MAGNESIUM FLUORIDE AS ENERGY-STORAGE MEDIUM FOR SPACECRAFT SOLAR THERMAL POWER-SYSTEMS

Magnesium fluoride (MgF2) was investigated as a phase change energy storage material for low Earth orbit power systems using solar heat to run thermal cycles. It provides a high heat of fusion per unit mass at a high melting point (1536 K). Theoretical evaluation showed the basic chemical compatibility of liquid MgF2 with refractory metals at 1600 K, though transient high pressures of H-2 can occur in a closed container due to reaction with residual moisture. The compatibility was tested in two refractory metal containers for over 2000 h. W-25Re showed no deterioration, while there was evidence that the fluoride reacted with hafnium in ASTAR-811C (Ta-8W-1Re-0.7Hf-0.025C). Corollary tests showed that the MgF2 supercooled by 10-30 K in the W-25Re and 50-90 K in the ASTAR-811C.