Combustion synthesis of NbN powders and their superconducting transition temperatures.

An investigation has been made on the synthesis of superconducting NbN and (Nb, Mo)N powders by a combustion reaction of nitriding their metal elements under high nitrogen pressures up to 10MPa. Nearly stoichiometric B1-NbN powders were prepared by the combustion reaction of Nb powders with a proper amount of NbN addition under 1-10MPa nitrogen pressures. The characteristics of this combustion reaction were investigated by measuring the propagating velocity of combustion for the packed reactants with various amounts of diluent. The superconducting transition temperature of NbN powders obtained was 16.9K, which is very close to the highest value of 17.3K reported for NbN up to now. On the other hand, Nb powders with 5, 10, and 20wt% Mo were burned under 10MPa nitrogen pressure in order to synthesize (Nb, Mo)N solid solutions. The combustion product of Nb with 5wt% Mo consisted of B1 phase only, while those with 10 and 20wt% Mo contained a small amount of residual Mo. The transition temperatures in the Nb-Mo-N system were 12.7-15.9K, which were lower than that of pure NbN obtained in this study.