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

“…11(b)). This is analogous to the mechanism proposed, based upon STM experiments, for O2 dissociation on TiO2 (110). 122 AIMD movies also show that N2 dissociation on VN(001) can be directly followed by a Nad/Nsurf desorption reaction promoted by momentum transfer to the surface from the incident gas molecule.…”

“…The ideal gas law provides an estimate of the nitrogen gas pressure to which the surface is exposed. The simulated nitrogen pressure, ~10 MPa (~75x10 3 Torr), is several orders of magnitude larger than N2 pressures typically used during reactive magnetron sputter deposition (≤ 20 mTorr), 109 while comparable to that employed for the synthesis of stoichiometric VN by nitridation of vanadium at temperatures ranging from 1400 to 2000 K [104,110] and for producing bulk cubic VNx (0.74 ≤ x ≤ 1) samples by float-zone and zone-annealing techniques at 2300 K. 111 High nitrogen pressures allow us to follow, over feasible simulation times, N and N2 gas/surface reactions (both N and N2 species are present in the plasma during reactive sputter deposition 112 ). Thus, N adatom migration on VN(001) occurs preferentially along atop-V FFH atop-V pathways.…”

Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN

“…Experimental measurements of atomic diffusivities generally employ isotopes. Although scattered over several orders of magnitude, values for nitrogen defect diffusion parameters in TM nitrides have been reported in various experimental works [8,28,29]. In contrast, experimental data for metal-element self-diffusivities are lacking for most of these ceramics.…”

Simulation of Precipitates Evolution in Steels

“…Since there is no published information on diffusion parameters in vanadium carbonitrides, we used correspondingly coefficients of C and N diffusion in vanadium carbide and nitride. The coefficient of tracer diffusion of C in the carbide was taken from [14], and that of N in the nitride was calculated based on the data of its chemical diffusion from [15] according to the formulas by Anderson and Ågren, relating coefficients of tracer and chemical diffusion [16].…”