Dissolution of Calcium Silicates in Molten CaCl2

Electrodeposition of silicon in molten CaCl2 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 CaSiO3 and Ca2SiO4 in molten CaCl2 at temperatures from 1047 K to 1233 K (774 degrees C to 960 degrees C). It is found that Ca2SiO4 dissolves more and faster. The formation of Ca2SiO3Cl2 or Ca3SiO4Cl2 as an intermediate product is observed. Metasilicate (SiO32-) and orthosilicate (SiO44-) are detected as soluble anions in the melt when dissolving CaSiO3 and Ca2SiO4, respectively. Mass transfer of these ions plays an important role in determining the overall dissolution rate. The polymeric chain-like structure of SiO32- is the reason for the slower dissolution of CaSiO3 compared with that of Ca2SiO4, which dissolves as SiO44- with a completely isolated tetrahedral structure in the melt. Phase relationships in the CaCl2-CaSiO3 and CaCl2-Ca2SiO4 binary systems at the CaCl2-rich side are clarified. These results should be useful for manipulating the electrochemical reactions of Si (IV) anions on purposes.