Preparation of Refractory Compounds

Salinger (44) reported the successful conversion of methyltrichloro-silanes to silicon carbide in a 50-kW RF plasma torch. The liquid methyltrichlorosilanes were fed to the tail flame of various plasmas and the solid products were recovered in an acid-resistant bag filter. Up to 85% recovery of theoretical solid product was reported, which was subsequently heated at 500°C to remove elemental carbon. Under the best condition (20-25% vol. hydrogen in argon plasma at 36 kW), up to 70%o conversion to j3-SiC was obtained with ca. 10% conversion to amorphous SiC. Salinger suggested that the good crystallinity of the )3-SiC so obtained meant that the reaction occurred in a gas temperature range in which )3-SiC was the stable crystalline form (i.e., 2300°C). 

MacKinnon and Wickens (37) have prepared boron carbide in the form of submicron particles by injecting BCI3, Hg, and CH into the tail of a 20-kW argon RF plasma. A 3 factorial experiment to study the effect of operating variables was carried out a preliminary analysis of which gave the following indications  

The maximum percentage boron conversion to B4C (93%) was obtained at a low (20 liters/min) BCI3 flow rate, at a relatively high (8 1) H2/BCI3 ratio, and at a stoichiometric (1 4) CH4/BCI3 ratio. 

The complete set of results obtained indicated the presence of interactions among the effects of BCI3 flow rate, H2/BCI3 ratio, and CH4/BCI3 ratio on the conversions obtained. 

Hartl (26) treated vanadium, chromium, and titanium nitrides in argon-5% acetylene RF plasmas. The nitrides were dropped as powders into a vertical plasma torch in which the gas stream was flowing upwards. Product was collected both as wall deposits and as loose powder at the lower end of the torch (i.e., the plasma gas inlet). In the case of vanadium, the products collected were identified by X-ray analysis to be a mixture of vanadium nitride and carbide (VC-VN) with 

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Yu. I. Krasnokutsky, V.G. Verecshak, Poluchenie tugoplavkikh soedinenii v plazme (Preparation of refractory compounds in plasma) Vischa Schola, Kiev, 1987 (in Russian). 

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