Preparation And Properties Of Oxalic Acid Dihydrate-ammonium Alum Eutectic/Zirconia Modified Expanded Graphite Composite Phase Change Material

Hydrated salts or hydrates with the features of high phase change latent heat,constant phase change temperature and low cost can be used as phase change materials（PCMs）in heat storage of heat exchanger,industrial waste heat utilization and other applications.However,they have shortcomings such as high melting point of single PCM,supercooling,easy leakage,low thermal conductivity,and poor compatibility with porous substrate.Therefore,in this paper,oxalic acid dihydrate（H₂C₂O₄·2H₂O）and ammonium aluminum sulfate dodecahydrate（Ammonium alum,NH₄Al（SO₄）₂·12H₂O）as the main PCMs,the preparation of H₂C₂O₄·2H₂O-NH₄Al（SO₄）₂·12H₂O binary eutectic PCM was discussed.On this basis,using zirconia（ZrO₂）as hydrophilic modifier,modified expanded graphite（MEG）obtained as component of PCM shape-stability and thermal conductivity,the structure and thermal properties of binary eutectic/MEG composite phase change material（CPCM）were investigated.（1）H₂C₂O₄·2H₂O-NH₄Al（SO₄）₂·12H₂O binary eutectic was prepared by melting blending process.The effects of different H₂C₂O₄·2H₂O mass fractions on the thermal properties of the obtained binary eutectic were discussed.The composition and crystal structure of the eutectic were analyzed.The supercooling degree and thermophysical properties of eutectic under different nucleating agent types and contents were studied.The results showed that the mixture of 25%H₂C₂O₄·2H₂O and 75%NH₄Al（SO₄）₂·12H₂O could form binary eutectic with excellent latent heat（246.9J·g-1）and suitable phase transition temperature（56.13℃）.Analysis of XRD and FT-IR indicated that H₂C₂O₄·2H₂O and NH₄Al（SO₄）₂·12H₂O in binary eutectic were only physically bonded.After addition of 3%Ni SO₄·6H₂O,the obtained eutectic PCM had latent heat of 229.2J·g-1,phase transition temperature of 55.97℃and supercooling degree of 2.92℃.The observation under optical microscope revealed that Ni SO₄·6H₂O could promote the crystallization of eutectic.（2）Expanded graphite（EG）was modified by ZrO₂sol which prepared by sol-gel process.The hydrophilicities of MEG with different ZrO₂mass fractions were analyzed,and the adsorption capacity of MEG on binary eutectic PCM was discussed.The results showed that10%ZrO₂could significantly reduce the contact angle of MEG to water（37.95°）,and the adsorption rate of the obtained MEG to binary eutectic PCM was markedly increased.The form-stability and thermal-physical properties of H₂C₂O₄·2H₂O-NH₄Al（SO₄）₂·12H₂O/MEG CPCMs with different MEG mass fractions were investigated.The results indicated that the addition of 24%MEG could achieve the shape-stabilization of CPCM（without leakage）.The obtained CPCM had a phase transition temperature of 54.10℃,latent heat of 168.5J·g^-1,thermal conductivity of 4.318W·m^-1·k^-1as well as an excellent thermal reliability.The micro-morphology and pore structure analysis of CPCM revealed that the binary eutectic entered the pores of MEG,resulting in the partial disappearance of MEG pores,and the significant decrease in the specific surface area and pore volume.The new CPCM has a promising application in solar energy phase change heat exchanger.