Premelting, Melting, and Degradation Properties of Molten Alkali Nitrates: LiNO₃, NaNO₃, KNO₃, and Binary NaNO₃-KNO₃

A simultaneous thermal analyzer (STA) was used to observe the transition and degradation events of LiNO₃, NaNO₃, KNO₃, and binary NaNO₃-KNO₃ salts for potential use as phase change materials (PCMs) and heat transfer fluid (HTF). Samples were heated from 50 °C to 800 °C at 10 °C/min scanning rate in three atmospheres (argon, air, and oxygen) using an STA to observe decomposition behavior. Thermal stability increased for all salts at high partial pressure of O₂ (\( P_{{{\text{O}}_{2} }} \)= 1.0) compared to inert argon (\( P_{{{\text{O}}_{2} }} \)= 0). O₂, N₂, NO, N₂O, and NO₂ were main evolved gases during nitrate decomposition. NO and O₂ started to evolve at approximately the same temperature after melting, indicating that primary and secondary decomposition reactions were concurrent and overlapping. The solid-solid transition, liquidus and solidus temperatures, heat of transition, heat of melting, and heat of solidification were obtained at various heating-cooling rates (1, 2, 4, 5, 6, 8, 10, and 15 °C/min) using an STA. At all heating-cooling rates, a small gap exists between liquidus and solidus temperatures for all samples due to the salts exhibiting supercooling phenomena. This study showed that the degradation point depends on the blanket atmosphere top of the molten salts and that heating rates have a minor effect on transition events (peaks height, peaks width, and transition enthalpies).