Study On Preparation And Properties Of Composite Phase Change Material Of Expanded Graphite/Eutectic Salt

Thermal energy storage technology is an important technology to solve the contradiction between energy supply and demand,so as to improve energy efficiency.The core of this technology is the development of phase change materials?PCMs?with excellent performance and low cost.In many industries such as fruit and vegetable preservation,beer industry,chemical industry,medicine,cold chain logistics,etc.,the PCMs with phase change temperature below 0°C have a very large application space.The eutectic salt is the most important type of low temperature PCMs,it has the advantages of large latent heat,large heat storage density and low price.However,it also generally has problems such as large supercooling degree,phase separation,and low thermal conductivity.For solving these problems,combining eutectic salt with porous supporting materials is one of the most effective methods.Expanded graphite?EG?is a functional carbon-based material with high thermal conductivity and high porosity,which can be used to encapsulate PCMs to produce high thermal conductivity shaped composite PCMs.However,since the hydrophobic nature of EG leads to its poor compatibility with eutectic salts,it is of great significance to find a suitable method to increase the compatibility of EG with the eutectic salts.In this work,the surface modification of EG was first studied.The surface of EG was coated with hydrophilic Al₂O₃ coating via a heterogeneous nucleation technique and subsequent heat treatment.The effect of different Al₂O₃ coating amount on the hydrophilicity of EG was explored.The results showed that when the coating amount of Al₂O₃ was 4.4 wt%,the contact angle of MEG with water was 33.9°,which was much smaller than that of unmodified.Meanwhile,the molecular dynamics?MD?simulation study on the hydrophilic properties of EG and MEG was carried out,the values of the contact angle for EG and MEG obtained through simulation are very close to that obtained from experiment.In addition,the distribution situation of Al₂O₃ on the surface of the EG was characterized by FE-SEM and it was found that a uniform distribution of Al₂O₃ on the EG surfaces.XRD analysis indicated that the Al₂O₃ coating on the EG surfaces were essentially amorphous.According to XPS and FTIR analysis,it was known that Al₂O₃ was well-bonded to EG substrate via chemical interactions between the O atoms in Al₂O₃ and C atoms on the EG surfaces.Moreover,the thermogravimetric analysis demonstrated that the oxidation resistance of the MEG is obviously enhanced.In this work,the eutectic salt solution was prepared by mixing 6 wt%K₂HPO₄·3H₂O,6wt%NaH₂PO₄·2H₂O,6 wt%Na₂S₂O₃·5H₂O and 82 wt%deionized water.The nucleating agent was added to reduce supercooling degree of the eutectic salt,and the effect of different mass fraction nucleating agents on its supercooling degree was explored.The experimental results show that when 1 wt%Na₂B₄O₇·10H₂O?borax?was added,the supercooling degree of the eutectic salt can be lowered to 1.84?.The composite PCM was prepared by adsorbing K₂HPO₄·3H₂O–NaH₂PO₄·2H₂O–Na₂S₂O₃·5H₂O–H₂O eutectic salt by MEG block.Within120 minutes,the adsorption capacity of MEG to eutectic salt reached 80.71 wt%,which was much higher than 4.84 wt%of unmodified EG,indicating that the surface modification of EG greatly enhances its adsorption performance to eutectic salt.In addition,FT-IR analysis showed that the MEG did not undergo a chemical reaction during the adsorption of the eutectic salt.The composite PCM has a phase change temperature of-5.30?,a latent heat of161.8 J/g,and a supercooling degree of 1.83?,the thermal conductivity of the composite PCM is 8.90 W/?m·K?,which is 13.3 times as large as that of the eutectic salt.Moreover,the400 times thermal cycle tests demonstrated excellent thermal reliability of the composite PCM.Suitable phase change temperature,high latent heat,high thermal conductivity,low supercooling degree and excellent thermal reliability endow K₂HPO₄·3H₂O–NaH₂PO₄·2H₂O–Na₂S₂O₃·5H₂O–H₂O eutectic salt/MEG composite PCM with broad application prospects in the fields of beer industry,air conditioning refrigeration,refrigerator refrigeration and cold chain logistics.