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Masashi Mikami and Ryoji Funahashia) National Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda, Osaka 563-8577, Japan; and CREST, Japan Science and Technology Agency, Ikeda, Osaka 563-8577, Japan

Daniel Chateigner CRISMAT-ENSICAEN Laboratory, UMR CNRS 6508, 14050 Caen Cedex, France (Received 12 October 2004; accepted 19 January 2005)

Bi–Ca–Co–O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi1.8Ca2Co2Ox and Bi2.5Ca2.5Co2Ox, revealed the latter is preferentially formed at high temperature (850 °C). The increase in sintering time was shown by growth of large platelike grains (up to 50 ␮m in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi2.5Ca2.5Co2Ox phase were observed by x-ray diffraction, thermogravimetry– differential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified. I. INTRODUCTION

Thermoelectric (TE) power generation is expected to provide a new energy source in the next few decades. Although intermetallic compounds, such as Bi2Te3 or PbTe, are actually used efficiently for practical applications, their technological scope is limited to low temperatures because of their low chemical stability at high temperatures. In contrast, new misfit-layered cobaltites, such as NaCo2O4,1 Ca3Co4O9,2–4 or Bi2Sr2Co2Ox,5 have attracted much attention in the last seven years due to their promising TE properties and high melting or decomposition temperatures. These systems have been deeply studied in terms that Seebeck coefficient (S), electrical resistivity (␳), and thermal conductivity (␬) were almost independent of one another due to deviation of the conventional band picture, indicating that each parameter can be independently controlled for improvement of TE properties. In addition to these well-known cobaltites, the high potential of the Bi–Ca–Co–O system6,7 for practical applications has recently been highlighted. Maignan et al.6 reported sintered specimens with large S values

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0131 1002

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 4, Apr 2005 Downloaded: 26 Jun 2014

of ∼140 ␮V/K, low electrical resistivity (␳ values of 40–60 m⍀cm), and small ␬ of ∼1 W/mK at 300 K. Itahara et al.7 investigated texturing of bulk materials and succeeded in reducing ␳ values to 20 m⍀cm, thus leading to figure of merit values close to 0.13 at 1060 K. In this study, the Bi–Ca–Co–O system was inves