EFFECTS OF PROCESS PARAMETERS ON ULTRAFINE SIC SYNTHESIS USING INDUCTION PLASMAS

A study is reported of the formation of ultrafine SiC powder through t he reaction of elemental silicon and CH4 in an induction plasma. The r eaction route used involved in the first place the vaporization of a f ine elemental silicon powder axially injected into the center of the d ischarge followed by the carburization reaction through the coinjectio n of CH4. The powder obtained was composed of a mixture of alpha- and beta-SiC with varying amounts of free carbon and free silicon. The par ticle size distribution was typically in the range of 40-60 nm with a corresponding specific surface area of 30-50 m(2)/g. A parametric stud y showed that the quality of the powder obtained varied with the plasm a plate power and the position of the injection probe. The plasma gas composition employed was found to influence the proportions of alpha- and beta-SiC in the synthesized SiC powder. With an Ar/N-2 mixture as the plasma gas, the ratio of the alpha to beta phases was less than 1. 0, whereas the ratio was greater than 1.5 when using a mixture of Ar/H -2 as plasma gas. The Si powder feed rate and the input C/Si molar rat io in the injected reactants significantly affected both the formation of the SiC and the free Si and free C content in the synthesized powd er. Lining the cylindrical reactor wall with graphite resulted in impr oved conversion of Si to SiC. The weight fraction of the powder collec ted at different sections of the reactor system varied with the reacto r operating conditions. The experimental results support the view that the formation mechanism for ultrafine SiC is dominated by the reactio n of Si vapor with the thermal decomposition products of CH4.