Independent Control of Electrical Conductivity and Thermopower by Chemically Manipulating Ni2+/Ni0 and Ti4+/Ti3+ Redox Pairs in Ni-Doped SrTiO3

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Thermoelectric materials efficiently convert thermal energy (temperature gradient) into electrical energy via the Seebeck and Peltier effects in solids. The energy conversion efficienty of a material is evaluated by the dimensionless figure-of-merit defined as ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity of the material. Since both S and σ are a function of the carrier concentration in the opposite way, it is usually impossible to control S and σ independently, leading to a difficulty to find excellent thermoelectric materials.Strontium titanate SrTiO3 (STO) has been known to show a fairly good thermoelectric performance as an oxide upon n-type doping under reducing conditions. Our previous studies on Cu- and Ni-containing oxides exsolving metallic Cu and Ni nanoparticles, respectively, under strongly reducing conditions [1,2] inspired us to explore a possibility of fine distributions of highly conductive metallic nanoparticles in bulk oxides, which enables formation of nanosized conduction paths and phonon scattering centers at the same time. Here we report exsolution of Ni nanoparticles from Ni-doped STO and an improved thermoelectric performance of the oxide via independently controlled carrier mobility and carrier concentration by chemical pathways such as hydrogen reduction.We synthesized Sr1-x (Ti0.8Nb0.2)1-y Ni y O3 (x,y = 0, 0.05, 0.10) by solid state reaction of starting materials of SrCO3, TiO2, Nb2O5 and NiO at the prescribed molar ratios. After sinteing in air at 1420 °C, the samples were reduced under 20% H2/N2 or 100% H2 at 1350 °C. Whereas the electrical conductivity σ of the samples showed metallic behavoir and significantly varied from 200 to 1900 S/cm at room temperature, the Seebeck coefficient S at the same temperature was within a narrow range of –50 to –80 μV/K, being unexpected from a fundamental theoretical relation between σ and S as a function of the carrier concentration.The molar fractions of Ni0, f(Ni0), out of total Ni amounts in the samples were obtained by the XPS spectra from the ratios of Ni2+/3+ and Ni0 2p3/2 peak areas. The molar fractions of Ti3+, f(Ti3+), out of total Ti were determined from the changes in the lattice parameters taking the difference in the ionic radii of Ti3+ and Ti4+ into account. An striking result was that f(Ni0) widely varies from 0.5 to 55% depending on the reducing conditions such as the H2 concentrations in the reaction atmosphere, while f(Ti3+) stays within 16 – 18% for all the samples. This analysis clearly explains the discrepant behaviour of σ and S, the former being tuned by the amount of the metallic Ni particles forming conduction paths, while the latter being governed by the carrier concentrations in the STO matrix. A highest ZTof 0.37 at 800 °C was achievced by the sample with the largest amount of Ni0. These results strongly suggest that the different chemical tendency of the Ni2+/Ni0 and Ti4+/Ti3+ redox pairs in Ni-doped SrTiO3 against reduction by hydrogen can be exploited to manipulate σ and S independently. Acknowledgment This work was financially supported by JSPS KAKENHI JP22H01779.