Macro-porous ceramics: processing and properties

Macro-porous ceramics: processing and
properties

T. Ohji* and M. Fukushima

Porous ceramics are now expected to be used for a wide variety of industrial applications from
filtration, absorption, catalysts and catalyst supports to lightweight structural components. During
the last decade, tremendous efforts have been devoted for the researches on innovative
processing technologies of porous ceramics, resulting in better control of the porous structures
and substantial improvements of the properties. This article intends to review these recent
progresses of porous ceramics. Because of a vast amount of research works reported in this field
these days, the review mainly focuses on macro-porous ceramics whose pore size is larger than
50 nm. Followed by giving a general classification of porous ceramics, a number of innovative
processing routes developed for critical control of pores are described, along with some
important 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 most
conventional technique for making porous ceramics, has been substantially sophisticated in
recent years. Very homogeneous porous ceramics with extremely narrow size distribution have
been successfully prepared through sintering combined with in situ chemical synthesis. Carefully
tailored micro-structure (size, morphology and orientation of grains and pores, etc.) of porous
ceramics has led to unique mechanical properties, which cannot be attained even in the dense
materials. Various types of the sacrificial fugitives have been examined for obtaining well-tuned
shape and size of pores. The freeze-drying techniques using water or liquid as fugitive materials
have been most frequently studied in recent years. Controlling growth of ice during freezing has
led to unique porous structures and excellent performances of porous ceramics, e.g. excellent
mechanical behaviour for highly porous lamellar hydroxyl-apatite scaffolds. Numerous
approaches on the replica templates have been developed in order to produce highly porous
ceramics having interconnected large pores and sufficiently strong struts without cracks. Natural
template approaches using wood, for example, as positive replica, have been intensively studied
in these years and have realised highly oriented porous open-porous structure with a wide range
of porosity. As for the direct foaming technique, a variety of novel techniques which stabilise the
bubbles 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 briefly
review porous ceramics with hierarchical porosity (incorporation of macro-, meso- and micro-
pores), which have attracted much attention in both academic and industrial fields. Finally the
article gives the summary and discusses the issues to be solved for further activating the potential
of porous ceramics and for expanding their applicability.

Keywords: Ceramics, Pores, Processing, Properties, Permeability, Mechanical property, Foam, Review

National Institute of Advanced Industrial Science and Technology (AIST), Introduction
Anagahora 2266-98, Shimo-shidami, Moriyama-ku, Nagoya 463-8560,
Japan For structural applications of brittle ceramic materials,
pores are generally what to be eliminated because they
*Corresponding author, email [email protected] act as fracture defects and degrade the structural
reliability, and therefore, ceramic engineers tried to
sinter ceramics to full density to attain high mechanical

ß 2012 Institute of Materials, Minerals and Mining and ASM International International Materials Reviews 2012 VOL 57 NO 2 115
Published by Maney for the Institute and ASM International
DOI 10.1179/1743280411Y.0000000006

Ohji and Fukushima Macro-porous ceramics: processing and properties

1 Classification of porous materials by pore size and corre- large like filtration and micro-filtration, the separation is
principally made by sieving effect where matters whose size
sponding typical applications and fabrication processes is larger than the pore size is trapped. In ultra filtration,
nano-filtration, and reverse osmosis where pore size is small,
strength. On the other hand, there have been various fluid permeability depends on the affinity of solute and
industrial applications where pores are taken advantage solvent with the porous materials as well.
of positively, from filtration, absorption, catalysts and
catalyst supports to lightweight structural components Because of a vast amount of research works reported in
and thermal insulator.1,2 In these decades, a great deal of this field these days, the review mainly focuses on macro-
research efforts have been devoted for tailoring deliber- porous ceramics; micro- and meso-porous ceramics
ately sizes, amounts, shapes, locations and connectivity whose pore size is below 50 nm are not included here.
of distributed pores, which have brought improved or Representative applications of macro-porous ceramics
unique properties and functions of porous ceramics.3–12 are briefly described. Ceramic filters are now widely
The merits in using porous ceramics for these applica- loaded in diesel engines to trap particulate matters in the
tions are generally combination of intrinsic properties of exhaust gas stream, so called, diesel particulate filters
ceramics themselves and advantages of dispersing pores (DPFs). Since the high combustion efficiency and low
into them. The former include heat and corrosion carbon dioxide emission of diesel engines, the demand of
resistances, wear and erosion resistance, unique electro- DPF is also expected to further increase over the
nic properties, good bioaffinity, low density, and high world.15–17 Ceramic water purification filters are used
specific strength, and the latter are low density, low for eliminating bacillus coli and suspension from waste
thermal conductivity, controlled permeability, high sur- water, because of their higher flux capability, sharper
face area, low dielectric constant, and improved piezo- pore size distribution, better durability and higher
electric properties.13,14 damage tolerance than those of organic hollow fibres.18
Ceramic foam filters have been used for removing
This article intends to review these recent progresses of metallic inclusions from molten metals such as cast iron,
porous ceramics. Porous materials are classified into three steel, aluminium, as well as rectifying flow of the molten
grades depending on the pore diameter d: macro-porous metals.19 Since the metallic inclusions result in defects in
(d.50 nm), meso-porous (50 nm.d.2 nm) and micro- cast metals, this filtration process substantially improves
porous (d,2 nm), according to the nomenclature of the performance of the products. Porous ceramics with
IUPAC (International Union of Pure and Applied high specific surface area are employed for absorptive and
Chemistry). Figure 1 shows this classification along with catalytic applications, where large area is required for
typical applications and fabrication processes specific to contacts with reactants, particularly in high-temperature
the pore diameters. One of the most representative or corrosive atmospheres. Bioreactors are devices or
applications of porous materials is filtration or separation systems that provide a biologically active environment,
of matters in fluids. Filtration is roughly classified into where micro-organisms and enzymes are immobilised and
several grades depending on pore diameter d and mole- biochemical reactions are performed in porous beds, and
cular weight cut-off of the matters (MWCO); filtra- porous ceramics are often used as such bio-reactor beds
tion (typically d.10 mm), micro-filtration (10 mm.d.100 due to chemical stability of ceramics and accommodative
nm), ultra filtration (100 nm.d.1 nm, MWCO5103–106), function of porous structure.20 Recently, porous biocera-
nano-filtration (d<1–2 nm, MWCO5200–103), and reverse mics with open pore structures have attracted great
osmosis (d,1 nm, MWCO<100). When the pore size is attention for bio-implant applications including of bone
regeneration.21 Bone cells are impregnated through the
open pores and grow on their biocompatible walls
resulting in bone in-growth. Many electrodes used in
electro-chemical devices including gas purifiers, gas
sensors, fuel cells, and chemical analysers are porous
ceramics.22 Some porous electrodes require two mode
distributions of pore sizes; small pores are for the
electrochemical reactions while large pores are for flow
paths of reactants. Properties of electro-ceramics also
depend substantially on the porosity content and
morphology and therefore porous ceramics are also
applied or expected to be used in various electro-devices.
For example, porous piezoelectric ceramics have im-
proved piezoelectric property and are good candidates
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 damage resistance (against thermal spalling).24,25
On the other hand, some porous materials of conductive
ceramics like zirconia and silicon carbide have been
utilised in heat exchangers and heaters.13

