Enhancing the thermoelectric power factor of nanostructured ZnCo 2 O 4 by Bi substitution.

Bi x ZnCo 2- x O 4 (0 ≤ x ≤ 0.2) nanoparticles with different x values 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 ZnCo 2 O 4 lattice till the x values of ≤0.1 and the secondary phase of Bi 2 O 3 is formed at higher x value ( 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 x ZnCo 2- x O 4 decreased with x value and it fell to 4 × 10 -2 Ω m for the sample with x value ≤ 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till the x value 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 0.1 ZnCo 1.9 O 4 shows the high-power factor ( α 2 σ 550 K ) of 2.3 μW K -2 m -1 and figure of merit of 9.5 × 10 -4 at 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 ZnCo 2 O 4 .