Sol-Gel Processing for Porous Ultra-High Temperature Ceramics

Reference	Presenter	Authors (Institution)	Abstract
14-093	Guo Jun Zhang	Zhang, G.J.(Donghua University); Li, F.(Donghua University); Huang, X.(Shanghai Institute of Ceramics);	Ultra-high temperature ceramics (UHTCs) are a class of materials that have melting point over 3000?C typically including borides, carbides and nitrides of early transition metals for potential applications at ultra-high temperatures. With the combined of their intrinsic properties and the advantages of porous ceramics, such as lightweight, low thermal conductivity, high specific strength and excellent thermal insulation, UHTC based porous ceramics may be applied as the promising candidate materials in thermal protection systems. Sol-gel routes enable the processing of materials in various desirable shapes, such as monoliths, films, fibers etc. Here, in this report, we present our recent work on the synthesis of UHTC based porous ceramics via sol-gel routes, i.e., sol-gel processing accompanied by direct foaming and phase separation. Ultra light, highly porous, closed- or open-cell structured ZrC/C foam can be produced by direct foaming of zirconia sols and phenolic resin followed by pyrolysis at 1400~1600°C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with cell sizes ranging from 40 to 300 ?m. The foams also display excellent thermal stability up to 2400°C. Recently, a sol-gel process accompanied by phase separation is established to produce highly macroporous UHTCs. This approach is based on the polymerization of furfuryl alcohol and gelation of inorganic sols. This sol-gel route has been demonstrated to be rather general and is possible to synthesize porous CrB2, ZrB2, TiB2, Cr3C2/CrB, ZrB2/ZrC, and ZrC/SiC monoliths with porosities ranging from 65 to 85 % and pore sizes ranging from 0.5 to 40 ?m. In each case, a porous hybrid monolith is obtained by directly drying under ambient pressure. After calcination, the corresponding boride or boride/carbide monoliths are acquired.
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