Reinhold Dipl Chem Dr Brink

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

Abstract An alternative to silicon tetrachloride mostly used for silicon nitride production by gas phase ammonolysis is CH 3 SiCl 3 . While powders resulting from ammonolysis of CH 3 SiCl 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 4 , CH 4 and H 2 . Quantitative mass spectrometric analysis of the exhaust gases allowed to balance the reactions. 29 Si and 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°C followed by a crystallization step at 1500°C results in crystalline α-Si 3 N 4 which is equivalent to powders obtained by ammonolysis of SiCl 4 .

Gas-phase synthesis of amorphous silicon nitride — reaction paths and powder characteristics

The gas phase reaction between SiCl4 and NH3 is investigated in the temperature range between 525 and 800°C at atmospheric pressure and at conditions typical for powder synthesis. By means of mass spectrometric in-situ measurements it was possible to detect the gaseous compounds H2NSiCl3, H2NSiCl2NH2, Cl3SiNHSiCl3, NH2Cl2SiNHSiCl3, (SiCl2NH)3 and Si3(NH)3Cl5NH2. The reactions taken place in the gas phase are very fast and result in the formation of a fine, chlorine containing product. Powders sampled at a reaction temperature of 800°C have an average molar ratio Si : N : Cl of 1 : 1, 33 : 0.28. Based on the proved gaseous intermediates and the composition of the powders reaction paths resulting in the formation of powders are derived. α-Si3N4 powders with a high sintering activity are obtained after thermal dechlorination of the synthesis products in ammonia atmosphere followed by a crystallization process between 1200 and 1500°C.