The effect of heat-treatment on the grain-size of nanodisperse plasmathermal silicon nitride powder

Nanodisperse silicon nitride has been synthesized by vapor phase reaction o f silicon tetrachloride and ammonia in a thermal plasma reactor and crystal lized at temperatures 1250, 1350, 1450 and 1500 degrees C. The average grai n-size and the dislocation density of the samples were determined by the re cently developed modified Williamson-Hall and Warren-Averbach procedures fr om X-ray diffraction profiles. A new numerical method provided log-normal g rain-size distributions from the size parameters derived from X-ray diffrac tion profiles. It has been shown that the average grain-size in the amorpho us phase is lower than that observed in the crystalline fraction. On the ot her hand, the average grain-size in the crystalline fraction decreases up t o 1450 degrees C while it increases during heat-treatment at 1500 degrees C . The size distribution and the area-weighted average grain-size obtained b y X-rays were in good agreement with those determined by TEM and from the s pecific surface area, respectively. The dislocation density was found to be of the order of 10(14) and 10(15) m(-2). (C) 2000 Kluwer Academic Publishe rs.