| Encapsulated phase change materials have wide application prospects in energy storage and conversion,device cooling,intelligent temperature control and so on.In the application of device cooling,due to the current development of electronic device chips and avionics towards high integration,high packaging density,small volume,etc.,the rapid increase of heat flux density of electronic devices causes the sharp rise of working temperature,which has serious effect on the performance of these electronic devices.Therefore,it is very important to improve the thermal response of phase change microcapsules in the working fluid,and thus can achieve high-speed cooling to remove the heat generated in time.To improve the thermal response of the encapsulated phase change materials,this work studies the phase change microcapsules from two aspects:reducing the particle size and improving the thermal conductivity of the shell.(1)For the particle size,the phase transition nanocapsules with smaller particle size are promising.Fatty acid phase change nanocapsules with respectively stearic acid(SA)and lauric acid(LA)as core and SiO2 as shell were successfully prepared by sol-gel process.The optimal phase change enthalpy of SA/SiO2 and LA/SiO2 nanocapsules was respectively 169.4J/g and 165.6J/g,while the encapsulation ratio was respectively as high as 71%and 85.9%.(2)Based on this,the influence of pH on the synthesis process is further explored,and the key steps to control the formation of phase change nanocapsules with the the formation mechanism was proposed:the key step for the combination of fatty acid and SiO2 is the coagulation process between SiO2 cluster and core emulsion droplet,and the monomer consumption rate in the silica sol forming SiO2 cluster is considered to be the key to the coagulation process.By adjusting the main conditions affecting the key factor of the preparation process in our experiments,the optimal particle size of fatty acid/SiO2 phase change nanocapsules was determined to be 340nm.(3)To further improve the thermal conductivity of the shell,fatty acids coated with high thermal conductivity metallic silver shell were synthesized in this study.The prepared SA/Ag,palmic acid(PA)/Ag and LA/Ag phase change nanocapsules have good thermal performance,and after the the fatty acids coated with Ag shell,the thermal conductivity of nanocapsules has been significantly improved,specially,the thermal conductivity of PA/Ag phase change nanocapsules reached 4.072W/(m·k).(4)The temperature response of composite phase change nanocapsules was finally studied by establishing a model.Based on the three-dimensional unsteady heat transfer,the heat conduction process of the phase-change nanocapsules in the fluid is simulated.In this paper,combined with the nano size effect and the micro scale heat transfer phenomenon,the change of the thermal conductivity of the materials based on the nano size is determined.The results show that the thermal response time of phase change nanocapsules with a particle size of 350nm is in nanosecond level,which is 5-6 orders of magnitude higher than that of microcapsules with a particle size of 100 ?m,and the thermal response time of SiO2 shell is 63ns later than that of silver shell with a particle size of 200nm,and the delay rate is 94.5%.Therefore,it is confirmed that reducing the capsule size and increasing the thermal conductivity of the shell material play an important role in improving the temperature response of the composite phase change capsule.In conclusion,this study will provide an important theoretical basis and technical support for the application of composite phase change nanocapsules. |
PDF Full Text Request