t. ohji and m. fukushima macroporous ceramics processing and properties.pdf

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Macro-porous ceramics: processing andpropertiesT. Ohji* and M. Fukushima

Porous ceramics are now expected to be used for a wide variety of industrial applications fromfiltration, absorption, catalysts and catalyst supports to lightweight structural components. Duringthe last decade, tremendous efforts have been devoted for the researches on innovativeprocessing technologies of porous ceramics, resulting in better control of the porous structuresand substantial improvements of the properties. This article intends to review these recentprogresses of porous ceramics. Because of a vast amount of research works reported in this fieldthese days, the review mainly focuses on macro-porous ceramics whose pore size is larger than50 nm. Followed by giving a general classification of porous ceramics, a number of innovativeprocessing routes developed for critical control of pores are described, along with someimportant properties. The processes are divided into four categories including (i) partial sintering,(ii) sacrificial fugitives, (iii) replica templates and (iv) direct foaming. The partial sintering, the mostconventional technique for making porous ceramics, has been substantially sophisticated inrecent years. Very homogeneous porous ceramics with extremely narrow size distribution havebeen successfully prepared through sintering combined with in situ chemical synthesis. Carefullytailored micro-structure (size, morphology and orientation of grains and pores, etc.) of porousceramics has led to unique mechanical properties, which cannot be attained even in the densematerials. Various types of the sacrificial fugitives have been examined for obtaining well-tunedshape and size of pores. The freeze-drying techniques using water or liquid as fugitive materialshave been most frequently studied in recent years. Controlling growth of ice during freezing hasled to unique porous structures and excellent performances of porous ceramics, e.g. excellentmechanical behaviour for highly porous lamellar hydroxyl-apatite scaffolds. Numerousapproaches on the replica templates have been developed in order to produce highly porousceramics having interconnected large pores and sufficiently strong struts without cracks. Naturaltemplate approaches using wood, for example, as positive replica, have been intensively studiedin these years and have realised highly oriented porous open-porous structure with a wide rangeof porosity. As for the direct foaming technique, a variety of novel techniques which stabilise thebubbles in ceramic suspension have been developed to suppress large pore formation, e.g.evaporation of emulsified alkane droplets and use of surface-modified particles. We also brieflyreview porous ceramics with hierarchical porosity (incorporation of macro-, meso- and micro-pores), which have attracted much attention in both academic and industrial fields. Finally thearticle gives the summary and discusses the issues to be solved for further activating the potentialof porous ceramics and for expanding their applicability.Keywords: Ceramics, Pores, Processing, Properties, Permeability, Mechanical property, Foam, Review

IntroductionFor structural applications of brittle ceramic materials,pores are generally what to be eliminated because they

act as fracture defects and degrade the structuralreliability, and therefore, ceramic engineers tried tosinter ceramics to full density to attain high mechanical

National Institute of Advanced Industrial Science and Technology (AIST),

Anagahora 2266-98, Shimo-shidami, Moriyama-ku, Nagoya 463-8560,Japan

* Corresponding author, email [email protected]

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strength. On the other hand, there have been variousindustrial applications where pores are taken advantageof positively, from ltration, absorption, catalysts andcatalyst supports to lightweight structural componentsand thermal insulator. 1,2 In these decades, a great deal of research efforts have been devoted for tailoring deliber-

ately sizes, amounts, shapes, locations and connectivityof distributed pores, which have brought improved orunique properties and functions of porous ceramics. 312

The merits in using porous ceramics for these applica-tions are generally combination of intrinsic properties of ceramics themselves and advantages of dispersing poresinto them. The former include heat and corrosionresistances, wear and erosion resistance, unique electro-nic properties, good bioafnity, low density, and highspecic strength, and the latter are low density, lowthermal conductivity, controlled permeability, high sur-face area, low dielectric constant, and improved piezo-electric properties. 13,14

This article intends to review these recent progresses of porous ceramics. Porous materials are classied into threegrades depending on the pore diameter d : macro-porous(d . 50 nm), meso-porous (50 nm . d . 2 nm) and micro-porous ( d , 2 nm), according to the nomenclature of IUPAC (International Union of Pure and AppliedChemistry). Figure 1 shows this classication along withtypical applications and fabrication processes specic tothe pore diameters. One of the most representativeapplications of porous materials is ltration or separationof matters in uids. Filtration is roughly classied intoseveral grades depending on pore diameter d and mole-cular weight cut-off of the matters (MWCO); ltra-tion (typically d . 10 mm), micro-ltration (10 mm . d . 100

nm), ultra ltration (100 nm . d . 1 nm, MWCO 5 103

106

),nano-ltration ( d < 12 nm, MWCO 5 20010 3), and reverseosmosis ( d , 1 nm, MWCO < 100). When the pore size is

large like ltration and micro-ltration, the separation isprincipally made by sieving effect where matters whose sizeis larger than the pore size is trapped. In ultra ltration,nano-ltration, and reverse osmosis where pore size is small,uid permeability depends on the afnity of solute andsolvent with the porous materials as well.

Because of a vast amount of research works reported inthis eld these days, the review mainly focuses on macro-

porous ceramics; micro- and meso-porous ceramicswhose pore size is below 50 nm are not included here.Representative applications of macro-porous ceramicsare briey described. Ceramic lters are now widelyloaded in diesel engines to trap particulate matters in theexhaust gas stream, so called, diesel particulate lters(DPFs). Since the high combustion efciency and lowcarbon dioxide emission of diesel engines, the demand of DPF is also expected to further increase over theworld. 1517 Ceramic water purication lters are usedfor eliminating bacillus coli and suspension from wastewater, because of their higher ux capability, sharperpore size distribution, better durability and higherdamage tolerance than those of organic hollow bres. 18

Ceramic foam lters have been used for removingmetallic inclusions from molten metals such as cast iron,steel, aluminium, as well as rectifying ow of the moltenmetals. 19 Since the metallic inclusions result in defects incast metals, this ltration process substantially improvesthe performance of the products. Porous ceramics withhigh specic surface area are employed for absorptive andcatalytic applications, where large area is required forcontacts with reactants, particularly in high-temperatureor corrosive atmospheres. Bioreactors are devices orsystems that provide a biologically active environment,where micro-organisms and enzymes are immobilised andbiochemical reactions are performed in porous beds, and

porous ceramics are often used as such bio-reactor bedsdue to chemical stability of ceramics and accommodativefunction of porous structure. 20 Recently, porous biocera-mics with open pore structures have attracted greatattention for bio-implant applications including of boneregeneration. 21 Bone cells are impregnated through theopen pores and grow on their biocompatible wallsresulting in bone in-growth. Many electrodes used inelectro-chemical devices including gas puriers, gassensors, fuel cells, and chemical analysers are porousceramics. 22 Some porous electrodes require two modedistributions of pore sizes; small pores are for theelectrochemical reactions while large pores are for owpaths of reactants. Properties of electro-ceramics alsodepend substantially on the porosity content andmorphology and therefore porous ceramics are alsoapplied or expected to be used in various electro-devices.For example, porous piezoelectric ceramics have im-proved piezoelectric property and are good candidatesfor ultrasonic transducers, etc. 23 A variety of porousceramics have been applied as materials for refractorybricks of kilns and furnaces in various industrial elds,due to their low thermal conductivity and high thermalshock damage resistance (against thermal spalling). 24,25

