Macro-porous ceramics: processing and properties

Abstract

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 Nano-SiC as Thermal Insulator: Wisdom on Balancing Thermal Stability, High Strength and Low Thermal Conductivity

“…[5][6][7][8] Partial sintering is used to produce homogenous porous ceramics with narrow pore size distribution; sacrificial fugitives targets at porous ceramics with well-tuned shape and size of pores, which is controlled by the characteristics of the sacrificial filler; replica templates is developed to obtain highly porous ceramics with interconnected large pores/channels and open cell walls; direct foaming usually utilizes chemical or physical blowing agent to generate pores with bubble shape in ceramic slurries. [9,10] As a result of the strong Si-C covalent bonding and low self-diffusion coefficient, SiC skeleton is difficult to be densified below 2,150°C. [11] In order to lower the sintering temperature, oxides such as Al 2 O 3 , MgO, SiO 2 and Y 2 O 3 are usually used as sintering additives to promote densification.…”

Fabrication Of Porous Ti By Thermal Decomposition And Sintering Of PMMA/TiH2 Powder Compact

“…Porous materials with controlled pore characteristics have been produced by several ways such as partial sintering of powder compacts, sacrificial fugitives as pore forming agents, replica templates and mold casting in gas atmosphere [4]. Among them, the process of sacrificial fugitives is useful particularly for obtaining desired pore morphologies and high porosity, because the porosity, pore shape and size can be easily controlled by the amount, shape and size of the pore forming agents [5,6].…”

Porous Ceramics Obtained with the Use of Aluminum Hydroxide Powder

“…Materials of aluminum oxide and composites based on it [6][7][8][9] have the greatest ability to preserve structure and properties when exposed to aggressive environments without degradation of properties, corrosion and chemical resistance, high strength. Al 2 O 3 and ZrO 2 -Al 2 O 3 ceramics with predefined porosity, size and shape of the pores, properly complies with the requirements to catalyst carriers, filters and bioimplants [9][10][11][12] . To date, there are many technological approaches that ensure necessary porosity in ceramics, among which the most common is the introduction of organic pore-forming additives.…”