As is known from Fig. 1, the representative processes
for making macro-porous ceramics are (i) partial
sintering, (ii) sacrificial fugitives, (iii) replica templates,

116 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

2 Representative fabrication processes of macro-porous the size of starting powders and degree of partial
ceramics sintering respectively. Generally, in order to provide
the desired pore size, the size of raw powder should be
and (iv) direct foaming. While the recent review articles geometrically in the range two to five times larger than
on porous ceramics9,11,12 have placed emphasis on che-
mical approaches related to the latter three, this article that of pore. Porosity decreases with increased forming
intends to overview the macro-porous ceramics fabri- pressure, sintering temperature and time. In addition,
cated through each of these four routes in below sections processing factors such as the type and amount of
(‘Partial sintering’, ‘Sacrificial fugitives’, ‘Replica tem- additives, green densities, and sintering conditions
plates’ and ‘Direct foaming’). A number of innovative (temperature, atmosphere, pressure, etc.) also greatly
techniques which have been developed recently for critical affect the micro-structures of porous ceramics.31 The
control of pores are introduced, divided into these four mechanical properties depend largely on degree of neck
categories, together with some important properties of growth between grains, as well as porosity and pore
porous ceramics obtained in these processes. It should be size. Green and coworkers32,33 found that before any
noted however, that a lot of new approaches for macro- densification occurs, the formation of necks between
porous ceramics such as phase separations26–30 have been
developed other than the processes shown here. Figure 2 touching particles by surface diffusion can increase
shows schematic illustrations of these processes, each of the elastic modulus to 10% of the fully dense value. The
which will be interpreted in its section. We then discuss porosities of porous materials obtained by partial sin-
gas permeability of these porous ceramics in with tering are usually below 50%. In industry, this method
different pore sizes and structures in the section on ‘Gas has been utilised for various applications including mol-
permeability’, and briefly review porous ceramics with
hierarchical porosity (incorporation of macro-, meso- ten metal filters, aeration filters (gas bubble generation
and micro-pores) in the section on ‘Hierarchically porous in wastewater treatment plants),13 and water purification
ceramics’, which have attracted much attention in both membranes.18
academic and industrial fields. Finally the article gives
summary and discusses the issues to be solved for further Several processing approaches have been developed to
realising the potential of porous ceramics and for
expanding their applicability. enhance grain bonding and improve strength of porous
ceramics. Oh et al.,34 Jayaseelan et al.35 and Yang et al.36
Partial sintering fabricated porous Al2O3 and Al2O3 based composites by
the pulse electric current sintering (PECS) technique and
Partial sintering of powder compact is the most found that the strength was substantially improved due
conventional and frequently employed approaches
to fabricate porous ceramic materials. Particles of to the formation of thick and strong necks. During
powder compact are bonded due to surface diffusion sintering the discharge is thought to take place between
or evaporation–condensation processes enhanced by the particles and to promote the bridging of particles by
heat treatments, and a homogeneous porous structure neck growth in the initial stages of sintering. This strong
forms when sintering is terminated before fully densified neck growth leads to substantially high strength com-
(see Fig. 2a). Pore size and porosity are controlled by
pared 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. y100 MPa at 30%
porosity35 (see Fig. 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. The PECS which rapidly heats diato-

mite powder successfully bonds the particles together
into relatively strong porous bodies, without signifi-
cantly destroying the internal pores of the diatomite
powder. The micro-structural features showed that
consolidation proceeds by the formation of necks at
temperatures around 700–750uC, which is followed by
significant melt phase formation around 850uC, resulting
in porous ceramics with a relatively high strength.

Deng et al.38,39 tried to obtain strong grain bonding
through 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 identi-

fied for ZrO2 porous ceramics fabricated by adding
Zr(OH)4.40

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Ohji and Fukushima Macro-porous ceramics: processing and properties

4 Micro-structure of porous CaZrO3/MgO composite fabri-
cated via in-situ reaction synthesis, exhibiting three-
dimensional network structure with strong grain necking43

(Reproduced with permission of John Wiley and Sons)

3 Flexural strength as a function of porosity for alumina/ with superior resistance against oxidation. In such a

3 vol.-% zirconia (AZ) fabricated via PECS and conven- process, the powder compacts are heated in air instead

tionally sintered alumina (top) and micro-structure of of an inert atmosphere. Because of the occurrence of
AZ (bottom).35 Strong neck growth of the AZ results in
surface oxidation at the heating stage, SiC particles are
substantially high strength compared to those of con-
bonded to each other by the oxidation-derived SiO2
ventional porous materials (Reproduced with permis- glass. The mechanical strength is strongly affected by

sion of John Wiley and Sons) particle size; the flexural strength attained as high as

Partial sintering through reaction bonding techniques 185 MPa at a porosity of 31%, when using fine a-SiC
have been frequently used for making porous ceramics,
where reaction products form or precipitate epitaxially powder (0?6 mm), while it was 88 MPa at 27% porosity
on grains, resulting in well-developed neck growth
between grains.41,42 In combination with the reactive for coarse powder (2?3 mm). The oxidation-bonding
sintering process, Suzuki et al.43,44 synthesised a
CaZrO3/MgO porous ceramics with three-dimensional technique has been applied to other materials including
grain net-work structure by using reactive sintering of silicon nitride,48 SiC/mullite composites,49 and SiC/
highly pure mixtures of natural dolomite [CaMg(CO3)2] cordierite composites.50
and synthesised zirconia powders. CaMg(CO3)2 decom-
poses into CaCO3, MgO and CO2 (g) at y500uC, and Partial sintering technique has been also applied for
CaCO3 then reacts with ZrO2 to form CaZrO3 and CO2
(g) at y700uC. Through liquid formation via LiF making porous silicon nitride with fibrous grains of high
doping, these reactions and liberated CO2 gas result in aspect ratios.51 Compared with oxide ceramics, the
formation of a homogeneous open-pore structure with
strong grain bonding as shown in Fig. 4. The pore size densification of silicon nitride ceramics is difficult
distribution is very narrow (with typical pore size:
y1 mm), and the porosity was controlled (about 30– because of strong covalent bonding between silicon
60%) by changing the sintering temperature. The
relatively high flexural strength (y40 MPa for 47% and nitrogen atoms. This difficulty of sintering silicon
porosity) was observed over the temperature range of
room temperature to 1300uC. The similar approach has nitride ceramics is beneficial for controlling density or
been applied other materials systems such as CaAl4O7/
CaZrO3 and CaZrO3/MgAl2O4 composite systems.45,46 porosity through adjusting the additives and the

