Synthesis of porous silicon nitride via the carbothermal reduction and nitridation of silica/PRF gel composite

Abstract

This research studied the synthesis of porous silicon nitride, one of the most promising structural materials for applications in filter at high-temperature, from silica/phenol-resorcinol-formaldehyde (PRF) gel composite which was prepared by combining sol-gel and polymerization of phenol, resorcinol and formaldehyde using sodium carbonate as catalyst and 3-aminopropyl trimethoxysilane (APTMS) as silica precursor. After aging of the silica/PRF gel for 72 hours, t-butanol was used in the solvent exchange process for 24 hours and repeated for 3 times. Then the sample was dried in freeze drying processes at -40°C for 3 days. Next, the silica/PRF composite was pyrolyzed under nitrogen gas and pre-calcined first under air at temperature in the range of 400-500°C for 2 hours to convert the PRF structure into porous carbon and partially remove carbon from the composite, respectively. Finally, the composite was subjected to the carbothermal reduction and nitridation at 1450°C for 10 hours followed by the removal of residual carbon by calcination at 800°C for 8 hours. From the results, differences in molar ratio of phenol-to-resorcinol, silica-to-carbon and pre-calcination temperature result in difference in porosity of both silica/carbon composite and that of silicon nitride product. The pre-calcination is the major effect to improve the porosity, which has an effect on porosity of silicon nitride product. Moreover, it was also found that the use of pre-formed silica sol, which is another way to decrease violent interaction between silica precursor and the PRF solution, has an effect on porosity and crystallite size of silicon nitride product.