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 micropores), 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.

Porous ceramics with complex pore structure were synthesized by a freeze-dry process. Freezing-in of a water-based ceramics slurry was done while controlling the growth direction of the ice. Sublimation marks of the ice were generated by drying under reduced pressure. Porous ceramics having a complex pore structure were obtained by sintering the green body: aligned macroscopic open pores contained micropores in their internal walls. The pore structure was substantially affected by the starting slurry concentration and sintering temperature. The pore formation mechanism is discussed in relation to these effects.

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze‐drying process. Freezing of a water‐based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze‐drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si3N4 had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si3N4 matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.

Vertically aligned ZnO nanotubes were prepared by etching ZnO rod arrays in aqueous solution, which were previously developed by chemical bath deposition method. The morphological, structural, photoluminescence, as well as photocatalytic properties of the ZnO nanotubes were examined with respect to the pH values of chemical bath solution. The morphology of the products was found to be sensitive to the pH values and chemical bath temperatures. The nanotubes synthesized at a low pH value (5.82) exhibited a strong UV emission and a weak defect-related visible emission. The highest photocatalytic efficiency was also observed at pH = 5.82. The possible mechanism for the difference of photocatalytic efficiency was discussed.

The mechanical properties of Al 2 O 3 -based porous ceramics fabricated from pure Al 2 O 3 powder and the mixtures with Al(OH) 3 were investigated. The fracture strength of the porous Al 2 O 3 specimens sintered from the mixture was substantially higher than that of the pure Al 2 O 3 sintered specimens because of strong grain bonding that resulted from the fine Al 2 O 3 grains produced by the decomposition of Al(OH) 3 . However, the elastic modulus of the porous Al 2 O 3 specimens did not increase with the incorporation of Al(OH) 3 , so that the strain to failure of the porous Al 2 O 3 ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH) 3 particles. Fracture toughness also increased with the addition of Al(OH) 3 in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.

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