On the other hand, some porous materials of conductiveceramics like zirconia and silicon carbide have beenutilised in heat exchangers and heaters. 13

As is known from Fig. 1, the representative processesfor making macro-porous ceramics are (i) partialsintering, (ii) sacricial fugitives, (iii) replica templates,

1 Classication of porous materials by pore size and corre-sponding typical applications and fabrication processes

Ohji and Fukushima Macro-porous ceramics: processing and properties

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and (iv) direct foaming. While the recent review articleson porous ceramics 9,11,12 have placed emphasis on che-mical approaches related to the latter three, this articleintends to overview the macro-porous ceramics fabri-cated through each of these four routes in below sections(Partial sintering, Sacricial fugitives, Replica tem-plates and Direct foaming). A number of innovative

techniques which have been developed recently for criticalcontrol of pores are introduced, divided into these fourcategories, together with some important properties of porous ceramics obtained in these processes. It should benoted however, that a lot of new approaches for macro-porous ceramics such as phase separations 2630 have beendeveloped other than the processes shown here. Figure 2shows schematic illustrations of these processes, each of which will be interpreted in its section. We then discussgas permeability of these porous ceramics in withdifferent pore sizes and structures in the section on Gaspermeability, and briey review porous ceramics withhierarchical porosity (incorporation of macro-, meso-and micro-pores) in the section on Hierarchically porousceramics, which have attracted much attention in bothacademic and industrial elds. Finally the article givessummary and discusses the issues to be solved for furtherrealising the potential of porous ceramics and forexpanding their applicability.

Partial sintering Partial sintering of powder compact is the mostconventional and frequently employed approachesto fabricate porous ceramic materials. Particles of powder compact are bonded due to surface diffusionor evaporationcondensation processes enhanced by

heat treatments, and a homogeneous porous structureforms when sintering is terminated before fully densied(see Fig. 2 a). Pore size and porosity are controlled by

the size of starting powders and degree of partialsintering respectively. Generally, in order to providethe desired pore size, the size of raw powder should begeometrically in the range two to ve times larger thanthat of pore. Porosity decreases with increased formingpressure, 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 greatlyaffect the micro-structures of porous ceramics. 31 Themechanical properties depend largely on degree of neckgrowth between grains, as well as porosity and poresize. Green and coworkers 32,33 found that before anydensication occurs, the formation of necks betweentouching particles by surface diffusion can increasethe elastic modulus to 10% of the fully dense value. Theporosities of porous materials obtained by partial sin-tering are usually below 50%. In industry, this methodhas been utilised for various applications including mol-ten metal lters, aeration lters (gas bubble generationin wastewater treatment plants), 13 and water puricationmembranes. 18

Several processing approaches have been developed toenhance grain bonding and improve strength of porousceramics. Oh et al. ,34 Jayaseelan et al. 35 and Yang et al.36

fabricated porous Al 2O3 and Al 2O3 based composites bythe pulse electric current sintering (PECS) technique andfound that the strength was substantially improved dueto the formation of thick and strong necks. Duringsintering the discharge is thought to take place betweenthe particles and to promote the bridging of particles byneck growth in the initial stages of sintering. This strongneck growth leads to substantially high strength com-pared to those of the conventional porous materials. Forexample, the exural strength of porous alumina-based

composites via PECS reached 250 and 177 MPa, with 30and 42% porosity respectively, which are considerablyhigh compared to those of porous alumina fabricated byconventional partial sintering, e.g. y 100 MPa at 30%porosity 35 (see Fig. 3). Using PECS, Akhtar et al. 37 alsofabricated porous ceramic monoliths from diatomitepowders, which are known as a cheap and renewable,natural resource. The PECS which rapidly heats diato-mite powder successfully bonds the particles togetherinto relatively strong porous bodies, without signi-cantly destroying the internal pores of the diatomitepowder. The micro-structural features showed thatconsolidation proceeds by the formation of necks attemperatures around 700750 u C, which is followed bysignicant melt phase formation around 850 u C, resultingin porous ceramics with a relatively high strength.

Deng et al. 38,39 tried to obtain strong grain bondingthrough combination of partial sintering and powderdecomposition. A mixture of a-Al 2O 3 and Al(OH) 3 wasused as the starting powder to make porous Al 2O3ceramics, and because Al(OH) 3 experiences a 60%volume contraction during decomposition and producesne Al 2O3 grains, the fracture strength of obtainedporous Al 2O3 was substantially higher than that of thepure Al 2O 3 sintered specimens because of strong grainbonding that resulted from the ne Al 2O3 grainsproduced by the decomposition of Al(OH) 3. Similar

improvement of mechanical properties was also identi-ed for ZrO 2 porous ceramics fabricated by addingZr(OH) 4.

40

2 Representative fabrication processes of macro-porousceramics


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Partial sintering through reaction bonding techniqueshave been frequently used for making porous ceramics,where reaction products form or precipitate epitaxiallyon grains, resulting in well-developed neck growthbetween grains. 41,42 In combination with the reactivesintering process, Suzuki et al.43,44 synthesised aCaZrO 3/MgO porous ceramics with three-dimensionalgrain net-work structure by using reactive sintering of highly pure mixtures of natural dolomite [CaMg(CO 3)2]and synthesised zirconia powders. CaMg(CO 3)2 decom-poses into CaCO 3, MgO and CO 2 (g) at y 500u C, andCaCO 3 then reacts with ZrO 2 to form CaZrO 3 and CO 2(g) at y 700u C. Through liquid formation via LiFdoping, these reactions and liberated CO 2 gas result information of a homogeneous open-pore structure withstrong grain bonding as shown in Fig. 4. The pore sizedistribution is very narrow (with typical pore size:y 1 mm), and the porosity was controlled (about 30 60%) by changing the sintering temperature. Therelatively high exural strength ( y 40 MPa for 47%porosity) was observed over the temperature range of room temperature to 1300 u C. The similar approach hasbeen applied other materials systems such as CaAl 4O7/

CaZrO 3 and CaZrO 3/MgAl 2O4 composite systems.45,46

She et al. 47 used an oxidation-bonding process for thelow-temperature fabrication of porous SiC ceramics

with superior resistance against oxidation. In such aprocess, the powder compacts are heated in air insteadof an inert atmosphere. Because of the occurrence of surface oxidation at the heating stage, SiC particles arebonded to each other by the oxidation-derived SiO 2glass. The mechanical strength is strongly affected byparticle size; the exural strength attained as high as185 MPa at a porosity of 31%, when using ne a-SiCpowder (0 ?6 mm), while it was 88 MPa at 27% porosityfor coarse powder (2 ?3 mm). The oxidation-bondingtechnique has been applied to other materials includingsilicon nitride, 48 SiC/mullite composites, 49 and SiC/cordierite composites. 50

