Synthesis of α-silicon nitride powder by gas-phase ammonolysis of CH3SiCl3

An alternative to SiCl4 mostly used for Si3N4 production by gas phase ammonolysis is CH3SiCl3. While powders resulting from ammonolysis of CH3SiCl3 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 nonstoichiometric, chlorine and carbon containing Si3N4 intermediates and NH4Cl are obtained. Gaseous reaction products are HCl, SiCl4, CH4 and H2. Quantitative mass spectrometric analysis of the exhaust gases allowed the reactions to be balanced. 29Si and 13C 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 NH3 at 900 C followed by a crystallisation step at 1500 C results in crystalline alpha-Si3N4 which is equivalent to powders obtained by ammonolysis of SiCl4. 25 refs.