Thermal conductivity enhancement of paraffins by increasing the alignment of molecules through adding CNT/graphene

Abstract

Molecular dynamics simulations were utilized to investigate the relationship between the structure of paraffins in solid and liquid states and its thermal conductivity. We observe that upon crystallization, a nanocrystalline paraffin structure develops and the value of thermal conductivity doubles, in agreement with experimental data. The introduction of carbon nanotubes or graphene layers leads to liquid ordering and associated thermal conductivity enhancement. More prominently, carbon nanofillers provide a template for directed crystallization and lead to even greater thermal conductivity increases. Our results indicate that introducing carbon nanotubes and graphene into long-chain paraffins leads to a considerable enhancement in thermal conductivity, not only due to the presence of a conductive filler, but also due to the filler-induced alignment of paraffin molecules.

Introduction

Introduction of nanoscale highly-conductive fillers into phase change materials (PCM) in order to enhance their thermal conductivity has attracted significant attention in recent years [1], [2], [3], [4], [5], [6], [7], [8]. For well-dispersed suspensions of spherical nanoparticles, the thermal conductivity is described adequately by the effective medium theory and significant enhancement is not exhibited [9]. However, through introduction of carbon nanotubes (CNT), carbon nanofibers, graphite nanoplatelets and graphene sheets into materials, considerable augmentation of thermal conductivity for both liquid and solid phases has been observed [3], [4], [5], [6], [7], [8], [10], [11], [12], [13]. Considering the high aspect ratio of these fillers, such enhancement is also predicted from the effective medium theory [14]. However, the magnitude of the enhancement can be greatly reduced by high interfacial thermal resistance between CNT/graphene and matrix material [15], [16], [17], [18].

The filler–matrix interface can also have a potentially positive effect on thermal conductivity due to the ordering of the atomic structure at the solid–liquid interface [19]. This idea is motivated by the fact that crystalline solids exhibit much greater thermal conductivity when compared to their amorphous counterparts [20], [21] due to lack of polarized thermal waves (phonons) in amorphous materials as well as in liquids. However, molecular simulations of simple, small molecule liquids at solid interfaces indicate that interfacial order in the liquid has little effect on thermal transport [22].

In this work, we use molecular dynamics (MD) simulation to study bigger molecules in their liquid and solid phases, specifically n-alkane (n-paraffins) molecules [23], [24], [25], [26], [27], [28], in order to investigate the effect of solid fillers-induced structures on thermal conductivity of such materials. The simulation methodology and thermal conductivity calculation method are described in Section 2. Model structures and their preparation are described in Section 3. We report the results of calculated thermal conductivity in Section 4. Finally, a summary and conclusions are presented in Section 5.