We report the carbothermal synthesis of sturdy, highly porous (&gt;85%) SiC and Si3N4 monolithic aerogels from compressed compacts of polyurea-crosslinked silica xerogel powders. The high porosity in those articles did not pre-exist pyrolysis, but 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 mm in size, 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 nano-thin layer of carbonizable polyurea derived from the reaction of an aromatic triisocyanate (TIPM: triisocyanatophenyl methane) with the innate –OH, deliberately added –NH2 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 Si3N4 artifacts by pyrolysis at 1500 oC under Ar or N2, 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. The final ceramic objects were chemically pure, sturdy, with compressive moduli at 37±7 and 59±7 MPa for SiC and SiN4, respectively, and thermal conductivities at 0.163±0.010 and 0.070±0.001 W m-1 K-1, respectively. The synthetic methodology has been extended to the synthesis of other ceramic and metallic aerogels including Fe, Co and Cu.

Sturdy, monolithic SiC and Si3N4 aerogels from compressed polymer-crosslinked silica xerogel powders

256th National Meeting (2018)

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