Thermoelectric properties of Mg 2 Si 1-x C x crystals grown by the vertical Bridgman method
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Thermoelectric properties of Mg2Si1-xCx crystals grown by the vertical Bridgman method Takashi Sakuma, Yoshinori Higuchi, Masayasu Akasaka, Tsutomu Iida, and Yoshifumi Takanashi Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan ABSTRACT Multicrystalline Mg2Si crystals were grown using a Brigdman method combined with a die-casting growth technique. Although molten and/or vapor-phase Mg at elevated growth temperatures exhibit high chemical reactivity with the surrounding crucible materials, the use of an alumina crucible with a BN coating allowed single phase crystal growth and Mg2Si of good crystalline quality. Incorporation of carbon into the Mg2Si was observed to form Mg2Si1-xCx. For x=0.01 and 0.03, a lower thermal conductivity and higher figure of merit than those of Mg2Si were observed in the temperature range from room temperature to 773 K. The maximum figure of merit was 0.62 at 773 K for a Mg2Si0.99C0.01 sample.
INTRODUCTION Magnesium silicide (Mg2Si) is a narrow-band-gap semiconductor with an indirect band gap of 0.78 eV. The expected figure of merit for Mg2Si compared with other thermoelectric materials exhibiting the same conversion temperature range, such as PbTe and CoSb3, may be slightly smaller, but it is still believed to be adequate for realizing thermoelectric power generation [6-7]. More important aspects of Mg2Si are that it has been identified as an environmental-friendly material, its constituent elements are abundant in the earth’s crust, and it is non-toxic. Although Mg2Si is the only stoichiometric silicide in the Mg-Si phase diagram, the conditions for growth of high-quality single crystal are not yet well-established. Because of the considerable handling difficulties due to the chemical reaction of Mg at the high temperatures (≈1400 K) required to obtain single crystal Mg2Si of good quality by solution growth, it has been difficult to obtain sufficient data characterizing the growth kinetics and the electrical and optical properties of this material. The high reactivity of molten and/or vapor-phase Mg at elevated growth temperatures causes it to stick to the crucible, leading to increased growth instability and contamination of the grown ingot. We have used the vertical Bridgman growth method to grow non-sticking single-crystal Mg2Si in crucibles made of conventional graphite or alumina, either with or without a boron nitride (BN) coating. Additionally, incorporation of carbon was performed to form Mg2Si1-xCx. For some compositions, the grown crystals of Mg2Si1-xCx resulted in an increase in the thermoelectric properties, such as the Seebeck coefficient and the power factor, and a decrease in the thermal conductivity, compared with the sample with no carbon doping.
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EXPERIMENTAL The vertical Bridgman method was used to grow Mg2Si crystals. The most common approach to making large crystals of inter-metallic compounds is to use a melt growth technique; however, the congruent melting point
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