Thermoelectric Properties of Phase Separated Composite of Ln-Doped SrTiO 3 and TiO 2 Micro Crystals
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1044-U06-07
Thermoelectric Properties of Phase Separated Composite of Ln-Doped SrTiO3 and TiO2 Micro Crystals Kiyoshi Fuda1, Kenji Murakami1, and Shigeaki Sugiyama2 1
Mater. Proc. & Appl. Chem. for Environments, Akita University, 1-1, Tegatagakuen-Cho, Akita City, 010-8502, Japan 2
Akita pref. Ind. Tech. Center, Akita City, 010-1623, Japan
ABSTRACT We prepared Sr(Ln)TiO3 / TiO2 (Ln = La, Y, Dy) mixed oxide systems composed of ultra fine particles aiming at good n-type oxide thermoelectric materials. The composites prepared by combination of wet process and SPS-sintering method showed mosaic textures composed of SrTiO3 (abr. as STO) and defective TiO2 fine particles. The samples showed relatively large n-type thermoelectric response, depending on the STO faction. Remarkable depression of thermal conductivity was observed at low temperatures when compared with those of STO single crystals. As a result, relatively high figure of merits were given to the samples. The maximum value of ZT was 0.15 at 800 for a sample containing La.
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INTRODUCTION Development in thermoelectric (TE) energy conversion technology has attracted much attention from a viewpoint of sustainability of global economy and environment. Although a variety of materials have been studied, oxide materials have recently drawn a special interest due to thermal stability as well as environmental friendliness. Layered cobaltites such as NaCo2O4 make a promising group of materials with 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]. Difficulty in developing high-performance TE materials seems to lie in finding low thermal conductivity in such oxides. The purpose of this study is to find a possibility to make an n-type TE oxide bulk material having low thermal conductivity and excellent TE properties as well. For this purpose, we fabricated and examined a series of composites constructed of TiO2 and Ln-doped SrTiO3 fine crystals. EXPERIMENTALS The composites were prepared via two processing steps: (1) precursor oxide preparation by wet processes; (2) sintering by using a spark plasma sintering (SPS) apparatus. The precursor oxide was prepared by starting hydrolysis of Ti-tetra-isopropoxide. The obtained titanium oxide gel containing hydroxide was suspended in mixed cation solution containing rare-earth and Sr ions in a proper proportion. This suspension was further hydrolyzed with NaOH addition to form Sr and Ln hydroxide layers on the titanium oxide. The precipitate was separated by filteration, washed with deionized water repeatedly, and dried in air at 70 °C. This precipitate was further calcined at 1000 °C for 1 hour, giving mixed oxide powder consisted of TiO2 (anatase) and Ln-doped SrTiO3 fine crystals. The specimens for the thermoelectric and the thermal conductivity measurements were prepared by the SPS sintering at 1350 °C under 50 MPa for 5
min. into pellets with di
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