Growth of CaSi2 single-phase polycrystalline ingots using the phase relationship between CaSi2 and associated phases

The phase relationship between CaSi2 and associated phases was studied in order to fabricate CaSi2 single-phase polycrystalline ingots. The phase diagram of the Ca–Si binary system around CaSi2 (CaxSi2; 0.9 ⩽ x ⩽ 1.1) was obtained by identifying the phases in samples of varying composition and differential thermal analysis of the phase transitions. CaSi2 was treated as an incongruent-melting material that precipitates through two peritectic reactions, namely CaSi + liquid → Ca14Si19 and Ca14Si19 + liquid → CaSi2, according to the currently accepted phase diagram. However, at the CaSi2 stoichiometric composition, the stable peritectic reaction Ca14Si19 + liquid → CaSi2 occurs only in the presence of impurities such as Ta, B or P. In contrast, CaSi + liquid → CaSi2 occurs as the metastable peritectic reaction in a high-purity melt. The undercooling required for CaSi2 precipitation is less than 5 °C. Therefore, single-phase CaSi2 could be produced from a high-purity stoichiometric CaSi2 melt with a small amount of undercooling, and a rod-like coupled structure of CaSi2 and CaSi phases formed by peritectic coupled growth in a high-purity Ca-rich ingot. The present study also investigates the fabrication process for CaSi2 single-phase polycrystalline ingots. These can be obtained by high-cooling-rate casting (1–100 °C min−1) of high-purity, stoichiometric CaSi2 melts using new Ta crucibles.