講演情報
[7p-N202-6]Phase decomposition of thermoelectric Ba1/3CoO2 films at high temperatures
〇(DC)Kungwan Kang1, Yuqiao Zhang2, Chuchu Yang3, Bin Feng3, Yuichi Ikuhara3, Ahrong Jeong4, Takashi Endo4, Yasutaka Matsuo4, Hiromichi Ohta4 (1.IST-Hokkaido U., 2.Jiangsu U., 3.U. Tokyo., 4.RIES-Hokkaido U)
キーワード：
Oxide thermoelectric material、Decomposition、Percolation
R-SPE-grown Ba1/3CoO2 (BCO) epitaxial films exhibit a relatively high thermoelectric figure of merit (ZT) of 0.55 at 600 °C in air, which is comparable to that of commercially available p-type PbTe. For high-power thermoelectric energy conversion, bulk thermoelectric materials with low cost fabrication such as ceramics would be required. However, the thermoelectric ZT of reported BCO ceramics is very low compared to the epitaxial films. Here, we systematically investigated the phase decomposition behavior of BCO epitaxial films at high temperatures. We analyzed crystalline phases after thermal annealings at several temperatures, as well as measurement of thermoelectric properties at room temperature. After annealing above 650 °C, BCO epitaxial films decomposed into BaCoO3−x (x = 0 and 0.4) epitaxial films and Co3O4 non-oriented polycrystalline films. Note that BaCoO2.6 (x = 0.4) is a semiconductor and BaCoO3 (x = 0) is an insulator. Figure a shows the annealing temperature dependence of electrical conductivity (RT) after annealing. After annealing below 650 °C, the conductivity does not change, while above 650 °C, the conductivity exponentially decreases with annealing temperature. To clarify the origin of the degradation of electrical conductivity, we calculated the volume fraction of semiconducting BaCoO2.6 in the films composed of BaCoO2.6, BaCoO3, and Co3O4, by analyzing in-plane X-ray Bragg diffraction peaks. The volume fraction of semiconducting BaCoO2.6 decreased gradually with increasing annealing temperature. Figure b shows the electrical conductivity and thermopower of the annealed films as a function of BaCoO2.6 volume fraction. Although data at each endpoint are unavailable, the electrical conductivity and thermopower appear to follow standard percolation theory. These results indicate that high temperature sintering of BCO ceramics result in the decomposition into BaCoO2.6, BaCoO3, and Co3O4, and degradation of thermoelectric properties.