She et al.47 used an oxidation-bonding process for the sintering process. In order to suppress densification,
low-temperature fabrication of porous SiC ceramics
oxides with high melting point and high viscosity such as

Yb2O3 are frequently used as sintering additives. The
addition of Yb2O3 also is known to accelerate the
fibrous grain growth of b-Si3N4.52 Figure 5 shows
micro-structure and mechanical properties of porous

silicon nitrides sintered at different temperatures by
using a-Si3N4 and 5 wt-% Yb2O3.52,53 The fracture
strength and fracture toughness were determined by

three-point flexure and chevron-notched beam tests

respectively. While the porous structure sintered at
1600uC consists of equiaxed a-grains, those are trans-
formed from equiaxed to fibrous when increasing the
temperature to 1700uC. Further increase in sintering
temperature gives rise to micro-structural change from

fine to coarse grains, while maintaining the porosity

around 40–45%. The fibrous micro-structure is advanta-

geous for the strengthening effects of grain bridging and

pullout. In actuality, both the fracture strength and
fracture toughness of the materials sintered at 1700uC
or higher are markedly improved, compared to those
at 1600uC. For examples, the strength for the sample
of equiaxed micro-structure sintered at 1600uC is
y40 MPa, while that at 1700uC is y380 MPa with

118 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

5 Micro-structures and mechanical properties (flexural strength and fracture toughness) of porous silicon nitrides sintered
with 5 wt-%Yb2O3 at 1600, 1700, 1800 and 1850uC. ‘P’ denotes porosity52 (Reproduced with permission of Elsevier)

the fibrous grain formation. As the sintering tempera- nitrogen pressure of 1 MPa. The texture of porous

ture further increases, the micro-structure becomes silicon nitride with porosity of 14% is shown in Fig. 6.

coarser and the strength decreases with increasing The fibrous grains of silicon nitride are well aligned

fracture toughness (though the toughness values are toward the casting direction, and the pores, whose
still below 4 MPa m1/2). Porous b-Si3N4 ceramics were
also fabricated by carbothermal reaction between silica shapes are mostly plate-like along the same direction,
and carbon.54 Micro-structure is controlled by varying
particle size of the carbon in this case. Tuyen et al.55 exist among the grains. The anisotropic (fibrous grain-

fabricated porous reaction-bonded silicon nitride by aligned) porous silicon nitrides showed excellent
nitridation process at 1350uC and post-sintering at
1550–1850uC, which provides similar fibrous micro- mechanical behaviour, when a stress is applied in the
structure and high porosity. The duration time for alignment direction.53,57 Figure 7 shows the fracture

sintering had a significant effect on the micro-structure strength and fracture toughness of the anisotropic
porous silicon nitrides as a function of porosity,53,57 in
and grain morphology. The fabrication process is
comparison with those of the isotropic porous silicon
advantageous due to the low cost of Si raw powder. nitrides that were sintered at 1800uC by using a-Si3N4
and 5 wt-%Yb2O3.53,58 The anisotropic materials exhib-
One of the unique processing routes for porous ited very high strength above 1?5 GPa, and very high
fracture toughness above 17 MPa m1/2 in the porosity
ceramics with anisotropic micro-structure is tape-casting
range below 5%. It should be noted that the toughness of
fibrous seed crystals or whiskers. For porous silicon
the porous materials with porosities below 10% is
nitrides, b-Si3N4 seed crystals were mixed with sintering
additives as starting powders, and the green sheets somewhat higher than that of the dense one (0%

formed by tape casting were stacked and bonded under porosity). These excellent mechanical properties were
pressure.56 Sintering was performed at 1850uC under a
due to enhanced crack shielding effects (bridging and

pull-out) of aligned fibrous grains. Debonding was

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Ohji and Fukushima Macro-porous ceramics: processing and properties

6 Micro-structures of anisotropic porous silicon nitride 8 SiOC ceramic foam using a sacrificial template consti-
prepared by tape-casting fibrous seed crystals (poros- tuted by PMMA micro-beads8 (Reproduced with permis-
ity: 14%)56 (Reproduced with permission of John Wiley
and Sons) sion of Elsevier)

promoted by the existence of pores, and aligned grains controlled by the amount of the agents, and pore shape
bridging the crack or interlocking each other were and size are also affected by the shape and size of the
elastically deformed or drawn apart without breaking, agents respectively when their sizes are large in compar-
leading to the above crack shielding. It has been also ison with those of starting powders or matrix grains. This
revealed that the anisotropic porous silicon nitrides are approach is useful particularly for obtaining high open
much superior even to the dense materials, for both the porosity. The agents, however, need to be mixed with
thermal shock fracture and damage resistances, which ceramic raw powder homogeneously for obtaining uni-
are known to be in antagonistic relation.53 form and regular distribution of pores. Solid fugitives such
as organic materials are usually removed through
Sacrificial fugitives pyrolysis, which requires long-term heat treatment and
generates a great deal of vaporised, sometimes harmful,
Porous ceramics can be obtained by mixing appropriate byproducts.
amounts of sacrificial fugitives as pore forming agents
with ceramic raw powder and evaporating or burning Polymethylmethacrylate (PMMA) beads and micro-
out them before or during sintering to create pores (see beads have been frequently employed for sacrificial
Fig. 2b). Frequently used pore forming agents include fugitives.8,59–64,77,79,82–85 For example, Colombo and his
polymer beads, organic fibres, potato starch, graphite, co-workers8,59–61 fabricated SiOC ceramic foam as
charcoal, salicylic acid, carbonyl, coal and liquid para- shown in Fig. 8, by dry mixing the silicon resin powder
ffin. The pore forming agents are generally classified into with a sacrificial template constituted by PMMA micro-
synthetic organic matters (polymer beads, organic fibres, beads, and subsequent heat treatments. Cruz et al.62 used
etc.),59–89 natural organic matters (potato starch, cellulose, a colloidal processing technique with PMMA sacrificial
cotton, etc.),67,68,90–105 metallic and inorganic matters templates, to fabricate macro-porous yttria-stabilised
(nickel, carbon, fly ash, glass particles, etc.),49,106–115 and zirconia ceramics. Descamps et al.63,64 produced macro-
liquid (water, gel, emulsions, etc.).116–156 Porosity is porous b-tricalcium phosphate (TCP) ceramics by using
PMMA. An organic skeleton, which was formed by inter-
7 Porosity dependence of fracture strength and fracture connecting the PMMA balls through a chemical super-
toughness, KIC of anisotropic and isotropic porous sili- ficial dissolution, was impregnated by the TCP slurry.
con nitrides. Stress is applied parallel to alignment The PMMA was eliminated by a thermal treatment at
direction for anisotropic material low temperature, and sintering was carried out to obtain
final porous structure. This process allows a total control
of the porous architecture; the porous volume can vary
from 70 to 80% and the interconnection size from 0?2 to
0?6 times the average macro-pore size.

