Effect of Surface Functionalization and Physical Properties of Nanoinclusions on Thermal Conductivity Enhancement in an Organic Phase Change Material.

We probe the role of surface functionalization and physical properties of nanoinclusions in thermal conductivity enhancement during liquid-solid phase transition in a hexadecane-based phase change material (PCM). Hexadecane-based PCM is loaded with six different nanoinclusions: carbon black nanopowder (CBNP), nickel nanoparticles (NiNPs), copper nanoparticles, silver nanowires (AgNWs), multiwalled carbon nanotubes, and graphene nanoplatelets (GNPs). The nanoinclusions CBNP, NiNP, AgNW, and GNP are surface-functionalized with oleic acid. Nanoinclusion-loaded PCM showed a large enhancement in thermal conductivity, which was more prominent in the solid state. Interestingly, a maximum thermal conductivity enhancement of ∼122% was observed in the solid state for the PCM loaded with 0.01 wt % CBNP. Higher thermal conductivity enhancement in the solid state is attributed to the formation of a nanocrystalline network structure during freezing of the PCM, consisting of a needlelike microstructure, which is confirmed by optical phase contrast microscopy. During solidification, the nanoinclusions are driven toward the grain boundaries, thereby forming a quasi-two-dimensional network of percolating structures with high thermal transport efficiency due to the enhancement of phonon-mediated heat transfer and near-field radiative heat transfer. Thermal conductivity increases with the increased loading of the nanoinclusions due to the formation of more interconnecting aggregates. Among the carbon-based nanoinclusions, the highest thermal conductivity enhancement is obtained for the PCM loaded with CBNP, which is attributed to the low fractal dimensions and volume-filling capability of CBNP aggregates. In the case of metallic nanoinclusions, the highest thermal conductivity enhancement is obtained for the PCM loaded with AgNW, which is due to the large aspect ratio of AgNW. The carboxylic group of oleic acid attached to the nanoinclusions is found to provide better steric stability with insignificant aggregation and improved thermal stability, which are beneficial for practical applications. Our results indicate that the initial thermal conductivity of carbon-based nanoinclusions has an insignificant role in the thermal conductivity enhancement of the PCM but the volume-filling capability of the nanoinclusion has a prominent role. The findings from the present study will be beneficial for tailoring the properties of nanoinclusion-loaded organic PCM for thermal energy storage and reversible thermal switching applications at room temperature.