AC, DC conduction and dielectric behaviour of solid and liquid phase sintered Al2O3-15Â mol% V2O5 pellets

Microstructural and dielectric characterization of porous Al2O3-V2O5 pellets sintered under both solid (600 °C) and liquid phase (850 °C) conditions are reported. The low temperature solid state sintering (SSS) leaves a very porous network of dense alumina particles surrounded by smaller facetted vanadia, whereas under liquid phase sintering (LPS), the V2O5 melts and recrystallizes over the relatively inert Al2O3, leading to a connected network structure. The ac conductivity and dielectric parameters of the pellets investigated from room temperature (RT) to 400 °C exhibited both universal dielectric (diffusive) behaviour at low frequencies and nearly constant loss (sub-diffusive) regimes at low temperatures. Activation energies calculated for dc/ac conduction at different frequencies suggests a composite conduction mechanism controlled by the Vanadia phase: at low frequencies, the calculated energies (Ea≈0.5 eV) compare with ionic diffusion of vanadium in V2O5, while at high frequencies and low temperatures an additional (Ea≤0.16 eV) polaronic hopping mechanism is seen. This cross-over frequency is significant for the LPS specimen indicating that the vanadia connected network assists conduction by providing a continuous (but disordered) diffusion route. Impedance and modulus spectral analysis show the presence of distributed time constants arising from electrodes, pore/matrix and phase interfaces and GBs.