Thermoelectric Properties of Nb-Doped SrTiO 3 / TiO 2 Multiphase Composite
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1166-N03-05
Thermoelectric Properties of Nb-Doped SrTiO3 / TiO2 Multiphase Composite Kiyoshi Fuda1 Kenji Murakami1, Tomoyoshi Shoji1 and Shigeaki Sugiyama2 1
Department of Applied Chemistry for Environments, Akita Univ., Akita 010-8502, JAPAN, Akita pref. Ind. Tech. Center, Akita, 010-1623, JAPAN [email protected] 2
ABSTRACT In this study, we fabricated and examined a series of multiphase type composites constructed of Nb-doped SrTiO3 / TiO2 fine particles. The composition of the composites and the sintering temperatures were selected in a two-phase region where a perovskite SrTiO3 and a rutile TiO2 phases coexist. The composites obtained here were found to commonly have a mosaic type texture constructed of TiO2 and SrTiO3 fine particles with a typical size of about 500 nm. In some samples we also found additive phases such as Sr6Ti7Nb9O42. The thermal conductivity values measured for the most samples with different contents are ranged between 2 and 5 Wm1 -1 K . The values are apparently lower than the value for single crystal SrTiO3 samples presented in literature. A sample with rather low relative density of about 80% showed a quite low thermal conductivity, about 1 Wm-1K-1. Taking account the other TE data, e.g. Seebeck coefficient and electrical conductivity, we calculated dimensionless figure of merit, ZT, to be at maximum 0.24 at 600°C.
INTRODUCTION Oxide thermoelectric (TE) materials have recently drawn a special interest due to thermal stability as well as environmental friendliness. Layered cobaltites such as NaCo2O4 have been studied as a promising materials group having high performance of p-type TE property. However, it seems that no satisfactory TE property has been found in n-type oxide bulk materials even though Al-doped ZnO and La-doped SrTiO3 have high thermoelectric (TE) responses [1,2]. Most recently, Ohta et al. demonstrated that super-lattice materials stacking Nb-doped SrTiO3 and non-doped SrTiO3 layers could give very large Seebeck coefficients due to two-dimensionally confined electrons [3]. This finding is quite suggestive for materials design of bulk thermoelectric materials. If we construct a bulk oxide with high density of interface between SrTiO3 and another phase, the composite oxide could give a high TE property. In addition, the interface between different phases would also cause a high thermal resistivity due to effective phonon scattering at such interface. The purpose of this study is to show a possibility of synthesis of n-type TE oxide bulk materials having low thermal conductivity and excellent TE properties as well. For this purpose, we fabricated and examined a series of composites containing two or more oxide phases such as TiO2 and Nb-doped SrTiO3.
EXPERIMENTAL The composite specimens were prepared by two successive steps: (1) preparation of precursor oxide by wet processes; and (2) sintering by using a spark plasma sintering (SPS) apparatus. The preparation of the precursor oxide was started with a hydrolysis of Ti-tetraisopropoxide in the presence of NbCl5 in p
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