Vapor Phase Growth of β-SiC Whiskers with Fluorosilicate Melt

State of Products at Various Temperatures

Abstract

The vapor phase growth of β-SiC whiskers from gaseous species evolved by the reaction of fluorosilicate melts with carbon has been studied in wide temperature range with two methods, in which the reactant gases were introduced from evolution chamber to reaction zone by a carrier gas or suction. In flow method, Ar gas was used as the carrier gas of SiO, that evolved by the reaction of SiO₂-NaF melt with carbon at 1350°C or 1400°C. When the SiO gas reacted with CO gas on a substrate of graphite at the temperatures ranging 1350°C to 800°C, β-SiC whiskers formed in the wide temperature range. At the same time, carbon deposited at the intermediate temperature zone and silicon at the lower temperature zone at the higher flow rate of the Ar by the disproportionation of CO and SiO respectively. Moreover, glassy material deposited in the lowest temperature range.
In the case of suction method, SiO gas was produced by the reaction of SiO₂-Na₃AlF₆ melt with carbon at 1450°C. In this case, AlF₃ gas should have evolved above atmospheric pressure from the inside of the melt, and then might have promoted the evolution of the SiO gas. By the reaction of the SiO gas with CO gas, a large amount of β-SiC whiskers were produced in the reaction zone of the temperature range from 1350° to 1190°C under the following conditions: the molar ratio of SiO₂ to Na₃AlF₆ was about unity, and suction was carried out with weakly reduced pressure. The results obtained have been discussed in detail from thermochemical data.