Space-charge theory applied to the grain boundary impedance of proton conducting BaZr0.9Y0.1O3 - δ

The specific grain interior and grain boundary conductivities, obtained from impedance spectroscopy and the brick layer model, are reported for BaZr0.9Y0.1O3 (-) (delta) as a function of po(2) and temperature. po(2)-dependencies were indicative of dominating ionic and p-type electronic conduction for the grain interior under reducing and oxidizing conditions, respectively, while the grain boundaries showed an additional n-type electronic contribution under reducing conditions. Transmission electron microscopy revealed enrichment of Y in the grain boundary region. These findings indicate the existence of space-charge layers in the grain boundaries. A grain boundary core-space-charge layer model is therefore applied to interpret the data. Using a Mott-Schottky approximation, a Schottky barrier height of 0.5-0.6 V and an effective grain boundary width of 810 nm (=2x space-charge layer thickness) is obtained at 250 degrees C in wet oxygen. Finite-element modelling of the complex impedance over a grain boundary with a space-charge layer depletion of protons yields a distorted semicircle as observed in the impedance spectra. (C) 2010 Elsevier B.V. All rights reserved.