Design And Construction Of Multilevel Titanium Dioxide Phase Change Material Microcapsules And Its Application

Phase change materials（PCMs）are widely known for their excellent heat storage and release properties during phase transitions.However,when solid-liquid transition occurs,PCMs are prone to leakage,which makes them limited to recycling.Moreover,due to their low thermal conductivity,most PCMs generate internal temperature gradients when heated,resulting in the decrease of heat conduction and thermal energy storage performance of PCMs.The packaging technology of inorganic materials as shell materials is a breakthrough technology to solve the above problems.It has the advantages of good thermal stability and high mechanical strength,and ensures good morphological stability while improving the thermal conductivity of phase change materials.Moreover,by designing multi-level shells for phase change microcapsules（Me PCMs）,the leakage of phase change materials can be prevented and their heat transfer performance can be improved,while expanding the application of phase change materials in different fields.（1）The challenges of fresh water scarcity and fossil energy storage make solar interface evaporation a promising and sustainable desalination technology.However,current interface evaporation systems mostly ignore the problems of intermittent solar irradiation and bacterial contamination.Therefore,a multi-level phase change microcapsule based on carbon black（CB）/chitosan（CS）modification was prepared as a photothermal material for desalination,and an interface evaporation system based on this multi-level phase change microcapsule was developed for efficient,antibacterial and salt-tolerant desalination.This multi-level phase change microcapsule uses n-docosane（n-22）PCM as the nucleus,and is wrapped by titanium dioxide（Ti O₂）inorganic shell material through interfacial polymerization to form a Ti O₂@n-22 microcapsule with a core-shell structure,and then modifies the CB/CS nanocomposite layer on the surface of Ti O₂.The results show that the multi-level Me PCM achieves a high latent heat energy of more than 140 J g^-1thanks to the dense Ti O₂inorganic shell coating.In addition,with the modification of CS and CB nanoparticles,multi-level Me PCM not only has95.04%sunlight absorption efficiency and good wettability,but also has excellent antibacterial and salt resistance.This innovative design enables the developed multi-level phase change microcapsule evaporation system to achieve a high evaporation rate of 2.58 kg m^-2h^-1and an evaporation efficiency of more than 90%under intermittent solar illumination.Compared with conventional evaporation systems without PCMs,the evaporation system developed in this study increased the total evaporated water volume by 1.03kg m^-2during 10 h of sunlight exposure in cloudy weather.The collected evaporated water has good vegetation compatibility and can meet the water quality requirements for crop growth in agricultural planting.This study provides a new approach for the development of high-performance interface evaporation systems with significant antibacterial and salt tolerance and sustained desalination capability under intermittent solar illumination.（2）The conventional core-shell structure of Me PCMs has certain limitations in terms of thermal conductivity and leakage prevention.To provide a solution to both of these problems,we designed a unique Pomegranate-like structure for Ti O₂@n-22 microcapsules to enhance their heat transfer and leakage resistance.The microcapsules were prepared by a controlled hydrolysis reaction in a non-aqueous emulsion template system and then the hydrolysates were polycondensation.In the synthesis process,the Pomegranate-like structure is derived from a two-layer emulsion system based on small micelles gathered inside and large spherical micelles outside,using a two-step emulsion technique.The hydrolytic polycondensation of TBT can be regulated by changing the droplet acceleration rate of deionized water,and Pomegranate-like Me PCM with high enthalpy of phase transformation is obtained.Literature search results show that its latent heat capacity is better than that of most Ti O₂-coated Me PCM reported in literature.Due to the unique Pomegranate structure of Me PCM,its heat transfer,thermal response and leakage resistance are improved.In addition,compared with the conventional core-shell structured Me PCM,the thermal conductivity of the obtained Pomegranate-like Me PCM was improved by 471%and the leakage rate was reduced by 66.5%.The novel Pomegranate-like Me PCM developed in this work has excellent thermal properties and has great potential for applications in thermal energy storage and management.