., the solvent is not seriously corrosive to the macromolecule materials), water is generally used. In chemical oxidation polymerization, the porous macromolecule materials make contact with the oxidants in solution, and then the compounds are provided for the chemical oxidation polymerization. Due to the contact with oxidants, a conductive macromolecule layer with thickness of 1 μm to tens of micrometers is formed on the resin parts. The ester polyurethane foam is dipped in the iron chloride solution, and a pyrrole polymer layer is formed on the resin, which makes contact with the pyrrole vapor. Compared to chemical plating, the electric plating has advantages like no pretreatment before plating, fast growth rate for the plated layer, easy fabrication, and greater mechanical strength of the coating.

Polymerization can also be used [45]. In the abovementioned method, the organic mcaromolecules are dipped in the monomer solution or vapor to form the conductive polymer with catalysis; whereas in the polymerization method, a conductive polymer suspension is prepared and then organic macromolecules are dipped in the suspension to finish the conduction treatment. For the conductive polymer, there are no other restrictions except that it can be removed in heating and that it can be those like polyaniline, polypyrrole, polythiophane, polyfuran, and their alkyl-, alkoxy-, and phenyl-derivatives.

Electric plating can be conducted via the traditional electroplating process. However, for electric plating of porous bodies, there will be a shortage of metal ions in the inner layer from the polarization with increasing the current density. The pulse current can be used to reduce the polarization. During plating, the solution in the pores is the same as that the outer solution in the plating bath due to the diffusional effect when the current is off. When the current is switched on again, the concentration of ions in the pores will decrease to the point where it impedes the plating process. If a pulse current is applied, the consumed ions are supplied again during the interval and then efficiency improves. The polarization is reduced to form a uniformly plated layer. The spraying of electroplating liquid can also reduce polarization by decreasing the concentration difference both inside and outside the pores. The fresh porous materials can be put on the plated porous materials to make the pores in the fresh one lack tension and deformation, as well as increasing the current density.

Continuous eletroplating is used by companies with large production capacities. Different porous metals or alloys can be prepared by changing the liquid metals, such as Ni, Cr, Zn, Cu, SN, Pb, Fe, Au, Ag, Pt, Pd, Al, Cd, Co, In, Hg, V, Tl, and Ga [43], and metal alloys, such as brass, bronze, Co-Ni alloys, Ni-Cr alloys, Cu-Zn alloys, and others. A special solution can be used for the metals that cannot use the aqueous solution. For example, the plating of Al and Ge can be electrolyzed by using organic solutions or molten salt solutions.

The porous metals can be either decomposed and sintered in the reduc-tive atmosphere with electroplated porous composites, or reductively sintered in a reductive atmosphere after burning off the organic solution in air. The thermal decomposition temperatures are determined by different organic substrates while considering the upper limit of the melting point of the metals for plating. The reduction temperatures are selected based on the oxide types and the annealing for the plated metals, also considering the upper limit of the melting point of the metals. For polyurethane-based porous materials, the decomposition temperature is in the range of 400°C–700°C (with the optimal range being 600°C–650°C), and the reduction temperature is 700°C–1100°C [45]. The warping increases if the substrate thickness is smaller than 3 mm, so the superposition of the single substrate is needed to reduce warping.Figure 2.21　The electroplated Ni layer:* (a) cross-section morphology; (b) surface morphology.

The Ni foams for electrode application were prepared using porous polyurethane plastics as the substrate via the electrodeposition method [46]. A carbon-based conductive glue was used to conduct electricity with the conventional Ni plating process. Ni plating with a fine and regularly layered structure was obtained (see Figure 2.21).

In the two-step process of burning organics and sintering, an Ni-plated body is pre-heated at 600°C in air for 4 min and then a thin layer of NiO is formed on the surface. A coarser layer is left behind due to the outward diffusion growth of Ni in NiO (NiO is a negative semiconductor oxide with metal deficiency), as shown in Figure 2.22b. The NiO layer will be reduced to Ni after sintering in the amminia-decomposed reductive atmosphere at 850°C–980°C after the organic base has burned off. A Ni foam product with increased grain size, dense structure, smooth surface, and no oxide residue forms after 40 min of heat treatment (Figure 2.23).

In the one-step process of sintering after plating, there is no formation and reduction of NiO, but thermal decomposition (formation of gas CH, HO) and the reduction of carbon (formation of gas CH, 424CH)do take place. The others are the same as that of the two-step 26process, and an Ni layer with similar structure and morphology is formed (Figure 2.24).Figure 2.22　The Ni layer after preheating at 600°C for 4 min: (a) cross section; (b) surface morphology.Figure 2.23　The Ni layer after reduction and sintering of preheated: (a) cross section; (b) surface morphology.Figure 2.24　The Ni layer formed after reduction and sintering of electroplated Ni: (a) cross section; (b) surface morphology.Figure 2.25　Ni foam prepared by electroplating: (a) the whole 3-D reticulated structure; (b) hollow cross-section structure.

The final product is the 3-D reticulated porous body after these two processes (Figure 2.25a) with a triangle hollow cross section (Figure 2.25b), in which the hollow is formed by organic decomposition.

The electroplated Ni layer on the organic porous body has a fine structure, but it has some obvious defects. The fine structure does not change with the formation of a NiO oxide file at 600°C for 2 min. A dense, smooth Ni layer forms with grown grains and a stable structure after sintering in an ammonia atmosphere at 980°C for 40 min. The Ni layer with a similar structure can be formed after sintering at 850°C, and it is enough to obtain the Ni layer by sintering at 850°C for 40 min. The 3-D reticulated Ni foam products with a hollow structure have good physical and mechanical properties.

One study [47] used a self-made delicate noncontact-type extensometer to measure accurately the ultimate tensile strength, yield strength, and Young's modulus for an open-cell nickel foam with an average pore size of 600 μm. This kind of extensometer can completely avoid any minor deformation that might be caused by the attachment of a conventional extensometer to the sample's surface prior to testing, and this function is based on the use of a laser camera that detects and records the dimensional changes as soon as the load is applied.

The Ni-Cr alloy foam can be prepared by the alternate plating of Ni and Cr and followed by heat treatment with diffusion of Ni and Cr in the plated layers. The obtained Ni and NiCr foams have plate 3thicknesses of 2–20 mm, and densities of .4–0.65 g/cm that are independent of the average pore size of the foam [5].

Some plated porous bodies can be overlapped to achieve the required thickness and then electroplated again to increase pore uniformity. The porous metals with uniform structures can also be prepared by overlapping, then sticking the individual porous body together, finally by vacuum plating or by spraying after the conduction treatment [43–45]. The sticking methods include fusion of the surface with a flame, with a binder and a hot adhesive. Porosity may be impaired by the film formed by the binder or adhesive, and so flame fusion is the best way to achieve good porosity.Ni Foam Preparation

The preparation and application of Ni foam was reported 40 years ago in the United States, and further research and development was conducted in Japan. With the successful application of Ni foam to the electrodes in alkaline batteries in the 1980s, its industrialization was accelerated [44].

It is the most common method used to prepare Ni foam by electrodeposition, achieving high quality at a reasonable cost [48, 49]. Polyurethane is used as the substrate for the plating, and it needs to be degreased in a chemical way; that is, cleaned in a solution of NaOH (30–40 g/L), NaCO (15–20 g/L), NaPO (30–40 g/L). A small 2334amount of detergent is added to the solution, and then it is heated to 40°C–50°C to facilitate emulsification. Another way is to soak the substrate in the water solution of acetone (1:4) for 5 min to eliminate surface tension and degrease and flatten the surface.

A degreasing process was recommended [48] in which the polyurethane substrate was soaked in a 20％ xylol solution for 30 min and then degreased in a solution of NaOH (25 g/L), NaCO (25 g/L), 23NaPO (25 g/L), and OP emulsifier (25 g/L) at 60°C–80°C for 10–30 34min. Some closed pores will be opened by degreasing, and the hydrophilic groups can be formed on the roughened pore surface that is advantageous to plating. Different roughing solutions need to be used for different plastics, and the most effective factors are the composition, concentration, roughing temperature, and time [48]. A polyether polyurethane base can be corroded with a strong oxidant in acidic conditions to wet the surface and increase the adherence of the plating through the micro-traces on the surface [49]. For example, roughing can be achieved with a solution of CrO (30 g/L) and HSO 324(20 mL/L) at 30°C for 3 min. Water cleaning is performed after roughing for the next sensitization.

