| With the increasing attention to environmental protection and the continuous implementation of national carbon-neutral strategy,power battery-driven ships have been gradually used in rivers,lakes.Power cells need to be kept at a suitable temperature because of their efficiency and safety.Paraffin wax is the main phase change material used in battery thermal management.It is easy to cause the thermal runaway of battery components because of its flammability.Polyethylene(PEG),which is similar to paraffin and non-flammable,may become the main phase change material for thermal management of batteries in the future.Therefore,this paper will focus on the issue of the leakage of PEG and the improvement of its comprehensive performance,through the study of the modification of PEG,to analyze the mechanism of leakage,heat conduction enhancement and so on.The main research contents of this paper are as follows:A new preparation method of composite phase change material to solve the leakage of PEG was proposed.A new type of shaped flame retardant phase change material was prepared by using the PEG1500·CaCl2 coordination polymer generated by PEG and CaCl2 in absolute ethanol as the support structure and the excess PEG1500 as the phase change heat storage unit.The optimum molar ratio between PEG1500 and CaCl2 is 1:3.1.A new preparation method of composite phase change material to solve the leakage of PEG is proposed,that is,a new shaped Flame Retardant phase change material is prepared by taking the PEG1500·CaCl2 coordination polymer generated by PEG and CaCl2 in absolute ethanol as the support structure and the excess PEG1500 as the phase change heat storage unit.The optimum molar ratio between PEG1500 and CaCl2 is 1:3.PEG is a kind of flame retardant material,the addition of filler improves the thermal stability of PEG1500·CaCl2 at high temperature,especially the two-dimensional lamellar structure of graphene can reduce the heat release rate during combustion of composite phase change materials,give play to the physical barrier effect,and further strengthen the flame retardant performance of PEG1500·CaCl2.2.The thermal conductivity enhancement and its mechanism of PEG1500·CaCl2 were studied.PEG1500·CaCl2 has a very low thermal conductivity,which seriously affects the heat dissipation efficiency of the battery thermal management system.Adding high thermal conductivity filler is one of the most simple and effective ways to enhance the thermal conductivity of materials.In order to understand the influence of filler size,material and shape on the thermal properties of PEG1500·CaCl2 and obtain the composite phase change material most suitable for battery thermal management system,three nano fillers,multiwalled carbon nanotubes,graphene and nano copper,were selected for comparative study.Multi-walled carbon nanotubes and graphene belong to carbonbased materials,and nano copper is a kind of metal material with high thermal conductivity,The changes of thermophysical properties before and after its action on PEG1500·CaCl2 shaped Flame Retardant phase change material were studied.The research shows that multi-walled carbon nanotubess,graphene and nano copper will not react with PEG1500·CaCl2,and the filler and substrate are physically mixed.The dispersion of graphene in PEG1500·CaCl2 is worse than that of multiwalled carbon nanotubess and nano copper,and certain agglomeration will occur,which will affect its thermal conductivity.However,the strong covalent bond of graphene can increase the thermal conductivity path,and graphene itself has a high thermal conductivity.Therefore,the improvement results of filler on the thermal conductivity of PEG1500·CaCl2 is:multi-walled carbon nanotubess>graphene>nano copper(the thermal conductivity of the composite increased from 0.23 to 0.78,0.52 and 0.39 W/(m·K)).In addition,the effect of carbon fiber on PEG1500·CaCl2 is studied.The composites containing carbon fiber felt have better thermal stability and higher thermal conductivity.However,due to the poor bonding ability between carbon fiber and PEG and large latent heat loss after multiple cycles,they are not suitable for practical application.3.The effects of filler size and material on phase transformation latent heat and temperature of composites were studied.The existence of fillers occupies the space of polymer phase change materials in 3D skeleton and affects the latent heat of composite phase change materials.With the increase of filler size,its latent heat decreases and the phase transition temperature increases.Because it is also a good nucleating agent,it can improve the recrystallization rate of PEG and offset the latent heat loss caused by their occupied space.The results show that the influence of several nano fillers on the latent heat of composite phase change materials is:nano copper>graphene>multi-walled carbon nanotubes.4.The phase change hysteresis of phase change materials and its relationship with battery temperature control were studied.The results show that the heat transfer of the composites in the solidification process is dominated by heat conduction,while the melting process is the synergistic effect of heat conduction and convection.The thermal diffusivity in the solidification process is less than that in the melting process,and the melting rate is faster than that in the solidification process.Under the combined action of thermal resistance,PEG1500·CaCl2 has an obvious phase transition lag phenomenon.The existence of phase change lag increases the solidification difficulty of phase change materials,destroys the uniform melting of phase change materials in the process of battery charge and discharge cycle,and also destroys the temperature equilibrium between its monomers;In addition,it will reduce the cycle life of the whole cooling system.5.The phase change hysteresis of phase change materials and its relationship with battery temperature control were studied.The results show that the heat transfer of the composites in the solidification process is dominated by heat conduction,while the melting process is the synergistic effect of heat conduction and convection.The thermal diffusivity in the solidification process is less than that in the melting process,and the melting rate is faster than that in the solidification process.Under the combined action of thermal resistance,PEG1500·CaCl2 has an obvious phase transition lag phenomenon.The existence of phase change lag increases the solidification difficulty of phase change materials,destroys the uniform melting of phase change materials in the process of battery charge and discharge cycle,and also destroys the temperature equilibrium between its monomers;In addition,it will also reduce the cycle life of the whole cooling system.6.The charge and discharge experimental platform of lithium-ion battery was built,and the temperature control effect of phase change materials used in lithiumion battery was studied.The results show that PEG1500·CaCl2 can effectively control the temperature rise of different types of lithium-ion battery packs,and the heat storage performance of phase change materials remains stable after multiple cycles.It is suitable for ships and other vehicles with long service life cycle.This study provides a theoretical basis for further exploring the relationship between flame retardant shaped composite phase change materials and phase change hysteresis and material properties in the future,and provides a design basis and new research and development ideas for phase change materials used in battery thermal management system. |
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