Sol–gel derived porous ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) are considered as a family of nonmetallic and inorganic materials that have melting point over 3000 ℃. Chemically, nearly all UHTCs are borides, carbides, and nitrides of early transition metals (e.g., Zr, Hf, Nb, Ta). Within the last two decades, except for the great achievements in the densification, microstructure tailoring, and mechanical property improvements of UHTCs, many methods have been established for the preparation of porous UHTCs, aiming to develop high-temperature resistant, sintering resistant, and lightweight materials that will withstand temperatures as high as 2000 ℃ for long periods of time. Amongst the synthesis methods for porous UHTCs, sol–gel methods enable the preparation of porous UHTCs with pore sizes from 1 to 500 μm and porosity within the range of 60%–95% at relatively low temperature. In this article, we review the currently available sol–gel methods for the preparation of porous UHTCs. Templating, foaming, and solvent evaporation methods are described and compared in terms of processing–microstructure relations. The properties and high temperature resistance of sol–gel derived porous UHTCs are discussed. Finally, directions to future investigations on the processing and applications of porous UHTCs are proposed.