The purpose of sensitization is to absorb a reductive layer of 2+2metal ions (Sn) on the surface [48]. There are SnCl, hydrochloric acid (HCl) and Sn bars in the sensitizing solution. The presence of HCl 2+is to maintain the stability of Sn and control the hydrolyzation of SnCl:2SnCl+HO→Sn(OH)Cl↓+HCl　(2-10)22SnCl+2HO→Sn(OH)↓2HCl　(2-11)222

The undissolved resultants after hydrolyzation deposit on the surface and act as the absorbing layer for the next activation process. 4+The presence of Sn bars can prevent the oxidation of SnCl to Sn in 2air by this reaction:04+2+Sn+Sn→2Sn　(2-12)

The sensitizingconditionin[49]isasfollows:asolutionof(SnCl8g/L)+ 2(HCl 20 mL/L) for 4 min. In [47], it is (SnCl 15 g/L)+(HCl 20 mL/L)+ 2(Sn bar) at 40°C for 3–5 min.

The activation is a process that forms a catalysis metal layer of Ag, Au, Pt, and Pd on the surface. Ag and Pd are the most widely used metals, and they have the following reactions [48]:+2+4+2[Ag(NH)]+Sn→2Ag+Sn+4NH(Agfor the activation center)　323(2-13)2－2+4+－2－(PdCl)+Sn→Pd+Sn+4Cl ((PdCl) for the Pd nucleus)　44(2-14)

ThePdactivationprocessisconductedinasolutionof0.4 g/L+5mL/L HCl for 2–5 min at 25–40°C.

If the sensitization and activation is conducted simultaneously, the process can take place in a solution of [(PdCl 0.25 g/L)+(NaCl 250 mL/2L)+ (SnCl 0.5–5 g/L)+(NaSnO 0.5 g/L)+(HCl 10 mL/L)+(Urea50 g/223L): (pH 0.7–0.8)] for 6 min [45].

The formed Pd has colloid features and can absorb the Sn ions. Then peptization is conducted to facilitate the Pd deposition on the surface by cleaning in 100 ml/L HCl or soaking in 50 g/L NaOH for 1 min. If cleaned in 3％ sodium hypophosphite, the water-cleaning process need not be conducted before the chemical Ni plating [48]. Some other results [49] indicated that the substrate can be chemically plated without the Pd activation process.

The chemically plated Ni is actually an amorphous or multicrystalline Ni-P alloy, and the plating liquid can be NiSO 20 g/L24+NaPO (30 g/L), sodium citrate (10 g/L)+ammonium chloride (30 g/25L). The pH value is adjusted to be 8.5–9.5, the plating is conducted in the liquid at 35°C–45°C for 3–5 min. The liquid can also be [(CuSO·45HO 10 g/ L)+(NiCl 2 g/L)+(CHKNaO 50 g/L)+(NaOH 8 g/L)], and 22446the plating time is 30 min.

The electroplating then can be conducted after the conduction treatment, and no brightening agent is used in the plating liquid. The liquid is [NiSO (250g/L)+NaSO (30g/L)+NaCl (10g/L)+Mg SO (40g/24244L)+HB (35 g/L)] with pH value of 5–5.5. The plating is conducted at 220°C–35°C with a current density of 0.8–1.5 A/dm and the time depends on the required areal density. The electroplate liquid with composition of [NiSO (250 g/L)+NiCl (40 g/L)+HBO (40 g/4233L)+CH–〈benzene ring〉–SONa] also can be used. The plating 12253temperature is 55°C, and the current density is2

2.0 A/dm [49]. The anode is Ni sheet or plate. The liquid should be filtered regularly, and the compositions also should be adjusted.Cu Foam Preparation

Cu foam has good electrical conductivity and ductility, so it is used as electrode materials in batteries. However, the application of Cu foam is limited due to its poor corrosion resistance compared to Ni foam. The preparation process is as follows: (1) soaking in polyurethane (with a thickness of 2.4 mm); (2) cleansing; (3) roughing; (4) a second roughing; (5) sensitization; (6) activation; (7) chemical deposition; (8) electrodeposition; (9) burning; (10) thermal reduction; (11) Cu foam is created [50]. The two-step roughing process, with the first step involving [(KMnO 8.0 g/L)+(HSO(d.1.84) 5.0 mL/L)] for 10 424min and the second step involving [(CrO 3g/L)+ (HSO(d .1.83) 4 mL/324L)] for 24 h, is effective. A water film can be absorbed easily on the rough surface.

The SnCl solution is used as the sensitizer after roughing with 2the compositions of [(SnCl·2HO 20 g/L)+(HCl(36％) 40 mL/L)]+Sn 22powder. The porous body is soaked in the sensitizer for 5 min and then rinsed with flowing water. The resulting reactions are:SnCl+HO→Sn(OH)Cl↓+ HCl　(2-15)22SnCl+HO→Sn(OH)↓HCl　(2-16)222Sn(OH)+Sn(OH)CL→Sn(OH)Cl↓　(2-17)23

The Sn(OH)Cl is in the form of gel, which has little solubility in 23water, and an absorbed layer of Sn(OH)Cl on the porous body plays 23a key role in the chemical deposition within this layer.

The sensitized surface can absorb a layer of metal particles with a catalyzing effect, and then it can be the catalysis activation center for the chemical deposition. The activating liquid is [(PdCl 0.2 g/2L)+(HCl(36％) 1.0 mL/L)], and it takes 5 min to produce the following reaction:2+2+Pd+Sn(OH)Cl→Pd↓+Sn　(2-18)23

Sn(OH)Cl is still a gel, and the formed simple Pd is absorbed into 3the gel layer. The activating liquid can be used repeatedly.

Pd is covered by the gel layer, and this layer should be removed before the chemical deposition to expose the Pd atoms. A 3 mL/L HCHO (pH 8–9) HCHO water solution can be used to remove the gel 3at room temperature in 5 min.

HCHO can be a reducer on the catalysis-activated surface, and a Cu layer is then deposited on the surface of the porous body:2+－－Cu+2HCHO+4OH=Cu↓+H+2HCOO+2HO　(2-19)22

Due to the catalysis effect of the newly formed Cu by the reaction in Eq. (2-19), the deposition can be continued until the required thickness is reached. The liquid is prone to decomposition, and then the stabilizer need to be used; CuSO and HCHO are added after the 4stabilizer has been used 10 times. The optimal liquid is [(CuSO·5HO 4212.0 g/L)+(EDTA 42.0 g/L)+(NaSO 20.0 g/L)+(stabilizer 4.0 g/24L)+(HCHO 20.0 mL/L)], with a pH of 12.5–13.0. The deposition time is 10 min, and the temperature is 25°C.

The solution of [(CuSO ·5HO 70 g/L)+(NaCl 0.60 g/42L)+(polyethylene glycol 0.03 g/L)+(sodium dodecylsulfate 0.05 g/L)+(HSO 25 mL/L)] is used to deposit the Cu layer with a uniform 24structure and ductility. The current increases while the voltage decreases within the intial 2–3 min of plating due to the depositon of the Cu layer with poor electrical conductivity, followed by improved electrical conductivity and the formation of crystallized Cu.

Therefore, a short period of pre-plating until stable electrical conduction occurs is necessary for the preparation of the uniformly structured products. For example, preplating is conducted for 3 min 2with 5 V to reach an optimum current density of 0.3 A/cm for 25 min of electrical deposition of Cu.

If the current density is higher, H will be released to reduce 2efficiency; while if the current density is lower, the needed deposition time will be longer. The deposition temperature has some influence on the internal stress of the layer, the dispersivity of liquid, and the deposition rate. Increasing the temperature will lead to reduced internal stress, no cracking, and an increased deposition rate, but low dispersivity. Therefore, the temperature of 40°C–50°C is selected for the deposition of Cu.

Based on this discussion, the best parameters for the electrodeposition are pre-plating at 5 V for 3 min, followed by 2deposition with a current density of 0.3 A/cm at 40°C–50°C for 25 min. The organic foam in the Cu foam can be eliminated by burning it off after drying. The surface color of Cu foam will become dark with the formation of CuO after burning off, and the Cu foam is then reduced in N/H(1:3) at 700°C to increase the strength and ductility. With 22increasing the reduction temperature till 700°C, the tensile strength increase accordingly, while it will decrease with continue increasing temperature above 700°C.

