Nanocomposite phase change materials (PCM) exhibit high latent heat and excellent thermal-chemical stability, rendering them promising candidates for a wide range of applications such as thermal energy storage and thermal management. Three-dimensional Metal-organic Frameworks (MOFs) with ultrahigh surface area, large pore volume, and tunable pore environment can be rationally designed as support materials for PCM. The thermal conductivity of phase change materials before and after composite formation significantly impacts their practical application performance. This study aims to investigate the variation in thermal conductivity of n-eicosane infiltrated into various spatial topologies (crb, dft, gis, lon, lta, pcl, sra, unc, unh, uni, unj) of Zeolitic Imidazolate Frameworks (ZIFs, a sub-class of MOFs). Equilibrium molecular dynamics (MD) simulation was performed to obtain the thermal conductivity of these n-eicosane/ZIFs based on the Green-Kubo method, in which the interaction between n-eicosane and ZIFs, Phonon Density of States (PDOS), and the location of n-eicosane carbon chains were also calculated to clarify the influence on thermal conductivity. MD simulation results reveal that n-eicosane, at mass fractions below 10%, enhances the thermal conductivity of ZIFs by 23%-196%. Notably, the thermal conductivity increment of ZIF-uni, ZIF-unc, and ZIF-crb is significantly larger than other typologies, which increased from 0.28, 0.29, 0.28 to 0.83, 0.71, 0.61 W/m·k. It was suggested that the relatively small pore size (5.43, 5.26, and 5.84 Å) and straight channel limited the movement of n-eicosane in the ZIFs pore with moderate interaction (about -40 kJ/mol) benefit to the thermal conduction of such three ZIFs-based PCM. The proximity of n-eicosane carbon chains to cell boundaries (<20 Å) primarily dictates this enhancement, contingent upon the curvature within ZIFs. It was also demonstrated that the straight n-eicosane carbon chains rather than the twisted carbon chains were favorable for the increase of thermal conductivity. Vibrational Density of States (VDOS) and material interaction analyses indicate significant low-frequency peaks at the end-carbon atoms of n-eicosane, influencing PCM phonons in composite phase change materials. Moreover, the enhancement of thermal conductivity exhibited quite obvious anisotropy, determined by the direction of n-eicosane carbon chains. These findings elucidate trends in thermal conductivity and influencing factors post-composite with porous materials and n-eicosane, offering a theoretical basis for designing and optimizing composite PCMs with specific thermal conductivity.

Key words: Composite phase change material, Zeolitic imidazolate frameworks, Molecular dynamic simulation, Thermal conductivity