Enhanced Thermoelectric Performance of Porous SrTiO3 with Exsolved Ni Nanoparticles

Abstract

Nanocomposite structuring is one of the key directions to improve the thermoelectric performance of bulk materials by enabling a significant reduction of the thermal conductivity and a possible enhancement in the power factor at the same time. In this paper, we report a strongly enhanced thermoelectric performance of porous SrTiO3 (STO) doped with Nb and Ni, which subsequently contained Ni nanoparticles formed after an exsolution reaction caused by a reducing post-treatment.Samples with a representative composition of Sr0.95(Ti0.8Nb0.2)0.95Ni0.05O3 (STNNO) were prepared by conventional solid state reaction of SrCO3, TiO2, Nb2O5, and NiO. The mixed raw powders were calcined once at 1273 K and twice at 1623 K in air to achieve the single phase of the STO structure. The resulting powder was mixed with a graphite powder and heated in air to remove graphite, forming loosely sintered bodies with a porous structure. The samples were finally heated at 1693 K for 10 h in air for sintering, and subsequently reduced at 1623 K for 20 h in 0% - 20% H2/N2.While the XRD patterns remained the same as the single phase of the STO structure even after the reducing treatment, SEM/EDS observation of the cross section of the samples revealed discretely dispersed Ni nanoparticles of ca. 50 nm in diameter inside of the sintered bodies. Whereas the electrical conductivity, σ, of the sample reduced in 20% H2 was significantly higher than that reduced in 5% H2, the Seebeck coefficient, S, of the former was just negligibly lower than the latter, being very insensitive to the sintering temperature and reducing atmosphere. However, the lattice thermal conductivity, κ ph, was the lowest for the sample reduced in 20% H2, resulting in ZT = 0.6 at 1073 K as the highest value among the STNNO-based oxides so far reported. These results imply that the reducing treatment caused the electron doping in STO as well as the exsolution of Ni nanoparticles simultaneously, resulting in the increased σ and the suppressed κ ph. The barely noticeable changes in S might be related to the large effective mass of electrons in the Ti 3d orbitals.