2.6.3 Reaction Deposition

In reaction deposition, an open-cell porous body is put into a container with the gaseous metal compounds and then heated to the decomposition temperature of the metal compounds. At that temperature, the decomposed metals will be deposited on the porous body to form a layer of metal. Finally, a porous metal is produced by sintering the open-cell metal coated porous body. For example, Ni foam with hollow sectional threads can be prepared by using nickel carbonyl [51].

This process can be realized by an effective thermal decomposition reaction at low temperatures. Nickel carbonyl is one kind of gas, and the reaction formed is Ni+4CO→Ni(CO). Ni and CO 4can be decomposed at temperatures of above 120°C. A solid Ni layer is plated when porous plastics pass through nickel carbonyl gas. If the process is repeated, the Ni layer will become thicker. If heated by the infrared ray, the polymer base will be stable at the decomposition temperature of the nickel carbonyl. A hollow reticulated metal is obtained after the substrate is removed by heat treatment or chemical reatment.

2.7 HOLLOW BALL SINTERING

Porous metals prepared by the hollow ball method have the following features: low density, good energy absorption, thermal exchange capability, and a high ratio of strength to weight [52]. These kind of porous materials are different from other traditional open-or closed-cell metal foams, in that the closed pores take a certain volume fraction and there are gap pores between each sintered ball. Therefore, these metals feature a mixture of pores with open and closed cells.

2.7.1 Preparation of Hollow Balls

The hollow balls made from different kinds of metals and alloys, like the Ti alloy, Ni alloy, and stainless steel, are prepared by the slurry and gas atomization methods. Strict controls can be executed in the process.Slurry Method

Slurry injection for the preparation of metals, intermetallics, and metal hydride powders was performed by the coaxial nozzle-injecting process by taking advantage of surface tension to form the balls. Alternatively, the polystyrene balls were sprayed and coated with the metal hydride slurry. The semi-dense hollow balls can be obtained by either process, and metal hollow balls then are obtained after heat treatment [52]. The nominal particle sizes of the hollow balls are 1 μm for both processes, and the semi-finished products become fully dense metals by reduction in the H atmosphere at a high temperature. The metal balls then were sintered together, either after or during reduction.

When the polymer balls are taken as the carrier, the slurry can be a suspension of the binder and metal powders, or the balls are deposited with any kind of metals by chemical and electic deposition. Finally, the balls are burned to obtain a dense metal shell by removing the polymer [5].

The microballs of metals, intermetallics, and metal hydrides prepared by the coaxial nozzle injection can be dried in the burette and then sintered [5]. A low-oxygen partial pressure is needed in the annealing process for the transformation of iron oxide to stainless steel balls [14].Atomization

The hollow balls can also be formed during molten metal atomization by using the proper parameters. Ar can be captured in the liquid drops during the atomization of the metals and alloys. During the throwing process of the liquid drops, Ar will expand untill the drops solidify, and then the hollow powders are obtained [14]. The hollow balls can be separated from the powders by the floatation method.Figure 2.26　Preparation process for hollow-ball porous metals [5].

2.7.2 Preparation of Porous Bodies

The hollow balls can be combined by isostatic or liquid sintering [14]. This process is very important since the property of the porous body is sensitive to the amount of the contact area for each ball.

The hollow balls of Cu, Ni, and Ti metals and their alloys with high porosity can be obtained by the sintering process shown in Figure 2.26 [5]. The light porous materials made by the hollow balls have pores with open and closed cells; porous materials with open-cell pores are obtained by sintering the agglomerated hollow balls, and the sintered neck can be generated on the neighboring balls. When the force is applied to the balls during sintering, the balls can deform into the shape of polyhedrons lead to the increased contacting surface area and also reduced open-cell porosity. The contact of the balls can be improved by using a binding slurry.

The closed-cell pores can be obtained by filling the gap between the balls with metal powder and then sintering. Liquid metals also can be used to fill the gap, and it is better to exert force and preheat the balls to make sure that the gap is totally filled. For the preparation of the closed-cell pores, it is enough to use thin-walled balls. The sandwiched structure can be obtained by sintering a hollow ball between two plates; both the combination of the balls and the combination of balls and plates are included.

One of the advantages for the hollow structure is the nonrandom distribution of the pore sizes, and the pore sizes can be adjusted by proper selction of the hollow balls. The mechanical and other physical properties of the hollow balls are more predictable than those of “actually obtained porous materials” with randomly distributed pore sizes. Another advantage is that the hollow ball process is applicable to all materials prepared by the powder metallurgy process, like superalloy, Ti alloys, and intermetallics. Therefore, the hollow balls can be used at high temperatures.

2.7.3 Fe-Cr Alloy Porous Products

The balls with an outer diameter of 2 mm are coaxial nozzle-injected with metal hydride slurry by using Fe-and Cr oxide powders, and then returned to a semi-dense state as they fall [52]. After that, the balls turn into FeCr base alloys with a porosity of less than 2％ after being reduced in hydrogen. The obtained metals actually are 405 ferritic stainless steel (Fe-12Cr) afterreduction, with a grain size of 10–20 μm. The grain size is much smaller than the wall thickness (0.1 mm), and then it can be completely reduced. Due to the effect of gravity and inertial forces in the coaxial nozzle injection process, the variation of the wall thickness is about 50％.

The reduced balls were then poured into the mold vibrationally and sintered at 1, 350°C for 48 h until full bonding from the diffusional 3effect is reached. The density of the porous body is 1.4 g/cm and corresponds to a relative density of 0.16.

2.8 PREPARATION OF THE DIRECTIONAL POROUS METAL

2.8.1 Solid-Gas Eutectic Solidification

Solid-gas eutectic solidification, also known as GASAR, is a newly developed technology for the preparation of porous metals. It is a casting method [5, 53]. In this process, the liquid metal with the dissolved gas (H) solidifies at the eutectic temperature (with a low co-2melting point), and H is separated due to low solubility of solidification. 2Then the metal solidifies and the pore's nucleus forms simultaneously at the eutectic temperature. The eutectic temperature depends on the system pressure, and the porosity can be adjusted by controlling the H pressure in the cast cavity. The temperature has a great effect on 2the H solubility in the liquid metal, so the melting temperature and the H pressure before solidification must be adjusted to match the dissolved H in the melt at the eutectic temperature. If the process is not performed with the proper temperature and pressure, and not at a eutectic temperature, some deputy eutectic phases will be formed with nonuniform microstructures. The pore size can be adjusted by the cooling rate, and small pores will be formed with an increasing cooling rate to reduce the H diffusional distance. GASAR has been used to produce porous metals like Ni, Cu, Mg, Al, Mo, Be, Co, Cr, W, bronze, steel, and stainless steel with a pore size of 5 μm–10 mm and porosity of 0.05–0.75. The porosity depends on the solubility of Hin the melt, and theporesizedepends on thediffusional coefficientof H in the melt. A single cast can be produced with alternate overlapping of dense and porous layers by GASAR. Sandwich-structured metals with honeycombs or foam cores can be produced without bonding, and they can be used as transportation parts due to its high shock resistance and energy absorbing ability.

GASAR can be performed in the sealed pressure vessel [8, 53]. The gas can dissolve into the molten metal to some degree, and the solubility will increase with increased pressure and temperature. The metals and gas will go through the eutectic solidification process and then form metal foam when the gas reaches its solubility limit. The isotropical and anisotropical metals with high porosity and different pore structures can be developed by controlling the system pressure, cooling rate, and thermal gradient direction (radiation direction) [8, 54]. For example, the pores with longitudinal or radial arrangement can be prepared with cooling of the mold [54] from the bottom or side. The porous honeycomb structure, similar to wood, can be obtained by arranging cylinderlike pores with a high shape ratio (i.e., the ratio of height to width, which is about 10 in this case); see Figure 2.27. Its bending rigidity is as high as that of the highest-quality engineering materials.

