Carbide formation of Va-group metals (V, Nb and Ta) in a solar furnace

Abstract

In our recent series of solar carbide synthesis for Si and d-group transition metals (Ti, Zr; Cr, Mo, W) using graphite (G) or amorphous carbon (aC) as carburising medium, we observed some evidences indicative of involvement of photochemical effects in the reaction process. For example, (1) graphitisation of aC was accelerated during carburisation of W under solar radiation; (2) high-temperature mono-carbide phase of Mo (α-MoC 1− x ) formed at a temperature appreciably lower than the lower-threshold temperature of the stability range of the α-MoC 1− x phase given in the available equilibrium phase diagram for Mo–C system. In this work, we investigated solar carburisation reactions for Va-group metals (V, Nb, Ta) in Ar or N 2 gas environment. For this group of metals, only carbide phases formed even in N 2 atmosphere without forming carbonitride like the cases with VIa-group metals (Cr, Mo, W). For carburisation of Ta with aC in Ar, an unexpected formation of sub-carbide Ta 2C besides mono-carbide TaC took place, while Ta 2C phase did not form through carburisation of Ta with aC in N 2 or carburisation of Ta with G in either Ar or N 2 atmosphere. No graphitisation promotion for aC was detected during the carburisation of none of the Va-group elements.