Supercooling Suppression and Thermal Conductivity Enhancement of Na₂HPO₄·12H₂O/Expanded Vermiculite Form-Stable Composite Phase Change Materials with Alumina for Heat Storage

Abstract

In the heat storage applications, Na2HPO4·12H2O phase change materials (PCM) show significant defects including form instability, high supercooling degree and low thermal conductivity. Aiming at these drawbacks, the Na2HPO4·12H2O-alumina/expanded vermiculite (EVM) form-stable composite phase change materials (NEA fs-CPCMs) with supercooling suppression and heat transfer enhancement were prepared. The favorable wettability of NEA fs-CPCMs was beneficial to their form-stabilization (encapsulation mass fractions above 59.7 wt %). The supercooling degree of NE5.3 was reduced to 1.4 °C after adding 5.3 wt % alumina as the nucleating agent (decreased by 90%). The surface effects of EVM and alumina and high wettability of PCM-alumina were responsible for the supercooling suppression of PCM. Large specific surface area of EVM could provide nucleation sites for crystallization of PCM. The surface electronegativity of alumina increased the affinity ability and diffusion rate of ions, thus effectively increasing the probability of nucleation. The heat transfer of NEA fs-CPCMs was significantly enhanced by the alumina as the thermal conductivity enhancement filler. The thermal conductivity of NE5.3 reached 0.418 W/(m K). Thermal energy storage behavior analysis indicated that the NEA fs-CPCMs showed large heat storage capacity (melting process: 97–151 J/g; solidification process: 60–89 J/g). The thermal reliability of NEA fs-CPCMs was effectively improved by coated paraffin.