Porous Ceramic Materials

Chapter 11.2.2

Porous Ceramic Materials Tatsuki Ohji National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan

Chapter Outline 1. 2. 3. 4.

Introduction Partial Sintering Sacrificial Fugitives Replica Templates

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1. INTRODUCTION Porous ceramics are now used for wide variety of industrial applications from filtration, absorption, catalysts, and catalyst supports to lightweight structural components. In these decades, a great deal of research efforts has been devoted for tailoring deliberately sizes, amounts, shapes, locations, and connectivity of distributed pores, which have brought improved or unique properties and functions of porous ceramics [1e12]. The merits in using porous ceramics for these applications are generally a combination of intrinsic properties of ceramics themselves and advantages of dispersing pores into them. The former include heat and corrosion resistances, wear and erosion resistance, unique electronic properties, good bioaffinity, low density, and high specific strength, and the latter are low density, low thermal conductivity, controlled permeability, high surface area, low dielectric constant, and improved piezoelectric properties [13,14]. This chapter intends to deals with the recent progress of porous ceramics. Porous materials are classified into three classes depending on the pore diameter, d: macroporous (d > 50 nm), mesoporous (50 nm > d > 2 nm), and microporous (d < 2 nm), according to the nomenclature of IUPAC (International Union of Pure and Applied Chemistry). Figure 1 shows this classification along with typical applications and fabrication processes specific to the pore diameters. One of the most representative applications of porous materials is filtration or separation of matters in fluids. Filtration is roughly classified into several classes depending on pore diameter, d, and molecular weight

Handbook of Advanced Ceramics. http://dx.doi.org/10.1016/B978-0-12-385469-8.00059-9 Copyright Ó 2013 Elsevier Inc. All rights reserved.

5. Direct Foaming 6. Gas Permeability 7. Summary References

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cut-off of the matters (MWCO); filtration (typically d > 10 mm), microfiltration (10 mm > d > 100 nm), ultrafiltration (100 nm > d > 1 nm, MWCO ¼ 103e106), nanofiltration (d z 1e2 nm, MWCO ¼ 200e103), and reverse osmosis (d < 1 nm, MWCO z 100). In filtration and microfiltration where pore size is relatively large, the separation is principally made by the sieving effect where matters whose size is larger than the pore size is trapped. On the other hand, in ultrafiltration, nanofiltration, and reverse osmosis where pore size is relatively small, fluid permeability depends on the affinity of solute and solvent to the porous materials as well. Because of the great deal of research work reported in this field these days, this chapter mainly focuses on macroporous ceramics; micro- and mesoporous ceramics whose pore size is below 50 nm are not included here. Representative applications of macroporous ceramics are briefly described. Ceramic filters are now widely loaded in diesel engines to trap particulate matters in the exhaust gas stream, so called, diesel particulate filters (DPFs). Since the high combustion efficiency and low carbon dioxide emission of diesel engines, the demand of DPF is also expected to further increase the world over [15e17]. Ceramic water purification filters are used for eliminating bacteria and suspension from wastewater, because of their higher flux capability, sharper pore-size distribution, better durability, and higher damage tolerance than those of organic hollow fibers [18]. Ceramic foam filters have been employed for removing metallic inclusions from molten metals such as cast iron, steel, and aluminum, as well as rectifying flow of

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Handbook of Advanced Ceramics

devices. For example, porous piezoelectric ceramics show good piezoelectric property and are expected to be used for ultrasonic transducers, etc. [23] A variety of porous ceramics have been applied as materials for refractory bricks of kilns and furnaces in various industrial fields, due to their low thermal conductivity and high thermal shock resistance [24,25]. On the other hand, some porous materials of conductive ceramics like zirconia and silicon carbide have been utilized in heat exchangers and heaters [13]. As is known from Figure 1, the representative processes for making macroporous ceramics are (1) partial sintering, (2) sacrificial fugitives, (3) replica templates, and (4) direct foaming. A number of innovative techniques which have been developed recently for critical control of pores are introduced, divided into these four categories, together with some important properties of porous ceramics obtained in these processes. It should be noted however, that many new approaches for macroporous ceramics such as phase separations [26e30] have been developed other than the processes shown here. Figure 2 shows schematic illustrations of these processes, each of which will be interpreted in its section. We then discuss gas permeability of these porous ceramics with different pore sizes and structures in FIGURE 1 Classification of porous materials by the pore size and corresponding typical applications and fabrication processes.

the molten metals [19]. Since the metallic inclusions results in defects in cast metals, this filtration process substantially improves the performance of the products. Porous ceramics with high specific surface area are adopted for absorptive and catalytic applications, where larger contact area with reactants is preferred, particularly in high temperature or corrosive atmospheres. Bioreactors are devices or systems that provide a biologically active environment, where microorganisms and enzymes are immobilized, and biochemical reactions are performed in porous beds, and porous ceramics are often used as such bioreactor beds due to chemical stability of ceramics and accommodative function of porous structure [20]. Recently, porous bioceramics with open-pore structures have attracted great attention for bioimplant applications including of bone regeneration [21]. Bone cells are impregnated through the open pores and grow on their biocompatible walls resulting in bone ingrowth. Many electrodes used in electro-chemical devices including gas purifiers, gas sensors, fuel cells, and chemical analyzers are porous ceramics [22]. Some porous electrodes require two mode distributions of pore sizes; small pores are for the electro-chemical reactions while large ones are for flow paths of reactants. Properties of electroceramics also depend substantially on the porosity content and morphology and therefore porous ceramics are also applied or expected to be used in various electro-

FIGURE 2 Representative fabrication processes of macroporous ceramics.

