Improvement of Bi2Sr2Co1.8Ox thermoelectric properties by laser floating zone texturing

Thermoelectric performances of cobaltite ceramics can be improved raising the electrical conductivity. This can be performed by increasing the size of grains and aligning them along a preferential direction by the Laser Floating Zone (LFZ) method. In this work, Bi2Sr2Co1.8Ox ceramics have been directionally grown at three different speeds, i.e. 15, 30 and 50 mm/h. Their microstructure and thermoelectric properties are significantly influenced by the LFZ growth conditions. In all cases, the microstructure exhibits alternate cobaltite layers with secondary phases and small CoO inclusions, particularly when the growth is performed at high speeds (30 and 50 mm/h). The thermopower is, nevertheless, higher than usual while electrical resistivity values are similar to those obtained by conventional solid state routes, leading to relatively high power factor values. This result is probably linked to the stacking of different BiSrCoO layers and/or to oxygen vacancies.