Thermophysical property degradation and structure evolution of the NaNO3-KNO3-Ca(NO3)2 molten salts after long-term high-temperature heat treatment

The degradation mechanism of the thermophysical property for the NaNO₃-KNO₃-Ca(NO₃)₂ molten salts after long-term operation remains unclear. To analyze the long-term service characteristics, the thermophysical property and strucutral changes of the NaNO₃-KNO₃-Ca(NO₃)₂ molten salts were investigated after subjected to isothermal treatment at 480.0 °C for 15 days in air and argon. It was demonstrated that argon atmosphere facilitated the formation of nitrite (NO₂⁻), leading to reduced melting point (with a maximum decrease of 7.9 °C), diminished specific heat capacity and lowered viscosity. Accelerated decomposition under argon atmosphere was identified as the primary cause of higher mass loss (2.5 %), attributed to the intensified decomposition reactions promoted by the dynamic argon environment. In contrast, decomposition was suppressed in air flow (1.8 % mass loss), where CO₂ participation in the formation of CaCO₃ and CO₃²⁻ species was confirmed. However, viscosity elevation in air-treated sample was observed due to progressive impurity accumulation. The structure evolution was furthered characterized by the variable-temperature Raman spectroscopy and X-ray diffraction. The coordination network of NO₃⁻ was disrupted by the bent configuration of NO₂⁻, resulting in weakened interionic interactions. The peak broadening and red-shift indicate extended N–O bonds and structural disorder. These atomic-scale modifications directly explain the reduced specific heat capacity and altered thermal stability.