Partial sintering technique has been also applied formaking porous silicon nitride with brous grains of high

aspect ratios.51

Compared with oxide ceramics, thedensication of silicon nitride ceramics is difcultbecause of strong covalent bonding between siliconand nitrogen atoms. This difculty of sintering siliconnitride ceramics is benecial for controlling density orporosity through adjusting the additives and thesintering process. In order to suppress densication,oxides with high melting point and high viscosity such asYb 2O 3 are frequently used as sintering additives. Theaddition of Yb 2O3 also is known to accelerate thebrous grain growth of b-Si3N 4.52 Figure 5 showsmicro-structure and mechanical properties of poroussilicon nitrides sintered at different temperatures byusing a-Si3N 4 and 5 wt-% Yb 2O3.52,53 The fracturestrength and fracture toughness were determined bythree-point exure and chevron-notched beam testsrespectively. While the porous structure sintered at1600 u C consists of equiaxed a-grains, those are trans-formed from equiaxed to brous when increasing thetemperature to 1700 u C. Further increase in sinteringtemperature gives rise to micro-structural change fromne to coarse grains, while maintaining the porosityaround 4045%. The brous micro-structure is advanta-geous for the strengthening effects of grain bridging andpullout. In actuality, both the fracture strength andfracture toughness of the materials sintered at 1700 u Cor higher are markedly improved, compared to those

at 1600 u C. For examples, the strength for the sampleof equiaxed micro-structure sintered at 1600 u C isy 40 MPa, while that at 1700 u C is y 380 MPa with

4 Micro-structure of porous CaZrO 3/MgO composite fabri-cated via in-situ reaction synthesis, exhibiting three-dimensional network structure with strong grain necking 43

(Reproduced with permission of John Wiley and Sons)

3 Flexural strength as a function of porosity for alumina/ 3 vol.-% zirconia (AZ) fabricated via PECS and conven-tionally sintered alumina (top) and micro-structure ofAZ (bottom). 35 Strong neck growth of the AZ results in

substantially high strength compared to those of con-ventional porous materials (Reproduced with permis-sion of John Wiley and Sons)


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the brous grain formation. As the sintering tempera-ture further increases, the micro-structure becomescoarser and the strength decreases with increasingfracture toughness (though the toughness values arestill below 4 MPa m 1/2 ). Porous b-Si3N 4 ceramics werealso fabricated by carbothermal reaction between silicaand carbon. 54 Micro-structure is controlled by varyingparticle size of the carbon in this case. Tuyen et al. 55

fabricated porous reaction-bonded silicon nitride bynitridation process at 1350 u C and post-sintering at15501850 u C, which provides similar brous micro-structure and high porosity. The duration time forsintering had a signicant effect on the micro-structureand grain morphology. The fabrication process isadvantageous due to the low cost of Si raw powder.

One of the unique processing routes for porousceramics with anisotropic micro-structure is tape-castingbrous seed crystals or whiskers. For porous siliconnitrides, b-Si3N 4 seed crystals were mixed with sintering

additives as starting powders, and the green sheetsformed by tape casting were stacked and bonded underpressure. 56 Sintering was performed at 1850 u C under a

nitrogen pressure of 1 MPa. The texture of poroussilicon nitride with porosity of 14% is shown in Fig. 6.The brous grains of silicon nitride are well alignedtoward the casting direction, and the pores, whoseshapes are mostly plate-like along the same direction,exist among the grains. The anisotropic (brous grain-aligned) porous silicon nitrides showed excellentmechanical behaviour, when a stress is applied in thealignment direction. 53,57 Figure 7 shows the fracturestrength and fracture toughness of the anisotropicporous silicon nitrides as a function of porosity, 53,57 incomparison with those of the isotropic porous siliconnitrides that were sintered at 1800 u C by using a-Si3N 4and 5 wt-%Yb 2O3.

53,58 The anisotropic materials exhib-ited very high strength above 1 ?5 GPa, and very highfracture toughness above 17 MPa m 1/2 in the porosityrange below 5%. It should be noted that the toughness of the porous materials with porosities below 10% issomewhat higher than that of the dense one (0%

porosity). These excellent mechanical properties weredue to enhanced crack shielding effects (bridging andpull-out) of aligned brous grains. Debonding was

5 Micro-structures and mechanical properties (exural strength and fracture toughness) of porous silicon nitrides sinteredwith 5 wt-%Yb 2O3 at 1600, 1700, 1800 and 1850

C. P denotes porosity 52 (Reproduced with permission of Elsevier)




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promoted by the existence of pores, and aligned grainsbridging the crack or interlocking each other wereelastically deformed or drawn apart without breaking,

leading to the above crack shielding. It has been alsorevealed that the anisotropic porous silicon nitrides aremuch superior even to the dense materials, for both thethermal shock fracture and damage resistances, whichare known to be in antagonistic relation. 53

Sacrificial fugitivesPorous ceramics can be obtained by mixing appropriateamounts of sacricial fugitives as pore forming agentswith ceramic raw powder and evaporating or burningout them before or during sintering to create pores (seeFig. 2 b). Frequently used pore forming agents includepolymer beads, organic bres, potato starch, graphite,charcoal, salicylic acid, carbonyl, coal and liquid para-fn. The pore forming agents are generally classied intosynthetic organic matters (polymer beads, organic bres,etc.), 5989 natural organic matters (potato starch, cellulose,cotton, etc.), 67,68,90105 metallic and inorganic matters(nickel, carbon, y ash, glass particles, etc.), 49,106115 andliquid (water, gel, emulsions, etc.). 116156 Porosity is

controlled by the amount of the agents, and pore shapeand size are also affected by the shape and size of theagents respectively when their sizes are large in compar-ison with those of starting powders or matrix grains. Thisapproach is useful particularly for obtaining high openporosity. The agents, however, need to be mixed withceramic raw powder homogeneously for obtaining uni-form and regular distribution of pores. Solid fugitives suchas organic materials are usually removed throughpyrolysis, which requires long-term heat treatment andgenerates a great deal of vaporised, sometimes harmful,byproducts.

Polymethylmethacrylate (PMMA) beads and micro-beads have been frequently employed for sacricialfugitives. 8,5964,77,79,8285 For example, Colombo and hisco-workers 8,5961 fabricated SiOC ceramic foam as

shown in Fig. 8, by dry mixing the silicon resin powderwith a sacricial template constituted by PMMA micro-beads, and subsequent heat treatments. Cruz et al. 62 useda colloidal processing technique with PMMA sacricialtemplates, to fabricate macro-porous yttria-stabilisedzirconia ceramics. Descamps et al.63,64 produced macro-porous b-tricalcium phosphate (TCP) ceramics by usingPMMA. An organic skeleton, which was formed by inter-connecting the PMMA balls through a chemical super-cial dissolution, was impregnated by the TCP slurry.The PMMA was eliminated by a thermal treatment atlow temperature, and sintering was carried out to obtainnal porous structure. This process allows a total controlof the porous architecture; the porous volume can varyfrom 70 to 80% and the interconnection size from 0 ?2 to0?6 times the average macro-pore size.