Chapter | 11.2.2

Porous Ceramic Materials

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Section 6. Finally, a summary will be given with the issues to be solved for further realizing the potential of porous ceramics and for expanding their applicability.

2. PARTIAL SINTERING Partial sintering of powder compact is one of the most conventional and frequently employed approaches to fabricate porous ceramic materials. Particles of powder compact are bonded due to surface diffusion or evaporationecondensation processes enhanced by heat treatments, and a homogeneous porous structure forms when sintering is terminated before being fully densified (see Figure 2 (a)). Pore size and porosity can be controlled by the size of starting powders and degree of partial sintering, respectively. Generally, in order to provide the desired pore size, the size of raw powder should be geometrically in the range two to five times larger than that of the pore. Porosity decreases with increased forming pressure, sintering temperature, and time. In addition, processing factors such as the type and amount of additives, green densities, and sintering conditions (temperature, atmosphere, pressure, etc.) also greatly affect the microstructures of porous ceramics [31]. The mechanical properties depend significantly on degree of neck growth between grains, as well as porosity and pore size. For example, the formation of necks between touching particles by surface diffusion without densification can increase the elastic modulus to 10% of the fully dense value [32,33]. The porosities of porous materials obtained by partial sintering are usually below 50%. In industry, this method has been utilized for various applications including molten metal filters, aeration filters (gas bubble generation in wastewater treatment plants) [13], and water purification membranes [18]. Several processing approaches have been developed to enhance neck growth between grains and improve strength of porous ceramics. Oh et al. [34], Jayaseelan et al. [35], and Yang et al. [36] fabricated porous Al2O3 and Al2O3-based composites by the pulse electric current sintering (PECS) technique and found that the strength was substantially improved due to the formation of thick and strong necks. During sintering, the discharge between the particles is thought to promote the bridging of particles by neck growth in the initial stages of sintering. This strong neck growth leads to substantially high strength compared to those of the conventional porous materials. For example, the flexural strength of porous alumina-based composites via PECS reached 250 and 177 MPa, with 30% and 42% porosity, respectively, which are considerably high compared to those of porous alumina fabricated by conventional partial sintering, e.g., ~100 MPa at 30% porosity [35] (see Figure 3). Using PECS, Akhtar et al. [37] also fabricated porous ceramic monoliths from diatomite powders, which are known as a cheap and renewable, natural resource.

FIGURE 3 Flexural strength as a function of porosity for alumina/ 3 vol.% zirconia (AZ) fabricated via PECS and conventionally sintered alumina (top) and microstructure of AZ (bottom) [35]. Strong neck growth of the AZ results in substantially high strength compared to those of the conventional porous materials. (Reprinted with permission of John Wiley and Sons. All rights reserved)

PECS that rapidly heats diatomite powder successfully bonds the particles together into relatively strong porous bodies, without significantly destroying the internal pores of the diatomite powder. The microstructural studies revealed that consolidation proceeds by the formation of necks at temperatures around 700e750 C, which is followed by significant melt phase formation around 850 C, resulting in porous ceramics with a relatively high strength. Deng et al. [38,39] tried to obtain strong grain bonding through the combination of partial sintering and powder decomposition. A mixture of a-Al2O3 and Al(OH)3 was used as the starting powder to make porous Al2O3 ceramics, and because Al(OH)3 experiences a 60% volume contraction during decomposition and produces fine Al2O3 grains, the fracture strength of obtained porous Al2O3 was substantially higher than that of the pure Al2O3 sintered specimens because of strong grain bonding that resulted from the fine Al2O3 grains produced by the decomposition of Al(OH)3. Similar improvement of mechanical properties was also identified for ZrO2 porous ceramics fabricated by adding Zr(OH)4 [40]. Partial sintering through reaction bonding techniques have been frequently used for making porous ceramics,

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FIGURE 4 Microstructure of porous CaZrO3/MgO composite fabricated via in-situ reaction synthesis, exhibiting 3-dimensional network structure with strong grain necking [43]. (Reprinted with permission of John Wiley and Sons. All rights reserved)

where reaction products form or precipitate epitaxially on grains, resulting in well-developed neck growth between grains [41,42]. In combination with the reactive sintering process, Suzuki et al. [43,44] synthesized a CaZrO3/MgO porous ceramic with three-dimensional grain network structure; using reactive sintering of highly pure mixtures of natural dolomite (CaMg(CO3)2) and synthesized zirconia powders. CaMg(CO3)2) decomposes into CaCO3, MgO, and CO2 (g) at ~500 C, and CaCO3 then reacts with

FIGURE 5 Microstructures of two porous silicon carbides fabricated via an oxidation-bonding process, and their pore-size distributions determined by mercury porosimetry (V: intrusion volume) [47]. (a) From fine (0.6 mm) powder. Porosity is 31%. (b) From coarse (2.3 mm) powder. Porosity is 27%. (Reprinted with permission of John Wiley and Sons. All rights reserved)


