Direct evidence of potential barriers at grain boundaries in Y-doped BaZrO₃ from dc-bias dependence measurements

Y-doped BaZrO₃ (BZY) is an outstanding proton conductor under wet atmosphere at relatively low temperatures (<600 °C). Dense polycrystalline ceramics of this material used in intermediate-temperature solid oxide fuel cells as solid electrolytes, however, suffer from uniquely high resistance across grain boundaries in the materials. It has long been speculated that such high resistance at the grain boundaries is originated from potential barriers which may be formed at the grain boundaries. Here we provide direct experimental evidence that Schottky-type potential barriers indeed exist at grain boundaries in BZY. The results of our study on dc-bias dependence of grain-boundary impedance in 5, 3 and 2 mol% Y-doped BaZrO₃ (BZY5, BZY3, and BZY2, respectively) clearly show that the relation between the current across a grain boundary and the applied dc-bias is ohmic where the dc-bias is lower than the thermal voltage, while that becomes superohmic at sufficiently high biases. In addition, it is found that the capacitance of the grain boundary decreases with the bias. These observations are consistent with what is expected from the thermionic emission mechanism for a current across a potential barrier. Hence we unambiguously verified that Schottky-type potential barriers are formed at grain boundaries in BZY to serve as extra internal resistors, and are responsible for high grain-boundary resistance observed in polycrystalline BZY ceramics. The barrier heights estimated for BZY5, BZY3, and BZY2 are 0.51, 0.85, and 0.95 V, respectively.