Crystallization and sintering characteristics of chemically vapor deposited silicon nitride powders
- PDF / 857,851 Bytes
- 5 Pages / 612 x 828 pts Page_size
- 55 Downloads / 189 Views
F. Orgaz Centro de Investigation de ERCROS, S.A., Ronda de Valdecarrizo, 23. Tres Cantos, 28760 Madrid, Spain (Received 14 February 1992; accepted 12 June 1992)
High purity silicon nitride powders have been obtained by Chemical Vapor Deposition (CVD), through the reaction of SiH4 and NH 3 gas mixtures at 1000 °C in a quartz reactor. The crystallization characteristics of the powder have been followed by infrared and x-ray diffraction analysis. The material shows a transition from amorphous to the a-phase after a thermal treatment at about 1300 °C for 1 h, while the /3-phase starts to appear at 1725 °C. The sintering properties of the amorphous and crystalline phases were evaluated by measuring the dilatometric curves of compacted powders.
I. INTRODUCTION Among the ceramic materials, silicon nitride occupies a prominent place. This is due to its outstanding characteristics: it is a relatively low density refractory compound, resists attack by molten metals, shows an extremely low expansion coefficient and hence a good resistance to thermal shock, and possesses a 70% covalent bond with a short distance. These properties make the silicon nitride one of the hardest and toughest materials but, in turn, difficult to sinter.1'2 The sintering difficulties have led many researchers to investigate the production routes that yield the best powder characteristics, i.e., a low grain size with a narrow distribution and an equiaxed morphology, a low impurity content with controlled amounts of oxygen, and a high proportion of the a-phase. The production routes available in a mass scale are the carbothermal nitridation of silica, the direct nitridation of silicon powder, and the liquid-phase (imide process) and gas-phase (CVD process) reactions. From these, the liquid and gas phase reactions are perhaps the ones giving the purest product, since the starting liquids or gases can be selected with a high degree of purity. Advantageously, these two methods yield a very fine particle size (in the submicron range), usually not needing further processing steps.2 However, despite its commercial interest, the literature concerning the liquid or gas phase reactions is rather scarce. Since the pioneering work of Galasso et al. ,3 there followed some other papers, either on the deposition of ceramic coatings (pyrolytic Si 3 N 4 ) 4 ' 5 or in the preparation of powders.6 Recent papers include the use of a cold plasma7 or a thermal plasma discharge.8 In this paper we have focused our interest in the study of the crystallization and sintering characteristics of silicon nitride powders. The powder was obtained by 2864
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 10, Oct 1992
Downloaded: 15 Mar 2015
the chemical vapor deposition technique (CVD), starting from silane (SiH4) and ammonia (NH3) mixtures, by reacting them at 1000 °C. We have followed this route, instead of the most conventional one based on SiCl4, as it shows a high deposition rate. In addition, the CVDproduced Si3N4 is very pure, and the by-products of the reaction a
Data Loading...