ZrO2 to form CaZrO3 and CO2 (g) at ~700 C. Through liquid formation via LiF doping, these reactions and liberated CO2 gas result in formation of a homogeneous open-pore structure with strong grain bonding as shown in Figure 4. The pore-size distribution is very narrow (with typical pore size: ~1 mm), and the porosity was controllable (~30e60%) by changing the sintering temperature. The relatively high flexural strength (~40 MPa for 47% porosity) was observed over the temperature range of R.T.1300 C. The similar approach has been applied other materials systems such as CaAl4O7/CaZrO3 and CaZrO3/MgAl2O4 composite systems [45,46]. She et al. [47] used an oxidation-bonding process for the low-temperature fabrication of porous SiC ceramics with superior resistance against oxidation. In such a process, the powder compacts are heated in air instead of an inert atmosphere. The heating temperature was kept below 1300 C in order to suppress formation of cristobalite. Figure 5 shows microstructures of two porous silicon carbides fabricated through the oxidation-bonding process (1300 C, 1 h), using fine (0.6 mm) and coarse (2.3 mm) a-SiC powders, together with their pore-size distributions. Because of the occurrence of surface oxidation at the heating stage, SiC particles are bonded to each other by the oxidation-derived SiO2 glass. The difference of the poresize distributions arises from the different starting powders. Mechanical strength is strongly affected by particle size; the flexural strength attained as high as 185 MPa at a porosity of 31%, when using the fine powder (Figure 5 (a)),

Chapter | 11.2.2


while it was 88 MPa at 27% porosity for the coarse powder (Figure 5 (b)). The oxidation-bonding technique has been applied to other materials including silicon nitride [48], SiC/mullite composites [49], and SiC/cordierite composites [50]. The partial sintering technique has been also applied for making porous nonoxide ceramics such as porous silicon nitride with fibrous grains of high aspect ratios [51e58]. Compared with oxide ceramics, the densification of silicon nitride ceramics is difficult because of strong covalent bonding between silicon and nitrogen atoms. This difficulty of sintering silicon nitride ceramics is beneficial for controlling density or porosity through adjusting the additives and the sintering process. In order to suppress densification, oxides with high melting point and high viscosity such as Yb2O3 are frequently used as sintering additives [52]. Depending on the sintering temperature, the addition of Yb2O3 also accelerates the fibrous grain growth of b-Si3N4, which substantially affects the mechanical properties of porous silicon nitrides [52,53]. Due to differences in the melting points and the preferential absorption sites of cations, the porous structure is substantially affected by the types of sintering additives as well [54]. Yang et al. [55] directly synthesized porous b-Si3N4 ceramics by carbothermal nitridation of silica, using carbon black as the carbon source and a-Si3N4 as seeds. The complete reaction results in a large weight loss and high porosities (about 65e70%) after sintering. Fine elongated fibrous b-Si3N4 grains were developed in the seeded samples while only large equiaxial grains were observed in the seed-free samples, as shown in Figure 6. The former sample exhibited relatively high flexure strength close to 40 MPa for the high porosity of ~65%, which is five times higher than that of the latter one. Tuyen et al. [56] fabricated porous reaction-bonded silicon nitride (RBSN) by nitridation process at 1350 C and post-sintering at 1550e1850 C, which provides similar fibrous microstructure and high porosity. The sintering time had a significant effect on the microstructure and grain morphology, and porous structure with fibrous grains of high aspect ratios was obtained by adjusting the time even at the comparatively low temperature (1550 C). These techniques offer the possibility of synthesizing highly porous and strong Si3N4 materials at considerably lower cost. One of the unique processing routes for porous ceramics with anisotropic microstructure is tape-casting fibrous seed crystals or whiskers. For porous silicon nitrides, b-Si3N4 seed crystals were mixed with sintering additives as starting powders, and the green sheets formed by tape casting were stacked and bonded under pressure, followed by sintering at 1850 C under a nitrogen pressure of 1 MPa [57,58]. The microstructural observation for the porous silicon nitrides revealed that the fibrous grains are aligned toward the

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FIGURE 6 Microstructures of two porous silicon nitrides fabricated via carbothermal nitridation of silica [55]. (a) Seeded sample with fine elongated fibrous grains. (b) Seed-free sample with large equiaxial grains. (Reprinted with permission of Elsevier. All rights reserved)

casting direction, and the plate-like pores exist among the grains. Because of the enhanced crack shielding effects of aligned fibrous grains, the anisotropic porous silicon nitrides showed excellent mechanical behavior, when a stress is applied in the alignment direction. More detailed discussion on the microstructure changes and mechanical properties (strength, fracture toughness, and thermal shock resistances) for the isotropic and anisotropic porous silicon nitrides will be given in the Chapter “Microstructural Control and Mechanical Properties.”

