The influence of grain boundary impedances on the p-type conductivity of undoped BaTiO3 ceramics

Impedance spectroscopy is used to deconvolute the dc conductivity (sigma) of undoped BaTiO3 ceramics (similar to 95% of the theoretical Xray density) into bulk (sigma(b)) and grain boundary (sigma(gb)) components at two oxygen partial pressures, P-O2 similar to 10 Pa (N-2) and similar to0.21 MPa (air). At 900degreesC, sigmasimilar tosigma(b) in both atmospheres, however, at lower temperatures Z* plots are dominated by the grain boundary component and sigmasimilar tosigma(gb). The temperature of the switch from sigmasimilar tosigma(b) to sigmasimilar tosigma(gb) is different in the two atmospheres and occurs at similar to850degreesC in air and similar to650degreesC in N-2. Isothermal plots of log sigma(b) VS log P-O2 in the temperature range 450-900degreesC show the expected oxygen partial pressure dependence with a gradient of + 1/4. In contrast, sigma(gb) is relatively insensitive to P-O2 and log sigma(gb) VS log P-O2 plots have gradients < + 1/4 with values as low as similar to + 1/14.0. In general, isothermal log sigma vs log pO(2) plots have gradients < + 1/4 as sigma is dominated by the grain boundary component. This may explain the wide range of gradients (similar to 1/4-1/9) reported in the literature for isothermal dc conductivity measurements on polycrystalline BaTiO3 in the p-type regime. (C) 2002 Elsevier Science Ltd. All rights reserved.