Carbothermal Reaction of Silica–Phenol Resin Hybrid Gels to Produce Silicon Nitride/Silicon Carbide Nanocomposite Powders

A carbothermal reaction of silica–phenol resin hybrid gels prepared from a two‐step sol–gel process was conducted in atmospheric nitrogen. The gels were first pyrolyzed into homogeneous silica–carbon mixtures during heating and subsequently underwent a carbothermal reaction at higher temperatures. Using a gel‐derived precursor with a C/SiO 2 molar ratio higher than 3.0, Si 3 N 4 /SiC nanocomposite powders were produced at 1500°–1550°C, above the Si 3 N 4 –SiC boundary temperature. The predominant phase was Si 3 N 4 at 1500°C, and SiC at 1550°C. The Si 3 N 4 and SiC phase contents were adjustable by varying the temperature in this narrow range. The phase contents could also be adjusted by changing the starting carbon contents, or by its combination with varying reaction temperature. A two‐stage process, i.e., a reaction first at 1550°C and then at 1500°C, offered another means of simple and effective control of the phase composition: the Si 3 N 4 and SiC contents varied almost linearly with the variation of the holding time at 1550°C. The SiC was nanosized (∼13 nm, Scherrer method) formed via a solid–gas reaction, while the Si 3 N 4 has two morphologies: elongated microsized crystals and nanosized crystallites, with the former crystallized via a gaseous reaction, and the latter formed via a solid–gas reaction. The addition of a Si 3 N 4 powder as a seed to the starting gel effectively reduced the size of the Si 3 N 4 produced.