Porous Materials: Processing and Applications

By Peisheng Liu and Guo-Feng Chen

Engineers and scientists alike will find this book to be an excellent introduction to the topic of porous materials, in particular the three main groups of porous materials: porous metals, porous ceramics, and polymer foams. Beginning with a general introduction to porous materials, the next six chapters focus on the processing and applications of each of the three main materials groups. The book includes such new processes as gel-casting and freeze-drying for porous ceramics and self-propagating high temperature synthesis (SHS) for porous metals. The applications discussed are relevant to a wide number of fields and industries, including aerospace, energy, transportation, construction, electronics, biomedical and others. The book concludes with a chapter on characterization methods for some basic parameters of porous materials. Porous Materials: Processing and Applications is an excellent resource for academic and industrial researchers in porous materials, as well as for upper-level undergraduate and graduate students in materials science and engineering, physics, chemistry, mechanics, metallurgy, and related specialties.

• A comprehensive overview of processing and applications of porous materials – provides younger researchers, engineers and students with the best introduction to this class of materials
• Includes three full chapters on modern applications - one for each of the three main groups of porous materials
• Introduces readers to several characterization methods for porous materials, including methods for characterizing pore size, thermal conductivity, electrical resistivity and specific surface area

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 12, 2014
• ISBN: 9780124078376

Peisheng Liu

P.S. Liu is a professor in the College of Nuclear Science and Technology at Beijing Normal University (BNU), Beijing, China. He graduated at Chinese Academy of Science in 1998, and received his doctor's degree in materials science and engineering. He has ever served as the committeeman of the academic committee of the Key Laboratory of Beam Technology and Material Modification of Ministry of Education of China, and the first director of Material Physics Division and then the vice director of Nuclear Physics Research Institute at BNU. Investigating on porous materials and high-temperature coatings for quite a number of years, he has published extensively in the area of materials science and engineering as the first author, including about 60 SCI papers, more than 40 EI articles, and 6 academic books. In addition, he holds 8 Chinese invention patents as the first originator.

Book preview

materials.

Preface

Porous materials are a class of functional-structural 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. 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 successful conclusion.

P.S. Liu; G.F. Chen, October 2013

Chapter One

General Introduction to Porous Materials

Abstract

Because porous materials are a new type of engineering materials, people are not generally aware of them. But they are worthy of note due to their valuable attributes of function and structure. This chapter gives a brief overview of the topic so that readers might have a rudimental understanding of it. Some primary concepts are introduced for porous materials, including porous metals, porous ceramics, and polymer foams, and their main groups and chief features.

Keywords

Porous materials

Primary concepts

Porous metals

Porous ceramics

Polymer foams

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 open-cell 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 types of oils and 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 powder-sintering 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 powder-sintering 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 second-generation 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.

Figure 1.11 Micrograph of a porous material fabricated by metallic fiber sintering [ 11 ].

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 1.12 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.

Figure 1.14 A lotus-type porous metal formed by gas-metal eutectic directional solidification [ 18 ].

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 three-dimensional, 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.

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.

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.

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.

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. Low-foamed or high-density polymer foams have a density of more than 0.4 g/cm³ 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 middle-density foams have a density of 0.1–0.4 g/cm³ and an expansion ratio of 1.5 – 9.0. High-foamed or low-density foams have a density of less than 0.1 g/cm³ 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, the density with the value of 0.4 g/cm³ 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 excellent 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 using porous materials and greatly opens 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 can be applied to simple and convenient forms of heat insulation, and the effect is just a little inferior to that of more expensive and difficult varieties. Low-rigidity 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.

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Chapter Two

Making Porous Metals

Abstract

There are a number of methods available for making porous metals. This chapter deals with the main processing technologies, including powder metallurgy, metallic fiber sintering, metallic melt foaming, gas injection into metallic melts, infiltration casting, investment casting, metal deposition, and hollow sphere sintering. In addition, the preparation of porous metal composites and porous metals with directional pores are introduced, as well as some specific processes for making porous metals.

Keywords

Porous metals

Processing methods

Powder metallurgy

Metallic melt foaming

Metal deposition

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.

Table 2.1

Preparation Methods for Metal Powders [6,7]

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.

Figure 2.1 Schematic diagram of the molten metal atomization process.

Figure 2.2 Stainless steel spheric powder created by gas atomization (× 300).

Mechanical 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 2.3 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, H2 + N2, or both are used to produce W, Mo, Fe, Cu, Co, and Ni powders. Transformed natural gas (H2 or CO) can be used for the preparation of Fe powders. And 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)n] that are either in the liquid state (which tend to evaporate), or in the solid state (which are easy to sublimate). For instance, Ni(CO)4 is a colorless liquid with melting point of 43 °C, Fe(CO)5 is an amber liquid with melting point of 103 °C, and Co2(CO)8, Cr(CO)6, W(CO)6, and Mo(CO)6 are all crystals of easy sublimation. Also, these carbonyl compounds have the tendency to decompose into metal powders and CO. The reaction of carbonyl compounds is

   (2-1)

For instance, nickel carbonyl can be formed by

   (2-2)

The decomposition of carbonyl compound is

   (2-3)

and nickel carbonyl can decompose into

   (2-4)

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 is

   (2-5)

For instance,

   (2-6)

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

CO, SO2, H2S, and H2 can be used as the reductant in solution in the gas reduction method, in which H2 is more popularly used. The reaction is

   (2-7)

For example,

   (2-8)

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