Effects of sintering temperature on microstructure and giant dielectric properties of (VÂ +Â Ta) co-doped TiO2 ceramics

The influences of sintering temperature on the microstructure and giant dielectric properties of a new co-doped TiO2 system, i.e., V and Ta co-doped TiO2, were investigated. The grain size of (V1/2Ta1/2)0.01Ti0.99O2 ceramics was enlarged with increasing sintering temperature. Dense microstructure and homogeneous dispersion of dopants were achieved in the ceramics sintered at 1400-1500 °C for 5 h. The dielectric permittivity in the frequency range 40-106 Hz of the (V1/2Ta1/2)0.01Ti0.99O2 ceramics significantly increased with the mean grain size, while the dielectric loss tangent was reduced to 0.033 at 102 Hz. Furthermore, the high-temperature stability of the dielectric permittivity was improved with increasing mean grain size. The electrically heterogeneous microstructure consisting of semiconducting grains and insulating grain boundaries and/or surface layers was confirmed using impedance spectroscopy. The conduction inside the semiconducting grains was attributed to electron hopping between Ti4+ and Ti3+, which was confirmed by X-ray photoelectron spectroscopy. Very high resistivity with a large conduction activation energy of the insulating parts was suggested as the primary cause of the giant dielectric permittivity with low loss tangent.