Chemical transport reactions in the vanadium-silicon-oxygen system and the ternary phase diagram

Abstract

Vanadium and silica glass react at 800 °–1200 °C to form V 3Si(s) and VO(s) if the reaction vessel contains gaseous vanadium chlorides or iodides causing a chemical transport of the metal to the glass. In the presence of chloride species, V 3Si and VO are chemically transported from the hot to the cooler portions of the reaction vessel. The chemical reactions accounting for the transport are proposed. The following equilibrium tie-lines in the ternary V-Si-O phase diagram were determined: SiO 2-VO, SiO 2-V 3Si, V 3Si-VO, V 3Si-V 2O, and V 3Si-V 4O; and the phase diagram is presented. Considerations of the phase data and the heats of formation of VO(s) and SiO 2(gl) showed that − ΔH o f (V 3Si,s) must be >9 kcal/mole. Relative to one another, the set of values for the heats of formation of V 3Si, V 5Si 3, and VSi 2 as given in the literature was found to be thermodynamically inconsistent with V-Si-O phase data.