Andersson and Bergstrom 82 used expandable micro-spheres as a sacricial template to produce macro-porous ceramic materials by a gel-casting process. Themicro-spheres consist of a co-polymer shell and are lledwith a blowing agent (isobutane), which allows rapidand facile burn-out. By controlling the amount and sizeof the expandable micro-spheres, it is possible to tunethe porosity up to 86% and the pore size distributionfrom 15 to 150 mm. As low amounts as 12 wt-% of themicro-spheres are required to create a nal porosityabove 80 vol.-%. Expandable micro-spheres as sacricial

templates, rather than other templates such as PMMAmicro-beads, are advantageous because of lower levelsof gaseous byproducts generated during pyrolysis, and

7 Porosity dependence of fracture strength and fracture

toughness, K IC of anisotropic and isotropic porous sili-con nitrides. Stress is applied parallel to alignmentdirection for anisotropic material

8 SiOC ceramic foam using a sacricial template consti-tuted by PMMA micro-beads 8 (Reproduced with permis-sion of Elsevier)

6 Micro-structures of anisotropic porous silicon nitrideprepared by tape-casting brous seed crystals (poros-ity: 14%) 56 (Reproduced with permission of John Wileyand Sons)


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lower cost of the overall materials. Kim and hisco-workers 83,84 used hollow micro-spheres as sacricialtemplates to make porous silicon carbide ceramicssynthesised from carbon-lled polysiloxane and others.Using preceramic polymer and organic micro-spheresfor fabricating porous ceramics allows use of the low-cost and/or near-net shaped processing techniques likeextrusion and direct casting. They reported relativelyhigh exural strength for porous SiC ceramics (e.g. 60and 45 MPa at 40 and 50% porosity, respectively) andvery low thermal conductivity (2 W m

2 1 K2 1, at y 70%

porosity). 84

Song et al. 85 produced micro-cellular silicon carbideceramics with a duplex pore structure by using expand-able micro-spheres and PMMA spheres; which resultedin the large pores and the small windows in the strutarea respectively. This porous ceramics showed excel-lent air permeability as shown in the section on Gaspermeability.

Diaz et al.93,94 fabricated porous silicon nitrideceramics by using a fugitive additive, corn starch(particle size: 518 mm). In order to obtain homoge-neous dispersion of the fugitives, the mixture slurry waskept in agitation by 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 corn starch to(Ba, Sr) TiO 3 powder to obtain (Ba, Sr) TiO 3 porousceramics. They found that depending on the porosity,the PTCR effect was 12 orders of magnitude improvedin comparison with the dense reference.

Chen et al.66 developed porous silicon nitride of equiaxed a-grains by using phosphoric acid (H 3PO 4) asthe pore-forming agent and pressureless sintering of relatively low temperatures techniques (10001200 u C).On the other hand, Li et al. 80 fabricated porous silicon

nitride with brous b-grain structure, using naphthalenepowder as the pore-forming agent and gas-pressuresintering of high temperatures above 1700 u C. Thebending strength of the former materials was 50 120 MPa in porosity range of 4263%, while that of the latter was 160220 MPa in porosity of 5054%. Thissubstantial difference in strength is most likely due tothe micro-structural difference (equiaxed versus brous),which was similar to what we observed in Fig. 5.

Ding et al. 49 used graphite as the pore-former tofabricate mullite-bonded porous silicon carbide ceramicsin air from SiC and a-Al 2O3 through in situ reactionbonding technique. Graphite is burned out to producepores and the surface of SiC is oxidised at hightemperatures to SiO 2, which, at further increasedtemperatures, reacts with a-Al 2O3 to form mullite(3Al 2O3.2SiO 2). SiC particles are bonded by the mulliteand oxidation-derived SiO 2.

Long bres such as cotton thread, 96 natural tropicalbre 97 and metal wires 109 are often used as pore formingagents for obtaining porous ceramics of throughchannels. Zhang et al.96 produced porous aluminaceramics with unidirectionally aligned continuous pores(diameter: y 160 mm) via the slurry coating of mer-cerised cotton threads. The pore size can be adjusted byusing cotton threads of different diameters, and theporosity can be controlled by changing the solids

concentration of the slurry. In this case excellentpermeability can be achieved for porous ceramics withunidirectional through channel pores, because gas can

ow directly through the pores. However, the prepara-tion of such ceramics is complex because handling longbres such as thin wire or cotton thread is difcult.Using short bres or whiskers as the pore-forming agentis an alternative that combines the advantages of partially sintered porous ceramic and those of unidirec-tional pores. Yang et al. 65 demonstrated formation of rod-shaped pores in silicon nitride ceramics, using slipcasting of aqueous slurries of silicon nitride powder andsintering additives with 060 vol.-% fugitive organicwhiskers. Rheological properties of slurries were opti-mised to achieve a high degree of dispersion with a highsolid-volume fraction. Samples were heated at 800 u C inair to remove the whiskers and sintered at 1850 u C innitrogen atmosphere to consolidate the matrix. Porositywas adjusted in 045% by changing the whisker contentin 060 vol.-%. The obtained porous silicon nitridecontained uniform rod-shaped pores with randomdirections, exhibiting relatively high gas permeabilityin comparison with porous silicon nitride containingequiaxed pores. 157 Isobe et al. 81,110 and Okada et al.86,87

used carbon bres (14 mm diameter and 600 mm length)or Nylon 66 bres (9 ?543 mm diameter and 800 mmlength) for pore-forming agent, and tried to align themby extrusion technique to produce porous alumina 81,110

and mullite 86,87 ceramics with unidirectionally-orientedpores. They showed that the pore sizes and porositiescan be controlled by varying the bre diameter and brecontent. The obtained samples showed better airpermeability than the conventional porous materialsused for lter applications. 110 This technique can allowthe production of highly oriented porous ceramics byindustrially favoured extrusion method.

Liquid phases such as water and oil, which are readilysublimated or evaporated, are often used as pore

forming agents.116157

One of the most frequentlystudied approaches in recent years is freeze-drying thewater or liquid-based slurry to produce porous ceramicsof unique structure. 119155 Figure 9 shows a schematicillustration of the procedures which was employed byFukasawa et al.119121 , and a porous silicon nitride bodyobtained thereby. When the bottom part of the slurry isfrozen, ice grows macro-scopically in the verticaldirection, and pores are generated subsequently bysublimation of the ice. Through sintering this greenbody, a porous ceramics with unidirectionally alignedchannels can be obtained; these channels contain smallerpores in the internal walls (Al 2O 3)

119,120 or brous grainsprotruding from them (Si 3N 4).

121 This method hasseveral advantages, including simple sintering processwithout materials to be burnt out, a wide range of porosity (30 to 99%) controlled by the slurry concentra-tion, applicability to various types of ceramics andenvironmental friendliness without emitting harmfulproducts. In particular, porous scaffolds with ice-designed channel-like porosity have been intensivelystudied for a wide variety of applications includingbiomedical implants and catalysis supports.

The porosity of the porous materials obtained by usingthis technique is a replica of the original ice structure. Theporous channels run from the bottom to the top of thesamples, and the pores most frequently exhibit an

anisotropic morphology in the solidication plane.Deville et al.128131 investigated freeze casting of ceramicslurries, and particularly the relationships between the




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freezing conditions and the nal porous structures, formoderate to highly concentrated suspensions. It has beenclaried that the morphology of the porous structures,i.e. the content, dimensions, shape and orientation of porosity, was adjusted by varying the initial slurrycompositions and the freezing conditions. For highlyconcentrated solutions, the particleparticle interactionspresumably lead to the formation of ceramic bridgesbetween two adjacent lamellae. They used the freeze-drying technique to make sophisticated porous andlayered-hybrid materials. The nacre-like structure withlamellar dendrites was obtained with a lamellar templateassembled by ice crystals, as shown in Fig. 10. Propercontrol of the freezing conditions resulted in a porousmultilayered ceramics with compressive strengths up tofour times higher than those of materials currently usedfor implantation. Munch et al.132 emulated naturestoughening mechanisms in aluminium oxide and poly-methyl methacrylate composites by using porous ceramicstructure fabricated by freeze-drying techniques, andsucceeded in obtaining toughness more than 300 times (inenergy terms) that of their constituents.

