Nanosized Si-C-N-powders by polysilazane pyrolysis and Si3N4/SiC-composite materials thereof

Nanosized silicon carbonitride powders have been synthesized by gas phase pyrolysis of evaporable precursors at temperatures between 600 and 1000 degrees C. These synthesis have been carried out by using of mixtures of cyclic polysilazanes, which were prepared by ammonolysis of dimethyldichlorosilane in an aprotic organic solvent, or of hexamethyidisilazane as starting materials. The spherical powder particles are X-amorphous. They demonstrate a particle size from 100 to 600 nm and a specific surface area from 10 to 350 m(2)/g. The Si-C-N-powders crystallize upper 1400 degrees C to Si3N4- and SiC-phases. The chemical composition, the specific surface area and the particle size distribution of the Si-C-N-powders are controlled by changing the starting materials, the synthesis and the pyrolysis conditions. The carbon content of the powders consists of bonded carbon in the SiC and of free carbon. By pyrolysis or post heating in reactive ammonia the carbon content can be diminished. On-line coupling of thermogravimetry with evolved gas analysis/mass spectrometry have been applied to characterize the behaviour of powders during annealing processes. The investigations indicate, that after the pyrolysis organic species are adsorbed onto the surface of the powders, which can be removed by suitable posttreatment. From the synthesized Si-C-N-powders Si3N4/SiC-composite materials have been obtained by sintering. In these composites SiC nanograins ave arranged both in the intra- and in the inter-type and hinder the growth of the Si3N4-grains. (C) 1998 Elsevier Science Limited. All rights reserved.