Thermal and electrochemical properties of a mixture of lunar regolith simulant (FJS-1) and CaF2

To establish the electrochemical reduction process of lunar regolith as an in-situ resource utilization technology on the Moon, it is crucial to prepare a high-temperature electrolytic melt that has a composition suitable for electrolysis. In this study, we propose a mixed melt consisting of lunar regolith, which comprises metal-oxide compounds, and CaF₂, which can be collected from fluorapatite on the Moon's surface. To characterize the lunar regolith-CaF₂ system, the thermal and electrochemical properties of a mixture of a lunar mare regolith simulant (FJS-1) and CaF₂ were investigated. The differential thermal analysis curves measured for various compositions of FJS-1 and CaF₂ found that the eutectic temperature was 1275 K at FJS-1:CaF₂ = 90:10 wt%, which is lower than the melting point of FJS-1, 1393 K. By electrochemical impedance spectroscopic technique, the electrical resistance of the melts at 1673 K was found to be 43 Ω for the FJS-1 melt, while the mixed melt of FJS-1 and CaF₂ (80:20 wt%) was found to be 5 Ω. The XRD analysis of the solidified melts revealed that the mixed melts' unique physical properties were due to the formation of chemical bonding of F⁻ ions due to CaF₂ and metal ions due to FJS-1. Furthermore, the electrochemical behavior of the mixed melt was investigated, demonstrating the electrodeposition of metals such as Si and Al derived from FJS-1. The reported data will provide new guidelines for designing electrolytic systems on the Moon, expanding the possibilities for controlling the temperature and electrochemical operations.