Study On Preparation And Thermal Energy Storage Properties Of Copper-Based Phase Change Macro-Capsules By Introducing Cavity Strategy

The development of reliable phase change materials（PCMs）is important for the high-temperature thermal energy storage in various energy systems such as solar thermal power generation and industrial waste heat recovery.Metallic PCMs are highly desirable for their excellent thermal storage density and thermal conductivity.However,metallic PCMs are prone to leakage and corrosion during the phase change process,requiring them to be encapsulated in core-shell structures prior to use.However,developing high-temperature phase change capsules with excellent heat storage capacity and long-term cycling stability remains challenging due to the volume expansion of metallic PCMs during high-temperature phase changes,which can rupture the capsule shell.Therefore,in this thesis,the preparation of macrocapsules based on high-temperature copper-based PCMs were investigated.The material properties and thermophysical properties of the capsules were characterized.The thermal cycling stability of the capsules with cavities were evaluated.The effects of core-shell size and cavity volume on the thermal storage performance of the capsules were explored.The main work of the thesis is as follows.Firstly,Cu powders were used as raw material,and they were spherulitized into millimeter/centimeter-sized core balls by“powder spheroidizing and pressing method”.These core balls were then coated with Al₂O₃ green bodies by the same method.After two-step sintering heat treatment,the Cu@Al₂O₃ capsules with cavities between the shells and cores were successfully prepared.To optimize the heat storage density of Cu@Al₂O₃ capsules,the size of the core was increased,the thickness of the shell was reduced,and the core green body was isostatically pressed.Cu@MgO-Si O₂/Al₂O₃macrocapsules with a larger core,thinner shell and more suitable cavity were successfully prepared.Thermal analysis showed that the core had a melting point of about 1062°C and a latent heat of about 210 J/g.Melting-solidification thermal cycling tests showed that Cu capsules had excellent cycling stability.In addition,the Cu capsule had a high heat storage density,which the capsule with a size of 9@1.5-100MPa showed high values of 222 J/g and 842 MJ/m³ in the temperature range of 1000-1100℃,respectively.Secondly,in order to regulate the working temperature of the Cu capsule,Cu-Si PCMs were synthesized by in situ alloying formation and encapsulated.Cu and Si powders were mixed to obtain uniform Cu/Si powders,which were then spheroidized into millimeter/centimeter-sized Cu/Si powder balls.Subsequently,Cu/Si powder balls were coated with Al₂O₃ shells to form capsule green bodies.After two-step sintering heat treatment,Cu-Si@Al₂O₃ capsules suitable for 800℃were successfully prepared.The thermal storage capacity of Cu-Si capsules was optimized by adjusting the core-shell size and cavity volume.Melting-solidification cycle tests showed that the Cu-Si capsules had excellent cycle stability and could be reused for a long time.In addition,the Cu-Si capsules also exhibited excellent heat storage properties.The capsule with the size of 9@1.5-150MPa was 198 J/g and 816 MJ/m³ in the temperature range of 750-850°C.Finally,the phase change temperature can also be adjusted by blending Cu-Ge alloy,in addition to blending Cu-Si alloy.Using Cu and Ge powders as the core raw materials,Cu-Ge@Al₂O₃ macrocapsules suitable for 600℃were successfully prepared by“powder spheroidizing and pressing method”and two-step sintering heat treatment.The thermal storage capacity of the capsules was optimized by adjusting the core-shell size and reducing the cavity.The results of thermal cycle tests showed that Cu-Ge capsule had good thermal cycle stability.In addition,Cu-Ge capsule had high heat storage capacity,in which the capsule with the size of 9@1.5-100MPa was 211 J/g and719 MJ/m³ in the temperature range of 600-700℃.In conclusion,a series of copper-based macrocapsules suitable for 600-1100°C were successfully prepared by“powder spheroidizing and pressing method”and two-step sintering heat treatment in this thesis.The as-prepared capsule exhibit excellent thermal cycle stability and high heat storage density,contributing significantly to the development of heat storage technology in the high temperature field.