AbstractMagnesium silicide (Mg2Si) has been regarded as a candidate for advanced thermoelectric materials which is used in the temperature ranging from 500 to 800 K correspond to that of vehicle exhaust emission. Besides, Mg2Si has benefits such as abundance of constituent element of Mg2Si in the earth's crust and it's non-toxicity substances compared with other thermoelectric materials that operate in the conversion temperature range such as PbTe and CoSb3. The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S2ãT/Û, of its constituent thermoelectric material where S is the Seebeck coefficient, ã is the electrical conductivity, Û is the thermal conductivity, and T is the absolute temperature. For thermoelectric device operation, the use of a material with ZT more than unity is needed to realize a conversion efficiency of ∼10 %. The optimization of doping careers in Mg2Si is required in order to realize unity of ZT. In that way, we have grown Mg2Si crystals along with doping elements of Bi and Al using vertical Bridgman method.Mg (99.99 %) and Si (99.99999 %) with a stoichiometric Mg : Si ratio of 67 : 33 were mixed congruently and melt into Mg2Si. Prior to the growth, Bi (99.999 %) powder at the ratio from 0.5 to 3 at % for Mg2Si and the pre-synthesized polycrystalline Mg2Si powder were mixed, and Mg2Si crystals were grown at a rate of 3 mm/h by vertical Bridgman method. Grown samples were characterized by x-ray diffraction (XRD) patterns and electron-prove microanalysis (EPMA), and the results indicated that Mg2Si crystals were reproductively grown due to use of polycrystalline Mg2Si as a source material of growth. Hall carrier concentrations were evaluated at room temperature. The electrical conductivity, the Seebeck coefficient, and the thermal conductivity were estimated in the temperature range from RT to 850 K. The grown crystals exhibited n-type conductivity in undoped and all Bi doped conditions. All the Bi doped crystals showed high electrical conductivity and high carrier concentration compared with that of the undoped crystal. On the other hand, the thermal conductivity was lowered in the proportion of the amount of Bi. Consequently, the thermal conductivity for the crystal that was Bi doped at 3 at % was 0.021 W/cmK at 842K, and its ZT reached 0.99 at 842 K, which is near the unity of ZT that is regarded as a standard of practical use for thermoelectric materials. The solid solubility limit of Bi to Mg2Si was assumed to be around 3 at % from our findings, and thus Al was codoped besides Bi in order further to improve the thermoelectric properties. We will discuss the results, additionally.
Flash spark plasma sintering of magnesium silicide stannide with improved thermoelectric properties
