Porous ceramics have been widely used in many areas, such as filters, insulators, biomedical implants, catalytic supports, gas sensors, lightweight structure material, and absorbents. Recently, porous silicon carbide ceramic have been increasingly studied because they were proved to exhibit a unique combination of good oxidation resistance and thermal-shock resistance as well as excellent mechanical and chemical stability. However, due to the covalent nature of Si-C bonds, SiC ceramics normally needed to be sintered at high temperatures or/and with the addition of sintering agents, which have limited the application of porous SiC ceramics. New processing routes that overcome these problems are the preceramic polymer processes, during which the polymer precursors convert into ceramic materials.
Preceramic polymer polysiloxane were adopted as the starting materials for the fabrication of SiC ceramics. During the heat treatment process, polysiloxane experienced an organic-inorganic transformation at a low temperature of 1300℃.
Microcellular ceramics are defined as celllular ceramics with cell sizes ≤ 30㎛ and cell densities ≥ 109 cells/㎤. These advantages contribute to the high impact strength, high toughness, high fatigue life, high stiffness-to-weight ratio, high thermal stability, low thermal conductivity, high resistance to chemical corrosion and high surface area.
In this work, processing techniques for producing microcellular silicon carbide with cell densities greater than 109 cells/㎤ and cells smaller than 30㎛ have been developed by a reaction method that incorporates a polysiloxane and reactive fillers (carbothermal reduction). The strategy adopted for making microcellualr SiC ceramics involved the following steps: (1) fabricating preceramic foams by heating a mixture of polysiloxane, carbon black (used as a carbon source), Al2O3-Y2O3 (used as a sintering additive), expandable microspheres (used as sacrificial templates), and SiC (an optional inert filler) (4) synthesizing SiC by carbothermal reduction.
Highly porous, open-close cell, microcellular SiC ceramics have been fabricated by the expansion method using expandable microspheres. Carbon black can be used as carbon source for producing open-close cell, microcellular SiC by carbothermal reduction. The addition of inert filler was beneficial for increasing the porosity and for improving dimensional control. A higher sintering temperature resulted in an increased dense strut, but decreased the porosity at Comp. 2 and increased the porosity at Comp. 1. (3) transforming the polysiloxane by pyrolysis into silicon oxycarbide (2) cross-linking the polysiloxane in the foamed body