3. SACRIFICIAL FUGITIVES Porous ceramics are often fabricated by mixing appropriate amounts of sacrificial fugitives as pore-forming agents with ceramic raw powder and evaporating or burning out them before or during sintering to create pores (see Figure 2 (b)). Frequently used pore-forming agents are polymer beads, organic fibers, potato starch, graphite, charcoal, salicylic

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acid, carbonyl, coal, and liquid paraffin. The pore-forming agents are generally classified into synthetic organic matters (polymer beads, organic fibers, etc.) [59e89], natural organic matters (potato starch, cellulose, cotton, etc.) [67,68,90e105], metallic and inorganic matters (nickel, carbon, fly ash, glass particles, etc.) [49,78,106e114], and liquid (water, gel, emulsions, etc.) [115e155]. Porosity is controllable by the amount of the agents and pore shape and size are also affected by the shape and size of the agents, respectively, when their sizes are large in comparison with those of starting powders or matrix grains. The agents, however, need to be mixed with ceramic raw powder homogeneously for obtaining uniform and regular distribution of pores. Solid fugitives like organic materials are usually removed through pyrolysis, which often requires long-term heat-treatment and generates a great deal of vaporized, sometimes harmful, by-products. Polymethylmethacrylate (PMMA) beads and microbeads have been frequently employed for sacrificial fugitives [8,59e64,77,79,82e85]. Colombo and his co-workers [8,59e61] fabricated SiOC ceramic foam by dry mixing the silicon resin powder with a sacrificial template constituted by PMMA microbeads, and subsequent heat treatments (Figure 7). Descamps et al. [63,64] produced macroporous b-tricalcium phosphate (TCP) ceramics using PMMA. An organic skeleton was formed by interconnecting the PMMA balls through a chemical superficial dissolution, and was impregnated by the TCP slurry. PMMA was then eliminated by a thermal treatment at low temperature, followed by sintering for final porous structure. This process allows a total control of the porous architecture; the porous volume can vary from 70 to 80%

FIGURE 7 SiOC ceramic foam using a sacrificial template constituted by PMMA microbeads [8]. (Reprinted with permission of Elsevier. All rights reserved)


and the interconnection size from 0.2 to 0.6 times the average macro-pore size. Andersson et al. [82] used expandable microspheres as a sacrificial template to produce macroporous ceramic materials by a gel-casting process. The microspheres consist of a co-polymer shell and are filled with a blowing agent (isobutane), which allows rapid and facile burn out. By controlling the amount and size of the expandable microspheres, it is possible to tune the porosity up to 86% and the pore-size distribution from 15 to 150 mm. Up to 1e2 wt.% of the microspheres leads to a final porosity above 80 vol%. Expandable microspheres as sacrificial templates, rather than other templates such as PMMA microbeads, are advantageous because of lower levels of gaseous by-products generated during pyrolysis, and lower cost of the overall materials. Kim and his co-workers [83,84] used hollow microspheres as sacrificial templates to make porous silicon carbide ceramics synthesized from carbon-filled polysiloxane and others. Using preceramic polymer and organic microspheres for fabricating porous ceramics allows use of the low-cost and/or near-net-shaped processing techniques like extrusion and direct casting [84]. Song et al. [85] produced microcellular silicon carbide ceramics with a duplex pore structure using expandable microspheres and PMMA spheres, which resulted in the large pores and the small windows in the strut area, respectively. This porous ceramics showed excellent air permeability as shown in Section 6. Diaz et al. [93,94] fabricated porous silicon nitride ceramics using a fugitive additive, cornstarch (particle size: 5e18 mm). In order to obtain homogeneous dispersion of the fugitives, the mixture slurry was kept in agitation using a magnetic stirrer for a while, and then was frozen and dried under vacuum for sieving. Kim et al. [95] mixed various amounts of cornstarch to (Ba, Sr) TiO3 powder to obtain (Ba, Sr) TiO3 porous ceramics. They found that depending on the porosity, the PTCR effect was 1e2 orders of magnitude improved in comparison with the dense reference. Chen et al. [66] produced porous silicon nitride of equiaxed a-grains by using phosphoric acid (H3PO4) as the pore-forming agent and relatively low temperature (1000e1200 C) sintering. On the other hand, Li et al. [80] fabricated porous silicon nitride with fibrous b-grain structure, using naphthalene powder as the pore-forming agent and gas-pressure sintering of high temperatures above 1700 C. The bending strength of the former materials was 50e120 MPa in porosity range of 42e63%, while that of the latter was 160e220 MPa in porosity range of 50e54%. This substantial difference in strength is attributable to the microstructural difference (equiaxed vs. fibrous), similar to the case of Figure 5. Ding et al. [49] used graphite as the pore-former to fabricate mullite-bonded porous silicon carbide ceramics in air from SiC and a-Al2O3 through an in-situ reaction

Chapter | 11.2.2


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bonding technique. Graphite is burned out to form pores and SiC is oxidized at high temperatures to SiO2, which further reacts with a-Al2O3 to form mullite (3Al2O3$ 2SiO2). SiC particles are bonded by mullite and oxidationderived SiO2. Long fibers such as cotton thread [96], natural tropical fiber [97], and metal wires [109] are often used as poreforming agents for obtaining porous ceramics of unidirectional through channels. Zhang et al. [96] fabricated porous alumina ceramics with unidirectionally aligned continuous pores (diameter: ~160 mm) via the slurry coating of mercerized cotton threads. The pore size can be adjusted, using cotton threads of different diameter, and the porosity can be controlled by changing the solids’ concentration of the slurry. In this case, excellent permeability can be achieved for porous ceramics with unidirectional through channel pores, because gas can flow directly through the pores. However, the preparation of such ceramics is complex because handling long fibers such as thin wire or cotton thread is difficult. Using short fibers or whiskers as the pore-forming agent is an alternative that combines the advantages of partially sintered porous ceramic and those of unidirectional pores. Yang et al. [65] demonstrated formation of rod-shaped pores in silicon nitride ceramics, using slip casting of aqueous slurries of silicon nitride powder and sintering additives with 0e60 vol% fugitive organic whiskers. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800 C in air to remove the whiskers and sintered at 1850 C in nitrogen atmosphere to consolidate the matrix. Porosity was adjusted in 0e45% by changing the whisker content in 0e60 vol%. The obtained porous silicon nitride contained uniform rod-shaped pores with random directions as shown in Figure 8, and therefore exhibited relatively high gas permeability in comparison to porous silicon nitride containing equiaxed pores [156]. Isobe et al. [81,110] and Okada et al. [86,87] used carbon fibers (14 mm

