Characterization of the Urea-Sodium Nitrate Eutectic Mixture as a PCM: Phase Segregation

The eutectic mixture formed by urea and sodium nitrate has been previously identified by the authors as a PCM candidate for heating and domestic hot water applications. It presents a melting point of 85 degrees C, a melting enthalpy of 172 J/g, and a price of around 0.9 (sic)/kg. Eutectic mixtures are claimed as congruent melting materials in PCM literature. However, the studied mixture showed phase segregation when exposed to repeated melting-solidification cycles. Thus, the mixture was studied in order to assess the mechanisms that cause the phenomenon. Experiments consisting of thermal cycling of samples using different cooling rates were carried out. Subsequently, the segregated materials were analyzed by different techniques, including XRD, DSC, PLM, and SEM. The results established a relationship between the operating conditions with the resulting crystal structures, which explain the phase segregation in the eutectic mixture. Besides, a mitigation measure was determined.