THERMAL-BEHAVIOR OF SICN NANOPOWDERS ISSUED FROM LASER PYROLYSIS

The thermal behaviour of two SIGN nanopowders (C/N=0.87 and C/N=0.22), issued from laser pyrolysis, is studied under two different atmospher es (He, N-2) by means of themogravimetry coupled with mass spectrometr y, and X-ray, diffraction. In the 25-1200 degrees C temperature range, the same behaviour in both helium and nitrogen is observed for the tw o SiCN nanopowders, very little weight loss occurs because of the evol ution of absorbed water, residual synthesis gases and also methane, hy drogen and car bon monoxide. However, above 1200 degrees C, the signif icant weight loss associated with the release of nitrogen and oxide sp ecies, under helium, is related to the decomposition process; whereas under nitrogen the weight gain results fi om the nitriding process. Th ese two phenomena are more important for an amorphous powder. A compar ison of the hot-pressing of Si3N4 UBE and SIGN nanopowders with sinter ing aids (Y2O3, Al2O3) indicates that the a to beta Si3N4 transformati on rate is increased by the small grain size and by the presence of Si C in SiCN nanopowders. (C) 1998 Elsevier Science Limited. All rights r eserved prbence de SiC. O 1998 Elsevier Science Lintited. All rights r eserved.