Crystal Growth of Mg2Si by the Vertical Bridgman Method and the Doping Effect of Bi and Al on Thermoelectric Characteris
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1044-U06-13
Crystal Growth of Mg2Si by the Vertical Bridgman Method and the Doping Effect of Bi and Al on Thermoelectric Characteristics Masataka Fukano, Tsutomu Iida, Kenichiro Makino, Masayasu Akasaka, Yohei Oguni, and Yoshifumi Takanashi Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan ABSTRACT Mg2Si bulk crystals were grown using the vertical Bridgman melt growth method. As n-type dopants, Bi and Al were incorporated during the growth. Thermoelectric properties, such as Seebeck coefficient, electrical conductivity and thermal conductivity, were measured as a function of temperature, up to 850 K. The maximum value of the dimensionless figure of merit, ZT, was 1.08 with a 3 at% Bi doped specimen. For a Bi+Al doped sample, the influence of Al co-doping resulted in slight improvements in power factor and thermal conductivity at elevated temperatures. However, no remarkable increase in the figure of merit was observed.
INTRODUCTION As a possible solution to the Greenhouse effect, reduction in CO2 emissions is believed to require a drastic curtailment of the amount of material consumed in fossil-fueled power generators. Solid-state thermal-to-electric energy conversion from waste heat is effective in improving the potential conversion efficiency of caloric power generation, such as conventional heat engines and high temperature furnaces.1 Magnesium silicide (Mg2Si), a face-centered-cubic CaF2 type structure, has been identified as a thermal-to-electric energy-conversion material at operating temperatures ranging from 500 to 800 K.2-3 Mg2Si exhibits fascinating aspects, such as the abundance of its constituent elements in the earth’s crust and the non-toxicity of both Mg2Si and its processing by-products. This suggests safe handling and device operation for practical use, and results in freedom from care about increasing regulations on hazardous substances. 4-6 In order to realize a thermal-to-electric energy-conversion power generator, a well-defined n-type dopant material for Mg2Si is needed to realize efficient and stable operation in the temperature range of ~800 K. To date, the incorporation behavior of the donor impurity, bismuth (Bi), acting as a donor in bulk Mg2Si, has not been examined in a systematic manner, because of the difficulties in forming good bulk crystals of Mg2Si. In Mg2Si, aluminum (Al) is also known as donor impurity. In this report, we describe the results of thermoelectric characteristic measurements on melt-grown Mg2Si with the incorporation of Bi and/or Al during crystal growth.
EXPERIMENT Mg2Si appears as the only stoichiometric silicide in the Mg-Si phase diagram.7 The mixture of Mg (99.95 %) and Si(99.9999 %) was pre-synthesized in a stoichiometric Mg:Si ratio (at.%) of 67:33 to obtain polycrystalline Mg2Si. The congruent melting point of Mg2Si at 1358 K is only 5 K below the boiling point of pure Mg at 1363 K. This proximity makes
the growth difficult, because of the occurrence of significant evaporation and
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