CHROMIUM CATALYZED SILICON NITRIDATION

Silicon powder compacts were fabricated with various amounts of chromium (0-5 at %) deposited onto the surface of the silicon powder by a solution-deposition process. These compacts were heated to several maximum temperatures in the range 1100-1250 degrees C in a flowing 10% H-2/90% N-2 atmosphere to evaluate the effect of the chromium content on the silicon nitridation. It was observed that silicon compacts containing 5 at % Cr were fully nitrided in approximately 3 h at 1150 degrees C, while less than 8% nitridation was achieved for pure-silicon compacts (with 0 at % Cr) compacts under the same conditions. Single-crystal silicon wafers with a 50 nm chromium layer were also nitrided; this provided a planar geometry, which facilitated our study of the catalysis mechanism. The rate-controlling process was shown to be first order, which may be indicative of a nucleation-and-growth mechanism, which is commonly observed for alpha-silicon-nitride formation. This work demonstrates the feasibility of producing reaction-bonded silicon nitride at low temperatures using chromium catalysis, and it indicates the potential for fabricating fibre-reinforced silicon-nitride composites containing thermally sensitive fibres.