Sturdy, Monolithic SiC and Si₃N₄ Aerogels from Compressed Polymer-Cross-Linked Silica Xerogel Powders

We report the carbothermal synthesis of sturdy, highly porous ( > 85%) SiC and Si ₃ N ₄ monolithic aerogels from compressed polyurea-cross-linked silica xerogel powders. The high porosity in those articles was created via reaction of core silica nanoparticles with their carbonized polymer coating toward the new ceramic framework and CO that escaped. Sol-gel silica powder was obtained by disrupting gelation of a silica sol with vigorous agitation. The grains of the powder were about 50 µm in size and irregular in shape and consisted of 3D assemblies of silica nanoparticles as in any typical silica gel. The individual elementary silica nanoparticles within the grains of the powder were coated conformally with a nanothin layer of carbonizable polyurea derived from the reaction of an aromatic triisocyanate (TIPM: triisocyanatophenyl methane) with the innate -OH, deliberately added -NH ₂ groups, and adsorbed water on the surface of silica nanoparticles. The wet-gel powder was dried at ambient temperature under vacuum. The resulting free-flowing silica/polyurea xerogel powder was vibration-settled in suitable dies and was compressed to convenient shapes (discs, cylinders, donut-like objects), which in turn were converted to same-shape SiC or Si ₃ N ₄ artifacts by pyrolysis at 1500 °C under Ar or N ₂ , respectively. The overall synthesis was time-, energy-, and materials-efficient: (a) solvent exchanges within grains of powder took seconds, (b) drying did not require high-pressure vessels and supercritical fluids, and (c) due to the xerogel compactness, the utilization of the carbonizable polymer was at almost the stoichiometric ratio. Chemical and materials characterization of all intermediates and final products included solid-state ¹³ C and ²⁹ Si NMR, XRD, SEM, N ₂ -sorption, and Hg intrusion porosimetry. Analysis for residual carbon was carried out with TGA. The final ceramic objects were chemically pure, sturdy, with compressive moduli at 37 ± 7 and 59 ± 7 MPa for SiC and Si ₃ N ₄ , respectively, and thermal conductivities (using the laser flash method) at 0.163 ± 0.010 and 0.070 ± 0.001 W m ^-1 K ^-1 , respectively. The synthetic methodology of this report can be extended to other sol-gel derived oxide networks and is not limited to ceramic aerogels. A work in progress includes metallic Fe(0) aerogels.