Evolution of Generated Calcium Vanadates at Different Locations in the Vicinity of the V₂O₅/CaO Interface with Annealing Parameters

Abstract

V₂O₅/CaO diffusion couples were prepared to investigate the reaction characteristics between V₂O₅ and CaO at 1073 K to 1273 K in air. The distribution, species, and number of phases generated in the proximity of the V₂O₅/CaO interface were characterized by electron probe microanalysis (EPMA), Fourier transform–infrared (FT–IR) spectroscopy, and X-ray diffraction (XRD) that was capable of selecting the scanning location. The results show that CaV₂O₆ was quickly generated to become the matrix all over the V-rich layer at a relatively low temperature (1073 K to 1223 K); Ca₂V₂O₇ was then formed in the vicinity of the CaV₂O₆/CaO interface. The presence of Ca₂V₂O₇ near the CaV₂O₆/air interface was unexpected and indicated that the diffusion of oxygen from the air into the V-rich layer had promoted its formation. Upon increasing the annealing temperature, CaV₂O₆ disappeared gradually at 1223 K, Ca₂V₂O₇ became the matrix at 1223 K, and Ca₃(VO₄)₂ appeared near the Ca₂V₂O₇/CaO interface at 1173 K. Average interdiffusion coefficients in Ca₃(VO₄)₂ at 1173 K, 1223 K, and 1273 K were 1.66 × 10⁻¹⁰, 3.45 × 10⁻¹⁰, and 17.5 × 10⁻¹⁰ cm² s⁻¹, respectively. The diffusion activation energy calculated using the Arrhenius equation was 290.15 ± 32.29 kJ/mol. With increasing annealing time, the amount of the phases (Ca₂V₂O₇ at 1073 K to 1123 K and Ca₃(VO₄)₂ at 1173 K to 1273 K) in the vicinity of the V-rich layer/CaO interface increased. At 1173 K, the diffusion thickness of Ca₃(VO₄)₂ increased with increasing annealing time with a square root of time dependency, which suggested that the formation of Ca₃(VO₄)₂ in the Ca₂V₂O₇ mixture was governed by the solid–solid diffusion process.