In order to avoid the freezing process under theextremely cold temperature, Araki and Halloran 133135

used camphene, C 10 H 16 , as a vehicle for producingporous ceramics via freeze-drying process. Slurriescontaining ceramic powder in the molten camphene,which were prepared at 55 u C, were quickly solidied

(frozen) when they were poured into polyurethanemoulds at room temperature. The obtained porousceramic bodies have interconnected pore channels of

nearly circular cross-sections, unlike ellipsoidal ones inconventional aqueous freeze casting. The channels are rep-licas of entangled dendrites of frozen camphene. Employ-

ing a similar camphene-based freeze-casting approach, Kohand his co-workers fabricated highly porous Al 2O3,136138

SiC, 139,140 PZT-based ceramics, 141,142 hydroxyapatite, 143,144

glass ceramics, 145 and ZrO 2,146,147 etc., having intercon-

nected pore without noticeable defects.Combining freeze-drying process and gelcasting techni-

que has been also most frequently employed approaches tomake porous ceramics with rened micro-structure. 148156

These studies show the use of organic polymer in freezecasting route affects the pore size and morphology bycontrolling ice crystal growth during freezing. Chenet al. 148,149 used alumina slurries containing tert-butylalcohol (TBA) and acrylamide (AM) for the freeze-dryingprocess. TBA, which freezes below 25 uC and volatilisesrapidly above 30 u C, acts as the freezing vehicle andtemplate for forming pores. Polymerised in the slurry asthe gelation agent, AM strengthens the green bodiessubstantially. The sintered porous ceramics have goodmechanical strength (compression strength of 150 MPa atporosity of y 60%) because the pore channels formed bythe TBA template are surrounded by almost fully densewalls without any noticeable defects. Ding et al.150 alsoused a gel freeze-drying process to fabricate porous mulliteceramics with porosity up to 93%. Alumina gel mixed withultrane silica was frozen (isotropically), followed bysublimation of ice crystals. Porous mullite ceramics wereprepared in air at 14001600 u C due to the mullitisa-tion between Al 2O3 and SiO 2. Porous yttria-stabilisedziroconia 151 and porous alumina 152 were fabricated byfreeze-drying process with addition of polyvinyl alcohol,which prevents ice crystal growth and reduces the poresizes substantially. Porous alumina with oriented porestructures has been also prepared by freeze castingtechnique with a water-soluble polymer such as poly-ethylene glycolon. 153 Using precursor silica hydrogels,Nishihara et al. 154 fabricated ordered macro-porous silica(silica gel micro-honeycomb), by freeze-drying methodswhere micrometre-sized ice crystals are used as a template.The pore sizes can be controlled by changing theimmersion rate into a cold bath and the freezing

temperature. For example, the average pore size can bereduced to as small as 4 ?7 mm withthe rate of20 cm h2 1 at

77 K. It was also reported that the thickness of the

9 Schematic illustration of freeze-drying process formacro-porous ceramics and a porous silicon nitridebody obtained thereby 121 (Reproduced with permissionof John Wiley and Sons)

10 Multilayered porous alumina structures a with dendri-tic-like features b (detail) produced via freeze-dryingprocess 129 (Reproduced with permission of Elsevier)




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honeycomb walls was affected by the SiO 2 concentrationand the pore size.

Using a gel-freezing method, Fukushima et al.155,156

fabricated porous cordierite or silicon carbide ceramicswith porosity from 80 to 95%, where unidirectionallyoriented cylindrical channels are uniformly distributedover relatively large bulk samples (typically severalcentimetre). They used gelatin as the gelation agent,which was mixed with water for the freezing vehicleand raw powder. The gel was frozen at 2 10to 2 70u C andwas dried under vacuum, followed by degreasing andsintering. The cell size and cell wall thickness bothdecreased with decreasing the freezing temperature, rang-ing from 20 to 200 mm and from 3 to 20 mm respectively.The numbers of cells, for example, for the cordieritesample frozen at 2 50u C and sintered at 1400 u C was1500 cells mm 2 2 in the cross-section, which is a markedlylarge number in comparison with those of samplesobtained by extrusion method (12 cells mm

2 2). Micro-structural observation revealed dense cell walls as shownin Fig. 11, which leads to relatively high compressivestrength, for example 17 MPa for 86% porosity sample of silicon carbide.

Replica templatesMacro-porous ceramics having interconnected largepores, or channels, of high volume porosity and opencell walls have been frequently fabricated by the replicatechniques (Fig. 2 c). The rst step of a typical templateprocess is impregnation of a porous or cellular struc-ture with ceramic suspension, precursor solution, etc.Various synthetic and natural cellular structures can beused as templates. The templates need to have adequateexibility, shape recovery ability and homogeneous opencell structure.

The most frequently used synthetic template is porouspolymeric sponge such as polyurethane. They are soakedinto a ceramic slurry or precursor solution to impregnatethe templates with them, and the surplus is drained andremoved by centrifugation, roller compression, etc. Inthis process, the appropriate viscosity and uiditydepending on the cell size, etc. are required so that

uniform ceramic layer forms over the sponge walls. Theceramic-impregnated templates are dried and then heat-treated to decompose the organic sponges. Following

the pyrolysis, the ceramic layers are sintered at highertemperatures to densify. Porosity higher than 90% canbe obtained with cell sizes ranging from a few hundredmicrometres to several millimetres. The open cells areinterconnected, which allows uid to pass through thefoams with a relatively low pressure drop. However, dueto cracking the struts during the pyrolysis, the mechan-ical properties of ceramic reticulated foams are generallypoor. 158 In order to avoid the strut crack formation, avariety of approaches have been developed. In order toincrease the struts thickness and heal the strut cracks,Zhu et al.159 recoated repeatedly a reticulate porousceramic body with thinner slurry of the same composi-tion, after the green body coated with thicker slurrywas preheated to burn out the sponge. In addition, Vogtet al. 160 attempted the vacuum inltration of ceramicslurry to ll up the struts in the pre-sintered foam.The hollow struts caused by burnout of the polyur-ethane template could be completely lled up, whichresulted in a considerable increase in compressivestrength. Luyten et al.161 used a reaction bonded,modied replica technique to produce strong strutsof ceramic foam. Jun et al.162,163 produced hydroxya-patite scaffolds coated with bioactive glassceramicsusing the polymer foam replication method, to enhancetheir mechanical properties and bioactivities. Pleschet al. 164 fabricated a reticulated macro-cellular aluminafoam coated with TiO 2 for photocatalytic applications.They showed that the photocatalytic activity can beaffected by the pore size of the host foam due to theaccessibility of UV light. This study implies that thesurface of macro-porous ceramic can play a role toprovide the additional functionalities for a component.Highly porous ceramics can be derived also frompreceramic polymers after pyrolysis above 800 u C in

inert atmosphere.8

One of the typical methods isdissolving the silicone resin preceramic polymer into asuitable solvent and adding appropriate surfactants andcatalysts, followed by the pyrolysis. The advantagesinclude a very wide range of pore (cell) sizes (typically1 mm to 2 mm), well-dened open-cell structures andmacro-defect-free struts. 165168 Paper-based templatehas been also proposed; Travitzk et al.169 succeeded infabricating single-sheet, corrugated structures, andmultilayer ceramics by using various paper replicatemplates. Unique micro-structures such as alignedpores derived from bres in the paper, elongatedmorphology and multilayer stacking were observed,and they substantially affected the anisotropic mechan-ical properties.

