Enhancing the thermoelectric power factor of nanostructured ZnCo₂O₄by Bi substitution

Bi_xZnCo_2-xO₄(0 =x= 0.2) nanoparticles with differentxvalues have been prepared by the sol-gel method; the structural, morphological, thermal and thermoelectric properties of the prepared nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo₂O₄lattice till thexvalues of =0.1 and the secondary phase of Bi₂O₃is formed at higherxvalue (x> 0.1). The synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of temperature. The electrical resistivity of the Bi_xZnCo_2-xO₄decreased withxvalue and it fell to 4 × 10^-2O m for the sample withxvalue = 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till thexvalue of 0.1 and decreased for the sample with higher Bi content (x= 0.2) as the resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck coefficient and electrical resistivity, Bi_0.1ZnCo_1.9O₄shows the high-power factor (a²s_{550 K}) of 2.3 µW K^-2m^-1and figure of merit of 9.5 × 10^-4at 668 K respectively, compared with other samples. The experimental results reveal that Bi substitution at the Co site is a promising approach to improve the thermoelectric properties of ZnCo₂O₄.