Fabrication of porous silicon nitride specimen via carbothermal reduction and nitridation of cabonized rf/silica composite

Abstract

Introduction of silica into RF solution, which was prepared via the sol-gel polycondensation of resorcinol and formaldehyde using sodium carbonate as catalyst, was studied. 3-Aminopropyl trimethoxysilane (APTMS) was used as silica precursor. The reaction between RF solution and silica precursor is violent and extremely exothermic which results in rapid solidification of the mixture. To slow down the reaction, and to increase the amount of the silica precursor in the reaction, the reduction of temperature of RF gel and the addition of acetic acid, as inhibitor, in RF gel were used. After aging of the silica/RF 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 splitted into two parts for each drying process, i.e. convectional air drying at 110°C and freeze drying processes at -40°C. Next, the silica/RF composite was pyrolyzed under nitrogen gas to convert the RF structure into porous silica/carbon composite. Finally, the obtained product, i.e., porous silicon nitride, was obtained via carbothermal reduction and nitridation at 1450°C for 10 hours followed by the removal of excess carbon by calcination at 800°C for 8 hours. From the results, acetic acid could retard the reaction between RF gel and silica precursor because it breaks cross-linking network within RF gel, which prolongs the gelation time of the composite. However, the higher the acetic acid content is, the lower the composite porosity becomes. Without the addition of acetic acid, the composite became fragile and could not be formed into solid specimen. Aging of RF gel at low temperature also allowed more silica precursor to be added to the composite, because of the decrease in the polymerization reaction between RF and silica. Similarly, for the use of pre-formed silica sol, the silica became sol before being added into RF sol so that, the reaction was less violent.