A uniform melt filled with hydrogen can be obtained when the eutectic metal system is melted in a hydrogen atmosphere with a pressure of 50 atm [5, 55]. The melt can transform into the heterogeneous two-phase (solid+gas) system through the eutectic reaction that takes place when the temperature decreases. The precipitation reaction occurs at a certain temperature when the system compositions are close to the eutectic position. The outside pressure must be compatible with the H content since the eutectic compositions depend on system pressure. If the heat is removed in the designed solidification direction, directionally solidified materials can be obtained. The advancement of the solidified frontier has a rate of 0.05–5 mm/s, and bubbles will be produced just before the frontier (when hydrogen goes out of the solidified metal). The selection of the processing parameters is done to ensure that the bubbles float out of the liquid and are kept in the solidified metal [56]. The pore morphology strongly depends on the H content, pressure above the melt, radiation direction, rate, and the chemical compositions. Generally, the elongated pores depend on the radiation direction and show the shape of round in the vertical direction. The pore size is 10 μm–10 mm, the pore length is 100 μm–300 mm, the morphology ratio is 1–300, and the porosity is 5％–75％ [55]. The pore size range is so wide due to the merge of small and big pores. Sometimes the pores can take the shape of a taper or ripple.Figure 2.27　Cross section of a porous product with cylindrical pores prepared by GASAR and the natural porous body [54]: (a) transverse sectional porous Cu product (with relative density of 0.84); (b) longitudinal section of porous Cu; (c) transverse section of Norway spruce (with relative density of 0.3); (d) longitudinal section of Norway spruce.

A practical GASAR facility is shown in Figure 2.28 [5]. Hydrogen is filled in the pressure jar for the melt, and directional solidification is realized at the last stage of preparation.

The hydrogen tends to diffuse in the pores before the frontier of solidified metal, and the pores are elongated in the direction of the solidification [57]. In a single-directional cooling process, ball-like pores can be produced through the pulse of pressure in the cast cavity. Materials with gradient porosity and alternating solid/porous layers can be produced by changing the processing parameters. The pores that may be produced are shown in Figure 2.29, which demonstrates that pores can be in the grains and along the grain boundaries. The pores occupy more than 50％ of the surface of the grain boundaries and are not used to evaluate porosity. The elongated grains and pores are typical in the directional cooled metals. Moreover, the grain sizes for porous copper are notably smaller than that of solid copper.Figure 2.28　Schematic showing of the GASAR facility [5]: 1—gas supply; 2, 4—melt; 3—mold; 5—hot sink; 6—solid/gas eutectic solidification; 7—outer wall for compression; 8—internal cavity; 9—heating part; 10—insulator; 11—funnel.

2.8.2 Directional Solidification

The directionally solidified porous metals were prepared successfully with H, O, and N as foaming gases in the 1990s [58–22260]. The pore distribution is quite uniform, and the pores have a radius of 10 μm–10 mm with length not above 80 mm and porosity of not over 80％. It has special properties that are different from that of sintered bodies and foamed metals.

The principle for the directional solidification is similar to that of the solidgas solidification, andit isbased onthegas solubility difference in the melt and in the solid metals. The main difference is that there is no requirement for a eutectic system in directional solidification and no precipitation at eutectic temperatures during solidification compared to solid-gas eutectic solidification. The gas will exert pressure on the molten metals due to the solubility difference of gas, and this will increase the solubility of gas atoms in the melt. Finally, cooling is applied to make the melt solidify in the designated direction. The gas atoms will be oversaturated with reduced solubility in the melt during solidification, and then they escape in the form of bubbles. During the growth of the bubbles, the surface area and interface energy at the bubble/liquid phase increase. The contact area (interface energy) between the gas bubbles and theliquidphasedoesnotchangeifthesolidificationrateisthesameasthebubble growth rate. Hence, the bubbles stop growing or even float up from the solid and can grow only in the solidification direction. That is, the bubbles are elongated and form the cylindrical pores along the solidification [58]. In order to control the pore shape and number, the proper selection of the melting temperature, the mixture ratio of foaming gas to inert gas, gas pressure, and the solidification rate is needed. Furthermore, the thermal gradient and impurities shall be controlled carefully, and the convection of melt must be restricted in order to avoid the separation and escape of gas bubbles from the solid during growth, as well as the connection of the bubbles.Figure 2.29　Different pore morphologies by GASAR [57]: (a) spheric pores; (b) radial pores; (c) cylindrical pores; (d–f) overlayer of solid/porous body/solid (left to right, the spheric pore, the radial pore, and the cylindrical pore).

For the preparation of porous copper, highly pure copper is melted in the high-frequency induction furnace with careful control of the pressure of H and Ar (both partial pressure of H and Ar is in the range of 0–1.0 MPa) [59, 60]. After H is dissolved in the molten copper at 1, 523 K for 1, 800 s, the copper melt is poured into the mold with the bottom water cooling (Figure 2.30), and the melt solidifies upright, in a single direction. H has a much lower solubility in solids than in liquids, and most of the H in the copper melt cannot dissolve in solid copper. H cannot stay in the solid-liquid interface at a constant temperature and then form elongated pores in the direction of solidification. The obtained cast has a diameter of 30–35 mm with a maximum height of 80 mm. Figure 2.31 shows the transverse and longitudinal section structure of porous copper with the shape of lotus root in the transverse section (by electric spark cutting). The porosity is determined by measuring the weight and the volume of the sample, while the average pore radius is measured with the photo analysis system. The porosity decreases with the increase of the Ar partial pressure under constant H partial pressure, whereas the porocity increases and then decreases under more H pressure with constant Ar partial pressure. Pore size, direction, morphology, and porosity can be influenced by the melting temperature, H and Ar partial pressure, the pressure ratio of H to Ar, and the solidification rate. Thereofore, all kinds of porous metals can be prepared by directional solidification through controlling these parameters.Figure 2.30　Directional solidification facility for the preparation of porous metals [59].

The strength of porous metals when the pores are aligned in a specified direction is better than the strength of metals with randomly located pores [59]. If N is used as the foaming gas and the pore wall is nitrided during the formation, the metal is stronger than if it is prepared with H or O as the foaming gas [58]. Additionally, a good dampening effect is demonstrated for porous metals due to the increased friction and internal deformation of metals accompanying gas atom diffusion.Figure 2.31　Transverse (top) and longitudinal (down) sections of directionally solidified porous copper [59]: (a) hydrogen with a partial pressure of 0.8 MPa, argon with a partial pressure of 0, porosity 32.6％; (b) hydrogen with a partial pressure of 0.4 MPa, argon with a partial pressure of 0, porosity 44.7％.

A further surface nitriding process may be applied to increase the toughness, hardness, and wear resistance for the directionally solidified porous metals [58]. If N is used as the foaming gas to prepare the porous metals, good properties will be achieved due to the selfnitriding. Moreover, nitriding can increase the wear resistance of the porous body with good chemical stability. Therefore, directionally solidified porous metals can meet the requirements for use as human bone joints.

2.9 OTHER METHODS

2.9.1 Powder Melting Foaming

The powder melting foaming process is similar to metal powder sintering in the solid sintering process. The only difference is that the heating temperature is above the melting point of metals for liquid sintering, while the solid process is under the melting point of metals for solid sintering.

In this process, metal powders (single metal, alloy powder, or a mixture of metal powders) are mixed with the particles of the foaming agent and transformed into a near-dense semi-product. The obtained semi-product is heated to a temperature above, but close to, the melting point of the related alloys, and the foaming agent decomposes and releases gas to expand the semi-product and form the porous materials [5, 61]. The final product is usually a closed-cell foamed body, and the porosity mainly depends on a couple of key factors, including the content of the foaming agent, heattreatment temperature, and heating rate. The metal powders and foaming agent are mixed with the rolling mixer, and the gas-releasing agents can be distributed uniformly in the mixture [62]. Densification can be performed with powder extrusion, axial thermal pressing, powder rolling, or isothermal static pressing depending on the required shape. It is economical for the extrusion process, and the sheet can be rolled. The foaming agent particles must be buried in airtight base metals to prevent the released gas from escaping the connected pores before the expansion, so it has no effect on pore generation and growth. The foaming agents can also be poured into the hollow die in an appropriate shape and then heated to the required temperature, and the final parts can be manufactured in various shapes.

The time needed to reach full expansion depends on the temperature and the size of the preformed part and ranges from several seconds to a number of minutes. TiH and ZrH can be 22employed as the foaming agents for Zn and Al alloys, while SrCO is 3used for steel. The metal hydrides are used as the foaming agents, and less than 1％ should be enough.

Besides the preparation of Al and Al alloys, it can be used for the preparation of Sn, Zn, brass, Pb, Au, and other metals and alloys with the proper selection of the foaming agents and the process parameters. The commonly used foaming agent is pure Al or a precision cast alloy such as the 2×××or 6×××alloy. Cast AlSi7Mg(A356) and AlSi12 have low melting points and are also used as the foaming agents. Generally, the pore size distribution and shape of the obtained products are random.

