Effects of Surface Area, Polymer Char, Oxidation, and NiO Additive on Nitridation Kinetics of Silicon Powder Compacts

The oxidative stability of attrition-milled silicon powder under reaction-bonding processing conditions has been determined. The investigation focused on the effects of surface area, polymer char, preoxidation, nitriding environment, and a transitional metal oxide additive (NiO) on the nitridation kinetics of attrition-milled silicon powder compacts tested at 1230 and 1350 deg C for 4 hrs. Silicon powder was wet attrition-milled from 2 to 48 hrs to achieve surface area (SA's) ranging from 1.3 to 63 sq m/g. A silicon powder of high surface area (63 sq m/g) was exposed for up to 1 month to ambient air or for up to 4 days to an aqueous-based solution with the pH maintained at 3, 7, and 9. Results indicated that the high-surface area silicon powder showed no tendency to oxidize further, whether in ambient air for up to 1 month or in deionized water for up to 4 days. After a 1 day exposure to an acidic or basic solution, the same powder showed evidence of oxidation. As the surface area increased, so did the percentage nitridation after 4 hrs in N2 at 1250 or 1350 deg C. Adding small amounts of NiO significantly improved the nitridation kinetics of high-surface area powder compacts, but both preoxidation of the powder and residual polymer char delayed it. Conversely, the nitridation environment had no significant influence on the nitridation kinetics of a high-surface area powder. Impurities present in the starting powder, and those accumulated during attrition milling, appeared to react with the silica layer on the surface of silicon particles to form a molten silicate layer, which provided a path for rapid diffusion of N2 and enhanced nitridation kinetics.