NANOSIZED SI-C-N-POWDERS BY POLYSILAZANE PYROLYSIS AND SI3N4 SIC-COMPOSITE MATERIALS THEREOF/

Nanosized silicon carbonitride powders have been synthesized by gas ph ase pyrolysis of evaporable precursors at temperatures between 600 and 1000 degrees C. These synthesis have been carried out by using of mix tures of cyclic polysilazanes, which were prepared by ammonolysis of d imethyldichlorosilane in an aprotic organic solvent, or of hexamethyid isilazane as starting materials. The spherical powder particles are X- amorphous. They demonstrate a particle size from 100 to 600 nm and a s pecific surface area from 10 to 350 m(2)/g. The Si-C-N-powders crystal lize upper 1400 degrees C to Si3N4- and SiC-phases. The chemical compo sition, the specific surface area and the particle size distribution o f the Si-C-N-powders are controlled by changing the starting materials , the synthesis and the pyrolysis conditions. The carbon content of th e powders consists of bonded carbon in the SiC and of free carbon. By pyrolysis or post heating in reactive ammonia the carbon content can b e diminished. On-line coupling of thermogravimetry with evolved gas an alysis/mass spectrometry have been applied to characterize the behavio ur of powders during annealing processes. The investigations indicate, that after the pyrolysis organic species are adsorbed onto the surfac e of the powders, which can be removed by suitable posttreatment. From the synthesized Si-C-N-powders Si3N4/SiC-composite materials have bee n obtained by sintering. In these composites SiC nanograins ave arrang ed both in the intra- and in the inter-type and hinder the growth of t he Si3N4-grains. (C) 1998 Elsevier Science Limited. All rights reserve d.