Formation of NbC and TaC by solid-state reaction

The formation of NbC and TaC by solid-state reaction of Nb2O5 and Ta2O5 with carbon, respectively, was studied at 1000 to 1285 deg C by X-ray diffraction. Factors affecting the formation of the carbides, such as the nature of the reaction vessel, the molar ratio of carbon/oxide, the form of reactant carbon, etc., were examined. By covering the graphite crucible and increasing the molar ratio of carbon to oxide, complete formation of NbC and TaC could be achieved in about 60 min at low temperatures (1170 and 1225 deg C, respectively). The beneficial effect of covering the crucible is ascribed to the retention of CO which facilitates the reaction at low temperatures. The formation kinetics of NbC and TaC obey a first-order equation, with activation energies of 90 and 93 kcal mol-1, respectively. Possible mechanisms for the solid-state reactions are discussed. (author)