Solid-State Synthesis of Magnesium Base Thermoelectric Alloys via Bulk Mechanical Alloying
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0980-II04-02
Solid-State Synthesis of Magnesium Base Thermoelectric Alloys via Bulk Mechanical Alloying Tatsuhiko Aizawa1,2 and Renbo Song3 1 University of Toronto, Toronto, M5S 3E4, Canada 2 AsiaSEED Institute, Tokyo, 104-0033, Japan 3 University of Science and Technology Beijing, Beijing, 100085, China, People's Republic of
ABSTRACT Magnesium binary and ternary alloy systems have been popular as a thermoelectric lightweight alloy to be working in the medium temperature range. The solid-state reactivity via the bulk mechanical alloying is applied to yield these types of alloys. The diffusion-controlled solidstate synthesis is first stated as a process to yield a binary compound, Mg2X (X = Ge, Sn and Pb). This processing is further applied to directly synthesize several ternary thermoelectric alloys Mg2Si1-xGex, Mg2Si1-ySny and Mg2Sn1-zPbz for 0.0 < x, y, z < 1.0 and to evaluate their thermoelectric properties. Hot pressing is used to make full-dense billets and samples for thermoelectric measurement. The effect of germanium and tin contents on their Seebeck coefficient and band-gap is investigated to describe the p-n transition behavior and to understand the change of electric structure with solid solution formation. INTRODUCTION Pseudo binary alloy systems among Mg2Si, Mg2Ge, Mg2Sn and Mg2Pb have been explored to improve the absolute figure-of-merit ZT and to reduce the initial cost for fabrication of thermoelectric materials working in the middle range of temperature [1-3]. Difficulty in materials processing and manufacturing of these alloys becomes a main issue to be solved toward further research and development: high vapor pressure of magnesium, chemical reactivity of Mg, Si or Ge with crucible and vial and various contaminations in processing [4-5]. Solid state synthesis via bulk mechanical alloying (BMA) succeeded in fabrication of various intermetallic compounds and alloys in bulk [6-8]. This solid state synthesis is characterized by mechanically induced synthesis; nucleation-controlled reaction to synthesize Mg2Si from elemental powder mixture of Mg and Si and diffusion-controlled reaction to synthesize Mg2X for X = Ge, Sn and Pb [9]. This process is successfully applied to synthesize magnesium base pseudo binary alloys [10-12]. In the present paper, the fundamental characteristics of this solid state synthesis, is investigated by using the magnesium base binary and ternary alloy systems. The solid-state reactivity via BMA is discussed by using Mg-Sn system. Mg-Sn-Pb system is employed that the lead works as an additive to improve the Seebeck coefficient without loss of electrical conductivity. Both Mg-Si-Ge and Mg-Si-Sn systems are also employed to investigate the effect of solid solution formation on the electrical property. As a common feature to these two systems, p-n transition behavior is also investigated to discuss the steep transition at the specified content of germanium and tin in these systems.
EXPERIMENT Elemental powders of Mg (99.9 % in purity, 100 µm in diameter), Si (99.99 % in purity,
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