Fabrication And Heat Transfer Performance Of Ultrathin Flexible Heat Pipe With Vapor-liquid Integrated Wick

Ultrathin flexible heat pipe(UFHP),as an ideal solution to solve the heat dissipation problem of flexible electronics such as foldable smartphones and flexible thin-film phased array antennas,has become the focus of current industry and academia.At present,UFHPs generally have problems such as thick,low thermal conductivity,and poor bending performance.Optimizing the vapor channel structure,wick structure,and shell structure of UFHPs is the only way to improve its comprehensive performance.Inspired by the structure of Microsorum fortunei leaf,a vapor-liquid integrated wick(VLIW)is proposed in this paper.Taking UFHPs with VLIW as the research object,the structural optimization and corresponding mechanism analysis of UFHPs is conducted based on the surface morphology regulation of VLIWs and the functional structure design of the shell.The main research contents are as follows:(1)Fabrication and morphology control of VLIWInspired by the structure of Microsorum fortunei leaf,VLIW is fabricated by hightemperature sintering.Three layers of sintered copper mesh mimic vein structure are used to transport the liquid working medium.An array of porous columns mimics the spore structure to support the shell and form the vapor channel.The surface morphology of VLIW is adjusted by alkali-assisted oxidation and high-temperature hydrogen reduction.The effect mechanism of height,particle size distribution of the porous column,and surface topography of the wick on the capillary performances of VLIW are systematically studied.The influence of surface morphology on the boiling heat transfer performance of VLIW is studied and a theoretical relation model of capillary performance and boiling heat transfer performance of VLIW is established,which laid a solid foundation for the fabrication of high-performance UFHPs.(2)Design and fabrication of UFHPs with VLIWThe bending fatigue performance of VLIW and the effect of porous column arrangement on the deformation of the shell are studied.The high structural strength of the VLIW and the pressure of the shell on it ensure that the porous column does not fall off under repeated bending conditions.The arrangement of triangular porous columns is determined according to the mechanical model of shell deformation to reduce shell deformation.The optimization scheme of heat transfer and bending performance of UFHP is designed.The laser processing technology is used to remove the ONy layer outside the aluminum-plastic film to reduce the shell thermal resistance,thereby improving the heat transfer performance of UFHP.The corrugated structure is embossed in the middle of the shell to improve the bending performance.A fabrication method of UFHP based on hot pressing technology is developed.UFHPs with thicknesses of0.53～0.81 mm are fabricated by adjusting the thickness of the vapor channel.(3)Study on heat transfer mechanism and performance of UFHP with VLIWThe capillary limit heat transfer model and thermal resistance network model of UFHP are established,and the startup performance and heat transfer performance test system is built to study the effects of filling ratio,vapor channel thickness,and bending angle on the heat transfer performance of UFHP.Shell collapse during bending is the key factor leading to the decrease in the thickness of the vapor channel and the thermal conductivity of the UFHP.However,the thermal conductivity of the UFHP with a thicker vapor channel(0.45mm)remains almost unchanged during bending.Under the natural convection condition,compared with the copper sheet of the same thickness,the UFHP reduces the temperature of the heat source by about 15.6-19.6%.The thermal conductivity of the UFHP keeps almost unchanged within 15 days.(4)Effect of shell structure and thermal resistance on heat transfer performance of UFHPThe influence of corrugated structure and thermal resistance of shell on the heat transfer performance of UFHPs is studied.The results show that the thermal conductivity of the UFHP can be increased by 61-67.2% to 2333.1-2423.7 W/m·K by reducing the thermal resistance of the shell.The corrugated structure of the shell has a smaller elastic modulus,which absorbs part of the stress variable in the bending process,thereby avoiding the plastic deformation of the shell during the bending process.The thermal conductivity of the corresponding UFHP only attenuates 11.8-12.9% after repeated bending 50 times,while that of the UFHP without corrugated structures attenuates 73.2% under the same conditions.Under natural convection conditions,the optimized UFHP still maintains good temperature uniformity after bending 100 times,while the original UFHP shows an obvious low-temperature zone.