Synthesis and thermoelectric properties of antifluorite materials
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1044-U11-03
Synthesis and thermoelectric properties of antifluorite materials Xiunu Sophie Lin, Dongli Wang, Matthew Beekman, and George Nolas Department of Physics, University of South Florida, 4202 East Fowler Ave, Tampa, FL, 336205700 ABSTRACT The compounds Mg2X (where X=Si, Ge, Sn) crystallize in the antifluorite structure. They possess properties that are similar to that of the group IV elemental semiconductors thus they have long been recognized as good candidates for thermoelectric applications. In addition, their properties can be readily tuned by doping or alloying. However, optimal performance of these materials requires continued investigation. We present low-temperature transport properties measurements of Sb doped Mg2X. Structure-property relationships are reported while their thermoelectric properties are investigated systematically in order to elucidate their potential as thermoelectric materials. INTRODUCTION The search for thermoelectric materials with enhanced thermoelectric figures of merit continues to be a critical pursuit in energy conversion research [1, 2]. The efficiency of a material for thermoelectric application is determined by the figure of merit, which is defined as ZT=S2T/κρ. Here S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. Generally, κ is composed of two components: κe, the thermal conductivity due to charge carriers, and κL, the thermal conductivity due to lattice vibration. A traditional approach in improving the efficiency of a bulk material is to optimize the power factor, S2/ρ, by fine tuning the carrier density of a material through doping, and at the same time decrease κ by reducing κL through alloying [3, 4]. This approach works very well when κL is much larger than κe. So far, several ideas have proven effective in optimizing ZT, such as the “phonon glass and electron crystal” approach which aims to lower κL through “rattling” in materials with a “cage like” structure, or the low-dimensional materials approach, which aims to enhance the power factor through quantum-confinement [10]. For the Mg2X system, ZT optimization has been achieved through alloying [5].
One class of materials that has long been recognized as a promising candidate for thermoelectric applications are the intermetallic semiconductor compounds Mg2X (X=Si, Ge, Sn) [5-10] crystallizing in the antifluorite structure. Nevertheless, aspects of their thermoelectric properties remain to be improved if they are to be used in thermoelectric applications. Recent studies on the Mg2X system have shown ZT enhancement through isoelectronic substitution [11]. In this report, we present a systematic study on the effect of non-isoelectronic substitution on the Mg2Si and Mg2Ge system. By substituting trivalent Sb for tetravalent Si, we investigate the competing effects of electron doping and vacancy formation on the low temperature transport properties. Unlike the previous studies in which only a small concentration of Sb or Bi doping is investigated [12-16], our study
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