Dielectric Characteristic in V₂O₅ Crystal and Crystallized Glass 70V₂O₅⋅30P₂O₅

Abstract

A large dielectric relaxation was observed in the crystallized glass containing a large amount of V₂O₅. The main crystalline precipitate in the crystallized glass 70V₂O₅⋅30P₂O₅ was V₂O₅ crystal. To clarify the origin of the large dielectric relaxation, the dielectric properties of the crystallized glass were studied and compared with those of polycrystalline V₂O₅ obtained by two different methods. The ε′ of crystallized glass 70V₂O₅⋅30P₂O₅ is two or three times larger than that of the glass for the same composition. The sintered and oriented V₂O₅ polycrystalline materials show a large dielectric relaxation; the value of the static dielectric constant in the specimen sintered at 620°C for 5h is 1000. The ε′ of polycrystalline V₂O₅ changed with the sintering temperature and with the direction of the crystal axis in the oriented specimen. The results obtained were analyzed on the basis of the equivalent circuit model for a double-layer dielectric. According to this model, the ε′ and σ of the crystal (the direction of b-axis) of the specimen oriented perpendicular to the electric field were 63 and 7.06×10^-4(mho/cm), respectively. The σ of this sample is nearly equal to the value of the b-axis in the V₂O₅ single crystal. The σ of the grain boundary is 6.60×10^-7(mho/cm) and is equal to that of amorphous V₂O₅. In the sintered specimen, the ε′ values of the crystal are about 10-13; the ε′ value of the grain boundary layer increased with increasing sintering temperature. The change of ε′ in the sintered specimen is thus related to the thickness of the grain boundary. The ε′ of the precipitated V₂O₅ crystal in the crystallized glass is nearly equal to that of the crystals for the sintered V₂O₅ polycrystalline materials. The thickness corresponding to the grain boundary in the crystallized glass is larger than that of the sintered polycrystalline material, The ε′ in the sintered specimen and crystallized glass is influenced not only by the V₂O₅ crystal properties but also by the properties of the grain boundary between V₂O₅ crystals. The analysis by the double-layer dielectric model led to the conclusion that the large dielectric relaxation of the crystallized glass is not due to the contribution of the direction of the preferential crystal axis in V₂O₅ crystal, but should be attributed to the effect of the inter-surface polarization between the highly conducting crystal (V₂O₅) and the poorly conducting matrix.