Size effect on the thermal conductivity of octadecanoic acid: A molecular dynamics study

Abstract

Octadecanoic acid (OA) is well-known as an excellent phase change material (PCM) for heat storage due to its high energy density and narrow phase change temperature. In the practical application of PCMs, OA is often filled into the porous or microencapsulated matrix in micro/nanoscale forms. Generally, the thermal transport properties of OA are greatly subject to its size effect, and unfortunately only its macroscopic thermal transport properties are well studied in the composite field. To meet the demands of efficient OA-filled composite PCMs, OA’s nanoscale thermal transport properties are undoubtedly significant. In this study, an atomic simulation was conducted through a top-down approach based on equilibrium molecular dynamics to calculate the size-dependent thermal conductivity (κ) of OA in bulk, nanowire, and nanochain forms. The vibrational density of states and phonon overlap energy for the three forms of OA were compared to extract the size effect of κ. The largest κ value occurred for the bulk form while the smallest value occurred for the nanochain form. The limited sizes of nanowire forms reduce the atom vibration and weaken the phonon matching level, thus resulting in a lower κ than the bulk form. The curled structures of the nanochain forms exhibit higher molecular distances and more phonon scattering probabilities under weak van der Waals forces, leading to the lowest thermal transport. This study’s findings provide a clear understanding of the size dependence of thermal transport in OA for constructing OA-filled PCMs.