Preparation And Properties Of Paraffin-based Phase Change Heat Storage Materials

Due to the advantages such as stable heat transfer,compact structure,high energy storage density,and low cost,phase change heat storage technology is widely used for food preservation,lithium ion battery thermal management,building energy conservation and other fields.For lithium ion batteries,the huge heat generated during high-rate charge and discharge has become the main factor limiting the development of lithium batteries.Therefore,it is imperative to develop a suitable thermal management system to keep the operating temperature of the lithium battery within the optimal range.In this paper,paraffin(PA)/ expanded graphite(EG),which has appropriate phase change temperature,large latent heat,good thermal cycling stability and low cost,was selected as the research object.To further improve its thermal conductivity,the preparation conditions of EG were optimized,and addition of thermal conductivity enhancement materials such as copper and graphene were investigated.A composite phase change material with the best heat storage performance and cost was selected to evaluate the temperature control effect on 20 Ah lithium iron phosphate power battery.The main research contents and conclusions of this paper are as follows:1.To prepare modified expanded graphite(MEG)with excellent pore structure,small leakage of paraffin(PA)and good thermal conductivity,the commercial expandable graphite powder was modified by chemical oxidation,and the chemical oxidation conditions and expansion temperature were optimized.The results show that compared with expanded graphite(EG)without chemical oxidation treatment,the modified expanded graphite(MEG)prepared under the optimized conditions significantly improves the properties of composite phase change materials.The prepared 80%PA-20%MEG material exhibits latent heat of 203.9 J/g,and latent heat retention of 99.89% after 50 thermal cycles.In particular,its thermal conductivity increases to 4.089W/(m·K),which is 23.2% higher than that of 80%PA-20%EG material.The micro pore structure analysis reveals that the MEG powder obtained after chemical oxidation treatment has more pores,and the total pore volume and total pore area are increased by 18.4% and 16.4%respectively compared with EG,which increases the contact area with PA,thus reduces the leakage of PA of composite phase change material and improves the thermal conductivity.2.Using PA as the main material for phase change heat storage,MEG as the main thermal conductivity material and support material,and electroless copper-plated expanded graphite(CPMEG)as the thermal conductivity enhancement material,PA/MEG/Cu composite phase change materials were prepared,and the effects of CPMEG replacing part of MEG on the structure and heat storage properties of the composites were studied.The results show that a layer of metal copper can be uniformly coated on the surface of MEG by electroless copper plating to form a three-dimensional porous network structure with stronger thermal conductivity.Although the prepared 80%PA-14%MEG-6%CPMEG composite phase change material has a very low content of copper(0.768%),it can significantly improve the thermal conductivity without damage to the latent heat of phase change and thermal cycle stability.Its thermal conductivity is 17.8 times higher than that of paraffin.3.Using PA as the main material for phase change heat storage,MEG as the dominant thermal support material,and graphene aerogel(GA)as thermal conductivity enhancement material,PA/MEG/GA composite phase change materials were prepared,and the effects of GA replacing part of MEG on the structure and heat storage properties of the composites were investigated.The results show that graphene aerogel(GA)prepared by hydrothermal method and freeze-drying technology has a good three-dimensional porous network structure.The prepared80%PA-17%MEG-3%GA composite phase change material has low content of GA,but shows excellent thermal cycling stability and good thermal conductivity.Its thermal conductivity is increased to 5.336W/(m·K)without damage to phase change latent heat and thermal cycling stability,which is 30.5% higher than 80%PA-20%MEG.4.The 80%PA-14%MEG-6%CPMEG composite phase change material with low cost and excellent heat storage performance was selected to evaluate the temperature control effect on 20 Ah lithium iron phosphate power battery,and the battery was also compared with bare battery and 80%PA-20%MEG sandwiched battery.The results show that the heat dissipation capacity of 80%PA-14%MEG-6%CPMEG composite phase change material is the best.Under the ambient temperature of40 ℃ and 2 C rate charge-discharge,the temperature rise at the top(near the tab)of the bare battery is 13.2 ～ 13.5 ℃,and the temperature difference between the top and bottom of the cell is 4.9 ℃.However,the temperature rise near the tab of the 80%PA-14%MEG-6%CPMEG sandwiched battery is reduced to 4.8 ～ 5.8 ℃,and the temperature difference between the top and bottom of the battery is also reduced to2 ℃,showing a good temperature control effect.