Porosity and microstructure control of porous ceramics by partial hot pressing
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Guo-Jun Zhang and Tatsuki Ohji Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8687, Japan (Received 13 February 2001; accepted 6 April 2001)
Precise control of porosity and microstructure of porous ceramics is essential. This paper describes a method for the preparation of porous ceramics with porosity from 0.0 to 0.4. The ceramics were prepared via hot pressing a powder mixture to a definite dimension, and the porosity was easily adjusted by the powder amount. As examples, porous Si3N4 or SiC ceramics were produced by powder mixtures that contained Si3N4 or SiC and a sintering additive of oxide. It was demonstrated that the present method is advantageous for producing ceramics with controllable porosity and microstructure.
I. INTRODUCTION
Porous ceramic is a very useful candidate material for a variety of components, including filters, fuel-cell electrodes, battery separators, catalyst supports, and sensors. Recently, its potential use in structural parts has been recognized, taking advantage of its low Young’s modulus, light weight and good strain/damage tolerances.1–3 When fabricating porous ceramics, it is essential to precisely control the porosity. A common method of fabricating porous ceramic materials is adding fugitive agents, which are burned out to form pores.4 However, this method requires an extra heat treatment to remove the agents, and it is generally difficult to obtain the micropores. Another method to adjust the porosity is sintering powder compacts to a certain degree of densification, the so-called partially sintering process.5,6 With this method, pores with a refined size remain among the grains after the sintering. However, it is difficult to precisely control the porosity because the sintering is affected by many factors such as characteristics of starting powders, forming state of powder compacts, types and amounts of sintering additives (if any), sintering conditions, etc. Sintering temperature and time are usually used to control the porosity in the partially sintering.7 In another case, it has been reported that some ceramics with very low sinterability show little dependency of the density on the sintering temperature;8 in this case, the sintering additive seems to play the most important role in controlling the density. Adjusting the above factors to control the porosity causes a simultaneous change in grain size. As the grain size is an important factor affecting mechanical properties, a net relationship between mechanical properties and porosity 1916
J. Mater. Res., Vol. 16, No. 7, Jul 2001
is difficult to deduce from partially sintered samples. It is significant to find a sintering method by which the porosity can be controlled without changing the sintering conditions and microstructure. In this study, a novel method was developed to obtain ceramics of varied porosity with relatively high accuracy; the ceramics were prepared via hot pressing of a powder mixture to a definite dimension beyond which the press
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