Hole-trapping effect on thermoelectric power of mixed ionic electronic conductor BaTiO3

Abstract

The effect of hole-trapping on the thermopower of a mixed ionic electronic conductor, e.g., BaTiO₃, is analyzed in terms of irreversible thermodynamics by taking trapped holes as a fourth kind of electronic charge carrier in addition to free electrons, free holes and mobile oxide ions. It is found that the effect manifests itself in two ways: thermostatically in the ionic thermopower via the thermodynamic factor and dynamically in the electronic thermopower via the electrical conductivity contribution of the trapped holes. The thermopowers of both 99.995% pure, undoped and 1.8 m/o Al-doped BaTiO₃, that were measured against oxygen activity in the range of −18 < log a_O2 ≤ 0 at elevated temperatures of 800° to 1100 °C [H.-I. Yoo and C. R. Song, J. Electroceram., 2001, 6, 61, ], are reanalyzed by taking into account the hole-trapping for the doped case. It is found that while the reduced heats-of-transport of free electrons and holes are, respectively, close to their thermal energy k_BT (k_B being the Boltzmann constant), that of trapped holes is close to their migration energy that is essentially the same as the trapping energy onto the acceptors doped, 1.04 eV.