Ca-germanide and Ca-silicide crystals were synthesized by mechanical alloying (MA). The resulting powders made from the Ca and Si powders contain Ca2Si and/or Ca5Si3 phases with additional cubic phase crystals. The grown phases, namely Ca2Si and Ca5Si3, can be controlled by the Ca/Si ratio and the Si powder size. For the powders synthesized using the Ca and Ge powders, the cubic crystalline phase was synthesized. It was found that the powders synthesized from the Ca and Ge powders are covered by an amorphous layer that contains oxidized carbons. The Ca-based bulk tablets pressed from the powders show a p-type semiconducting behavior, though the powder surface is heavily oxidized, which might affect the transport property of the bulk.
Orthorhombic polycrystalline Ca2Si sintered compact bulk crystals were synthesized via a two-step treatment, and the experimental evaluation of the electrical and thermoelectric properties of Ca2Si is reported for the first time. First, single-phase Ca2Si powders were synthesized by the heat treatment of Mg2Si powders under a Ca vapor. Then Ca2Si sintered compacts were synthesized by the spark plasma sintering technique using the Ca2Si powders. It was found that single-phase Ca2Si sintered compacts were obtained when the sintering was carried out at temperatures lower than 673 K with high pressure of 300 MPa. The structural, electrical and thermoelectric properties of the Ca-silicide sintered compacts and their dependence on process conditions are discussed.
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