講演情報
[22a-13P-6]Difficulty of Bulk Ceramic Fabrication of Ba1/3CoO2 Showing High Thermoelectric ZT
〇(D)Kungwan Kang1, Diwen Chen1, Masato Imaizumi2, Akitoshi Nakano2, Chuchu Yang3, Yuqiao Zhang4, Yusaku Magari5, Takashi Endo5, Bin Feng3, Yasutaka Matsuo5, Yuichi Ikuhara3, Ichiro Terasaki2, Hiromichi Ohta5 (1.IST-Hokkaido Univ., 2.Nagoya Univ., 3.U. Tokyo, 4.Jiangsu U., 5.RIES-Hokkaido U.)
キーワード：
Thermaelectric material、Thermal stability、Free standing film
Recently, our research group demonstrated that Ba1/3CoO2 epitaxial films that were fabricated by ion exchange treatment of Na3/4CoO2 epitaxial films exhibited the highest thermoelectric ZT (0.55) among oxides at 600 °C in air [1,2]. Although the ZT of Ba1/3CoO2 epitaxial films is comparable to the practical p-type thermoelectric materials such as PbTe and SiGe, epitaxial films and extremely thin single crystals of Ba1/3CoO2 are not practical at all. Thus, a realization of bulk ceramics of Ba1/3CoO2 showing high ZT is crucial topic. However, in 2015, a research group from Shanghai Institute of Ceramics reported the ZT of bulk ceramic BaxCoO2 (x = 0.19) was only 0.21 at 800 K [3]. They fabricated the ceramics by SPS of BaxCoO2 powder at 400 °C under a pressure of 300 MPa. In order to find a way to fabricate bulk ceramics of Ba1/3CoO2 showing high ZT, in this study we studied (1) anisotropy in the thermoelectric properties and (2) change in the crystalline phase and thermoelectric properties upon high-temperature heating (>600 °C). We fabricated freestanding Ba1/3CoO2 single crystalline films [4] with two different crystallographic orientations and measured the anisotropy in the thermoelectric properties at room temperature (Table). The ZT along with the ab-plane of Ba1/3CoO2 is 0.047, higher than that in the cross-plane direction, revealing large anisotropy in the thermoelectric properties. We also found that the in-plane atomic arrangement of Ba ions changed from hexagonal-orthorhombic mixture to orthorhombic when the Ba1/3CoO2 epitaxial films were heated above 625 °C. Simultaneously, the electrical conductivity became worse (Figure) similar to that of Ca1/3CoO2 reported by Sugiura et al.[5] These results suggest that high crystallographic orientation and low sintering temperature to prevent the phase transition are crucial to realizing bulk ceramic Ba1/3CoO2 showing high thermoelectric ZT.