Experimental investigation of the sodium chloride–calcium chloride eutectic system and its thermophysical properties

The NaCl–CaCl 2 system is a promising phase change material (PCM) for high-temperature thermal energy storage around 500 °C. However, reported melting temperatures, enthalpies of fusion, and specific heat capacities vary widely due to differences in purity, composition definition, and evaluation protocols. This work provides a consistent thermophysical data set for NaCl–CaCl 2 compositions near the eutectic composition by applying a unified DSC/ c p /TGA methodology. Four technically pure NaCl–CaCl 2 mixtures (98–99 % purity) close to the eutectic composition and one high-purity reference were prepared under argon and characterised on a STA 449 F3 system. The investigated compositions were selected to assess compositions within the eutectic range reported in the literature and the influence of small composition deviations on the thermal response. Melting temperatures were determined by DSC using ISO 11357 onset criteria. Thermal events were evaluated by baseline-corrected peak integration using individually verified onset and integration limits, while thermal stability was assessed by TGA and the specific heat capacity c p ( T ) was determined using the sapphire method. The composition identified as eutectic in the present study is 52:48 mol% (36.3:63.7 wt%), and it exhibits a single, narrow melting peak at T m = 502.3 ± 2.0 °C with an enthalpy of fusion of 168.3 ± 5.3 J g −1 , in agreement with recent high-quality literature. Off-eutectic mixtures display more complex thermal events with higher apparent thermal event enthalpies (approximately 190 J g −1 ), consistent with overlapping eutectic and non-eutectic phase transitions. TGA confirms negligible mass loss between 600 and 700 °C, including a 96 h hold, indicating good thermal stability under inert atmosphere. The specific heat capacity of the eutectic composition S1 remains nearly constant at ≈ 0.7 J g −1 K −1 up to 300 °C, increases to ≈ 0.92 J g −1 K −1 between 300 and 350 °C, and stabilises near 0.90–0.95 J g −1 K −1 in the liquid state up to 700 °C. The results close existing data gaps for the NaCl–CaCl 2 system and provide experimentally determined thermophysical parameters, together with a transparent evaluation protocol, for the modelling and design of chloride-based latent heat storage materials.