Synthesis of alpha -silicon nitride powder by gas-phase ammonolysis of CH sub 3 SiCl sub 3

An alternative to silicon tetrachloride mostly used for silicon nitride production by gas phase ammonolysis is CH sub 3 SiCl sub 3 . While powders resulting from ammonolysis of CH sub 3 SiCl sub 3 at room temperature have a constant composition, high temperature reactions result in materials showing a strong dependence on the reaction conditions. Increasing reaction temperature leads to powders with increasing silicon content, while the chlorine content decreases. Mixtures of non stoichiometric, chlorine and carbon containing silicon nitride intermediates and ammoniumchloride are obtained. Gaseous reaction products are HCl, SiCl sub 4 , CH sub 4 and H sub 2 . Quantitative mass spectrometric analysis of the exhaust gases allowed to balance the reactions. exp 29 Si and exp 13 C CP-MAS-NMR powder characterisation was used to deduce the reactions taking place in the gas phase. An overlapping of radical forming and substitution reactions is probable. Dechlorination of the powders in ammonia at 900 deg C followed by a crystallization step at 1500 deg C results in crystalline alpha -Si sub 3 N sub 4 which is equivalent to powders obtained by ammonolysis of SiCl sub 4 .