Andersson and Bergstro¨ m82 used expandable micro-
spheres as a sacrificial template to produce macro-
porous ceramic materials by a gel-casting process. The
micro-spheres 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 micro-spheres, it is possible to tune
the porosity up to 86% and the pore size distribution
from 15 to 150 mm. As low amounts as 1–2 wt-% of the
micro-spheres are required to create a final porosity
above 80 vol.-%. Expandable micro-spheres as sacrificial
templates, rather than other templates such as PMMA
micro-beads, are advantageous because of lower levels
of gaseous byproducts generated during pyrolysis, and

120 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

lower cost of the overall materials. Kim and his flow directly through the pores. However, the prepara-
co-workers83,84 used hollow micro-spheres as sacrificial tion of such ceramics is complex because handling long
fibres such as thin wire or cotton thread is difficult.
templates to make porous silicon carbide ceramics Using short fibres or whiskers as the pore-forming agent

synthesised from carbon-filled polysiloxane and others. is an alternative that combines the advantages of
partially sintered porous ceramic and those of unidirec-
Using preceramic polymer and organic micro-spheres tional pores. Yang et al.65 demonstrated formation of
rod-shaped pores in silicon nitride ceramics, using slip
for fabricating porous ceramics allows use of the low- casting of aqueous slurries of silicon nitride powder and

cost and/or near-net shaped processing techniques like sintering additives with 0–60 vol.-% fugitive organic
whiskers. Rheological properties of slurries were opti-
extrusion and direct casting. They reported relatively mised to achieve a high degree of dispersion with a high
solid-volume fraction. Samples were heated at 800uC in
high flexural strength for porous SiC ceramics (e.g. 60 air to remove the whiskers and sintered at 1850uC in
nitrogen atmosphere to consolidate the matrix. Porosity
and 45 MPa at 40 and 50% porosity, respectively) and was adjusted in 0–45% by changing the whisker content
very low thermal conductivity (2 W m21 K21, at y70% in 0–60 vol.-%. The obtained porous silicon nitride
porosity).84 contained uniform rod-shaped pores with random
directions, exhibiting relatively high gas permeability
Song et al.85 produced micro-cellular silicon carbide
in comparison with porous silicon nitride containing
ceramics with a duplex pore structure by using expand- equiaxed pores.157 Isobe et al.81,110 and Okada et al.86,87
used carbon fibres (14 mm diameter and 600 mm length)
able micro-spheres and PMMA spheres; which resulted or Nylon 66 fibres (9?5–43 mm diameter and 800 mm
length) for pore-forming agent, and tried to align them
in the large pores and the small windows in the strut by extrusion technique to produce porous alumina81,110
and mullite86,87 ceramics with unidirectionally-oriented
area respectively. This porous ceramics showed excel- pores. They showed that the pore sizes and porosities
can be controlled by varying the fibre diameter and fibre
lent air permeability as shown in the section on ‘Gas content. The obtained samples showed better air

permeability’. permeability than the conventional porous materials
Diaz et al.93,94 fabricated porous silicon nitride used for filter applications.110 This technique can allow
the production of highly oriented porous ceramics by
ceramics by using a fugitive additive, corn starch industrially favoured extrusion method.

(particle size: 5–18 mm). In order to obtain homoge- Liquid phases such as water and oil, which are readily
sublimated or evaporated, are often used as pore
neous dispersion of the fugitives, the mixture slurry was forming agents.116–157 One of the most frequently
studied approaches in recent years is freeze-drying the
kept in agitation by using a magnetic stirrer for a while, water or liquid-based slurry to produce porous ceramics
of unique structure.119–155 Figure 9 shows a schematic
and then was frozen and dried under vacuum for sieving. illustration of the procedures which was employed by
Kim et al.95 mixed various amounts of corn starch to Fukasawa et al.119–121, and a porous silicon nitride body
obtained thereby. When the bottom part of the slurry is
(Ba, Sr) TiO3 powder to obtain (Ba, Sr) TiO3 porous frozen, ice grows macro-scopically in the vertical
ceramics. They found that depending on the porosity,
direction, and pores are generated subsequently by
the PTCR effect was 1–2 orders of magnitude improved sublimation of the ice. Through sintering this green
body, a porous ceramics with unidirectionally aligned
in comparison with the dense reference. channels can be obtained; these channels contain smaller
Chen et al.66 developed porous silicon nitride of pores in the internal walls (Al2O3)119,120 or fibrous grains
protruding from them (Si3N4).121 This method has
equiaxed a-grains by using phosphoric acid (H3PO4) as several advantages, including simple sintering process
the pore-forming agent and pressureless sintering of without materials to be burnt out, a wide range of
relatively low temperatures techniques (1000–1200uC). porosity (30 to 99%) controlled by the slurry concentra-
On the other hand, Li et al.80 fabricated porous silicon tion, applicability to various types of ceramics and

nitride with fibrous b-grain structure, using naphthalene environmental friendliness without emitting harmful
products. In particular, porous scaffolds with ice-
powder as the pore-forming agent and gas-pressure designed channel-like porosity have been intensively
sintering of high temperatures above 1700uC. The studied for a wide variety of applications including
bending strength of the former materials was 50– biomedical implants and catalysis supports.

120 MPa in porosity range of 42–63%, while that of The porosity of the porous materials obtained by using
this technique is a replica of the original ice structure. The
the latter was 160–220 MPa in porosity of 50–54%. This porous channels run from the bottom to the top of the
samples, and the pores most frequently exhibit an
substantial difference in strength is most likely due to anisotropic morphology in the solidification plane.
Deville et al.128–131 investigated freeze casting of ceramic
the micro-structural difference (equiaxed versus fibrous), slurries, and particularly the relationships between the

which was similar to what we observed in Fig. 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 in situ reaction
bonding technique. Graphite is burned out to produce

pores and the surface of SiC is oxidised at high

temperatures to SiO2, which, at further increased
temperatures, reacts with a-Al2O3 to form mullite
(3Al2O3.2SiO2). SiC particles are bonded by the mullite
and oxidation-derived SiO2.