Compared to Al foam, steel foams have the following advantage [61]:(1) high strength and high ratio of stiffness to density; (2) low raw material cost; and (3) compatibility of the melting points to the structural steels. It can be prepared by the following processes: mixing the commercial steel powders (Fe-2.5Cr) with the particles of foaming agent, densification, and melting the densified part at 1, 300°C, which leads to the expansion of the foaming agent at the heating rate of 30°C/min. The total heating time will be 5 min. Due to the big difference in density between the steel and the foaming agent, it is significantly important to distribute the mixtures of powders uniformly. The foaming agent (SrCO or MgCO)of 0.2 wt％ and carbon of 2.5 wt％ are added 33to the steel powder to achieve a better sintered result.

The top priority in this task is to select the proper foaming agent for the steel foams [61]. The foaming agents shall be decomposed at temperatures close to T (the melting point of the alloy) and release menough gas to ensure that the foaming pressure is higher than the environmental pressure. T (decomposition temperature) shall be Dbetween the solidus and liquidus (1, 250°C–1, 350°C). SrCO and 3MgCO can be used as the foaming agents and they are decomposed 3as follows:

SrCO(T=1, 290°C)3DSrCO→SrO(s)+CO(g)　(2-20)32

MgCO(T=1, 310°C)3DMgCO→MgO(s)+CO(g)　(2-21)32

The ideal decomposition temperature TD of the foaming agent shall be compatible with the melting temperature of the alloyed steels. If TD is higher than the melting temperature of the steel, the foaming agent dissolves into the melt or floats on the surface of the melt, while the preformed part breaks from the high internal pressure so long as T is lower than the melting point. When the requirements are met, Dsteel foam with the required porosity will be produced by the careful control of heating and cooling.

The final density and quality of the foamed body will be strongly influenced by the compositions of the mixtures, including the content of the foaming agent and the carbon. The carbon content has a notable influence on the foaming behavior and the mechanical property of the foam body. The addition of 2％–3％ carbon improves the foaming property and the base metal strength, as well as decreasing the melting and foaming temperatures.

Expansion of the foam after melting is the key procedure in the process, and the mixing procedures also strongly affect the foam density and pore distribution. On the other hand, it has a negligible effect on the densification of the foam compared to pressure. At the melting point, the viscosity of the alloyed steel melt is relatively small, and the coarsening of pore comes quickly with the expansion of the foam. Therefore, the duration at the peak temperature is controlled over several minutes to reduce the possibility of pore coarsening while allowing the expansion of the foamed body. The relative density of the final foamed steel (Figure 2.32) is 0.41–0.45 with an average pore size of 1–1.3 mm.Figure 2.32　Pore structure of foamed steel [61]: (a) MgCO as foaming agent; (b) 3SrCO as foaming agent.3

2.9.2 Investment Casting

The investment casting process is illustrated in Figure 2.33. The plastic foam (polyurethane) is poured into the container with the designated shape, followed by the refractory slurry. The foam sponge is burned and removed after drying and hardening, and a preformed mold is formed with the original designed 3-D reticulated plastic [1, 5, 8, 9, 15]. The molten metals are then poured into the inlet of the preformed mold, the mold is removed after the solidification (with pressurized water), and finally the foamed metals can be obtained, representing the original polymer sponge structure. If the gap is not big enough for the liquid metal to flow just from gravity, pressure and heating may be applied [5].

Porous metals with low melting points like Al, Cu, Mg, Pb, Sn, and Zn and their alloys can be developed by this method. However, it is difficult to fill the filaments completely, to control the directional solidification, and to remove the mold materials without damaging the microstructure [5]. The obtained porosity is in the range of 2–16 /cm (5–40 ppi). The complex parts can be prepared with the premolded polymer foam. The density and morphology of the porous metal products with the porosity of 80％–97％ is determined by the premolded polymer.Figure 2.33　The process for investment casting of porous metals [5]: (a) polymer foam as precursor; (b) particle stacking as precursor.

2.9.3 Self-Propagating, High-Temperature Synthesis (SHS)

Self-propagating, high-temperature synthesis (SHS), also known as combustion synthesis, has developed as a technology for material preparation over the past 30 years [3, 63, 64]. Intermetallics and composite materials can be prepared by this method. The working principle behind this method is that the synthesis of the materials is maintained by self-made heat from the chemical reaction. The reactants change into the resultant during the burning that takes place after the reaction begins. Due to its high reaction rate and the high thermal gradient, a great density of defects in crystal lattice will be generated and then the porous frameworks are formed easily in a large surface area. It has the advantages of short production cycle, low energy consumption, simple process, and low cost.

The porous TiNi alloys have good potential applications in the medical field due to their superelastic and shape-memory properties, and they also have excellent biocompatibility, high strength, good shock resistance, and antiwear/anticorrosion property [65, 66]. A TiNi alloy with a porosity higher than 35％ cannot be prepared by the traditional casting process or by powder metallurgy [65, 67, 68]. On the other hand, SHS in the Ar atmosphere can be used to prepare TiNi alloys with higher porosity by mixing pure Ti and Ni powders with atom ratios of 1:1 and pressing into a green body of 65％ density [65]. The green body after the SHS process will be maintained, but with a length 3extension of 70％, an apparent density of 3.15 g/cm (whereas the 3density of dense TiNi is 6.45 g/cm), and a porosity of 51％, which is a significant increase over 35％ in the pressed green body before SHS. The maximum pore radius is 100–150 μm, and the relative 32permeability coefficient is 1750 m/(h.kPa.m). It is indicated that the TiNi porous alloys prepared by SHS has good interconnection of pores. The pore structures are more complex than that of sintered porous metals. The pore shapes are irregular, and there are two types of pores: large, opencell pores with a size of hundreds of micrometers and small, closed-cell pores of size less than 10 μm. The closed cells are mostly on the walls of the pores (Figure 2.34). The pore wall is mainly the collective body of the small particles with closed cells. The main phase is TiNi, and there are also small transition phases of TiNi 2and TiNi. No pure Ti and Ni single phases are found in the X-ray 34diffraction (XRD) results. This shows that Ti and Ni powders can combine within several seconds of reaction time.

There is a big difference between the counterdiffusion of Ti and Ni elements at high temperatures, and the unidirectional migration is demonstrated since the diffusion of Ni in Ti is much higher than vice versa [65]. Based on that fact, porous TiNi alloys can be prepared by SHS: the vacancy is left with the Ni diffusion away and the compound of Ni and Ti will be formed, leading to volume and porosity increases, along with the release of heat.Figure 2.34　SEM morphology of the porous TiNi pore structure [65]: (a) cross section; (b) fractured surface.

The effect of the main processing parameter (i.e., preheating temperature on the uniformity of pores in TiNi alloys) was investigated [66]. Ti (99.5％ purity) was mixed with Ni (99.6％ purity) with sizes of＜50 μm at an atom ratio of 1:1. Then the mixture is dried in the oven in vacuum at 100°C for 6 h and mixed for another 6 h. The mixed powders were put into the steel mold and pressed into a cylindrical green body of Φ20 mm×20–30 mm in a thermal press set at 7 MPa. The density of the green body is 45％ of the solid density; at that point, it is put into the graphite mold. After that, the mold is put in the SHS reactor with flowing Ar gas of 1 atm and heated to different preheating temperatures(0–600°C) at a heating rate of 100°C/min. The SHS process was ignited by a W wire, and the preheating temperature (T) oand the maximum temperature (T) are recorded by the X-Y recorder cand the W-Re thermocouple. It is found that the pores in the resultants are beltlike and distributed uniformly when the preheating temperature is lower than 250°C. The resultants are in a solid state or a semisolid state with small amount of liquid phases, and they have low porosity due to the fact that the gas does not expand fully.

