Scale effect on thermal properties and phase transition characteristics of the ZnCl2-NaCl-KCl mixed chloride salt

Abstract

Composite chloride salts have become molten salt heat storage materials with high development potential due to their excellent heat storage performance. Since chloride salts are prone to leakage, they need to be adsorbed onto porous materials to prepare composite phase change materials (CPCMs). Changes in the pore size of porous materials will lead to changes in the scale of chloride salts. Currently, there is little research on the impact of scale variation on the thermophysical and phase change characteristics of composite chloride salts, and the underlying mechanisms are not yet clear. With the ZnCl₂-NaCl-KCl (3:1:1 mol%) eutectic salt as the phase change material (PCM), this study integrates molecular dynamics (MD) simulation with an experimental approach to investigate the impact of scale on the thermophysical properties and phase change properties of CPCM. The inherent influence mechanism of scale effect is analyzed from a microscopic perspective. The results indicate that as the scale increases, the interaction energy between molecules within the system gradually increases, and the system structure becomes more compact. The thermal conductivity first increases and then decreases, reaching a maximum of 0.45 W/(m·K) at 8 nm; the volumetric thermal expansion coefficient gradually decreases. As the scale increases from 3 nm to 15 nm, the melting temperature rises by 8.1 %, while the solidification temperature decreases by 1.73 %, resulting in a significant increase in the degree of supercooling. The latent heat gradually increases with scale, which is consistent with the coordination number.