We have investigated the ball milling effects for thermoelectric Bi1− x Sr x CuSeO materials. The characteristic rotation speed R for the ball milling exists, at which the crystallite size of the starting powder suddenly decreased. The grain size in the bulks sintered using ball-milled powders also decreased and thermoelectric properties were enhanced mainly by the induced carriers, rather than by the reduction in thermal conductivity. The ball milling effects of carrier doping on thermoelectricity are discussed.

We have fabricated the BiCuSeO bulks using raw materials with Bi or Se deficiencies in the nominal composition and investigated crystallographic, chemical compositional, and thermoelectric properties. Owing to the Bi or Se deficiencies in the starting composition, excessive elements and related compounds were deposited as impurity phases and the matrix phase is nearly the stoichiometric BiCuSeO phase. The electrical resistivity, q(T), of the bulks decreases and thermoelectric power, S(T), also decreases with increasing the contents of Bi or Se deficiencies in the starting composition in spite of the stoichiometric matrix phase. These results strongly suggest that, from the X-ray photoelectron spectroscopy measurements, the actual oxidation states of Bi and Cu deviate from the formal valences of stoichiometric Bi 3þ Cu 1þ Se 2À O 2À . The introduction of a small amount of Bi and Se vacancies is also suggested. As a result, mobile carriers are introduced and the q and S values are changed. The maximum thermoelectric dimensionless figure of merit of ZT ¼ 0.60 was achieved at 773 K for the Bi 1-x CuSeO samples (x ¼ 0.025 and 0.05) in the starting composition. These results are in clear contrast with the reported results for the Cu deficiency bulks. Using these results, we propose charge valence equations and the origin of the carriers in the present BiCuSeO bulks and discuss the influence of created carriers on the thermoelectric properties.Published by AIP Publishing. https://doi.

Li y CoO2 has a similar layered structure to Na y CoO2, which is a typical p-type oxide thermoelectric material, and the average Co valence of 3 + y is controlled by the Li content y. We investigated the thermoelectric properties of LiCo1− x M x O2 samples (M = Cu, Mg, Ni, Zn) for the first time at high temperatures, in which Co3+ was substituted by the divalent M2+ ions, and the average Co valence of 3 + x can be controlled similarly to the Li content y in Li y CoO2. The substitution of the M2+ ions for the Co site was found to show thermoelectric properties similar to those of Li y CoO2 with the same average Co valence. The Mg-doped sample showed the highest thermoelectric performance at high temperatures in this study; the thermoelectric power factor P is 2.38 × 10−4 W m−1 K−2 at 1173 K and the dimensionless figure of merit ZT is 0.024 at 876 K. The thermoelectric potential of LiCo1− x M x O2 is discussed and compared with those of Li y CoO2 and Na y CO2 systems.

We have grown Bi 0.9 Sr 0.1 CuSeO epitaxial thin films on MgO and SrTiO 3 (STO) single-crystal substrates by pulsed laser deposition (PLD) under various growth conditions, and investigated the crystal orientation, crystallinity, chemical composition, and thermoelectric properties of the films. The optimization of the growth conditions was realized in the film grown on MgO at the temperature T s = 573 K and Ar pressure P Ar = 0.01 Torr in this study, in which there was no misalignment apart from the c-axis and no impurity phase. It was clearly found that the higher crystal orientation of the epitaxial film grown at a higher temperature under a lower Ar pressure mainly enhanced the thermoelectric power factor P (= S 2 /ρ), where S is the Seebeck coefficient and ρ is the electrical resistivity. However, the thermoelectric properties of the films were lower than those of polycrystalline bulk because of lattice distortion from lattice mismatch, a low crystallinity caused by a lower T s , and Bi and Cu deficiencies in the films.

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