Effect of Nitrogen to Niobium Atomic Ratio on Superconducting Transition Temperature of δ-NbN_x Powders

摘要

Powders of cubic niobium nitride δ-NbN_x with a particle size of below 20 μm were prepared by reactive diffusion at T= 1455- 1475°C under nitrogen pressures of P1(N2) = 0.1-3 MPa and P2(N2) = 25 MPa. For these powders, the values of the stoichiometric coefficient x, lattice parameter a, and the superconducting transition temperature T_c were measured and the a(x), T_c(x) and T_c(a) functions were analyzed. The T_c values were found to linearly grow with increasing a (decreasing structure imperfection). A maximum value of T_c (15.8 K) corresponded to a maximum value of a (4.3934 A). Maxima in the a(x) and T_c(x) curves were found to correspond to a slightly substoichiometric nitride with x = 0.98. Having synthesized cubic niobium nitrides with 0.892 < x < 1.062, we managed to measure the dependences of a and T_c onx all over the almost entire homogeneity range for δ-NbN_x. Our a(x) and T_c(a) functions were found to reasonably agree with those previously reported for SHS-produced 8-NbN_x powders.