Long fibres such as cotton thread,96 natural tropical
fibre97 and metal wires109 are often used as pore forming

agents for obtaining porous ceramics of through
channels. Zhang et al.96 produced porous alumina

ceramics with unidirectionally aligned continuous pores
(diameter: y160 mm) via the slurry coating of mer-
cerised cotton threads. The pore size can be adjusted by

using cotton threads of different diameters, 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

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Ohji and Fukushima Macro-porous ceramics: processing and properties

10 Multilayered porous alumina structures a with dendri-

tic-like features b (detail) produced via freeze-drying
process129 (Reproduced with permission of Elsevier)

9 Schematic illustration of freeze-drying process for nearly circular cross-sections, unlike ellipsoidal ones in

macro-porous ceramics and a porous silicon nitride conventional aqueous freeze casting. The channels are rep-
body obtained thereby121 (Reproduced with permission
licas of entangled dendrites of frozen camphene. Employ-
of John Wiley and Sons)
ing a similar camphene-based freeze-casting approach, Koh
freezing conditions and the final porous structures, for and his co-workers fabricated highly porous Al2O3,136–138
moderate to highly concentrated suspensions. It has been SiC,139,140 PZT-based ceramics,141,142 hydroxyapatite,143,144
clarified that the morphology of the porous structures, glass ceramics,145 and ZrO2,146,147 etc., having intercon-
i.e. the content, dimensions, shape and orientation of nected pore without noticeable defects.
porosity, was adjusted by varying the initial slurry
compositions and the freezing conditions. For highly Combining freeze-drying process and gelcasting techni-
concentrated solutions, the particle–particle interactions
presumably lead to the formation of ceramic bridges que has been also most frequently employed approaches to
between two adjacent lamellae. They used the freeze- make porous ceramics with refined micro-structure.148–156
drying technique to make sophisticated porous and
layered-hybrid materials. The nacre-like structure with These studies show the use of organic polymer in freeze
lamellar dendrites was obtained with a lamellar template
assembled by ice crystals, as shown in Fig. 10. Proper casting route affects the pore size and morphology by
control of the freezing conditions resulted in a porous
multilayered ceramics with compressive strengths up to controlling ice crystal growth during freezing. Chen
four times higher than those of materials currently used et al.148,149 used alumina slurries containing tert-butyl
for implantation. Munch et al.132 emulated nature’s
toughening mechanisms in aluminium oxide and poly- alcohol (TBA) and acrylamide (AM) for the freeze-drying
methyl methacrylate composites by using porous ceramic process. TBA, which freezes below 25uC and volatilises
structure fabricated by freeze-drying techniques, and rapidly above 30uC, acts as the freezing vehicle and
succeeded in obtaining toughness more than 300 times (in template for forming pores. Polymerised in the slurry as
energy terms) that of their constituents.
the gelation agent, AM strengthens the green bodies
In order to avoid the freezing process under the
extremely cold temperature, Araki and Halloran133–135 substantially. The sintered porous ceramics have good
used camphene, C10H16, as a vehicle for producing
porous ceramics via freeze-drying process. Slurries mechanical strength (compression strength of 150 MPa at
containing ceramic powder in the molten camphene, porosity of y60%) because the pore channels formed by
which were prepared at 55uC, were quickly solidified the TBA template are surrounded by almost fully dense
(frozen) when they were poured into polyurethane walls without any noticeable defects. Ding et al.150 also
moulds at room temperature. The obtained porous
ceramic bodies have interconnected pore channels of used 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 1400–1600uC due to the mullitisa-
tion between Al2O3 and SiO2. Porous yttria-stabilised
ziroconia151 and porous alumina152 were fabricated by

freeze-drying process with addition of polyvinyl alcohol,

which prevents ice crystal growth and reduces the pore

sizes substantially. Porous alumina with oriented pore

structures has been also prepared by freeze casting

technique 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 methods

where micrometre-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. For example, the average pore size can be
reduced to as small as 4?7 mm with the rate of 20 cm h21 at

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

122 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

11 Micro-structures of porous silicon carbide prepared the pyrolysis, the ceramic layers are sintered at higher
temperatures to densify. Porosity higher than 90% can
by freeze-drying technique using gelatin as gelation be obtained with cell sizes ranging from a few hundred
agent frozen at 210uC156 (Reproduced with permission micrometres to several millimetres. The open cells are
of Elsevier) interconnected, which allows fluid to pass through the
foams with a relatively low pressure drop. However, due
honeycomb walls was affected by the SiO2 concentration to cracking the struts during the pyrolysis, the mechan-
and the pore size. ical properties of ceramic reticulated foams are generally
poor.158 In order to avoid the strut crack formation, a
Using a gel-freezing method, Fukushima et al.155,156 variety of approaches have been developed. In order to
fabricated porous cordierite or silicon carbide ceramics increase the struts thickness and heal the strut cracks,
with porosity from 80 to 95%, where unidirectionally Zhu et al.159 recoated repeatedly a reticulate porous
oriented cylindrical channels are uniformly distributed ceramic body with thinner slurry of the same composi-
over relatively large bulk samples (typically several tion, after the green body coated with thicker slurry
centimetre). They used gelatin as the gelation agent, was preheated to burn out the sponge. In addition, Vogt
which was mixed with water for the freezing vehicle et al.160 attempted the vacuum infiltration of ceramic
and raw powder. The gel was frozen at 210 to 270uC and slurry to fill up the struts in the pre-sintered foam.
was dried under vacuum, followed by degreasing and The hollow struts caused by burnout of the polyur-
sintering. The cell size and cell wall thickness both ethane template could be completely filled up, which
decreased with decreasing the freezing temperature, rang- resulted in a considerable increase in compressive
ing from 20 to 200 mm and from 3 to 20 mm respectively. strength. Luyten et al.161 used a reaction bonded,
The numbers of cells, for example, for the cordierite modified replica technique to produce strong struts
sample frozen at 250uC and sintered at 1400uC was of ceramic foam. Jun et al.162,163 produced hydroxya-
1500 cells mm22 in the cross-section, which is a markedly patite scaffolds coated with bioactive glass–ceramics
large number in comparison with those of samples using the polymer foam replication method, to enhance
obtained by extrusion method (1–2 cells mm22). Micro- their mechanical properties and bioactivities. Plesch
structural observation revealed dense cell walls as shown et al.164 fabricated a reticulated macro-cellular alumina
in Fig. 11, which leads to relatively high compressive foam coated with TiO2 for photocatalytic applications.
strength, for example 17 MPa for 86% porosity sample of They showed that the photocatalytic activity can be
silicon carbide. affected by the pore size of the host foam due to the
accessibility of UV light. This study implies that the
Replica templates surface of macro-porous ceramic can play a role to
provide the additional functionalities for a component.
Macro-porous ceramics having interconnected large Highly porous ceramics can be derived also from
pores, or channels, of high volume porosity and open preceramic polymers after pyrolysis above 800uC in
cell walls have been frequently fabricated by the replica inert atmosphere.8 One of the typical methods is
techniques (Fig. 2c). The first step of a typical template dissolving the silicone resin preceramic polymer into a
process is impregnation of a porous or cellular struc- suitable solvent and adding appropriate surfactants and
ture with ceramic suspension, precursor solution, etc. catalysts, followed by the pyrolysis. The advantages
Various synthetic and natural cellular structures can be include a very wide range of pore (cell) sizes (typically
used as templates. The templates need to have adequate 1 mm to 2 mm), well-defined open-cell structures and
flexibility, shape recovery ability and homogeneous open macro-defect-free struts.165–168 Paper-based template
cell structure. has been also proposed; Travitzk et al.169 succeeded in
fabricating single-sheet, corrugated structures, and
The most frequently used synthetic template is porous multilayer ceramics by using various paper replica
polymeric sponge such as polyurethane. They are soaked templates. Unique micro-structures such as aligned
into a ceramic slurry or precursor solution to impregnate pores derived from fibres in the paper, elongated
the templates with them, and the surplus is drained and morphology and multilayer stacking were observed,
removed by centrifugation, roller compression, etc. In and they substantially affected the anisotropic mechan-
this process, the appropriate viscosity and fluidity ical properties.
depending on the cell size, etc. are required so that
uniform ceramic layer forms over the sponge walls. The Natural resources of porous structures such as woods,
ceramic-impregnated templates are dried and then heat- corals, sea sponge, etc. have been also used as replica
treated to decompose the organic sponges. Following templates. The woods are transformed to carbonaceous
preforms by heat-treatment under inert atmosphere. They
are then infiltrated with oxides and non-oxides that react
to form porous ceramics. The frequently used infiltrations
include molten metals,170–180 gaseous metals,175,181–186
alkoxide solutions,187–189 and others.190,191 The benefits
are 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 woods
and Si infiltrated samples are shown in Fig. 12. Par-
ticularly, the oriented vessels of the woods provide unique

