Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V2N and δ-VN1−x develop. The morphology of the β-V2N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V2N and δ-VN1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V2N) and 2.93 eV (δ-VN1−x ). By using δ-VN1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V2N is formed on the surface of the couple as a result of out-diffusion of nitrogen.