The porous body is not ideal since the beltlike pores have a low bearing capability. When the preheating is conducted at 250°C –400°C, the pores will be spheric and uniformly distributed. Many irregular or spheric particles are found on the inner wall of the pores by SEM examination. It is indicated that the results produced at the maximum reaction temperature are in the coexisting zone of the liquid and solid phases, with moderate content in the liquid phase. The viscosity is low, and the gas expands more completely and leads to the formation of spheric pores at this temperature. With increasing the preheating temperature, the reaction temperature also increases and pore sizes enlarge. However, at this time, there are still more solid phases, and the viscosity is still high. The gas expansion cannot break the walls of the pores, but it restricts the floating and agglomeration of pores, leading to the uniform distribution of the pores. The liquid phase fraction is about 20％–35％ at the reaction temperature; and when the preheating tempera-ture is 400°C, the resultants have a porosity of 70％. When preheating is conducted between 400°C and close to 600°C, the pores in the resultants are still spheric but no longer uniformly distributed. The pores are mostly in the upper part of the product and have smooth inner walls. The liquid phases are dominant at this reaction temperature with low viscosity, and the resultant cannot restrict the floating and agglomeration of pores to generate nonuniform distribution of the pores. In the meanwhile, the porosity also decreases due to the floating and expelling of the gas.Figure 2.35　Morphologies of porous TiNiFe shape memory alloys made by SHS with different porosities [16]: (a) 56.8％, (b) 59.6％, and (c) 62.3％.

Figure 2.35 shows several porous TiNiFe(at％) shape 50482memory alloys fabricated through combustion synthesis, which can be promising porous implant candidates.

2.10 PREPARATION OF POROUS METAL COMPOSITES

The preparation of porous metal composites is subject to reprocessing or combined processing based on the abovementioned methods. Examples of this include the redepositing or filling (casting) of a porous metal with other metals, alloys, or nonmetals; welding and bonding of porous metals with other structural metal parts; or making porous bodies by mixing metal powders, fibers, and other materials (like composite porous electrode materials with metal and carbon fibers); and sintering of Ni powder and fiber with addition of a pore-forming agent (NH)CO).423

The porous Al and Al alloy composites with metal reinforcements were prepared by die-casting and squeeze casting in 1980s, and they were used as the pistol materials in internal combustion engines [69]. Pistols are produced as follows: The die is heated to 200°C–400°C and a metal reinforcement [(1.0–20.0)Cr-(4.0–30.0)Ni-(0–3.0)Mo-(0–3.0)C-(0–8.0)Cu -(0–3.0) Si-(0–9.0)Mn-Fe, with minimum pore sizes of＜3.0 μm] is heated to 400°C–750°C. Next, molten Al alloy of 680°C–820°C is poured into the die. Then it is pressed with a pistol-like punch head. The punch head is pressed with its own weight until the Al alloy solidifies, and then the pressure is increased to 2, 000 bar (1 bar .0.1 MPa≈1 atm). At last, the molten Al alloy is pressed into the designated structure withthe pore reinforcement. The Ni-Al alloy is formed at the interface. The composite is taken out from the die after complete solidification and then machined into the final product. The thermal and mechanical loadingproperty is greatly improvedforthe Alalloycomposite.

In the 1990s, a method [70] was invented to prepare porous metal composite material used for gas sensors, electrodes in fuel cells, and chromatography separators with the purpose of reducing costs by plating a noble metal layer onto low-cost porous base materials, while not reducing the activity of the metal. It includes the processes of metallization on the porous base materials and the oxidation and reduction of the metal layer. The oxidation of the metal layer is realized by the oxidative plasma and the reduction is achieved by reductive plasma in the cold state. The oxidation and the reduction processes for the metal layer can increase the numbers of pores in the layer, the microroughness, and the active surface area. The vacancies are created by removing the original oxygen atoms from oxides in the reduction process and the O atoms react with the H atoms. The ceramic or polymer base materials that are not reactive to the plasma can also be metalized in gas, and the plated metals can be Pt, Pd, Ag, Ni, and their alloys. The ratio of metal to base materials must be less than 1:1, and preferably less than 1:100. Oxygen plasma is used for the oxidation, while ydrogen plasma is used for reduction.

Inorganic film is well suited to the processes of microfiltration, ultrafiltration, gas separation, and film reaction due to its good heat resistance and chemical stability [71]. The traditional inorganic porous film has a ceramic supporting base, and it is prone to be damaged during use. Moreover, the sealing and joining of the ceramic composites are very difficult to accomplish at high temperature and under high pressure. These difficulties can be easily overcome by using a porous metal substrate. A SiO membrane was prepared on 2the porous Ti base by the sol-gel method using TEOS. The membrane has been found to crack easily during preparation or use due to the differences in temperature and large thermal expansion coefficient (TEC) between the base metal and membrane materials. The porous metal-SiO composite membrane can be prepared by filling the pores 2with gel particles and by forming much smaller pores in a controlled depth of the base metal surface filled with sol. The whole sol-gel, drying, and buring process is repeated 8–10 times.

The key technical points for the membrane filling process are as follows: (1) the sol with different particles can be prepared by the sol-gel method under different conditions by multiple fillings of the micropores; (2) the penetration depth can be controlled by using the proper organic solvent; (3) membrane cracking can be prevented by improved aging and burning processes. The SiO sol is prepared by 2the hydrolysis and polycondensation of TEOS in aqueous alcohol solution with the addition of a certain amount of diethanol amine and CMC. The hydrolysis of TEOS can be accelerated with catalyzers (acidic or alkalescence). A linear molecular polymer with low molecular weight tends to be formed with the acidic catalyzer. Common acidic catalyzers are HCl, HNO, HAc, and HF. The pH value shall be in the 3range of 7.8–8.0 if HCl is used. The overfiltration of sol into the substrate may affect the flux of gas through the membrane. Therefore, the substrate can be immersed in an organic solvent (heavy hydrocarbon or haloogenated hydrocarbon) to prefill the pores in the substrate, and then the substrate with membrane is immersed into the sol with the proper immersion depth and time. A gel layer is formed in the pores on the surface of the base through the change of sol to gel by the dissolution of ethanol into the organic solvent. In order to prevent the gel layer from cracking, a steam bathing process at a constant temperature can be performed by putting the substrate with membrane above the bath with a water temperature of 50°C for 10 h, and then drying in air at room temperature for 24 h. For the burning process, the substrate with membrane is heated to 773 K at a heating rate of 1 K/min and then held for 300 min to make the gel layer change into SiO2 film. Finally, the mixture is cooled to room temperature at the same rate. The process may need to be repeated to obtain a membrane without defects, and the ratio of water to ester is adjusted to match the gel particle size and pore size in the substrate.

A porous metal oxide layer was prepared on the inner surface of porous iron-base materials by the sol-gel, dipping, or perfusing method to improve the catalyzing activity on the inner surface of the carrier. The oxides can be alumina, silica, and titania. These oxides can be the carrier of the active catalyzers and used for the heterogeneous multiphase catalysis in the gas reaction. If an alumina layer is needed for the Al-containing porous metal substrate, high-temperature oxidation can be conducted to obtain the alumina layer, and then another layer of alumina is produced by the sol-gel process to improve the combined strength of the different layers.

Other than the sol-gel method, the anode oxidation, chemical vapor deposition, and nanoparticle deposition methods can be used to deposit an internal oxide layer in the surface of porous metal substrate [72]. This is characterized by the wide range of composition selections, pore factors, and easy surface designs for the sol-gel method, and it can be adjusted by sol composition and processing.

The open-cell metal-organic composite materials can be used in catalysis, separation, and gas storage [73]. It is reported that a metal-organic reticulated composite materials can be synthesized with good stability at a heating temperature of 300°C [74]. The 3-D reticulated structure with a higher observable surface area and larger pore volume than that of most porous crystal zeolite can be obtained through binding the two-carboxyl coated with single-carboxylate to form the super-tetrahedron composite structure via the metal carboxylate chemical process.

The biomaterials used to make synthetic human bone joint can have a lifetime of 25 years or more by maintainng lubrication through the porous metal composite yielding layer [75]. The sintered gradient stainless steel substrate with a porosity range of 10％–35％ is prepared to combine the yielding layer with the cuplike metallic body. The gradient structure can be obtained using different sizes of particles and pressing processes. A composite layer with a higher torsion bearing capability can be prepared by combining the polymer and the porous metal substrate by the traditional impregnating and pouring technology. The composite structure by the mechanical bonding of polymer in the gap of metal substrate can effectively increase the interface bonding strength in the composite and prevent the disastrous fracture of polymer after the interface failure. Therefore, the strain, loading, and the adhesion property for the porous substrate at failure is higher than that of the dense materials. The total porosity and related permission rate for a 316 L porous sintered body are influenced by the particle size, pressure, and sintering temperature. If products are sintered in an Ar atmosphere, the corrosion resistance is higher than that of products sintered in a vacuum or in 75％ H: 25％ N 22atmosphere, but they will be less hard.

