Determination of the eutectic concentration between Mn(NO3)2·6H2O and NaNO3 using the DSC peak sharpness method

Abstract

While inorganic eutectic phase change materials (PCM) present the advantage of a high volumetric melting enthalpy and a high thermal stability under repeated heating/cooling cycles, there is a lack of known materials in the temperature range of 20–24 °C, interesting for building applications. The DSC peak sharpness method is employed to determine the eutectic concentration between Mn(NO3)2·6H2O and NaNO3, which is 96.5 mass% Mn(NO3)2·6H2O and 3.5 mass% NaNO3. Thereby, the DSC peak sharpness method is demonstrated to yield the same result independently of the two chosen heating/cooling rates of 0.5 and 2 K min−1. The eutectic PCM has an onset temperature of 22.1 °C and a melting enthalpy of (126 ± 6) J g−1. Even though the same eutectic concentration is obtained for both heating/cooling rates, the heating/cooling rate of 0.5 K min−1 reveals that the melting peak of Mn(NO3)2·6H2O is composed of two overlapping melting peaks. The second melting peak supposedly corresponds to the eutectic between Mn(NO3)2·6H2O and Mn(NO3)2·4H2O, which melts at a slightly lower temperature than Mn(NO3)2·6H2O (difference of −0.6 °C). This assumption is in accordance with the water content of the purchased Mn(NO3)2·6H2O measured via Karl Fischer titration being (0.66 ± 0.2) mass% too low.