LiNO3–NaNO3–KNO3 salt for thermal energy storage: Thermal stability evaluation in different atmospheres

The thermal stability of the eutectic LiNO3–NaNO3–KNO3 salt was investigated by simultaneous differential scanning calorimetry, thermogravimetry and mass spectrometry (DSC/TG–MS). The work was carried out between room temperature and 1000 °C in blanket gas atmospheres of argon, nitrogen, oxygen and air. The stability of the salt, as measured by the gases evolving from the melt, was influenced by the atmosphere. Evolution of the main gaseous species NO was detected at 325 °C in an atmosphere of argon, at 425 °C in an atmosphere of nitrogen, at 475 °C in an atmosphere of air and at 540 °C in an atmosphere of oxygen. Prior to melting, the eutectic underwent endothermic (α/β) solid–solid type transformation at 87 °C. The melting point was 121 °C, and the solidification point 98 °C. Under-cooling of the salt coincided with the onset of the (α/β) solid–solid transformation upon heating. At a temperature of 500 °C in air, TG analysis showed that the long-term stability of the salt was limited and this was confirmed by DSC. Uncertainty analysis indicated that the measurement of temperature is accurate to ±2.7 °C.