EFFECT OF PRESSURE ON THE COMPOSITION AND SUPERCONDUCTING T-C VALUE OF NBN PREPARED BY COMBUSTION SYNTHESIS

The synthesis of niobium nitride by combustion of packed niobium powde r under nitrogen atmosphere was studied in the pressure range 3.5-700 bar without solid-phase dilution. Surface melting of niobium was only detected at 30 bar. The reaction products consist of delta-NbN above 1 00 bar and of gamma-Nb4N3 +delta-NbN at lower pressures. The lattice p arameter of delta-NbN vs. the applied pressure presents a maximum corr esponding to the transition from a predominantly defective nitrogen su blattice (N/Nb<1) to a predominantly defective niobium sublattice (N/N b>1). Bulk metal-nitride components were obtained using a 'chemical ov en' configuration and combining the high pressure with the high combus tion temperatures of the niobium powder. Thick nitride films of approx imate to 50 mu m with a critical superconducting temperature up to 17. 2 K were prepared. The films consist of a delta-NbN external layer, of a gamma-Nb4N3 intermediate layer and of a beta-Nb2N inner layer. (C) 1998 Elsevier Science S.A.