Natural resources of porous structures such as woods,corals, sea sponge, etc. have been also used as replicatemplates. The woods are transformed to carbonaceouspreforms by heat-treatment under inert atmosphere. Theyare then inltrated with oxides and non-oxides that reactto form porous ceramics. The frequently used inltrationsinclude molten metals, 170180 gaseous metals, 175,181186

alkoxide solutions, 187189 and others. 190,191 The benetsare a wide variety of obtained porous structures (de-pending on the type of wood selected), low-cost start-ing materials, near-net and complex shape capabilities,and relatively low temperature manufacturing process.

Examples of porous structures of the pyrolysed woodsand Si inltrated samples are shown in Fig. 12. Par-ticularly, the oriented vessels of the woods provide unique

11 Micro-structures of porous silicon carbide preparedby freeze-drying technique using gelatin as gelationagent frozen at 2 10 C156 (Reproduced with permissionof Elsevier)




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anisotropic porous structure of aligned unidirectionalchannels, which is suitable to the applications such asltration and catalysis supports. Porous biomimeticsilicon carbide produced through this approach has beenalso studied for the use as a medical implant material. 192

Biomorphic porous silicon nitride was produced fromnatural sea sponge via replication method. The spongeswere impregnated with silicon-containing slurry via dip-coating, and were heat-treated to delete the bio-polymers, leading to a Si-skeleton. Subsequent thermaltreatment under owing nitrogen promoted the nitrida-tion of the silicon, porous a/b-silicon nitride with aporosity of 88% and the original morphology of the seasponge. 193

Direct foaming In direct foaming techniques, ceramics suspension whichis foamed by incorporating air or gas is stabilised anddried, and subsequently is sintered to obtain consoli-dated structure (Fig. 2 d ). This technique allows low-cost and easy production of highly porous ceramicmaterials, up to more than 95% porosity. Porousceramics with unidirectional channels have been alsodeveloped recently by using continuous bubble forma-tion in ceramic slurry 194,195

However, due to the thermodynamic instability, thegas bubbles are likely to coalesce in order to reduce thetotal Gibbs free energy of the system, resulting in largepores in the nal porous bodies. It is, therefore, criticallyrequired to stabilise the air or gas bubbles in ceramicsuspension. One of the most frequently approaches forthe stabilisation is to use surfactants which reduce theinterfacial energy of the gasliquid boundaries. The poresize of the produced porous body ranges from below50 mm up to the mm scale, 196201 depending on howeffectively and rapidly the used surfactants work.

Surfactants used for the stabilisation are classied intoseveral types including non-ionic, anionic, cationic andprotein. A variety of effective surfactants have been

developed for the direct foaming of porous ceramics,and representative approaches have been reviewed byStudart et al. 9 and Sepulveda. 202

Barg et al.199,200 developed a novel direct foamingprocess with emulsifying a homogeneously dispersedalkane or airalkane phase in the stabilised aqueouspowder suspension. In contrast to the conventional directfoaming methods, foaming is made here by evaporationof the emulsied alkane droplet, leading to a time-dependent expansion of the emerging foam in a mould. Itwas possible to realise interconnected structures with cellsizes from 0 ?5 to 3 mm and porosities up to 97 ?5%. Thisautonomous foaming process also allows high exibilityin the production of ceramic parts with gradientstructures and complex shaping. Foaming proceeds as aconsequence of the evaporation of the alkane phaseresulting in the growth of the stabilised alkane bubblesand in a volume increase in the foam. The resultingfoamed green body possesses a tight cylindrical form witha cross-section corresponding to the mould. Figure 13shows typical three stages of the foaming process of anemulsied alumina powder suspension (using 5 ?5 vol.-%heptane and 0 ?83 vol.-% anionic surfactant):

(i) alkane emulsion in the powder suspension(ii) transition of emulsion to wet foam

(iii) formation of a polyhedral structure (transition tostable foam). While alkane droplets in the topregion evaporate and grow, new droplets aresimultaneously starting the foaming process inthe lower parts until the whole volume of theemulsion is converted into stable foam.

Similarly to replica template approach, preceramicpolymer solution has been used instead of ceramicsuspension for direct foaming. Colombo and Modesti 203

fabricated porous ceramics by dissolving preceramic

polymers (silicone resins) into a suitable solvent andadding blowing agent, surfactant, catalyst, etc., followedby pyrolysis at 10001200 u C in inert atmosphere.

12 Micro-structures of a different types of natural wood-

derived carbon performs172

and of b

biomorphous poroussilicon carbides from pinus silvestris 186 (Reproduced withpermission of Elsevier)

a alkane emulsion in the powder suspension; b transi-tion of emulsion to wet foam; c formation of polyhedral

structure (transition to stable foam)13 Three stages of foaming process of emulsied cera-mic powder suspension 199 (Reproduced with permis-sion of John Wiley and Sons)




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Expansion was achieved by high speed mixing (intro-duction of bubbles in the solution) and heat treatment at2540 u C. Because of the limited amount of defects in thestruts, the obtained porous ceramics showed higherstrength in comparison to conventional reticulatedfoams. 204

Kim et al. 205,206 developed porous ceramics with a neand uniformly distributed micro-cellular structure frompreceramic polymers using CO 2 as a blowing agent. Amixture of polycarbosilane and polysiloxane was satu-rated with gaseous CO 2 under a high pressure and then alarge number of bubbles were introduced using athermodynamic instability via a rapid pressure drop.The micro-cellular ceramics were obtained throughpyrolysis and optional, subsequent sintering.

It has been shown that particles with tailored sur-face chemistry can also be used efciently to stabilisegas bubbles for producing stable wet foams. 207210

Gonzenbach et al. 211214 have developed a novel directfoaming method that uses colloidal particles as foamstabilisers in order to obtain macro-porous ceramicswith smaller cell sizes than those of the foams prepared

with long-chain surfactants. Owing to the adsorption of partially hydrophobic particles to the air/water interface,the method allows for the fabrication of ultra-stable wetfoams which show neither bubble coalescence nordisproportionation over several days, as opposed toseveral minutes typically required for the collapse of thesurfactant-based foams. The attachment of colloidalparticles at the air/water interface is promoted byadjusting the wettability of the particle upon adsorptionof short-chain amphiphilic molecules on the surface.Because of its remarkable stability, the particle-stabi-lised foams can be dried directly in air without crackformation. The macro-porous ceramics obtained aftersintering exhibit porosities from 45 to 95% and cell sizesbetween 10 and 300 mm. The compressive strengths of the sintered foams with closed cells (for example 16 MPaat porosity of 88% in alumina foams) are substantiallyhigher than those of foams prepared with otherconventional techniques. The surface-modied particleswhich originally cover the air bubble in wet foamsbecome a thin surface layer of single grains aftersintering. Macro-porous ceramics with open porositycan be also prepared with this technique by simplydecreasing the concentration of stabilising particles.

