Effect of bismuth doping on thermoelectric properties of Sr2TiCoO6

Oxides are considered as one of the most promising materials for thermoelectric applications because of their environmental-friendly nature, lower processing cost and better stability at high temperature compared to other intermetallic compounds and alloys. Recently SrTiO3 based perovskites and double perovskites have shown promising thermoelectric properties especially at high temperature. In this report, the prospect of Sr2TiCoO6 based double perovskites modified by aliovalent substitution of Bi+3 in Sr-site has been evaluated for high temperature thermoelectric applications. BixSr2-xTiCoO6 (0 ≤ x ≤ 0.2) ceramics have been synthesized by solid state reaction method. Dense ceramic samples have been characterized to inspect their crystal structure and microstructure using X-Ray Diffraction and Scanning Electron Microscopy, respectively. The Seebeck coefficient (S) and electrical conductivity (s) variations with temperature for all the compositions have been measured in the temperature range from 323 K to 1073 K. The electrical conductivity has been increased to nearly double in magnitude with bismuth substitution in Sr2TiCoO6 as a result of more electronic defects created by aliovalent doping in these oxides. The maximum conductivity (∼ 3917 S/m) has been obtained for the composition Bi0.2Sr1.8TiCoO6 at 760 K resulting enhanced thermoelectric power factor in this composition. With increase in bismuth content, the semiconductor to metal transition (TMS) was found to be shifted towards higher temperature in these double perovskites. Furthermore, the charge transport mechanism in these compositions has been elucidated with respect to variable range hopping and small polaron hopping conduction models.