High Thermal Conductivity Shape-Stabilized Phase Change Materials
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High Thermal Conductivity Shape-Stabilized Phase Change Materials Min Xiao, Luyi Sun1, Chengya Huang, Yonghua Zhang, Kecheng Gong Polymer Structure & Modification Lab, South China University of Technology, Guangzhou, 510640, P. R. China 1 Department of Chemistry, University of Alabama, Tuscaloosa, AL 35487, U.S.A. ABSTRACT In this paper, shape-stabilized phase change materials (PCMs) based on paraffin and thermoplastic elastomer poly(styrene-butadiene-styrene) are prepared. The shape-stabilized PCMs can keep the same shape in a solid state even when their temperature is above the melting point of the paraffin. They exhibit same phase transition characteristics as paraffin and up to 80% of the latent heat of paraffin. Thermal conductivity of the shape-stabilized PCMs is increased significantly by the introduction of expanded graphite (EG). The time for complete solidification and complete melting of the composite P(80)/S(20)/EG(3) is two ninths and two fifths of paraffin, respectively. INTRODUCTION Thermal storage units that utilize latent heat storage materials have received great attention in the recent years because of their large heat storage capacity and isothermal behavior during the charging and discharging processes. Among the various kinds of PCMs of interest, paraffin has been found to exhibit many desirable characteristics such as high heat of fusion, negligible supercooling, low vapor pressure in the melt, chemically inert and stable, self nucleating, etc [1,2]. However, they also have an unacceptably low thermal conductivity. Therefore heat transfer enhancement techniques are required for thermal storage applications [3-5]. Paraffin has another drawback. Because they are solid-liquid PCMs, they give rise to extra problems in encapsulating them. Recently, a shape-stabilized PCM that can keep the same shape in a solid state even when its temperature is above its melting point has been developed [6-7]. Therefore, problems for encapsulating are solved by using this kind of shape-stabilized PCM. In this paper, shape-stabilized PCMs with high thermal conductivity were prepared and their thermal storage performances were investigated. MATERIALS AND METHODS Materials Technical grade paraffin (melting point Tm = 56-58°C, purchased from Maoming Petroleum Chemcal Industrial Corporation); styrene-butadiene-styrene copolymer (SBS) (SBS1401, provided by Yueyang Petroleum Chemcal Industrial Corporation); expandable graphite (kindly supplied by Bao Ding Lianxin Carbide Ltd.), which was prepared with H2SO4 as an intercalant and HNO3 as an oxidant, volumetric expansion ratio 210cm3/g. KK8.12.1
Preparation of expanded graphite(EG) Expandable graphite was first dried in a circulation oven at 60° C for 10 hours. Then heat treatment was performed by inserting a steel crucible containing about 3g of expandable graphite into the furnace held at a constant temperature of 800° C and being kept for 1 minute. Expansion and exfoliation occurred during the heat treatment. Preparation of shape-stabilized PCMs By slowly adding paraff
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