diameter and 600 mm length) or Nylon 66 fibers (9.5e43 mm diameter and 800 mm length) as a pore-forming agent, and tried to align them by an extrusion technique to produce porous alumina [81,110] and mullite [86,87] ceramics with unidirectionally oriented pores. The pore sizes and porosities can be controlled by varying the fiber diameter and fiber content, and the obtained samples showed better air permeability than the conventional porous materials used for filter applications [110]. This technique can allow the production of highly oriented porous ceramics by an industrially favored extrusion method. Liquids such as water and oils, which are readily sublimated or evaporated, are often used as pore-forming agents [115e156]. One of the most frequently employed approaches in recent years is freeze-drying the water- or liquid-based slurry to produce porous ceramics of unique structure [118e154]. Figure 9 shows a schematic

FIGURE 8 Porous silicon nitride with rod-shaped pores, prepared using 50 vol% fugitive organic whiskers of 33 mm diameter. The obtained porosity is ~40% and the mean pore diameter is ~22 mm.

FIGURE 9 Schematic illustration of the freeze-dry process for macroporous ceramics and a porous silicon nitride body obtained thereby [120]. (Reprinted with permission of John Wiley and Sons. All rights reserved)

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illustration of the procedures which Fukasawa et al. [118e120] employed, and a porous silicon nitride body obtained thereby. When the bottom part of the slurry is frozen, ice grows macroscopically in the vertical direction, and pores are generated subsequently by sublimation of the ice. Sintering this green body results in a porous ceramic with unidirectionally aligned channels, which contain smaller pores in the internal walls (Al2O3) [118,119] or fibrous grains protruding from them (Si3N4) [120]. The advantages of this approach include a simple process without materials to be burnt out, a wide range of porosity (30e99%) controllable by the slurry concentration, applicability to various types of ceramics, and environmental friendliness without emitting harmful products. Particularly porous scaffolds with ice-designed channel-like porosity fabricated by this method have been intensively studied for a wide variety of applications including biomedical implants and catalysis supports. The porosity of the porous materials obtained using this technique is a replica of the original ice structure. The porous channels run from the bottom to the top of the samples (when the bottom part is first frozen), and the pores most frequently exhibit an anisotropic morphology in the solidification plane. Deville et al. [127e130] investigated the relationships between the freezing conditions and the final porous structures in freeze casting of ceramic slurries. It has been clarified that the morphology of the porous structures including the dimensions, shape, and orientation of porosity are adjustable by varying the initial slurry compositions and the freezing conditions. For highly concentrated solutions, the particleeparticle interactions lead to the formation of ceramic bridges between two adjacent lamellae. Using this technique, they fabricated sophisticated porous and layered-hybrid materials such as nacre-like structure with lamellar dendrites and high compressive strength (four times higher than those of materials currently used for implantation). Munch et al. [131] emulated nature’s toughening mechanisms by combining two ordinary compounds, alumina and polymethyl methacrylate, into an ice-templated structure, and succeeded in obtaining toughness more than 300 times (in energy terms) that of their constituents. Araki and Halloran [132e134] used camphene, C10H16, as a vehicle for producing porous ceramics via a freezedrying process, to realize a freezing process at room temperature. Slurries containing ceramic powder in the molten camphene were prepared at 55 C, and were quickly solidified (frozen) when they were poured into polyurethane molds at room temperature. The obtained porous ceramics have pore channels of nearly circular cross sections (unlike ellipsoidal ones obtained via conventional aqueous freeze casting). Koh and his co-workers used a similar camphene-based freeze-casting approach to fabricate highly porous Al2O3 [135e137], SiC [138,139],


