Reaction diffusion and phase equilibria in the V-N system

Abstract

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 ЊC to 1700 ЊC and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of ␤-V 2 N and ␦-VN 1Ϫx develop. The morphology of the ␤-V 2 N/␣-V(N) interface depends on the saturation state of the ␣-V(N) core. If the nitrogen content in ␣-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the ␣-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 ␤-V 2 N and ␦-VN 1Ϫx were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (␤-V 2 N) and 2.93 eV (␦-VN 1Ϫx ). By using ␦-VN 1Ϫx 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., ␤-V 2 N is formed on the surface of the couple as a result of out-diffusion of nitrogen.

Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N2 Dissociative Chemisorption, N Adatom Migration, and N2 Desorption

“…The ideal gas law provides an estimate of the nitrogen gas pressure to which the surface is exposed. The simulated nitrogen pressure, ∼10 MPa (∼75 × 10 3 Torr), is several orders of magnitude larger than N 2 pressures typically used during reactive magnetron sputter deposition (≤20 mTorr) while comparable to that employed for the synthesis of stoichiometric VN by nitridation of vanadium at temperatures ranging from 1400 to 2000 K , and for producing bulk cubic VN x (0.74 ≤ x ≤ 1) samples by float-zone and zone-annealing techniques at 2300 K . High nitrogen pressures allow us to follow, over feasible simulation times, N and N 2 gas/surface reactions (both N and N 2 species are present in the plasma during reactive sputter deposition).…”