Comprehensive analysis of thermophysical properties of the NaNO3–KNO3 mixture with metastable phases

Abstract

This study focuses on the comprehensive analysis of the NaNO₃–KNO₃ system, a salt mixture that is extensively used as a heat transfer fluid and thermal energy storage material in concentrated solar power (CSP) plants. Despite its common application, discrepancies exist in the reported equilibrium phase diagrams, largely due to the formation of metastable phases influenced by experimental conditions. This work combines various thermophysical methods, including differential scanning calorimetry, thermomechanical analysis, laser flash analysis, and high-temperature X-ray diffraction, to resolve these discrepancies. The thermal and structural properties of a 50 mol % NaNO₃ – 50 mol % KNO₃ mixture have been determined. Significant differences in phase transition temperatures and volume were observed between the first and second heating cycles. In-situ HTXRD confirmed the formation of metastable solid solution phases upon cooling to room temperature after the first heating. The comprehensive analysis provided insights into the equilibrium and metastable states of the mixture, highlighting the importance of combining thermal analysis techniques with XRD for a thorough characterization of material properties.