PZT-based ceramics [140,141], hydroxyapatite [142,143], glass-ceramics [144], and ZrO2 [145,146], etc., having interconnected pore without noticeable defects. Many researchers have tried to combine the freeze-dry process and the gel-casting technique to produce porous ceramics with refined microstructure [147e155]. It has been shown that the use of an organic polymer in the freeze-casting route affects the pore size and morphology by controlling ice crystal growth during freezing. Chen et al. [147,148] used alumina slurries containing tert-butyl alcohol (TBA) and acrylamide (AM) for the freeze-dry process. TBA freezes below 25 C and volatilizes rapidly above 30 C, acting as the freezing vehicle and template for forming pores, while AM is polymerized in the slurry as the gelation agent, strengthening the green bodies substantially. The sintered porous ceramics have high compression strength (~150 MPa at 60% porosity) because the pore channels formed by the TBA template are surrounded by almost fully dense walls without any noticeable defects. Ding et al. [149] also employed a gel freeze-drying process to fabricate porous mullite ceramics with porosity up to 93%. Alumina gel mixed with ultrafine silica was frozen isotropically, followed by sublimation of ice crystals. Porous mullite ceramics were prepared in air at 1400e1600 C due to the mullitization between Al2O3 and SiO2. Porous yttria-stabilized zirconia (YSZ) [150] and porous alumina [151] were fabricated by the freeze-drying process with addition of polyvinyl alcohol (PVA), which suppresses ice crystal growth and reduces the pore sizes substantially. Porous alumina with oriented pore structures has been also fabricated by the freeze-casting technique with a water-soluble polymer such as polyethylene glycol (PEG) [152]. Using precursor silica hydrogels, Nishihara et al. [153] fabricated ordered macroporous silica (silica gel micro-honeycomb) using freeze-dry methods, where micrometer-sized ice crystals are used as a template. The pore sizes can be controlled by changing the immersion rate into a cold bath and the freezing temperature. The average pore size can be as small as 4.7 mm with the rate of 20 cm/h at 77 K, and the thickness of the honeycomb walls can be adjusted by the SiO2 concentration. Fukushima et al. [154,155] fabricated porous cordierite or silicon carbide ceramics with porosity from 80 to 95% using a gel-freezing method; unidirectionally oriented cylindrical channels are uniformly distributed over relatively large bulk samples (typically several centimeters). Gelatin was used as the gelation agent, which was mixed with water for the freezing vehicle and raw powder. The gel was frozen at e10 to e70 C, dried under vacuum, and degreased before sintering was carried out (at 1200e1400 C for cordierite and at 1800 C for silicon carbide). The cell size and cell wall thickness both decreased with decreasing the freezing temperature, from 200 to 20 mm and from 20 to 3 mm, respectively. The

Chapter | 11.2.2


FIGURE 10 Microstructures of porous silicon carbide prepared by the freeze-dry technique using gelatin as the gelation agent frozen at e10 C.

number of cells for porous cordierite frozen at e50 C and sintered at 1400 C was 1500 cells/mm2 in the cross section; this is remarkably large in comparison to those of samples obtained by the extrusion method: 1e2 cells/mm2. The dense cell walls (Figure 10) lead to relatively high compressive strength, for example, 17 MPa for 86% porosity sample of silicon carbide.

4. REPLICA TEMPLATES Macroporous ceramics having interconnected large pores, or channels, of high porosity have been frequently fabricated by the replica techniques (Figure 2 (c)). The first step is typically impregnation of a porous or cellular template with ceramic suspension, precursor solution, etc. Various synthetic and natural cellular structures are used as the templates. The most frequently used synthetic template is porous polymeric sponge such as polyurethane. They are soaked into a ceramic slurry or precursor solution to impregnate the templates with them, and the surplus is drained and removed by centrifugation, roller compression, etc. In this process, appropriate viscosity and fluidity depending on the cell size, etc., are required in order to obtain uniform ceramic layer over the sponge walls. The ceramic-impregnated templates are dried and then heattreated to decompose the organic sponges, followed by sintering the ceramic layers at higher temperatures. Porosity higher than 90% can be obtained with cell sizes ranging from a few hundred micron meters to several millimeters. The open cells are interconnected, which allows fluid to pass through the foams with a relatively low pressure drop. Figure 11 shows a typical example of alumina foam prepared by slurry infiltration of polyurethane templates, which has been reported by Faure et al.

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[157]. However, due to cracking the struts during the pyrolysis, the mechanical properties of ceramic reticulated foams are generally poor. In order to avoid strut crack formation, various approaches have been made [157e163]. For examples, Vogt et al. [159] employed vacuum infiltration of ceramic slurry to fill up the struts in the presintered foam. The hollow struts caused by burnout of the polyurethane template were completely filled up, resulting in a considerable increase of compressive strength. Jun et al. [161,162] produced hydroxyapatite scaffolds coated with bioactive glass-ceramics using the polymer foam replication method, to enhance their mechanical properties and bioactivities. Highly porous ceramics can be also obtained from preceramic polymers after pyrolysis above 800 C in inert atmosphere [8]. One of the typical methods is dissolving the silicone resin preceramic polymer into a suitable solvent and adding appropriate surfactants and catalysts, followed by pyrolysis. The advantages are a wide control of pore sizes (typically 1 mm to 2 mm), well-defined open-cell structures, and macro-defect-free struts [164e167]. Travitzky et al. [168] also succeeded in fabricating single-sheet, corrugated structures, and multilayer ceramics by using various paper replica templates. Natural resources of porous structures such as woods, corals, sea sponge, etc., can be also used as replica templates. The woods are transformed to carbonaceous preforms by heat-treatment in inert atmosphere. They are then infiltrated with molten metals [169e179], gaseous metals [174,180e185], alkoxide solutions [186e188], and others [189,190]. The advantages include a wide variety of obtained porous structures (depending on the type of wood selected), low-cost starting materials, nearnet and complex shape capabilities, and a relatively lowtemperature fabrication process. Locs et al. prepared porous SiC ceramics from pyrolyzed pine wood samples via impregnation with SiO2 sol and heat-treatment at 1600 C for 4 hours, as shown in Figure 12 [190]. The longitudinal pore size in SiC is 10e20 mm and the wall thickness is 3e5 mm. The oriented vessels of the woods provide unique anisotropic porous structure of aligned unidirectional through channels, which is suitable to applications such as filtration and catalysis supports. Porous biomimetic silicon carbides obtained through this approach have been also studied for medical implant materials [191]. Biomorphic porous silicon nitride was produced from natural sea sponge via replication method. The sponges were impregnated with silicon-containing slurry via dip coating, and were heat-treated to delete the bio-polymers, leading to a Si-skeleton. Subsequent thermal treatment under flowing nitrogen promoted the nitridation of the silicon, porous a/b-silicon nitride with a porosity of 88%, and the original morphology of the sea sponge [192].

