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.