Thermoelectric properties and microstructure of large-grain Mg 2 Sn doped with Ag
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1166-N03-26
Thermoelectric Properties and Microstructure of Large-Grain Mg2Sn Doped with Ag H. Y. Chen and N. Savvides CSIRO Materials Science and Engineering, Sydney 2070, Australia ABSTRACT Mg2Sn ingots, doped p-type by the addition of 0−1.0 at. % Ag, were prepared by the vertical Bridgman method at growth rates ~ 0.1 mm/min. The crystalline quality and microstructure of ingots were analyzed by x-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The single-phase Mg2Sn ingots consist of highly oriented large grains. Measurements of the Hall coefficient, Seebeck coefficient α, and electrical conductivity σ in the temperature range 80−700 K were conducted to study the dependence on the silver content, and to determine the thermoelectric power factor α2σ which reached a maximum value 2.4×10-3 W m-1 K-2 at 410 K for 1.0 at.% Ag content. INTRODUCTION Thermoelectric materials convert heat energy directly to electricity or electrical energy to refrigeration. Their conversion efficiency is determined by the figure of merit ZT = α2σ/κ where α is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Recent world-wide advances in thermoelectrics [1-3] have built on progress in the 1950s–1980s [4-7]. The commercial viability for thermoelectric power generation depends not only on the efficiency but also on the cost of materials and their toxicity. The compounds Mg2X (X = Si, Ge, Sn) and their solid solutions are promising candidates to satisfy these requirements [8-10] and also have low weight to power ratio. The high vapor pressure and chemical reactivity of Mg, however, has delayed the development of these compounds for thermoelectric applications. Recent work by Zaitsev et al. [11] has shown that ZT ≈ 1.1 can be achieved in Mg2Si1-xSnx solid solutions prepared by direct co-melting. Mechanical alloying [12], solid-state reaction [13], spark plasma sintering [14], and hot pressing [15, 16] have also been used. We have undertaken a detailed investigation of the crystal growth and thermoelectric properties of the Mg2X compounds [17, 18]. In this paper we report on the preparation and microstructure of Mg2Sn crystalline ingots grown from the melt using the Bridgman technique. The ingots were doped with silver (0–1.0 at. % Ag) to obtain p-type material, and their thermoelectric properties were studied as a function of Ag content. EXPERIMENT The investigated Mg2Sn ingots were grown from the melt using a vertical resistive tube furnace. High-purity Mg (4N), Sn (6N), and Ag (3N) were mixed with the desired atom ratio (Table 1), then placed into a high-purity boron nitride crucible (with screwed cover) and sealed in a quartz ampoule under a mixture of Ar and H2 gases at 0.8 MPa. The sealed quartz ampoule
was suspended in the furnace which has a sharp temperature gradient toward the bottom. Initially the ampoule was held at 1093 K for 1 hour, then lowered through the thermal gradient at a fixed velocity of 0.1 mm/min to achieve directional solidification.
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