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FIGURE 11 Alumina foam preparation by slurry infiltration of polyurethane templates [157]. (a) Polyurethane foam; (b) impregnated and dried foam; and (c) dense alumina foam. For color version of this figure, the reader is referred to the online version of this book. (Reprinted with permission of Elsevier. All rights reserved)

5. DIRECT FOAMING In direct foaming techniques, the ceramics suspension is foamed by incorporating air or gas and stabilized and dried, followed by sintering to obtain a consolidated structure (Figure 2 (d)). The advantage of this technique is low-cost and easy fabrication of highly porous ceramic materials (95% or higher porosity). Porous ceramics with unidirectional channels can be produced using continuous bubble formation in ceramic slurry [193,194]. However, due to the thermodynamic instability, the gas bubbles easily coalesce in order to reduce the total Gibbs free energy of the system, which results in undesirable large pores. It is, therefore, critically needed to stabilize the air or gas bubbles in ceramic suspension. One of the most frequently approaches for the stabilization is to use surfactants reducing the interfacial energy of the gaseliquid boundaries. Surfactants used for stabilization are classified into several types including nonionic, anionic, cationic, and protein, and the pore size of the produced porous body

ranges from below 50 mm up to the millimeter scale, depending on the used surfactants. A variety of effective surfactants have been developed for direct foaming of porous ceramics [195e201]. Barg et al. [198e200] developed a novel direct foaming process by emulsifying a homogeneously dispersed alkane or airealkane phase in the stabilized aqueous powder suspension. Foaming is made by evaporation of the emulsified alkane droplet, leading to high performance ceramic foams with porosities up to 90% and cell sizes ranging from 3 to 200 mm. This autonomous foaming process also allows high flexibility in the production of ceramic parts with gradient structures and complex shaping. Foaming proceeds as a consequence of the evaporation of the alkane phase resulting in the growth of the stabilized alkane bubbles and in a volume increase of the foam. Figure 13 shows an example of the alumina foam which was produced through sintering (1550 C/2 h) from high alkane phase emulsified suspensions containing 45 vol% particle content [200].

Chapter | 11.2.2


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FIGURE 13 Microstructure of sintered alumina foams (1550 C/2 h) produced from high alkane phase emulsified suspensions containing 45 vol% particle content. (a) Open interconnected porous cells and (b) Dense strut constituted by a monolayer of alumina particle [200]. (Reprinted with permission of Elsevier. All rights reserved)

FIGURE 12 Porous SiC ceramics prepared from pyrolyzed pine wood samples via impregnation with SiO2 sol and heat-treatment at 1600 C for 4 hours. (a) Cross-sectional view and (b) The walls’ outer layer is denser than the inner zone [190]. (Reprinted with permission of Elsevier. All rights reserved)

Preceramic polymer solution has been also used instead of ceramic suspension for direct foaming. Colombo et al. [202] produced porous ceramics by dissolving preceramic polymers (silicone resins) into a suitable solvent with blowing agent, surfactant, catalyst, etc., and heat-treating them at 1000e1200 C in inert atmosphere. Due to the suppressed defect formations in the struts, the strength of the obtained porous ceramics was relatively high in comparison to those of conventional reticulated foams [203]. Kim et al. [204,205] fabricated porous ceramics with a fine microcellular structure from preceramic polymers using CO2 as a blowing agent. A mixture of polycarbosilane and polysiloxane was saturated with gaseous CO2 under a high pressure and bubbles were introduced using a thermodynamic instability via a rapid pressure drop, followed by pyrolysis and sintering. It has been shown that particles with tailored surface chemistry can also be used efficiently to stabilize gas bubbles

for producing stable wet foams [206e214]. Gonzenbach et al. [210e214] have developed a novel direct foaming method that uses colloidal particles as foam stabilizers. The method is based on the in-situ hydrophobization of initially hydrophilic particles to enable their adsorption on the surface of air bubbles. In-situ hydrophobization is accomplished through the adsorption of short-chain amphiphiles on the particle surface. The obtained ultra-stable wet foams show neither bubble coalescence nor disproportionation over several days, as opposed to the several minutes typically required for the collapse of the surfactant-based foams. Because of their remarkable stability, the particlestabilized foams can be dried directly in air without crack formation. The macroporous ceramics obtained after sintering have porosities from 45% to 95% and cell sizes between 10 and 300 mm. The compressive strength of the sintered foams with closed cells is relatively high in comparison with those of foams prepared with other conventional techniques (for example, 16 MPa at a porosity of 88% in alumina foams). It has been observed that the surface-modified particles which originally cover the air bubble in wet foams become a thin surface layer of single grains after sintering. Macroporous ceramics with open porosity can be also fabricated using this technique when decreasing the concentration of stabilizing particles.