Gas permeabilityGas permeability is one of the most important propertiesof porous ceramics which are expected to be used for gas-lters such as DPF, since large pressure drops cannot betolerated in such applications. Highly porous ceramicswith aligned unidirectionally through pore channels,which were prepared by freeze-drying process, areexpected to provide excellent permeability. In this section,we discuss Darcian permeability of porous ceramicswith different pore sizes and structures. Figure 14 showsthe Darcian permeability as a function of pore sizefor porous ceramics fabricated by the freeze-dryingprocesses, 152,156 organic spherical fugitives, 61,85 extrudedorganic brous fugitives, 81,87 direct foaming, 196,215 andreplica templates. 215,216 The pore structures are classied

into three categories of Spherical (Connected),61,85,196,215

Cylindrical (Connected) 81,87 and Cylindrical, 152,156 asschematically shown. The Darcian permeability K is

determined from pressure drop and owrate of air by theDarcys law. 217 Based on the capillary model, K isexpressed by

K ~ wD p 2=C (1)where w is the porosity, Dp is the pore diameter and C is aconstant depending on the pore structure. 217,218 All the K values of Fig. 14 are adjusted at the porosity of 0 ?85 byusing equation (1) for comparison. Note that the inertialcontribution (non-Darcian permeability) was consideredin addition to the viscous one (Darcian permeability) inRefs. 61, 152, 215 and 216, which results in high values of K , compared to the case of neglecting the inertialeffect 81,85,87,156,196 (the ratio of viscous contribution intotal is 6090% 152 ). If the uid ows through theunidirectional cylindrical pores penetrating in parallel,C is 32 (Ref. 218), which is shown as the solid line(w5 0?85) in the gure. The porous ceramics fabricated bythe freeze-drying processes, 152,156 which have cylindricalthrough channels, showed the permeability very close tothis solid line, indicating the unidirectional alignment of the cylindrical pores. The permeability required for acommercially available DPF is 10

2 11 to 102 12 m2,219 and

most of the freeze-dry-processed materials exceed thiscriterion. The porous ceramics prepared with extrudedorganic brous fugitives 81,87 showed lower permeabilityvalues than those of the freeze-dry-processed ones, mostlikely because of limited contact area among the shortbres. As for the porous ceramics with a duplex porestructure, 85 the permeability increases with increasing the

amount of PMMA micro-beads and decreasing theaveraged pore size, since the number of the smallwindows in the strut area increases.

14 Darcian permeability as a function of pore size for porousceramics fabricated by freeze-drying processes, 152,156

in comparison with those of other processes includ-ing organic spherical fugitives, 61,85 extruded organicbrous fugitives, 81,87 direct foaming, 196,215 and replicatemplates. 215,216 Solid line indicates theoretical perme-ability K 5 wD p2/32 (w5 0?85) for case of unidirectionalcylindrical pores penetrating in parallel




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Hierarchically porous ceramicsPorous ceramics can be divided into three categories of macro-, meso- and micro-porous, according to its poresize, as described in the section on Introduction. Basedon very recent review article of Colombo et al.,12

hierarchical porosity can be dened as incorporationof two or three of these different porous structures,which can be provided by various methods of templat-ing, impregnation, emulsion, phase separation, coatingand etching. Porous ceramic components with suchhierarchical porosity have attracted a great deal of attention, due to the synergy effects of differentadvantages that each porous structure can provide.

A typical example is micro-porous hydrogen permse-lective silica membrane coated on the surface of meso-porous c-alumina or macro-porous a -alumina supports,which is expected to be widely used in future for hydrogenusage. In this case, thin micro-porous layer has a role of permselective of hydrogen gas and macro-porous mem-brane support provides a mechanical strength due toneck among alumina particles and mass transfer due to

interconnected macro-pores.220224

Generally, the macro-porous supports are fabricated by extrusion route or slipcast and following partial sintering, and then meso-porous or micro-porous membranes are coated onto thesupports by dip coating of sol solution through hydrolysisand condensation of silicon alkoxide, or chemical vapourdeposition. Meso-porous intermediate layers frequentlyput between membranes and supports. Yoshino et al.220

fabricated micro-porous silicamembranes and membranemodules and examined the gas permeation characteristicsand stability of substrate. Tubular type a-aluminasubstrate with 0 ?7 mm was prepared by an extrusionmethod, and c alumina intermediate layer with 60 nm

pore size was formed by dip-coating of boehmite sol overthe substrate, followed by another dip coating process of silica sol to make silica membranes on the top. Thealumina tube showed good heat-cycle stability forstrength and permeation ( , 5% changes after 100 cyclesof room temperature773 K). Excellent hydrogen perme-ability of 5 6 10

2 8 to 5 6 102 6 mol m

2 2 s2 1 Pa

2 1 withH 2/N 2 selectivity of 30300 was obtained at 873 Kwith the silica membranes. Gu et al.221 reported that adual-element silica-alumina composite membrane wasdeposited by chemical vapour deposition of tetraethy-lorthosilicate and aluminium-tri-sec-butoxid, on a-alu-mina macro-porous support with c-alumina meso-porousintermediate layers, as shown in Fig. 15. The meso-porous intermediate layers were formed by dip coating of boehmite sols with different particle sizes, resulting inmultiple graded structures without cracks. The supportedcomposite silicaalumina membrane showed high hydro-gen permeability in the order of 10

2 7 mol/m2 2 s

2 1 Pa2 1

and good stability against water vapour in comparison topure silica at 873 K. Besides gas separation membranes,similar processing approaches have been applied for avariety of ltrations, adsorptions and catalysts. 225229 Inaddition biomorphous method, 230,231 etching method 232

and PECS method 233,234 can also provide hierarchicallyporous ceramics.

The above fabrication processes comprise several

steps including forming, sintering, coating, etc. For themore simplied and efcient processes, several attemptsfor one-pot synthesis (single step pyrolysis) have been

made to realise hierarchically porous ceramics. The mostfrequent approach for the one-pot synthesis is to usepreceramic polymers. Preceramic polymers, includingorganic and inorganic polymer with continuous siliconenetwork and sol solution prepared by hydrolysis andcondensation of metallic alkoxide, can provide ceramicmaterials as residue through its pyrolysis. During heattreatment of preceramic polymer with silicone network,organic groups can be decomposed and be transferred toinorganic bonds such as SiO, SiC and SiN, andusually under inert atmosphere to prevent an oxidationof organic groups. When preceramic polymer is used asraw material to fabricate hierarchically porous ceramic,meso- or micro-porosity has been formed by varying thepyrolysis temperature of preceramic polymer, 235,236

using the block copolymer, 237242 and the addition of ller into the polymer. 236,243 Several macro-porousprocessing approaches including sacricial fugitives,replica templates and direct foaming have been utilisedin order to provide the macro-posority. Holland et al.237

reported that porous titania, zirconia, and aluminafrom the inltration of the corresponding alkoxidesinto the latex spheres as the templates for macro-pores and revealed the highly ordered micro-structures.Kim et al. 238 fabricated hierarchically porous aluminawith meso- and macro-pores, where meso-porous struc-ture was obtained using alkyl carboxylate as a chemicaltemplate, and macro-pore structure was developedthrough polystyrene beads or silica gels. Suzukiet al. 239 prepared hierarchically porous silica andalumina by dual templating method of surfactant andpolystyrene beads. They conrmed macro-pores around200 nm from the polystyrene sphere, and ordered meso-pores around 4 nm due to the triblock copolymer andmicro-pores of , 2 nm due to the tail of copolymer.Dacquin et al. 241 prepared macro-porousmeso-porousalumina by using 400 nm polystyrene beads for macro-pores and triblock copolymer for meso-pores, as shownin Fig. 16. The regular macro-porous skeleton obtainedfrom polystyrene template was observed (A, B) while

high resolution TEM study revealed the ordered,hexagonally packed meso-pores resulting from self-assembly of the block copolymer solution (C).

