Study On Effect Of Shell Structure On Thermophysical Properties And Applications Of Microencapsulated Phase Change Materials

Microencapsulated phase change materials(MEPCMs)are tiny particles with organic or inorganic shell and phase change materials(PCM)core.Because of the protection of the shell,MEPCMs can overcome the leakage and large volume change during the phase change process of PCM.Moreover,the large heat transfer area and small size of the MEPCMs can increase the charging and discharging rate.These characteristics make MEPCMs have broad application prospects in industrial waste heat recovery,building energy conservation,intelligent textile,electronic heat dissipation,solar energy utilization,etc.At present,most of the research on MEPCMs focuses on the preparation of microcapsules,and the study on the influence of shell structure on the properties is not comprehensive,which limits the development and application of MEPCMs.To solve the problems mentioned above,this paper focuses on the effect of shell structure on the thermal properties and the applications of MEPCMs.Preparation,characterization and application of phase change microcapsules with different shell structures,as well as the preparation and performance of new latent heat functional thermal fluids have been studied.The innovative work and research results obtained in this paper are summarized as follows:(1)For the first time,the dual microfluidic technology was combined with UV curable method for the controllable preparation of phase change microcapsules,and the control mechanism of microfluidic was investigated by numerical simulation.To reveal the effect of shell thickness on the phase change process and thermophysical parameters of phase change microcapsules,the phase change characteristics and thermophysical properties of MEPCMs with different shell thickness were studied.MEPCMs with particle size of 600?m and shell thickness of 30?m,75?m and 150?m were obtained by adjusting the flow ratios of the inner phase and middle phase to 1:1,1:2 and 1:3,respectively.When the capsule shell thickness was 30?m,the enthalpy of phase transition was 161.6 k J/kg,the encapsulation ratio was about 73.36%,and the thermal conductivity was 0.152 W/(m·K).When the shell thickness of the capsule was 150?m,the enthalpy of phase change decreased to 67.3 k J/kg,the encapsulation ratio was about30.64%,and the thermal conductivity increased to 0.169 W/(m·K).After 100 heat cycles,the capsules would not be deformed or damaged.Experimental and simulation results show that the thickness of capsule shell increases with the increase of velocity ratio between the middle phase and the innermost phase.With the increase of the thickness of capsule shell,the enthalpy of phase change of the microcapsule gradually decreased,while the thermal conductivity,thermal stability and heat fatigue performance of the capsule gradually increased.(2)By constructing the shell of phase change microcapsule with micro-nano structure and testing its various properties,the influence rule of the shell micronano structure on the thermo-physical properties and multifunctional application of phase change microcapsule was revealed.Using the pikering emulsion and anhydrous interface polymerization method,a multifunctional MEPCM with Cu O-doped organic shell and eicosane core was successfully prepared.Due to the protection of the shell,the thermal decomposition temperature of MEPCMs increased by 40?compared eicosane,and the microcapsules had better thermal stability.the addition of copper oxide nanoparticles made the MEPCM had higher photothermal conversion efficiency and ultraviolet shielding performance.Compared with the MEPCMs without copper oxide,the photothermal conversion efficiency of multifunctional phase change microcapsules was about 10 times higher,and the shielding effect of UVA rays was about 30%higher.The dendritic structure on the surface of the capsule made the multifunctional phase change microcapsule had a contact angle of 148.3°and showed super hydrophobic characteristics.The fabric coated with MEPCM had phase change latent heat of 36.8k J/kg.The thermal cycling experiment confirmed that the intelligent fabric had a good temperature regulating function.In addition,the contact angle of intelligent fabric was tested,and it was found that the intelligent fabric had the characteristics of super hydrophobic due to the existence of super hydrophobic microcapsules,and the contact angle was 141.6°.(3)Sol-gel method was used to prepare MEPCMs with inorganic shell,and their microstructures and thermo-physical properties were tested to reveal the influence of inorganic material shells on nucleation and heat transfer characteristics of MEPCMs.The MEPCM and CNT were added into deionized water to prepare the hybrid MEPCM/CNT suspension,and their dispersion stability,thermal property and heat transfer characteristics were studied to reveal the influence of dual system on the thermophysical properties and photothermal conversion characteristics of latent functional thermal fluids.The results showed that Ti O₂ shell enhanced the thermal properties of MEPCM,the thermal energy storage efficiency of MEPCM was over 98%.The thermal conductivity of the capsule was 0.49W/(m·K)higher than that of octadecane.Compared with pure octadecane,the thermal decomposition temperature of MEPCM was up to 20?.The capsules and CNTs had synergistic effect in the base solution,and the network structure formed by the CNTs in the base solution could delay the agglomeration and sedimentation process of the MEPCMs.Compared with the MEPCM suspension,the hybrid suspension had better dispersion stability and thermal properties.The thermal conductivity and specific heat of hybrid suspension were up to0.83 W/(m·K)and 6.57 J/(g·K),which was 19.7%and 56.4%higher than that of pure water.The hybrid suspension had a high spectral absorption rate in the whole band,and the photothermal conversion and energy storage efficiency of the hybrid suspension can reach up to 86%.(4)High temperature molten salt phase change microcapsules were synthesized by a sol-gel method in ethanol.The effect of Ti O₂shell on the thermal properties of MEPCMs was studied.The MEPCMs were mixed with high temperature thermal oil to prepare high temperature latent heat functional thermal fluid suitable for above 300?.The physical properties of the microcapsules were tested,and the influence of molten salt MEPCMs on heat transfer and rheological properties of high temperature latent heat functional thermal fluid was revealed.When the ratio of sodium nitrate to TBT was 3:2,the optimal high-temperature MEPCM can be obtained.The capsule's encapsulation ratio was about 74.60%,the thermal conductivity was higher than 0.78 W/(m·K),and the capsules had high thermal stability,the mass loss was less than 5%after 200 thermal cycles.DSC test results with different temperature rising rates showed that the actual charging and releasing rate of MEPCMS were greatly affected by the external temperature changing rate.High temperature changing rate could significantly improve the charging and releasing rate of MEPCMS,but this effect was rapidly weakened after exceeding 15?/min.The concentration of MEPCM in thermal oil can affect the dispersion stability of functional thermal fluid.When the mass concentration was less than 4 wt.%,the MEPCMS can be well dispersed in thermal oil.The thermal conductivity and specific heat capacity of the functional thermal fluid were significantly improved after the MEPCMS were added into the thermal oil.When the mass concentration of the capsule was 4 wt.%,the thermal conductivity and specific heat capacity of the functional thermal fluid were 18.4%and 131.7%higher than that of the pure thermal oil.However,the phase change microcapsule will lead to the increase of the viscosity of the base solution and shear thickening.