International Materials Reviews 2012 VOL 57 NO 2 123

Ohji and Fukushima Macro-porous ceramics: processing and properties

12 Micro-structures of a different types of natural wood- a alkane emulsion in the powder suspension; b transi-
derived carbon performs172 and of b biomorphous porous tion of emulsion to wet foam; c formation of polyhedral
silicon carbides from pinus silvestris186 (Reproduced with structure (transition to stable foam)
13 Three stages of foaming process of emulsified cera-
permission of Elsevier) mic powder suspension199 (Reproduced with permis-

anisotropic porous structure of aligned unidirectional sion of John Wiley and Sons)
channels, which is suitable to the applications such as
filtration and catalysis supports. Porous biomimetic developed for the direct foaming of porous ceramics,
silicon carbide produced through this approach has been and representative approaches have been reviewed by
also studied for the use as a medical implant material.192 Studart et al.9 and Sepulveda.202

Biomorphic porous silicon nitride was produced from Barg et al.199,200 developed a novel direct foaming
natural sea sponge via replication method. The sponges process with emulsifying a homogeneously dispersed
were impregnated with silicon-containing slurry via dip- alkane or air–alkane phase in the stabilised aqueous
coating, and were heat-treated to delete the bio- powder suspension. In contrast to the conventional direct
polymers, leading to a Si-skeleton. Subsequent thermal foaming methods, foaming is made here by evaporation
treatment under flowing nitrogen promoted the nitrida- of the emulsified alkane droplet, leading to a time-
tion of the silicon, porous a/b-silicon nitride with a dependent expansion of the emerging foam in a mould. It
porosity of 88% and the original morphology of the sea was possible to realise interconnected structures with cell
sponge.193 sizes from 0?5 to 3 mm and porosities up to 97?5%. This
autonomous foaming process also allows high flexibility
Direct foaming in the production of ceramic parts with gradient
structures and complex shaping. Foaming proceeds as a
In direct foaming techniques, ceramics suspension which consequence of the evaporation of the alkane phase
is foamed by incorporating air or gas is stabilised and resulting in the growth of the stabilised alkane bubbles
dried, and subsequently is sintered to obtain consoli- and in a volume increase in the foam. The resulting
dated structure (Fig. 2d). This technique allows low- foamed green body possesses a tight cylindrical form with
cost and easy production of highly porous ceramic a cross-section corresponding to the mould. Figure 13
materials, up to more than 95% porosity. Porous shows typical three stages of the foaming process of an
ceramics with unidirectional channels have been also emulsified alumina powder suspension (using 5?5 vol.-%
developed recently by using continuous bubble forma- heptane and 0?83 vol.-% anionic surfactant):
tion in ceramic slurry194,195
(i) alkane emulsion in the powder suspension
However, due to the thermodynamic instability, the (ii) transition of emulsion to wet foam
gas bubbles are likely to coalesce in order to reduce the (iii) formation of a polyhedral structure (transition to
total Gibbs free energy of the system, resulting in large
pores in the final porous bodies. It is, therefore, critically stable foam). While alkane droplets in the top
required to stabilise the air or gas bubbles in ceramic region evaporate and grow, new droplets are
suspension. One of the most frequently approaches for simultaneously starting the foaming process in
the stabilisation is to use surfactants which reduce the the lower parts until the whole volume of the
interfacial energy of the gas–liquid boundaries. The pore emulsion is converted into stable foam.
size of the produced porous body ranges from below Similarly to replica template approach, preceramic
50 mm up to the mm scale,196–201 depending on how polymer solution has been used instead of ceramic
effectively and rapidly the used surfactants work. suspension for direct foaming. Colombo and Modesti203
Surfactants used for the stabilisation are classified into fabricated porous ceramics by dissolving preceramic
several types including non-ionic, anionic, cationic and polymers (silicone resins) into a suitable solvent and
protein. A variety of effective surfactants have been adding blowing agent, surfactant, catalyst, etc., followed
by pyrolysis at 1000–1200uC in inert atmosphere.

124 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

Expansion was achieved by high speed mixing (intro- 14 Darcian permeability as a function of pore size for porous
duction of bubbles in the solution) and heat treatment at ceramics fabricated by freeze-drying processes,152,156
25–40uC. Because of the limited amount of defects in the
struts, the obtained porous ceramics showed higher in comparison with those of other processes includ-
strength in comparison to conventional reticulated ing organic spherical fugitives,61,85 extruded organic
foams.204 fibrous fugitives,81,87 direct foaming,196,215 and replica
templates.215,216 Solid line indicates theoretical perme-
Kim et al.205,206 developed porous ceramics with a fine ability K5wDp2/32 (w50?85) for case of unidirectional
and uniformly distributed micro-cellular structure from
preceramic polymers using CO2 as a blowing agent. A cylindrical pores penetrating in parallel
mixture of polycarbosilane and polysiloxane was satu-
rated with gaseous CO2 under a high pressure and then a determined from pressure drop and flowrate of air by the
large number of bubbles were introduced using a Darcy’s law.217 Based on the capillary model, K is
thermodynamic instability via a rapid pressure drop.
The micro-cellular ceramics were obtained through expressed by
pyrolysis and optional, subsequent sintering.
K ~wDp 2 =C (1)
It has been shown that particles with tailored sur-
face chemistry can also be used efficiently to stabilise where w is the porosity, Dp is the pore diameter and C is a
gas bubbles for producing stable wet foams.207–210 constant depending on the pore structure.217,218 All the K
Gonzenbach et al.211–214 have developed a novel direct
foaming method that uses colloidal particles as foam values of Fig. 14 are adjusted at the porosity of 0?85 by
stabilisers in order to obtain macro-porous ceramics
with smaller cell sizes than those of the foams prepared using equation (1) for comparison. Note that the inertial
with long-chain surfactants. Owing to the adsorption of
partially hydrophobic particles to the air/water interface, contribution (non-Darcian permeability) was considered
the method allows for the fabrication of ultra-stable wet
foams which show neither bubble coalescence nor in addition to the viscous one (Darcian permeability) in
disproportionation over several days, as opposed to
several minutes typically required for the collapse of the Refs. 61, 152, 215 and 216, which results in high values of
surfactant-based foams. The attachment of colloidal
particles at the air/water interface is promoted by K, compared to the case of neglecting the inertial
adjusting the wettability of the particle upon adsorption effect81,85,87,156,196 (the ratio of viscous contribution in
of short-chain amphiphilic molecules on the surface. total is 60–90%152). If the fluid flows through the
Because of its remarkable stability, the particle-stabi-
lised foams can be dried directly in air without crack unidirectional cylindrical pores penetrating in parallel,
formation. The macro-porous ceramics obtained after
sintering exhibit porosities from 45 to 95% and cell sizes C is 32 (Ref. 218), which is shown as the solid line
between 10 and 300 mm. The compressive strengths of
the sintered foams with closed cells (for example 16 MPa (w50?85) in the figure. The porous ceramics fabricated by
at porosity of 88% in alumina foams) are substantially the freeze-drying processes,152,156 which have cylindrical
higher than those of foams prepared with other
conventional techniques. The surface-modified particles through channels, showed the permeability very close to
which originally cover the air bubble in wet foams
become a thin surface layer of single grains after this solid line, indicating the unidirectional alignment of
sintering. Macro-porous ceramics with open porosity
can be also prepared with this technique by simply the cylindrical pores. The permeability required for a
decreasing the concentration of stabilising particles. commercially available DPF is 10211 to 10212 m2,219 and