6. GAS PERMEABILITY Gas permeability is one of the most important properties of porous ceramics which are expected to be used for gasfilters such as diesel particulate filter (DPF), since large pressure drops should be avoided in such applications.

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Highly porous ceramics with aligned unidirectionally through pore channels are expected to provide excellent permeability, and as already stated, the freeze-dry technique is one of the most representative processes for producing such porous ceramics. This section deals with Darcian air permeability of porous ceramics with different pore sizes and structures. Figure 14 shows the Darcian permeability as a function of pore size for porous ceramics fabricated by freeze-dry processes [151,155], organic spherical fugitives [61,85], graphite fugitives [215,216], extruded organic fibrous fugitives [81,87], direct foaming [195,217], and replica templates [217,218]. The pore structures are classified into three categories of “Spherical (Connected)” [61,85,195,215e218], “Cylindrical (Connected)” [81,87] and “Cylindrical” [151,155], as schematically shown in the figure. The Darcian permeability, K, is determined from pressure drop and flow rate of air by the Darcy’s law [219]. Based on the capillary model, K is expressed by K ¼ fD2p =C

(1)

where f is the porosity, Dp is the pore diameter, and C is a constant depending on the pore structure [219,220].

FIGURE 14 Darcian air permeability as a function of pore size for porous ceramics fabricated by the freeze-dry processes [151,155], in comparison to those of other processes including organic spherical fugitives [61,85], graphite fugitives [215,216], extruded organic fibrous fugitives [81,87], direct foaming [195,217], and replica templates [217,218]. The solid line indicates theoretical permeability K ¼ fD2p =32ðf ¼ 0:85Þ for the case of unidirectional cylindrical pores penetrating in parallel.

K values of Figure 14 are those adjusted from the reported values at the porosity of 0.85 using Eqn (1) for comparison. The inertial contribution (non-Darcian permeability) was considered in addition to the viscous one (Darcian permeability) in Ref. [61,152,217e219], which results in high values of K in comparison to the cases of neglected inertial effect [81,85,87,155,195,215] (the ratio of viscous contribution in total is 60e90% [151]). The solid line of the figure shows the case that the fluid flows through the unidirectional cylindrical pores penetrating in parallel (C is 32) [220]. The porous ceramics fabricated by the freeze-dry processes [151,155] showed permeability very close to this solid line, indicating the unidirectional alignment of the cylindrical pores. The permeability required for a commercially available DPF is 1011 to 1012 m2 [221], and most of the freeze-dry-processed materials exceed this criterion. The porous ceramics prepared with extruded organic fibrous fugitives [81,87] showed lower permeability values than those of the freeze-dry-processed ones, most likely because of limited contact area among the short fibers.

7. SUMMARY During the last decade, tremendous efforts have been devoted to research on porous ceramics, resulting in better control of the porous structures and substantial improvements of the properties. This chapter reviewed these recent progresses of porous ceramics. Because of the vast amount of research works reported in this field these days, the chapter mainly focused on macroporous ceramics whose pore size is larger than 50 nm. Followed by giving a general classification of porous ceramics, a number of innovative processing routes for critically controlling pores were described, along with some important properties. They were divided into four categories including (1) partial sintering, (2) sacrificial fugitives, (3) replica templates, and (4) direct foaming. The partial sintering, which is one of the most common techniques for making porous ceramics, has been substantially sophisticated in recent years. Very homogeneous porous ceramics with extremely narrow size distribution has been successfully prepared through sintering combined with in-situ chemical synthesis. Porous silicon nitrides with aligned fibrous grains and pores have demonstrated excellent mechanical properties, which are equivalent, or sometimes superior, to those of the dense materials. The sacrificial fugitives have an advantage that pore shape and size are controllable by the shape and size of the agents, respectively. The fugitives are generally removed through pyrolysis, generating a great deal of vaporized, sometimes harmful, by-products, and a lot of research has been conducted to reduce or eliminate them. The freeze-dry processes using water or liquid as fugitive agents are advantageous in this viewpoint and have been very intensively studied in recent years. Careful control of

Chapter | 11.2.2


ice growth leads to unique porous structures and excellent performances of porous ceramics. The replica template techniques have been widely used to fabricate highly porous ceramics with interconnected large pores. Porous polymeric sponge such as polyurethane is the most typical synthetic template used for this process. However, due to cracking struts during pyrolysis of the sponge, the mechanical property is generally low; a variety of approaches have been used to avoid strut crack formation. Natural template approaches using wood, for example, as positive replica, have been frequently studied in these years and have realized highly oriented porous open-porous structure with a wide range of porosity. The direct foaming technique is low-cost and an easy fabrication process of porous ceramics with high porosity volume. In order to suppress coalescence of gas bubbles in ceramic suspension that results in large pores in the final porous bodies, various methods which stabilize the bubbles have been developed; they include use of effective surfactants, evaporation of emulsified alkane droplets, and use of surface-modified particles. Finally, we discussed gas permeability (Darcian air permeability) of porous ceramics with different pore sizes and structures. It has been demonstrated that the freeze-dry-processed porous ceramics with cylindrical through channels have excellent permeability, which is close to the ideal case that the fluid flows through the unidirectional cylindrical pores penetrating in parallel.

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Chapter | 11.2.2


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