Effects of surface area, polymer char, oxidation, and NiO additive on nitridation kinetics of silicon powder compacts

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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 1250 and 1350°C for 4 h. Silicon powder was wet-attrition-milled from 2 to 48 h to achieve surface areas (SAs) ranging from 1.3 to 63 m2/g. A silicon powder of high surface area (63 m2/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, or 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 h in N2 at 1250 or 1350°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 the nitridation kinetics.

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