Development of low-melting point molten salts and detection of solid-to-liquid transitions by alternative techniques to DSC

The ‘Solar salt’ (60% NaNO₃-40% KNO₃, wt. %) is the most used heat transfer and storage material in high temperature CSP systems. The main drawback is its high melting temperature of 228 °C, which requires extra-energy to keep it in the liquid state and avoid damage to pipes at low temperatures. Multi-component salts are combinations of different cations and anions. The difference in size of the ions hinders the crystallization of the material and provides lower melting temperatures. Multi-component salts are considered in this study to replace simpler combinations, such as binary and ternary eutectic mixtures. Herein, we report on two novel six-component nitrates with a melting temperature of 60–75 °C and a thermal stability up to ~500 °C under a linear heating program in N₂ atmosphere. Properties such as the thermal conductivity in solid and molten state, heat capacity and vibrational spectra were evaluated. The study of the thermal behaviour of these materials using differential scanning calorimetry was insufficient, hence alternative and complementary techniques were used, such as: the three-omega technique, optical transmission and Raman spectroscopy. Multi-component salts were found to solidify as amorphous solids even at slow cooling rates and water was found to behave as a catalyst of crystallization.