Synthesis of cubic niobium nitride by reactive diffusion under nitrogen pressure

The synthesis of niobium nitride by reactive diffusion in a furnace at 1395–1475 °C and under nitrogen pressure in the range 2–25 MPa was investigated. In experiments, we used compacted Nb powder with a mean particle size of 43 μm. Phase transformations in the product as studied by electron probe microanalysis (EPMA) were found to proceed in the following order: Nb → α-Nb(N) → β-Nb 2N 1± x → γ-Nb 4N 3± x → δ-NbN 1± x . The size of niobium particles which could react with nitrogen to yield cubic niobium nitride was estimated (SEM analysis) from the dependence of the thickness Δ of the δ-NbN 1± x outer layer formed on the surface of Nb particles on the dwell time t dw at 1460–1473 °C. It was shown that Δ grew nearly proportional to t dw. At t dw = 30 min and P(N 2) = 2 MPa, Δ was found to attain a value of about 15.5 μm. Prolonged heating ( t dw ≈ 60 min) was found to result in decomposition of the single-phase cubic niobium nitride into a two-phase (multiphase) product. This was confirmed by XRD data and magnetic measurements which showed the occurrence of two different critical temperatures T c in the same sample. The maximum critical temperature T c was found to attain a value of 15.6 K.