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-Nb2N1±x → γ-Nb4N3±x → δ-NbN1±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-NbN1±x outer layer formed on the surface of Nb particles on the dwell time tdw at 1460–1473 °C. It was shown that Δ grew nearly proportional to tdw. At tdw = 30 min and P(N2) = 2 MPa, Δ was found to attain a value of about 15.5 μm. Prolonged heating (tdw ≈ 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 Tc in the same sample. The maximum critical temperature Tc was found to attain a value of 15.6 K.