The Dissolution of SiC and Other Materials in Molten Si

Abstract

The observed rate of dissolution of SiC filaments in molten Si at 1550~ is explained by a Fick's law calculation. In the absence of dissociation barriers, SiC, Si3N4, SiO2, carbon, and graphite are predicted to dissolve in molten Si at a rate proportional to the product of their solubilities in Si(l) and the molar volume of the non-Si species in the solid. Predicted dissolution rates are compared to experimental values from the literature.A number of refractory composites are currently of interest for high temperature structural applications. These include reaction-bonded silicon carbides and nitrides. One of these, General Electric Company's Silcomp TM silicon/silicon carbide composite, is made by reacting a carbonaceous preform with molten Si to form SiC (1). All of these materials are subject to microstructural changes which can occur rapidly when the silicon is molten due to high liquid diffusivities. Germane to any study of the Si + C or the Si + N reactions is the rate at which the compounds SiC and Si3N4 can dissociate under a given driving force.Various forms of carbon as well as SiC, Si3N4, and SiO2 have been considered for use as containment materials for molten Si (2-4). The erosion rates for these materials have been shown to be amazingly similar, certainly to within an order of magnitude of each other. This implies that the over-all dissociation/dissolution kinetics must be approximately the same for all of these materials.This paper shows that the rate of consumption of SiC in molten Si is simply limited by the diffusion of C away from the SiC surface and that there are no intrinsic reaction barriers to the dissociation of SiC. By an extension of the analysis used in studying SiC, the Chaney and Varker results (4) will be reinterpreted more generally. ExperimentalMaterials.--The SiC filaments were manufactured by AVCO Corporation by a chemical vapor deposition process. Optical and electron microscopy indicate good dimensional uniformity, a fine-grained microstructure, and a smooth surface. The as-received filament diameter was 143 ~m.The Si used was purchased from Materials Research Corporation. The only detectable impurity in the MARZ-Grade Si was oxygen at 1.7 X 10 -s mole/cm 3.