Synthesis of Niobium and Tantalum Nitrides via Nitridation of Powders during Calcium Reduction of Their Oxide Compounds

We have studied characteristics of powders prepared by nitriding products of calcium vapor reduction of Ta2O5, Nb2O5, Mg4Та2O9, and Mg4Nb2O9. The process was run in two steps: (1) reduction of the oxide compounds at 750, 800, 850, and 900°C for 1–6 h and (2) holding of the reduction products in a nitrogen atmosphere at the same temperatures for 1–5 h. The use of Ta2O5 as a precursor led to the formation of products containing, in addition to tantalum, the θ-TaN and Ta2N nitrides. Tantalum nitrides were difficult to synthesize from Mg4Та2O9 reduction products. If Nb2O5 was used as a precursor, the nitridation products contained, in addition to niobium, the NbN and Nb2N nitrides with hexagonal close-packed lattices. In the case of nitridation of Mg4Nb2O9 reduction products, the percentage of the nitrides was a factor of 6–10 smaller than in the case of reduction and nitridation of niobium pentoxide, which can be accounted for in terms of specific features of the pore structure of the powders. Raising the reduction temperature from 750 to 850°C reduced the specific surface area of the nitrided powders from 26.2 to 11.6–14.5 m2/g. The powders had a mesoporous structure.