[22a-13P-1~11] 9.4 Thermoelectric conversion

10:15 AM - 10:30 AM JST(1:15 AM - 1:30 AM UTC)Presentation by Applicant for JSAP Young Scientists Presentation AwardEnglish Presentation

[22a-13P-6] Difficulty of Bulk Ceramic Fabrication of Ba_1/3CoO₂ 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.)

Keywords:

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.