Macro-porous ceramics: processing and properties

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.