Development of high performance P-type sodium cobaltate and N-type strontium titanate thermoelectric materials

Abstract

Oxide thermoelectric materials have been drawing extensive attention as substitutes for conventional thermoelectric materials due to their low cost, nontoxicity, and high stability. Currently, the most promising p-type and n-type oxides are layered cobaltates (including NaxCoO2 and Ca3Co4O9) and donor-doped SrTiO3, respectively. Major enhancement of the thermoelectric properties of these materials relies on the reduction of thermal conductivity and the improvement of electrical conductivity through doping technology, compositing, nanotechnology, etc. We demonstrated the successful fabrication of p-type Na0.8Co1-xFexO2 and (1-x)Na0.77CoO2/xCa3Co4O9 composites by means of spark plasma sintering (SPS) technique. The thermoelectric properties were improved with small amount of Fe doping (x ≤ 0.01). A significant enhancement of Seebeck coefficient was achieved in (1-x)Na0.77CoO2/xCa3Co4O9 composites, approximately 17% higher than that of Ca3Co4O9 at 680 °C. The electrical resistivities of the composites were higher than the theoretical values. The increase in the Seebeck coefficient and the electrical resistivity of the composites is most likely associated with the compressive strain in Ca3Co4O9 grains due to the mismatch of thermal expansion coefficients between Na0.77CoO2 and Ca3Co4O9. Most importantly, the chemical stability of Na0.77CoO2 was improved by adding up to a 30 vol.% fraction of Ca3Co4O9 without deteriorating its thermoelectric performance. A typical thermoelectric generator consists of p-type and n-type semiconductors which form p-n junctions to convert the wasted heat from exhaust gases to useful electricity. Therefore, apart from the p-type thermoelectric oxides, we also investigated the thermoelectric performance of n-type SrTiO3. We examined the solubility of Y and La in Sr1-1.5xMxTiO3 (M = Y, La) and the corresponding thermoelectric properties. After determination of the proper amount of dopants (Y and La), the effects of Sr content on the phase composition, microstructure, and thermoelectric properties of SrxY0.04TiO3 (0.92 ≤ x ≤ 1) and SrxLa0.12TiO3 (0.82 ≤ x ≤ 0.90) were studied. Finally, the effects of Y and Nb co-doping on the thermoelectric properties of stoichiometric or TiO2 excess in the formula Sr0.96Y0.04Ti1+z-xNbxO3 were determined. SrO Ruddlesden-Popper phases that existed in Sr0.96Y0.04TiO3 and Sr0.88La0.12TiO3 could be eliminated by decreasing Sr content and increasing oxygen deficiency, thus achieving improved electrical conductivity. A further decrease in Sr content resulted in the formation of TiO2 Magnéli phases, which blocked the electrical conduction path. The absolute value of the Seebeck coefficient of Sr0.96Y0.04TiO3 with excess Sr was increased due to the decreased carrier concentration, whereas that of Sr0.88La0.12TiO3 with excess Sr was reduced possibly due to the deterioration of band degeneracy. The electrical conductivity and the Seebeck coefficient of both stoichiometric and TiO2 excess Sr0.96Y0.04Ti1-xNbxO3 increased with increasing Nb content. For a certain doping content, excess TiO2 improved the electrical conductivity, but showed no obvious influence on the Seebeck coefficient possibly due to its minor amount.