Dispersing different nanoparticles in paraffin wax as enhanced phase change materials: A study on the stability issue

摘要

Highly conductive nanoparticles were proposed to be dispersed into phase change materials (PCMs) such as paraffin wax for heat transfer enhancement. The mixture, often referred to as nanoparticle-enhanced phase change material (NePCM), has been studied extensively for latent heat energy storage but with conflicting results. This study attempts to understand this problem by investigating the stability of NePCMs under multiple thermal (melting-solidification) cycles, which has not been well explained in previous studies. We believe that stability of a NePCM is prerequisite for any experimental investigation of its thermal properties or application. In this study, paraffin wax was chosen as the base material. Three different types of nanoparticles were tested, i.e., multi-walled carbon nanotubes, graphene nanoplatelets, and aluminum oxide nanoparticles (Al2O3). The nanoparticles were dispersed into paraffin wax at varying mass fractions using mechanical dispersion methods (sonication, stirring) with and without different surfactants. Stability of different mixtures was investigated after consecutive thermal cycles performed in an environmental chamber. Significant coagulation and deposition of nanoparticles were found after a few thermal cycles regardless of the nanoparticle type, concentration, or dispersion method. Different boundary conditions in heating were also examined for their effects. None of these methods led to long-term stable NePCMs. The negative results from this study indicate that long-term stability of NePCM (at least for the paraffin wax and nanoparticles tested) remains a major challenge and requires further research with a multidisciplinary approach.