Spark Plasma synthesis and diffusion of Cu and Ag in vanadium mixed valence oxides

Abstract

Spark Plasma sintering (SPS) technique allows powders to be compacted at low temperature with a very short holding time. The powder loaded into a carbon die is heated via direct current pulses and simultaneously submitted to an uni-axial pressure of several MPa. Full density of the sample is achieved within minutes. This process is used to study Cu and Ag metals interactions with V₂O₅ oxide. Syntheses of M_xV₂O₅ phases (M = Cu, Ag) have been achieved within minutes. Thus Cu and Ag atoms penetrate microcrystals of V₂O₅ oxide at a high speed, shearing its crystal network and simultaneously rebuilding the crystal structures of the prototype networks β, β′, ε or δ M_xV₂O₅. To account for the formation of these phases identified by X-ray diffraction, structural mechanisms are proposed. Cu and Ag atomic diffusion parameters have been determined from energy dispersive X-ray spectroscopy (EDX) and electron micropobe analysis (EPMA) line scans. High values of diffusion coefficients have been determined. Cu atoms diffuse faster than Ag, D_Cu ≈ 4 × 10⁻⁸ m²/s and D_Ag ≈ 0.5–1 × 10⁻⁹ m²/s in ε and δ M_xV₂O₅ phases, respectively. Their formation may also be used as a model for further investigations into the diffusion mechanisms of atoms in solids and for a better understanding of the SPS process.