15 Micro-structure of hierarchically porous ceramics com-prising silica-alumina composite membrane, c-aluminamulti-intermediate meso-porous layers and aluminamacro-porous support 221 (Reproduced with permissionof Elsevier)




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The growth of nano-structure on macro-porous substratecan be also categorised as hierarchically porous ceramics.The freeze-drying process is one of the routes which give ussuch porous structures. 119121 As already stated, the macro-scopic through channels obtained by this process containsmall pores in the internal walls and protruding whisker-like

grains, which results in pore size distribution with two orthree peaks corresponding to the respective pores. Anotherapproach is the chemical process via preceramic polymer.As shown in Fig. 17, Vakifahmetoglu et al.244,245 developeda cellular SiOC ceramics with nano-wires using preceramicpolymer, foaming agent and metallic catalyst and showedhigh specic surface area of 110 m 2 g

2 1. They revealed themuch formationof long SiC or Si 3N 4 nano-wireson thewallof the cellular SiOC, and the effect of pyrolysis conditions(atmosphere and temperature) on the formation of nano-wires. SiC nano-wires on the macro-pores have been alsoformed by conventional pressing, 246 replica templates 247

and camphene dendrites, 140 where preceramic polymer andSiC powder as raw materials have been heated on the one-pot synthesis. It was suggested that the formation of nano-wires were due to iron impurities in the raw SiC powder.Some other approaches have been also proposed;Vanhaecke et al.248 and Edouard et al.249 fabricated SiCnano-wires on the surface of SiC foam by the reactionbetween carbon nano-bres and SiO gas, followed by theoxidation removal of residual carbon. Jayaseelan et al.prepared cordierite whiskers 250 and SiC nano-bres 251 onthe cordierite honeycombs via air sintering and carbother-mal reduction respectively, using mixture of cheap rawpowder such as kaolin, talc, alumina, carbon and silica.

Summary and future prospectiveDuring the last decade, tremendous efforts have beendevoted for the researches on innovative processing

technologies of porous ceramics, resulting in bettercontrol of the porous structures and substantialimprovements of the properties. This article reviewedthese recent progresses of porous ceramics. Because of avast amount of research works reported in this eldthese days, the review mainly focused on macro-porousceramics whose pore size is larger than 50 nm. Followedby giving a general classication of porous ceramics, anumber of innovative processing routes developed forcritical control of pores were described, along with someimportant properties. They were divided into fourcategories including (i) partial sintering, (ii) sacricialfugitives, (iii) replica templates, and (iv) direct foaming.

The partial sintering, the most conventional techniquefor making porous ceramics, has been substantiallysophisticated in recent years. Very homogeneous porousceramics with extremely narrow size distribution hasbeen successfully prepared through sintering combinedwith in situ chemical synthesis. Porous silicon nitrideswith anisotropic micro-structure (aligned brous grainsand pores) produced via tape-casting and partialsintering have exhibited excellent mechanical properties,which are equivalent, or sometimes superior, to those of the dense materials.

Advantage of the sacricial fugitives is that poreshape and size are controlled by the shape and size of theagents respectively. Various kinds of fugitive agentshave been used for obtaining desired porous structure.The fugitives are most frequently removed through

pyrolysis, generating a great deal of vaporised, some-times harmful, byproducts, and a lot of works have beenmade to reduce or eliminate them. The freeze-drying

16 Micro-structures of macro-porousmeso-porous aluminaprepared by using polystyrene for macro-pores and tri-block copolymer for meso-pores. (A) SEM and (B) TEMobservations for macro-porous skeleton, and (C) high-resolution TEM for meso-pores 241 (Reproduced with per-mission of American Chemical Society)

17 Micro-structure of SiOC foam (top), and SiC nano-wiresformed on its wall surface (bottom) 244 (Reproduced withpermission of John Wiley and Sons)




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processes using water or liquid as fugitive materials areadvantageous in this viewpoint and have been veryintensively studied in recent years. Controlling growth of ice during freezing leads to unique porous structures andexcellent performances of porous ceramics. For exam-ple, the highly porous lamellar hydroxyl-apatite scaf-folds via this approach are several times stronger thanmaterials currently used for implantation. Also thefreeze-dry-processed porous ceramics that have cylind-rical through channels demonstrated the excellentpermeability.

The replica template techniques have been widelyused to fabricate porous ceramics with interconnectedlarge pores of high volume porosity. Porous polymericsponge such as polyurethane is the most typicalsynthetic template used for this process. However, dueto cracking struts during pyrolysis of the sponge, themechanical reliability is substantially degraded; a varietyof approaches have been used to avoid the strut crackformation. Natural template approaches using wood,for example, as positive replica, have been frequentlyused in these years and have realised highly orientedporous open-porous structure with a wide range of porosity.

The direct foaming technique offers low-cost and easyproduction of highly porous ceramic materials. In orderto suppress coalescence of gas bubbles in ceramicsuspension that results in large pores in the nal porousbodies, various methods which stabilise the bubbles havebeen developed, including use of effective surfactants,evaporation of emulsied alkane droplets, and use of surface-modied particles. These novel processing routesalso lead to better mechanical properties. Then, thearticle briey reviewed porous ceramics with hierarchicalporosity (incorporation of macro-, meso- and micro-

pores), which have attracted much attention in bothacademic and industrial elds. Several attempts for one-pot synthesis (single step pyrolysis) such as preceramicpolymer-derived approaches have been made to producehierarchically porous ceramics in more simplied andefcient manner.

We now discuss several issues to be solved for furtheractivating the potential of macro-porous ceramics andfor expanding their applicability. As has been seen inthis review, a number of approaches have been alreadyused towards environmentally benign, resource-produc-tive, and inexpensive fabrication processes. This ten-dency will be of course enhanced in future, and a greateramount of efforts will be devoted to the followingtargets; fewer heat-treatments (pyrolysis, calcination,sintering, etc.) of shorter time and lower temperature,processing in air atmosphere and ambient pressure,complete elimination of harmful byproduct generation,use of abundant resources or recycled materials, near netshape forming and sintering, etc.

Another important issue is further improved perme-ability of porous ceramics even with smaller pore size,which will be strongly demanded in many applicationsof porous ceramics such as lters, membrane/catalystssupports, and reactor beds. This will be potentiallyattained by more precisely controlling pores themselves(size and its distribution, shape, location, orientation,

etc.) and matix micro-structures (grains, whiskers, bres,etc.) at different scale levels from nano to macro. Forexample, the DPF of next generation will be desired to

lter smaller pollutants in the exhaust gases with lesspressure drop. We presume that one of the effectivesolutions will be a hierarchically porous structureincorporating well controlled macro-scaled cylindricalpores and micro- or nano-scaled whiskers grown fromthe internal walls, which enable the uid permeation andpollutant capture respectively. 250,251

Finally, it should be remarked that porous ceramiccomponents are most frequently subjected to mechanicalloads and thermal shocks in their numerous applicationsand that better structural reliability will be criticallyneeded to further expand their applicability in futureindustries. Greater efforts will be made for ensuring themechanical reliability, such as enhancing neck growthamong matrix grains and avoiding crack formationduring fabrication. On the other hand, while pores aregenerally believed to deteriorate the mechanical proper-ties, this is not always true as seen in this review.Carefully tailored porous micro-structures have a greatpossibility to give rise to substantially improved orunique properties that are not attained even in densematerials.

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