Several unique properties can be demonstrated by the porous metal composite, and the sandwiched structure is an example of a simple, porous metal composite [76]. A composite structural material combining a dense shell with porous metals can have the optimized mechanical properties under a certain load [77–79]. It is well suited to applications in the automotive and aerospace industries due to its lightness, specific high level of stiffness, and good dampening performance [77, 80, 81].

It is easy to obtain a sandwiched panel by binding two plates of metal sheets to a porous metal core. A real metallic binding can be realized by rolling the Al or steel sheet onto foamable preformed materials, and a further deep drawing of the composite can be applied to deform the composite. Finally, the core is foamed and expanded, and the panel is maintained its dense state during heat treatment (see Figure 2.36). Porous Al can be made into the composite with steel or Ti and Al sheets. Al sheet melting during foaming can be avoided by selecting core materials and sheet metal with different melting points (e.g., the melting point for the sheet is higher than that of foaming materials).Figure 2.36　Sandwiched panel with a porous Al core (12 mmin thickness) and two steel sheets [5].

A tube or cylinder with a random shape can be filled with porous Al in different ways [5]: (1) the preformed rod for foaming can be inserted into the cylinder and then heated in the furnace to foam and fill it; (2) the hollow foaming materials are inserted into the cylinder while contacting the inner surface and then expanded centripetally. Another way to prepare the sandwiched composite is via thermal spraying of Al on the premolded porous Al, and then preparing the porous Al part with a dense outer shell. Of course, the application of this process is not limited to the tube parts.2

A sandwiched plate with area of 2, 500×1, 200 mm and thickness of 130 mm was produced by a German company in a recent study [82]. A flat sandwiched plate can be made into different shapes of products based on the requirements. The development is focused on the sandwiched structure with Al foam as the core.

With the progress of porous metal development, porous iron and stainless steel have attracted the most attention due to their low cost, high compression resistance, low TEC, and high thermal stability. It is also advantageous compared to foamed metals with low melting points because they have higher strength, improved energy absorption, and hightemperature capability, and they have great potential for applications in the automotive, shipbuilding, bridge construction, and transportation industries [77, 83–86]. Figures 2.37–2.39 show examples of sandwich-structured porous iron and stainless steel created with a simple process, and metallurgical bonding between the sheet and the core is demonstrated [87].

Iron or stainless steel foam is preferable to Al foam and its sandwich-structured products in terms of strength and weldability [84]. Iron foam, with its highly porous sandwiched structure, can be used as light and highly functional material in transportation, machining, and construction of structural parts [79, 83–86, 88, 89]. The reports on iron foam is not so popular due to the difficulty in processing, and no related sandwiched iron foam material has been reported until now.Figure 2.37　Sandwiched structure with porous iron and 304 sheets [87]: (a) iron foam; (b) bending sample of an iron foam sandwich structure; (c) plane sample of an iron foam sandwich structure.

2.11 SPECIAL PROCESSING OF POROUS METALS

The cutting process for the porous metal products needs to vary due to the required size and shape of the application. The traditional sawing, grinding, and drilling processes can be applied to porous metal products, but they have some problems, such as they distort the material to some degree and also damage metal foam with low density [90, 91]. For the highly required smooth surface of metal foam, electric spark cutting, chemical polishing, water jet cutting, or high-speed cutting can be applied.

What constitutes proper processing of metal foam depends on the quality requirements in place [91]. For example, a diamond saw is used to cut the intrinsically hard or hard-phase intensified metal foams, while electric spark cutting or chemical polishing is generally used for normal metal foams because other tools may damage the foam surface. The following sections describe the special machining process used for metal foams [92].Figure 2.38　Cross section of the interface at a plane/core of sandwiched iron foam [87]: (a) cutting-edge area; (b) cross-sectional part; (c) high magnification of (b).Figure 2.39　Stainless steel foam and the related sandwiched structure [87]: (a) 304 foam; (b) 304 foam/sheet sandwich structure.Numerical control (NC) Electric Spark Cutting

Electric spark cutting is applicable to metal foams made by powder metallurgy. The size of the material in this process is highly adjustable, and the surface quality is easily controlled. It has special advantages in the processing of small, ultrathin metal foams. It is indicated that pulse width is the main factor that influences surface roughness depending on the scale of the discharge pit. Therefore, reducing the discharge energy by narrowing the pulse width can lead to improved surface quality.Water Jet Processing

Wire-electrode cutting is not applicable to the composites of metal foam and nonmetals. In this case, sawing, milling, and water cutting are mostly used. Blade cutting may lead to damage like fracture, collapse, and stripping of the framework from the metal foam. High-pressure water cutting can prevent these issues. In high-pressure water jet cutting, water is pressurized and then jetted through a nozzle of very small diameter to produce a high-speed jet stream. Sand introduced into the water jet improve the cutting force. It is an ideal process for machining metal foam with a large area and resin-bonded metal foam. Water-jet cutting of metal foam is illustrated in Figure 2.40.

In this process, the surface quality is proportionally related to the cutting speed. A second or third cutting may be needed to achieve the desired surface quality. An obvious taper may be generated after a thick product has been cut, due to the expansion of the water flow, and the existence of tapering mostly affects the bottom of the product.Figure 2.40　Water-jet cutting of metal foam [92].Figure 2.41　Illustration of laser-cut metal foam [92].Laser Processing

The desired shape of products made by dense materials can be obtained with precision cutting with lasers. Specific process parameters are required to cut the metal foam due to limitations in the process. The surface of the metal foam is rougher than that of the dense metals after laser cutting, and uneven surfaces may be generated, as occurs with spherical cutting. A laser cutting of metal foam is illustrated in Figure 2.41.

It can be seen from Figure 2.41 that the surface is rough and uneven after laser cutting, and this is caused by a nonuniform thermal distribution. A layer of oxides forms on the pore wall in heating, and the oxides have a higher melting point than that of metals. Spherical cutting occurs due to the different melting rates for the surface and the interior of the foam framework. Metal foam with low density is easier to melt, so it also is much easier to cut into metal foam with small pores than large pores.

2.12 CONCLUDING REMARKS

There are many ways to make porous metals. The tendency is to create porous metals with high porosity, uniform structure, and good mechanical properties, and then expand their application areas. Most applications of porous metals demand higher porosities and higher specific surface areas based on the strength requirements, except for sandwiched structural materials and thermal insulation materials, which need a closed-cell structure. Therefore, large-scale production of 3-D reticulated porous metals is promoted. Currently, the applications of 3-D reticulated highly porous metals cover nearly all the applied areas for porous metals and even have expandeda little more, suchaswith filters; fluid mixers; heat exchangers; soundabsorbers; electromagnetic shieldmaterials; catalysts and their carriers; electrodes for NiCd, NiH, Li and fuel cells; cathodes for electrical synthesis and recycling of heavy metals; composite materials; and structural materials in the aerospace industry. The methods used to make porous metals are all applicable to the preparation of highly porous metallic materials except for metal deposition (electrodeposition), special powder metallurgy, and infiltration casting. It is clear that further development and preparation of highly porous metals are needed to explore theprospects of using these high-quality engineering materials.

REFERENCES

[1] Davies GJ, Shu Z. Metallic foams: their production, properties, and applications. J Mater Sci 1983; 18:1899–911.

[2] Liu PS, Liang KM. Functional materials of porous metals made by P/M, electroplating, and some other techniques. J Mater Sci 2001; 36:5059–72.

[3] Liu PS, Yu B, Hu AM, Liang KM, Gu SR. Techniques for preparation of porous metals. J Mater Sci Technol 2002; 18(4):299–305.

[4] Tang HP, Zhang ZD. Developmental states of porous metal materials. Rare Metal Mat Eng 1997; 26(1):1–6.

[5] Banhart J. Manufacture, characterization, and application of cellular metals and metal foams. Prog Mat Sci 2001; 46:559–632.

[6] Baoji Institute of Nonferrous Metal Research. Powder-metallurgical porous materials. Beijing: Metallurgical Industry Press; 1978.

[7] Huang PY. The principle of powder metallurgy. Beijing: Metallurgical Industry Press; 1997.

[8] Wang DQ, Shi ZY. The process, properties, and applications of metallic foam. J Dalian Rail Inst 2001; 22(2):79–86.

[9] Chen W, Liu ZH, Zhu CY, He FQ. Properties, applications, and preparation methods of metal foams. Nonferr Min Metall 1999; 1:33–6.

[10] Jiang B, Liu YH, Si YH. Properties of open-cell aluminum foams prepared by the spaceholder method. Heat Treat Metals 2007; 32(3):33–5.

