Effect of Rare-Earth Cation Doping on Enhancement of The Thermoelectric Power of Zinc Oxide
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Effect of Rare-Earth Cation Doping on Enhancement of The Thermoelectric Power of Zinc Oxide Kiyoshi Fuda1,2 and Shigeaki Sugiyama3 Research Institute of Materials & Resources, Akita Univ., Akita 010-8502, JAPAN, 2CREST, Japan Science and Technology Agency, 3Akita pref. Ind. Tech. Center, Akita, 010-1623, JAPAN 1
ABSTRACT We investigated the effects of doping of rare-earth ions (Ce, Pr, Nd, Sm, and Eu) on the thermoelectric properties of ZnO, especially Seebeck coefficient in the temperature range from 100 to 800 °C. The data were scattered more or less depending on the doping species, but it was commonly found that the rare-earth doped samples showed higher Seebeck coefficients with compared to those for Al-doped samples. The highest values were observed for Nd-doped one ranging from -360 to -400µVK-1 in the temperature range observed, the lowest ones for Pr-doped one ranging from -250 to -310 µVK-1, whereas those for Al-doped one ranging from -80 to -140 µVK-1. It should also be pointed out that the power factor at 100 °C for Pr-doped ZnO was twice or more as much as that for Al-doped one. From the results it seemed that rare earth doping is effective for enhancement of thermoelectric power of ZnO. INTRODUCTION Zinc oxide and the related materials have been studied as a candidate for high performance thermoelectric materials. Although it is known that Al-doped ZnO shows fairly high potentiality at high temperatures around 1000 K [1,2], further evolution in property of this system is required for practical purpose. In this concern, rare-earth (RE) cations seems to be interesting due to their unique properties such as magnetism or fairly large ionic radii and high valence as well. Since the zinc ions locate the center of a tetrahedron of oxide ions in ZnO lattice, the substitution by other cations is difficult even for Al3+ ions except at very low concentrations. In the case of larger dopant ions, it would make the other phases such as homologous phases ZnO-In2O3 [3,4], or segregate into two or more phases. It has been known that RE-doping into ZnO matrix often leads to deposit of RE-rich oxide at the grain boundary region, causing a non-linear resistance behavior as in a varistor. In such conditions, modification of the band structure of ZnO would occur and some kind of effect on the thermoelectric properties would also expected. Here we will report the thermoelectric properties of RE-lightly-doped ZnO as well as the microscopic texture of them. EXPERIMENTAL The RE-doped ZnO samples were prepared by a wet sol-gel process: mixed aqueous solution of zinc acetate and rare-earth metal chlorides with the molar ratio of 0.4 % was hydrolyzed at room temperature; the recovered mixed hydroxide was washed and calcined at 500 °C, followed by sintering at 1400 °C for 10 hrs. In the same way ZnO samples containing 0.4 mol % of Ce, Pr, Nd and Eu were prepared.
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The microscopic structure was examined by using scanning electron microscope (SEM; HITACHI S-4500 model) attached with an energy dispersive
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