Gas permeability most of the freeze-dry-processed materials exceed this

Gas permeability is one of the most important properties criterion. The porous ceramics prepared with extruded
of porous ceramics which are expected to be used for gas- organic fibrous fugitives81,87 showed lower permeability
filters such as DPF, since large pressure drops cannot be
tolerated in such applications. Highly porous ceramics values than those of the freeze-dry-processed ones, most
with aligned unidirectionally through pore channels,
which were prepared by freeze-drying process, are likely because of limited contact area among the short
expected to provide excellent permeability. In this section,
we discuss Darcian permeability of porous ceramics fibres. As for the porous ceramics with a duplex pore
with different pore sizes and structures. Figure 14 shows structure,85 the permeability increases with increasing the
the Darcian permeability as a function of pore size
for porous ceramics fabricated by the freeze-drying amount of PMMA micro-beads and decreasing the
processes,152,156 organic spherical fugitives,61,85 extruded
organic fibrous fugitives,81,87 direct foaming,196,215 and averaged pore size, since the number of the small
replica templates.215,216 The pore structures are classified
into three categories of ‘Spherical (Connected),61,85,196,215 windows in the strut area increases.
‘Cylindrical (Connected)’81,87 and ‘Cylindrical’,152,156 as
schematically shown. The Darcian permeability K is

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Ohji and Fukushima Macro-porous ceramics: processing and properties

Hierarchically porous ceramics 15 Micro-structure of hierarchically porous ceramics com-

Porous ceramics can be divided into three categories of prising silica-alumina composite membrane, c-alumina
macro-, meso- and micro-porous, according to its pore
size, as described in the section on ‘Introduction’. Based multi-intermediate meso-porous layers and alumina
on very recent review article of Colombo et al.,12 macro-porous support221 (Reproduced with permission
hierarchical porosity can be defined as incorporation
of two or three of these different porous structures, of Elsevier)
which can be provided by various methods of templat-
ing, impregnation, emulsion, phase separation, coating made to realise hierarchically porous ceramics. The most
and etching. Porous ceramic components with such frequent approach for the one-pot synthesis is to use
hierarchical porosity have attracted a great deal of preceramic polymers. Preceramic polymers, including
attention, due to the synergy effects of different organic and inorganic polymer with continuous silicone
advantages that each porous structure can provide. network and sol solution prepared by hydrolysis and
condensation of metallic alkoxide, can provide ceramic
A typical example is micro-porous hydrogen permse- materials as residue through its pyrolysis. During heat
lective silica membrane coated on the surface of meso- treatment of preceramic polymer with silicone network,
porous c-alumina or macro-porous a-alumina supports, organic groups can be decomposed and be transferred to
which is expected to be widely used in future for hydrogen inorganic bonds such as Si–O, Si–C and Si–N, and
usage. In this case, thin micro-porous layer has a role of usually under inert atmosphere to prevent an oxidation
permselective of hydrogen gas and macro-porous mem- of organic groups. When preceramic polymer is used as
brane support provides a mechanical strength due to raw material to fabricate hierarchically porous ceramic,
neck among alumina particles and mass transfer due to meso- or micro-porosity has been formed by varying the
interconnected macro-pores.220–224 Generally, the macro- pyrolysis temperature of preceramic polymer,235,236
porous supports are fabricated by extrusion route or slip using the block copolymer,237–242 and the addition of
cast and following partial sintering, and then meso- filler into the polymer.236,243 Several macro-porous
porous or micro-porous membranes are coated onto the processing approaches including sacrificial fugitives,
supports by dip coating of sol solution through hydrolysis replica templates and direct foaming have been utilised
and condensation of silicon alkoxide, or chemical vapour in order to provide the macro-posority. Holland et al.237
deposition. Meso-porous intermediate layers frequently reported that porous titania, zirconia, and alumina
put between membranes and supports. Yoshino et al.220 from the infiltration of the corresponding alkoxides
fabricated micro-porous silica membranes and membrane into the latex spheres as the templates for macro-
modules and examined the gas permeation characteristics pores and revealed the highly ordered micro-structures.
and stability of substrate. Tubular type a-alumina Kim et al.238 fabricated hierarchically porous alumina
substrate with 0?7 mm was prepared by an extrusion with meso- and macro-pores, where meso-porous struc-
method, and c alumina intermediate layer with 60 nm ture was obtained using alkyl carboxylate as a chemical
pore size was formed by dip-coating of boehmite sol over template, and macro-pore structure was developed
the substrate, followed by another dip coating process of through polystyrene beads or silica gels. Suzuki
silica sol to make silica membranes on the top. The et al.239 prepared hierarchically porous silica and
alumina tube showed good heat-cycle stability for alumina by dual templating method of surfactant and
strength and permeation (,5% changes after 100 cycles polystyrene beads. They confirmed macro-pores around
of room temperature–773 K). Excellent hydrogen perme- 200 nm from the polystyrene sphere, and ordered meso-
ability of 561028 to 561026 mol m22 s21 Pa21 with pores around 4 nm due to the triblock copolymer and
H2/N2 selectivity of 30–300 was obtained at 873 K micro-pores of ,2 nm due to the tail of copolymer.
with the silica membranes. Gu et al.221 reported that a Dacquin et al.241 prepared macro-porous–meso-porous
dual-element silica-alumina composite membrane was alumina by using 400 nm polystyrene beads for macro-
deposited by chemical vapour deposition of tetraethy- pores and triblock copolymer for meso-pores, as shown
lorthosilicate and aluminium-tri-sec-butoxid, on a-alu- in Fig. 16. The regular macro-porous skeleton obtained
mina macro-porous support with c-alumina meso-porous from polystyrene template was observed (A, B) while
intermediate layers, as shown in Fig. 15. The meso- high resolution TEM study revealed the ordered,
porous intermediate layers were formed by dip coating of hexagonally packed meso-pores resulting from self-
boehmite sols with different particle sizes, resulting in assembly of the block copolymer solution (C).
multiple graded structures without cracks. The supported
composite silica–alumina membrane showed high hydro-
gen permeability in the order of 1027 mol/m22 s21 Pa21
and good stability against water vapour in comparison to
pure silica at 873 K. Besides gas separation membranes,
similar processing approaches have been applied for a
variety of filtrations, adsorptions and catalysts.225–229 In
addition biomorphous method,230,231 etching method232
and PECS method233,234 can also provide hierarchically
porous ceramics.

