A study on quaternary mixed fluoride salts as heat storage materials

Abstract Mixed fluoride salts have shown great potential as heat storage materials. In this study, a series of these salts were prepared from NaF, KF, MgF2, and CaF2, and their physical and thermodynamic properties were systematically investigated. Upon melting, these quaternary fluoride salts formed new double salt phases, and different initial formulations resulted in different contents and compositions of the double salt phases. The five groups with the optimal formulations contained only the KCaF3 and KMgF3 double salt phases, while the two reference samples contained KCaF3, KMgF3, K2MgF4, and NaMgF3 double salt phases. The emergence of new phases decreased the latent heats of the salts. The formulation with the largest theoretical latent heat (NKMC-5) initially showed better performance; however, its high ΔT and ΔT′ values decreased its thermal cycling stability (ΔT is the difference between the melting onset and saturation point temperatures, whereas ΔT′ is the difference between the melting onset temperature and the temperature at which 90 % of the material is in the liquid phase). Microstructural analyses revealed that the structural compaction changed with different salt ratios owing to the formation of new phases and changes in the liquid content with temperature.