Mg ₂ Si _1-x Sn _x systems, an ecologically friendly semiconductor, are the perspective material for thermoelectric generators at temperatures range from 500 to 800K. The single phase of this system at the compositions range of 0.4 < x < 0.6 has not been reported until now. The single phase of Mg ₂ Si _0.5 Sn _0.5 has been successfully obtained by a Liquid-Solid reaction method and Hot-pressing method. The minimum value of thermal conductivity was identified at around x=0.5. The high thermoelectric performance can be attained by the controlling of carrier concentration for Mg ₂ Si _0.5 Sn _0.5 . In this present work, the thermoelectric properties for the single-phase of Sb-doped Mg ₂ Si _0.5 Sn _0.5 were investigated. Seebeck coefficient alpha, electrical resistivity p and thermal conductivity k were measured from room temperature to 850K. The sign of alpha showed negative for all samples and has been n-type conduction. The temperature dependency of alpha for non-and 5000ppmSb-doped samples increased up to the maximum value of -440muVK ^-1 at 440K and -328muVK ^-1 at 578K, respectively. The alpha of 7500ppmSb sample were increased linearly with temperature. For non- and 5000ppmSb-sample, the temperature dependency of the p showed semiconducting properties. On the other hand, the sample of 7500ppmSb or more showed metallic behavior. The difference of this behavior is result of the conduction mechanism changed by the increase of carrier concentration. The carrier component of thermal conductivity was increased, while the phonon component of thermal conductivity was decreased slightly with carrier concentration. The dimensionless figure of merit was showed markedly enhanced the maximum value of ZT=1.2 for Sb doped 7500ppm at 620K

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