Synthesis of metal silicides using alkali metals and their thermoelectrical properties

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Powder and bulk of thermoelectric metal silicides such as β-FeSi_2,MnSi_<1.7+δ>, CoSi, CrSi_2,and Mg_2Si were prepared by heating mixtures of Si and metal powders or Si powders and compact body of metal powders with Na at low temperatures (500-900℃). Thermoelectric properties of the prepared metal silicides were characterized. The existence of Na-Si melt above ca. 500℃ was revealed by the study of Na-Si phase diagram. It indicates that the metal silicides are prepared by a reaction of Si (liquid) in the melt with the metals (solid). Powder of high-melting point metal silicides such as α-MoSi_2and NbSi_2,and metastable phase β-MoSi_2were prepared at 800,627,and 600-C, respectively. NbSi_2layer, which has oxidation resistance, was fabricated on the surface of Nb plate by heating the Nb plate with Si powder and Na. It was demonstrated that the new preparation method using Na can be adopted for the low-temperature preparation of metal silicides.