Combustion synthesis of vanadium carbonitride from V-C powder compacts under nitrogen pressure

Abstract

An experimental investigation on the preparation of vanadium carbonitrides by self-propagating high-temperature synthesis (SHS) was conducted using compacted samples of the vanadium and carbon powder mixture ignited in gaseous nitrogen. Effects of the initial sample stoichiometry, nitrogen pressure, and dilution by vanadium nitride (VN) on the combustion characteristics and the degree of conversion were studied. Experimental observation shows that upon initiation the combustion process is characterized by the steady propagation of a self-sustained reaction front. The flame-front propagation velocity was found to decrease with an increase in the carbon content of the compact, due to the decrease of combustion temperature with increasing carbon content. For the reactants composed of V + 0.3C and V + 0.5C, after the passage of the flame front the nitridation continued lengthily in an afterburning stage. Because of a less amount of available vanadium to react with nitrogen, the compact of V + 0.5C achieved a higher degree of nitridation than did the V + 0.3C sample. However, the nitridation percentage of the V + 0.7C sample was lower than that of the V + 0.5C sample, mainly due to the absence of afterburning reaction in the V + 0.7C powder compact. Moreover, it was found that diluting the V + 0.3C sample by a small amount of VN as the sample of 0.9V + 0.3C + 0.1 VN effectively enhanced the degree of nitridation, but a further increase in the diluent concentration provided no additional improvement. The XRD analysis indicates the formation of single-phase carbonitrides V(C,N) slightly rich with vanadium from the reactant compacts of V + 0.5C, V + 0.7C, and 0.9V + 0.3C + 0.1VN. For the sample of V + 0.3C, due to the low degree of nitridation there exists an intermediate nitride phase of V 2N in addition to the dominant phase of V(C,N).