Enhanced Thermal Conductivity of Phase Change Materials Modified by Exfoliated Graphite Nanoplatelets
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1218-Z06-10
Enhanced Thermal Conductivity of Phase Change Materials Modified by Exfoliated Graphite Nanoplatelets Jinglei Xiang and Lawrence T. Drzal Department of Chemical Engineering and Materials Science Michigan State University, East Lansing, MI 48824-1226 ABSTRACT Composite phase change materials (PCM) were prepared by mixing exfoliated graphite nanoplatelets (xGnP) into paraffin wax. The two types of graphite nanoplatelets that were investigated were xGnP-1 having thickness of 10 nm and a diameter of 1 um and xGnP-15 having the same thickness with a platelet diameter of 15 um. Direct casting and two roll milling were used to prepare samples. Scanning electron microscopy images show that the nanofillers disperse very well in paraffin matrix without noticeable agglomeration. Paraffin/xGnP-15 PCM consistently exhibited higher thermal conductivity than xGnP-1 PCM. The improvement in thermal conductivity was as high as 5 fold for xGnP-15 composites and 2 fold for xGnP-1 composites at 4 vol%. The aspect ratio, particle orientation, and interface density between the conductive particles and the polymer matrix were found to be the critical parameters in determining the conductivities of the resulting nanocomposites. The thermal physical properties of the nanocomposites were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). It was found that the latent heat of nanocomposites was not negatively affected in the presence of xGnP particles and the thermal stability improved. INTRODUCTION Solid-liquid PCMs have been reported in the literature as useful for latent heat thermal energy storage applications because of their high energy density [1-3]. Examples include space heating and cooling in buildings, solar thermal heating, and thermal management in microelectronics. Among organic PCMs with low packaging cost, paraffin wax has attracted numerous attentions for its low cost, moderate energy densities, low vapor pressure, negligible super-cooling, and chemical inertness [4]. However, one of the intrinsic disadvantages associated with paraffin wax is its low thermal conductivity, which limits the rate of absorbing and releasing heat to and from the environment, restricting its wider applications. Various high thermally conductive fillers (e.g. metallic fins [5, 6], ceramic powder fillers, graphitic carbon fibers [7], carbon nanofibers [8], graphite particles [9] and exfoliated graphite [10]) have been reported to improve the effective thermal conductivity of PCMs. However, one usually needs a very high concentration of these traditional conductive fillers to achieve a noticeable improvement in thermal conductivity, resulting in increased density of the composites, processing difficulties, and increased cost. In addition, the filler material will replace the active matrix, reducing its thermal energy storage capacity. Exfoliated graphite platelets, produced from graphite intercalated compounds, are particles consisting of several layers of graphene sheets. They have a very high a
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