Effects of particle packing structure on molten salts migration behavior and thermal properties in composite phase change materials

Abstract

The high-temperature losses from migration and leakage of molten salts pose significant challenges to the thermal stability of chloride-based composite phase change materials (CPCMs). To overcome the challenges, NaCl-KCl eutectic salts and MgAl₂O₄ ceramic matrix with different particle sizes were employed to prepare the CPCMs via mixed sintering methods. In this study, the effects of particle packing on the microstructure were systematically investigated, along with their further impacts on molten salts migration and thermal properties. The results show that the continuous phase within the CPCMs changed from molten salts to MgAl₂O₄ ceramic matrix as the particle size of MgAl₂O₄ decreased. This change was attributed to the finer MgAl₂O₄ particles (≤0.01 mm), which segmented and encapsulated the molten salts, thereby effectively inhibiting their migration and leakage and resulting in a 72.3 % reduction in the mass loss of the CPCMs. Although the molten salts were segmented into multiple micro-regions, leading to an increased porosity and formation of 1–5 μm pores, the MgAl₂O₄ as the continuous phase endowed the CPCMs with enhanced cold compressive strength (48 MPa). After 100 thermal cycles, the minimum mass loss was 1.66 wt%. According to the Young-Laplace and Kelvin equations, reduced pore size from MgAl₂O₄ particle packing increased capillary pressure and decreased vapor pressure of the molten salts, thereby suppressing their migration and evaporation. This indicates that the pores formed by particle packing also play a critical role in inhibiting molten salts leakage. This work provides a new perspective to enhance the thermal stability of chloride-based CPCMs.