Dramatically Enhanced Thermoelectric Properties of Ca 3 Co 4 O y by Large Amount of RE Substitution
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Dramatically Enhanced Thermoelectric Properties of Ca3Co4Oy by Large Amount of RE Substitution Y. Sugiura1, S. Horii1, T. Kumagai1, T. Okamoto1, K. Otzschi1, J. Shimoyama1,2 and K. Kishio1 1 Department of Superconductivity, University of Tokyo, Tokyo 113-8656, Japan 2 PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
ABSTRACT We report the synthesis and thermoelectric properties of [(Ca1-xREx)2CoO3-δ]0.62CoO2 compounds (RE : rare earth element, RE = Y, Sm, Eu, Tb, Dy, Ho and Lu). From optimization of synthesis conditions, all the chosen RE in this study were able to be substituted for the Ca sites in the block layers up to x = 0.25. In the case of RE = Tb, at 1150 K, only κ was systematically decreased with x, while both Seebeck coefficient (S) and resistivity (ρ) were almost constant for the charge of x. Consequently, the dimensionless figure of merit (ZT) was increased with x and reached 0.2 at 1150 K for x = 0.25. Moreover, interestingly, it was found that S and ρ at 300 K were enhanced with ionic radius of the doped trivalent RE ions in spite of the doping of equivalent RE ions. This is probably because local structures of oxygen sites around Co ion in the CoO2 layers largely depend on kinds of the doped RE. INTRODUCTION Thermoelectric (TE) power generation using waste heat is one of the hopeful technologies to solve ecological issues, including the green house effect, because the efficiency of total power generation using this system is expected to be higher. In the case of high temperature heat waste such as exhaust gas from automobiles and smelting furnaces in steel plants and garbage incineration facilities, oxides are preferable materials, because of the high decomposition temperature and high chemical stability in air. Recently, a layered cobalt oxide, NaxCoO2, showed high thermoelectric properties comparable to that of a practical TE material in a Bi2Te3 system, in single crystalline samples [1]. This report has triggered research and development on layered cobaltites, and [Bi0.87SrO2]2[CoO2]1.82 [2,3,4] and [Ca2CoO3-δ]0.62CoO2 (~ Ca3Co4O9; Ca349) [5, 6, 7] with misfitted crystal structure were discovered and have also been used as TE materials at high temperatures. Especially for Ca349, single crystals of the co-doped compound by Bi and Sr showed a practical dimensionless figure of merit (ZT = S2ρ-1κ-1T, Z : figure of merit, S : Seebeck coefficient, ρ : resistivity, κ : thermal conductivity ) over 1 above 1000 K [8]. The Ca349 phase is stable up to approximately 1000°C in air, however, it showed large oxygen nonstoichiometry (0 < δ
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