Silicon for silicon nitride based products

Abstract

Previous studies have shown that the oxide layer on the surface of silicon particles can reduce the hydrogen evolution that comes from contacting silicon with water during slip casting, prior to nitridation. This study will investigate the effect of the oxide layer on the nitridation of silicon powder. Silicon powders have been stored at 10◦C and 22◦C humid atmospheres in two conditions; as received and after a thermal reduction to remove the surface oxide layer. After differ- ent storage times, the samples were nitrided using TGA and a large scale furnace. For references some samples were also nitrided in as received condition and after a thermal oxidation. The nitridation results showed that the samples that were thermally reduced before storage had a lower degree of nitridation than the samples that were stored directly, indicating that the oxide layer had a positive effect on the nitridation. The difference in nitridation between the two storage temperatures were slightly less evident, but results showed that the samples stored at 10◦C had a larger degree of nitridation than those stored at 22◦C. XRD analyses showed that most of the silicon nitride product was α-Si3N4, usually with an α/β-ratio between 0.8 and 0.9. A silicon powder of much higher purity was also nitrided for comparison. The results showed that the high purity silicon had a much lower degree of nitridation, indicating that the silicon nitride reaction is very susceptible to impurities in the silicon powder. The α/β-ratio was also lower in the pure silicon.