[11] Jee CSY, Ö zgüven N, Guo ZX, Evans JRG. Preparation of high-porosity metal foams. Metal Mat Trans B 2000; 31B:1345–52.

[12] Banhart J, Baumeister J, Weber M. Metal foams near commercialization. Met Powder Rep 1997; 52(4):38–41.

[13] Baumeister J, Schrader J. Methods for manufacturing foamable metal bodies. German patent No. DE 4101630, 1991.

[14] Evans AG, Hutchinson JW, Ashby MF. Multifunctionality of cellular metal systems. Prog Mat Sci 1999; 43:171–221.

[15] Wang F, Wang LC. The research and development of metallic foams. Res Stud Foundry Equip 2000; 1:48–51.

[16] Li YH, Qi GX, Li YH, Deng ZY, Wang CZ. Porous TiNiFe alloy fabricated by combustion synthesis and powder sintering. Rare Metal Mat Eng 2010; 39(S1):227–30.

[17] Neumann P. Porous metal structures made by sintering: processes and applications. Materialwiss Werkstofftech 2000; 31(6):422–3.

[18] Liu GT. Metal fibers and recent advances. Rare Metal Mat Eng 1994; 23(1):7–15.

[19] Morimoto T, Nakagawa F. Porous metallic material, porous structural material, and porous decorative sound absorbing material, and methods for manufacturing the same. U.S. patent 4828932, 1989-05-09.

[20] Fang ZC, Ma ZL. Manufacturing process for foamed metals. Dev Appl Mat 1998; 13 (2):35–9.

[21] Jia YQ, Qu FZ, Shi XJ, Sun SD. Preparation and mechanical properties of stainless steel fiber reinforced stainless steel porous materials. Chin J Nonferr Metals 1998; 8 (S2):33–5.

[22] Zhou W, Tang Y, Liu B, Song R, Jiang LL, Hui KS, et al. Compressive properties of porous metal fiber–sintered sheet produced by the solid-state sintering process. Mat Des 2012; 35:414–8.

[23] Wan ZP, Liu B, Zhou W, Tang Y, Hui KS, Hui KN. Experimental study on shear properties of a porous metal fiber–sintered sheet. Mat Sci Eng A 2012; 544:33–7.

[24] Liu PS, Liang KM, Gu SR, Yu Q, Fu C, Li TF, et al. Substrate materials of a porous metal electrode. Chin J Rare Metals 2000; 24(6):440–4.

[25] Coates D, Paul G, Daugherty P. Advances in lightweight nickel electrode technology. J Power Sour 1990; 29:521–9.

[26] Ridgeway JA. Cellarized metal and method of producing the same. U.S. patent 3297431, 1967.

[27] Akiyama S, Imagawa K, Kitahara A, Nagata S, Morimota K, Nishikawa T, et al. Foamed metal and method for producing the same. European patent EP 0210803A1, 1986, U.S. Patent 4712277, 1987.

[28] Kunze HD, Baumeister J, Banhart J, Weber M. P/M technology for the production of metal foams. Powder Metal Intl 1993; 25(4):182–5.

[29] Zhao ZD, Zhang Y, Li J. The study and application progress of porous metal. Light Alloy Fab Tech 1998; 26(11):1–4.

[30] Zhu ZG. Metallic foam materials. Physics 1999; 28(2):84–8.

[31] Wu J, Jia F, Wang MM, Chen HF. Preparation and application of metal foams. J Net Shape Form Eng 2011; 3(3):62–5.

[32] Song ZL, Ma LQ, He DP. Controlling pore structure during the foaming process of aluminium melt. Foundry 1997; 4:9–11.

[33] Jin I, Kenny LD, Sang H. Method of producing lightweight foamed metal. U.S. patent 4973358, 1990.

[34] Li YX. Comparison of aluminum foams produced by melt forming and gas injection processes. Spe Cast Nonferr All 2011; 31(12):1097–9.

[35] Simone AE, Gibson LJ. Aluminum foams produced by liquid-state processes. Acta Mater 1998; 46(9):3109–23.

[36] Chu SJ, Wu K, Niu Q, Yang TJ. Classification on the foaming behaviours in metallurgical melts. J Univ Sci Tech Beijing 1998; 22(1):20–6.

[37] Zhang WK, Li NZ, He DP. Preparation of porous Al with high porosity by the infiltration method. Chin J Nonferr Metals 2005; 15(8):1248–52.

[38] Yang SY. The casting process around granules for producing foam metals. Mat Mech Eng 1997; 21(4):41–3.

[39] Xu QY, Chen YY, Li QC. Porous Al-alloy of pressurized infiltration casting and its sound-absorbent property. Foundry 1998; 4:1–4.

[40] Masaaki H, Tetsuya N, Shono I, Furukawa M. The method for continuous manufacture of porous metals. Japanese patent 2795A, 1992.

[41] Chang FH, Zhang LQ. Preparation of an Al metal foam laser target. Atom Eng Sci Tech 1999; 33(4):309–13.

[42] Bray H. Design opportunities with metal foam. Eng Mat Des 1972; 16(1):19.

[43] Brannan JR, Bean AS, Vaccaro AJ, Stewart JJ. Continuous electroplating of conductive foams. U.S. patent 5098544, 1992.

[44] Yu GX. Development of foamed nickel substrate for storage battery. Batt Bimonth 1995; 25(3):140–8.

[45] Yutakako M, Takahaku K. The method manufacturing porous metals and the porous metals made by this method. Japanese Patent 1995; (109597A).

[46] Liu PS, Liang KM. Preparation and corresponding structure of nickel foam. Mater Sci Technol 2000; 16(5):575–8.

[47] Aly MS. Tensile properties of open-cell nickel foams. Mat Des 2010; 31:2237–40.

[48] Wo L, Li BH, Liu AH, Ji FM. The preparation of foamed nickel substrate for batteries. Batt Bimonth 1991; 21(6):9–11.

[49] Chen W, Liu ZH, Zhu CY, He FQ. Preparation of nickel foam without the Pd element. J Kunming Univ Sci Tech 1998; 23(6):16–8.

[50] Li BS, Niu YS, Liao KJ, Zhai YC, Shao ZC. Electrodeposition for manufacturing foamy copper. Eng Chem Metal 1998; 19(3):199–204.

[51] Babjak J, Ettle VA, Paserin V. Nickel foam. European patent 0402738 A2, 1990.

[52] Lim TJ, Smith B, McDowell DL. Behavior of a random hollow sphere metal foam. Acta Mat 2002; 50:2867–79.

[53] Shapovalov V. Method for producing shaped slabs of particle stabilized foam metals. U.S. patent 5181549, 1993.

[54] Simone AE, Gibson LJ. Efficient structural components using porous metals. Mat Sci Eng 1997; A229:55–62.

[55] Shapovalov VI. Porous and cellular materials for structural applications. In: Schuart DS, Shih DS, Evans AG, Wadley HNG, editors. MRS symp proc, vol. 521; 1998. p. 281.

[56] Apprill GM, Poirier DR, Maguire MC, Gutsch TC. MRS symp Proc, vol. 521; 1998. p. 291.

[57] Simone AE, Gibson LJ. The tensile strength of porous copper made by the GASAR process. Acta Mater 1996; 44(4):1437–47.

[58] Xiao HX, Chen G, Cui P. Prospect of making artificial bones with unidirectional solidification porous metals. Spe Cast Nonferr All 2001; 2:88–9.

[59] Nakajima H, Hyun SK, Ohashi K, Ota K, Murakami K. Fabrication of porous copper by unidirectional solidification under hydrogen and its properties. Coll Surf 2001; A179:209–14.

[60] Yamamura S, Shiota H, Murakami K, Nakajima H. Evaluation of porosity in porous copper fabricated by unidirectional solidification under pressurized hydrogen. Mat Sci Eng 2001; A318:137–43.

[61] Park C, Nutt SR. PM synthesis and properties of steel foams. Mat Sci Eng 2000; A288:111–8.

[62] Baumeister J, Banhart J, Weber M. Aluminum foams for transport industry. Mat Des 1997; 18(4/6):217–20.

[63] Zhao KY, Zhu XK, Su YS, Zhang JQ. A study on self-propagating high temperature synthesis of TiB2. Powder Metal Tech 1997; 15(1):26–8.

试读结束[说明：试读内容隐藏了图片]

下载完整电子书