Enhancement of thermoelectric figure of merit by incorporation of large single crystals in Ca 3 Co 4 O 9 bulk materials
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Toyohide Sano and Masaaki Kitawaki Osaka Electro-Communication University, Hatsu-cho, Neyagawa, Osaka 572-8530, Japan (Received 21 January 2003; accepted 11 April 2003)
Having recently succeeded in synthesizing large single crystals of (Ca2CoO3)CoO2 (Co-349) with superior thermoelectric properties using a modified flux method, we have prepared a composite material of Co-349 powder and single crystals and examined its thermoelectric properties. The electrical conductivity of this composite, which contained 20 wt.% single crystals, was higher than that of a sample without the single crystals. While the achievable effect has yet to be fully realized, improved grain alignment and the effect of current bypassing grain boundaries through the large single crystals in the composite are thought to cause the increasing , which consequently results in an enhanced thermoelectric figure of merit of about 0.56 at 973 K in air.
I. INTRODUCTION
Thermoelectric power generation is expected to develop as a valuable energy source in the next few decades. Central to the realization of thermoelectric power generation are thermoelectric materials that not only possess high thermoelectric figure of merit (ZT ⳱ S2T/; S, Seebeck coefficient; T, temperature; , electrical conductivity; and , thermal conductivity) upwards of 1.0, but also high chemical stability at high temperature in air and no content of harmful elements. Good thermoelectric performance at high temperature in air was recently reported for single crystalline whiskers of two such materials: Ca 3 Co 4 O 9 (Co-349) 1 and Bi2Sr2Co2O9 (BC-222).2,3 Though these whiskers are about 1.0–10-mm long, their cross-sectional area has thus far been limited to about 5.0 × 10−4 mm2. Bulk polycrystalline materials are therefore necessary to suppress electrical resistance and are indispensable for power-generation applications. Study of the Co-349 whiskers thus far has shown a two-dimensional electrical property that originates from their crystal structure (Fig. 1).4 The Co-349 phase consists of a layered structure in which electrical-conducting CoO2 layers and insulating Ca2CoO3 rocksalt layers alternate along the c axis. This a)
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J. Mater. Res., Vol. 18, No. 7, Jul 2003 Downloaded: 18 Dec 2014
causes Co-349 bulk materials with no preferred grain alignment to have several times lower than that of the whiskers and, consequently, ZT of the former is lower than the latter.5–7 It seems that is increased by grain alignment, which is to say, the arrangement of all Co-349 grains in the crystallographic ab plane. Moreover, the influence of grain boundaries, which have lower than the intragrain regions, should be reduced. Although hot pressing is indeed effective for improved grain alignment and increased , ZT of hot-pressed materials is 0.3 at most,7 and as such a different approach is required in the preparation of Co-349 bulk materials for the desired application. Here, the preparation and thermoelectric propertie
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