Study On The Preparation And Heat Storage Properties Of Al-based Metal Phase Change Composite Materials

In recent years,the rapid development of the heat storage field has led many scholars to research phase change materials（PCMs）.As an important energy storage medium,PCMs can effectively alleviate the problem of time and space imbalance encountered in the development and utilization of renewable energy.The research of PCMs in the high-temperature field is particularly important for solar heat storage and industrial waste heat recovery.As PCMs,metals and their alloys have the advantages of high thermal conductivity,large heat storage density and good stability,making them potential PCMs in the field of high-temperature heat storage at present and in the future.However,it can be shaped and packaged due to potential safety hazards such as liquid phase leakage and high-temperature corrosion during the phase transformation process.Even if the PCMs are packaged,they will leak due to the volume change of PCMs during the phase transition process.Therefore,how to develop metal phase change materials with high heat storage capacity and stable thermal cycle tolerance is still a big challenge.To sum up,in this thesis,the high-temperature Al-based metal PCMs were selected for molding and packaging from both micro and macro perspectives.Using the water boiling principle of Al-based metal and the appropriate finalization packaging method,the problem of metal leakage was solved.Al-based metal phase change composite materials were successfully prepared,and carry out the optimization of heat storage performance and heat storage characteristics.Firstly,Al microsphere powder was used as phase change material and Al（OH）₃powder was used as composite matrix material,the mixture of the two materials was subjected to boiling pretreatment,and a shell coating composed of Al OOH（boehmite）and Al（OH）₃ was formed on the surface of the Al microspheres.After that,cylindrical phase change composite materials（PCCMs）were formed by cold pressing and then subjected to high-temperature calcination treatment under oxygen.Successfully prepared the Al microcapsules with a core-shell structure,and the microcapsules were stably embedded into the Al₂O₃ matrix of shaped Al/Al₂O₃ PCCMs.In contrast,when raw materials were not subjected to boiling treatment or Al（OH）₃ was replaced with Al₂O₃,there were varying degrees of leakage during the preparation process.To optimize the enthalpy of heat storage and the mechanical strength of the material,the optimal powder mixing ratio,boiling,and cold pressing conditions were explored.The thermal analysis test results showed that the phase transition temperature of the Al/Al₂O₃ PCCMs was about 653℃,and the latent heat of phase transition was about68 J/g.It is suitable for 650℃high-temperature thermal storage systems.After 100thermal cycles,the micro-core-shell structure and macro morphology of the samples remained stable,and all test data remained good according to the past,with good thermal cycle stability.Secondly,to explore the application of PCCMs in different temperature ranges,and taking into account the better physical and chemical properties of Al-Si alloy,Al-25Si alloy microsphere powder was further selected as phase change material,Al₂O₃powder or Al（OH）₃ powder as composite matrix raw material,and the method of"boiling pretreatment,cold-pressing molding and high-temperature calcination"was adopted.The Al-25Si/Al₂O₃ PCCMs suitable for 550℃high-temperature heat storage systems were successfully prepared.The effects of boiling pretreatment,Al₂O₃ or Al（OH）₃ powder as matrix raw materials,cold-pressing molding pressure,and other conditions on the successful preparation of PCCMs were investigated experimentally.The results showed that the PCCMs obtained by using Al（OH）₃ powder as the matrix raw material had higher enthalpy of phase transition,and there were different degrees of leakage during the preparation process without boiling pretreatment or excessive shaping pressure.The thermal analysis test results showed that the oxidation resistance of the samples with higher shaping pressure was better.The optimized samples had a phase transition temperature of about 572℃,and the latent heat of phase transition was about 111 J/g.The melting-freezing thermal cycle experiment results showed that the PCCMs had excellent thermal cycle stability and could be used in high-temperature heat storage systems at 550℃for a long time.Finally,in addition to adjusting the temperature application range of PCCMs by changing the selection of PCMs,it is also possible to expand its application by preparing composites with different structural forms,such as porous foams.Using Al microsphere powder and Al（OH）₃ powder as raw materials,first underwent boiling pretreatment,and then added the binder aluminum dihydrogen phosphate（Al（H₂PO₄）₃）,dispersant polyacrylamide（PAA）,and sintering aid magnesium oxide（Mg O）to the boiled powder raw materials to make a mixed slurry,which was freezing-drying and shaped in a mold.Finally,it was subjected to high-temperature heat treatment under oxygen.Through optimizing and adjusting the material ratio and freezing-drying conditions,the porous ceramic foam embedded with Al phase change microcapsules was successfully prepared.The test results showed that the phase transition temperature of the porous foam composite was about 655℃,the latent heat of phase transition was about 71 J/g,and there was no obvious change after a long-term cycle.The microstructure of the Al microcapsule core-shell structure,the macroscopic porous structure,and the encapsulation state remained stable.This kind of porous foam with phase change and temperature regulation function has a promising application as a catalyst carrier and high-temperature heat insulation.In summary,a series of Al-based metal PCCMs with different structural forms suitable for high-temperature storage systems at 500-700℃have been successfully prepared using the method of"boiling-pretreatment,cold-pressing or freezing-drying molding,and high-temperature calcination".The prepared PCCMs have good latent enthalpy and thermal cycle stability,which play a certain role in promoting the application of Al-based Metals in the field of high-temperature heat storage.