Influence of Crystal Growth Cooling Conditions on Thermoelectric Properties of Aurivillius Phase Bi-V-O

Aurivillius phase Bi2VO5.5 is known as an oxygen ion conductor. In previous studies on Bi2VO5.5, the Seebeck coefficient of the sintered body was about 10 mVK−1 at 800 K. However, the resistivity was 103 Ω m at 800 K. It seemed that this high resistivity was caused by high grain boundary resistance because of cracks at boundaries. In this study, specimens have been prepared by a melting method, aimed at reducing the boundaries. The influence of crystal growth cooling conditions on the thermoelectric properties of Aurivillius phase Bi-V-O is discussed. The crystal growth cooling conditions investigated were slow cooling with cooling rate of 9 K h−1, furnace cooling, and quenching. The surface and cross-section of the sample were observed by scanning electron microscopy (SEM). The crystalline phase was identified by x-ray diffraction (XRD) analysis. The resistivity was measured by the direct current (DC) two or four terminals method. The Seebeck coefficient was measured by the small temperature difference method. The transgranular resistance and grain boundary resistance were evaluated by the complex impedance method. All samples consisted of layered grains. The grain thickness at cross section decreased with increasing cooling rate. The resistivity of the quenched and slowly cooled specimens was approximately 1000 times lower compared with the furnace cooled specimen and sintered body over the measured temperature range.