Thermoelectric Properties of Bi-substituted Ca 3 Co 4 O 9 Single Crystal
- PDF / 214,518 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 52 Downloads / 205 Views
S3.2.1
Thermoelectric Properties of Bi-substituted Ca3Co4O9 Single Crystal M. Mikami1, K. Chong2, and R. Funahashi1,3 1 CREST, Japan Science and Technology Agency, Ikeda, Osaka 563-8577, Japan 2 Osaka Electric-Communication Univ., Neyagawa, Osaka 572-0833, Japan 3 National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka 563-8577, Japan
ABSTRACT We have grown single crystals of Bi-substituted Ca3Co4O9 by a solution method. The cationic ratio (Ca, Bi)/Co of the grown crystals measured by an energy dispersive X-ray spectrometer tended to exceed that of the starting ratio (Ca, Bi)/Co=3/4. For instance, the average cationic composition of the grown crystals was Ca:Bi:Co=3.3:0.3:4, while that of the starting material was Ca:Bi:Co=2.7:0.3:4. So, the crystallographic structure of the obtained crystals may correspond to the Ca2Co2O5 phase rather than the Ca3Co4O9 phase. Thermoelectric properties in the direction of ab-axis were measured at various temperatures. Seebeck coefficient (S) of Ca3.3Bi0.3Co4O9+δ is positive and increases with increasing temperature from 130 to 200 µV/K in a temperature region of 300-973 K. The electrical resistivity (ρ) of the sample is about 1.5 mΩcm at whole temperature region of 300-973 K. This value is lower than that of non-substituted Ca3Co4O9. The thermoelectric power factor (S2/ρ) is improved by the Bi-substitution, resulting from the reduction of resistivity. INTRODUCTION The recent discovery of large thermopower coexisting with low electric resistivity in NaxCoO2 and Ca3Co4O9 has made layered-structural cobalt oxides recognized as promising candidates for thermoelectric materials [1-4]. The single crystal of these cobalt oxides exhibits good thermoelectric performance along the ab-axis, ZT ≈ 1 at 1000 K [2, 4]. In these two-dimensional cobalt-containing oxides, it is proposed that the high thermoelectric performance is caused by unique structural and transport properties, such as a misfitted structure and a strong electron-electron correlation [1, 5]. Therefore, the improvement of thermoelectric properties of these cobalt oxides is rather difficult because of its complicated electronic state,
S3.2.2
which could not be well described by the simple band theory. However, some attempts to improve the thermoelectric properties by partial element substitution have been reported [5-9]. For Ca3Co4O9 in particular, it is reported that the Bi-substitution for a part of Ca site enhances its thermoelectric properties in sintered polycrystalline material [8]. However, it is difficult to elucidate the cause of the improvement of thermoelectric figure-of-merit by polycrystalline sample because of its microstructure such as grain boundaries and grain alignment. Therefore, in order to investigate accurately the origin of the superior thermoelectric properties in Bi-substituted Ca3Co4O9, large single crystals are indispensable. In this study, we grew single crystals of Ca3-xBixCo4O9+δ (x=0.0 - 0.5) by a solution method. Thermoelectric properties in the ab-plane were measured at v
Data Loading...