The above fabrication processes comprise several
steps including forming, sintering, coating, etc. For the
more simplified and efficient processes, several attempts
for one-pot synthesis (single step pyrolysis) have been

126 International Materials Reviews 2012 VOL 57 NO 2

Ohji and Fukushima Macro-porous ceramics: processing and properties

16 Micro-structures of macro-porous–meso-porous alumina 17 Micro-structure of SiOC foam (top), and SiC nano-wires
prepared by using polystyrene for macro-pores and tri- formed on its wall surface (bottom)244 (Reproduced with
block copolymer for meso-pores. (A) SEM and (B) TEM
observations for macro-porous skeleton, and (C) high- permission of John Wiley and Sons)
resolution TEM for meso-pores241 (Reproduced with per-
mission of American Chemical Society) technologies of porous ceramics, resulting in better
control of the porous structures and substantial
The growth of nano-structure on macro-porous substrate improvements of the properties. This article reviewed
can be also categorised as hierarchically porous ceramics. these recent progresses of porous ceramics. Because of a
The freeze-drying process is one of the routes which give us vast amount of research works reported in this field
such porous structures.119–121 As already stated, the macro- these days, the review mainly focused on macro-porous
scopic through channels obtained by this process contain ceramics whose pore size is larger than 50 nm. Followed
small pores in the internal walls and protruding whisker-like by giving a general classification of porous ceramics, a
grains, which results in pore size distribution with two or number of innovative processing routes developed for
three peaks corresponding to the respective pores. Another critical control of pores were described, along with some
approach is the chemical process via preceramic polymer. important properties. They were divided into four
As shown in Fig. 17, Vakifahmetoglu et al.244,245 developed categories including (i) partial sintering, (ii) sacrificial
a cellular SiOC ceramics with nano-wires using preceramic fugitives, (iii) replica templates, and (iv) direct foaming.
polymer, foaming agent and metallic catalyst and showed
high specific surface area of 110 m2 g21. They revealed the The partial sintering, the most conventional technique
much formation of long SiC or Si3N4 nano-wires on the wall for making porous ceramics, has been substantially
of the cellular SiOC, and the effect of pyrolysis conditions sophisticated in recent years. Very homogeneous porous
(atmosphere and temperature) on the formation of nano- ceramics with extremely narrow size distribution has
wires. SiC nano-wires on the macro-pores have been also been successfully prepared through sintering combined
formed by conventional pressing,246 replica templates247 with in situ chemical synthesis. Porous silicon nitrides
and camphene dendrites,140 where preceramic polymer and with anisotropic micro-structure (aligned fibrous grains
SiC powder as raw materials have been heated on the one- and pores) produced via tape-casting and partial
pot synthesis. It was suggested that the formation of nano- sintering have exhibited excellent mechanical properties,
wires were due to iron impurities in the raw SiC powder. which are equivalent, or sometimes superior, to those of
Some other approaches have been also proposed; the dense materials.
Vanhaecke et al.248 and Edouard et al.249 fabricated SiC
nano-wires on the surface of SiC foam by the reaction Advantage of the sacrificial fugitives is that pore
between carbon nano-fibres and SiO gas, followed by the shape and size are controlled by the shape and size of the
oxidation removal of residual carbon. Jayaseelan et al. agents respectively. Various kinds of fugitive agents
prepared cordierite whiskers250 and SiC nano-fibres251 on have been used for obtaining desired porous structure.
the cordierite honeycombs via air sintering and carbother- The fugitives are most frequently removed through
mal reduction respectively, using mixture of cheap raw pyrolysis, generating a great deal of vaporised, some-
powder such as kaolin, talc, alumina, carbon and silica. times harmful, byproducts, and a lot of works have been
made to reduce or eliminate them. The freeze-drying
Summary and future prospective

During the last decade, tremendous efforts have been
devoted for the researches on innovative processing

International Materials Reviews 2012 VOL 57 NO 2 127

Ohji and Fukushima Macro-porous ceramics: processing and properties

processes using water or liquid as fugitive materials are filter smaller pollutants in the exhaust gases with less
advantageous in this viewpoint and have been very pressure drop. We presume that one of the effective
intensively studied in recent years. Controlling growth of solutions will be a hierarchically porous structure
ice during freezing leads to unique porous structures and incorporating well controlled macro-scaled cylindrical
excellent performances of porous ceramics. For exam- pores and micro- or nano-scaled whiskers grown from
ple, the highly porous lamellar hydroxyl-apatite scaf- the internal walls, which enable the fluid permeation and
folds via this approach are several times stronger than pollutant capture respectively.250,251
materials currently used for implantation. Also the
freeze-dry-processed porous ceramics that have cylind- Finally, it should be remarked that porous ceramic
rical through channels demonstrated the excellent components are most frequently subjected to mechanical
permeability. loads and thermal shocks in their numerous applications
and that better structural reliability will be critically
The replica template techniques have been widely needed to further expand their applicability in future
used to fabricate porous ceramics with interconnected industries. Greater efforts will be made for ensuring the
large pores of high volume porosity. Porous polymeric mechanical reliability, such as enhancing neck growth
sponge such as polyurethane is the most typical among matrix grains and avoiding crack formation
synthetic template used for this process. However, due during fabrication. On the other hand, while pores are
to cracking struts during pyrolysis of the sponge, the generally believed to deteriorate the mechanical proper-
mechanical reliability is substantially degraded; a variety ties, this is not always true as seen in this review.
of approaches have been used to avoid the strut crack Carefully tailored porous micro-structures have a great
formation. Natural template approaches using wood, possibility to give rise to substantially improved or
for example, as positive replica, have been frequently unique properties that are not attained even in dense
used in these years and have realised highly oriented materials.
porous open-porous structure with a wide range of
porosity. References

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bodies, various methods which stabilise the bubbles have 5. F. Schuth: Annu. Rev. Mater. Res., 2005, 35, 209–238.
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