Fabrication of Mg2Si thermal-to-electric energy conversion device using die-casting sysnthesis method

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Magnesium siliside (Mg2Si) crystals have been grown using the vertical Bridgman method in a non-wetting growth environment, achieved by the use of an anti-adhesion coating on the crucible wall. The minimized adhesion of highly reactive molten magnesium (Mg), silicon (Si) and Mg2Si permitted easy removal of the grown ingot from the crucible, and the external shape of the grown ingot followed the shape of inner wall of the crucible. The grown crystals were a single phase of polycrystalline Mg2Si. The grown crystals exhibited n-type conductivity, which could result from the residual impurities in the Mg source material used and unintentional impurity incorporation during growth. Since Mg2Si is a material candidate for thermal-to-electric energy-conversion, the thermoelectric properties such as Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured as a function of temperature up to 873K. Die-casting process induced characteristics of such thermoelectric prop … More erties are discussed. The maximum dimensionless figure-of-merit was estimated to be 0.17 at 656K. Electrode materials consisting of Cu, Ti and Ni were formed on Bi-doped n-type Mg2Si by means of a monobloc plasma-activated sintering (PAS) technique. Due to the difference in thermal expansion coefficients between Ti and Mg2Si, rather high residual thermal stresses gave rise to the introduction of cracks, which were mainly located in the Mg2Si layer, when Ti was used as the electrode material. In the case of the Cu electrodes, monobloc sintering could not be performed in a reproducible manner because Cu melts abruptly and effuses at around 973K, which is 100K lower than the sintering temperature that is required for Mg2Si of good crystalline quality. When compared with the results for Cu and Ti, the monobloc PAS process for Ni was both stable and reproducible. The room-temperature I-V characteristics of Ni electrodes were considered to be adequate for practical applications, with durable Mg2Si-electrode junction properties being realized at a practical operating temperature of 600K with delta-T=500K. Less