ELECTROCHEMICAL SYNTHESIS OF CERAMIC MATERIALS .3. SYNTHESIS AND CHARACTERIZATION OF A NIOBIUM NITRIDE PRECURSOR AND NIOBIUM NITRIDE POWDER

Anodic dissolution of Nb foil in an electrolyte solution containing liquid NH3 and NH4Br produces an insoluble precursor that can be calcined to yield phase-pure NbN. Infrared spectroscopy is consistent with a precursor having the general formula NbXn(NH2)5-n.pNH3. By changing the calcination conditions, the phase of the resulting NbN powder can be varied. For example, calcining in pure Ar yields the superconducting delta-NbN phase at 600 and 800-degrees-C but results in gamma-NbN at 1000-degrees-C. A mixed 85 % Ar/ 15 % H-2 calcining atmosphere also results in delta-NbN at 600-degrees-C, but yields gamma-NbN phases at 800-degrees-C. When the precursor powder is calcined in NH3 at 600, 800, or 1100-degrees-C gamma-NbN phases result, but subsequent calcining in Ar at 1450-degrees-C yields superconducting delta-NbN (T(c) = 14.75 K). The calcination ambient affects the chemical composition of the powder: in all cases more reducing conditions lower the level of Br contamination.