Dissolution of Calcium Silicates in Molten CaCl 2

Electrodeposition of silicon in molten CaCl 2 has been drawing increasing attention. Soluble silicate anions in the melt can be reduced and tailored into functional materials. To ensure a precise control of the electrochemical processes, a comprehensive knowledge on the soluble silicate anions in the melt is indispensable. Here, we clarify the formation behavior of soluble silicate anions by dissolving solid CaSiO 3 and Ca 2 SiO 4 in molten CaCl 2 at temperatures from 1047 K to 1233 K (774 °C to 960 °C). It is found that Ca 2 SiO 4 dissolves more and faster. The formation of Ca 2 SiO 3 Cl 2 or Ca 3 SiO 4 Cl 2 as an intermediate product is observed. Metasilicate (SiO 3 2− ) and orthosilicate (SiO 4 4− ) are detected as soluble anions in the melt when dissolving CaSiO 3 and Ca 2 SiO 4 , respectively. Mass transfer of these ions plays an important role in determining the overall dissolution rate. The polymeric chain-like structure of SiO 3 2− is the reason for the slower dissolution of CaSiO 3 compared with that of Ca 2 SiO 4 , which dissolves as SiO 4 4− with a completely isolated tetrahedral structure in the melt. Phase relationships in the CaCl 2 –CaSiO 3 and CaCl 2 –Ca 2 SiO 4 binary systems at the CaCl 2 -rich side are clarified. These results should be useful for manipulating the electrochemical reactions of Si (IV) anions on purposes.