High-temperature thermoelectric properties and X-ray photoemission spectra of layered Co-based oxides Bi_2-xAg_xSr₂Co ₂O_8-δ

Layered Co-based ceramics with a nominal composition of Bi _2-xAg_xSr₂Co₂O_8-δ(x= 0.0, 0.4, 0.8, denoted by BAC-222) were prepared using conventional solid state reaction method. X-ray photoemission spectroscopy(XPS) was used to investigate their electronic structures. The cobalt ions are existed in mixed states of Co³⁺ and Co⁴⁺. It is found that the n_Co4+/ n_Co3+ ratio increases with the Ag doping content. The O1s photoemission spectra show that there are lattice oxygen and chemical absorbed oxygen in all the samples. Meanwhile, the conductivity and Seebeck coefficient of Bi_2-xAg_xSr₂Co₂O _8-δ were studied. The substitution of Bi³⁺ by Ag⁺ results in dramatically increase of the electrical conductivity while the Seebeck coefficient is kept almost unchanged. This abnormal phenomenon means that Ag ⁺ ions play an important role in the electrical transport property of BAC-222. The power factor of Bi_1.6Ag _0.4Sr₂Co₂O_8-δ reaches 1.23×10^-4 W·m^-1·K^-2 at 1 123 K, which suggests that Bi_2-xAg_xSr₂Co ₂O_8-δ are good potential oxides for thermoelectric application.