Research On The Mechanism Of Heat And Mass Transfer At The Interface Of Vertically-Oriented Graphene For Heat-transfer Medium And Heat Recovery Application

With the continuous development of electronic devices in the direction of high power-density,efficient heat transfer and conversion technology is an important technical support to adapt to such development of miniaturization and integration of electronic devices.In recent years,graphene,a two-dimensional nanomaterial,has attracted extensive attention from researchers and industries due to its excellent electrical and thermal properties,stable chemical properties,and low cost.The heat and mass transfer mechanism of graphene-based materials in nanoscale plays a key role in its macroscopic performance in practical applications.However,the current research on its related mechanism and precise regulation in fabrication needs to be further in-depth researched.In this dissertation,the enhanced heat and mass transfer process at the interface of vertically-oriented graphene（VG）is carried out by various means of experiments and numerical simulations.By exploiting the unique orientation of VG,continuous heat flow channel is constructed in an efficiently thermally conductive medium.For the more microscopic interfacial process of heat transfer,the VG nanofin structure is applied to study the interfacial heat transfer.The VG nanofins are connected to the substrate through covalent bonds.The contribution of such structure in enhancing the overall system heat transfer is characterized by experimental method.In terms of technical application,the comprehensive advantages of VG in interfacial heat transfer and ion adsorption are exploited,and a thermo-induced electric double-layer capacitor device that can convert thermal energy into electrochemical energy is designed,and the energy conversion efficiency of the carbon-based electrode is improved.By growing vertically oriented graphene on an electrically insulating boron nitride film and adding polymethyl methacrylate（PMMA）,a thermal interface material（TIM）with both highly thermally conductive and electrical insulating properties is obtained.The test comparison shows that the vertically oriented graphene takes full advantage of the extremely high in-plane thermal conductivity of graphene,and together with the boron nitride film,a highly oriented thermal conductive framework can be constructed in the thermal interface material.This structure has a filling ratio of 29.3 vol%.The thermal conductivity of the polymer PMMA is increased from 0.2 W m^-1 K^-1 to 4.03 W m^-1 K^-1,effectively improving the thermal conductivity of graphene and other highly thermally conductive fillers.improved efficiency.Using the improved effective medium theory,the microscopic heat transfer process is analyzed,and the influence of related parameters such as the thickness of the interlayer film on the thermal conductivity of the overall composite is studied.And using the finite element model analysis method,it is found that the increase of nanosheet size in a certain range can promote the thermal conductivity of the overall composite material.Using VG as a basic material for heat dissipation of practical high power density devices,a good heat dissipation effect is obtained,proving the potential of VG in improving the thermal conductivity of polymers.In the process of heat transfer in mixed media,the thermal boundary resistance between internal materials has become one of the important factors hindering the thermal conductivity of the overall system.And it is difficult to measure the thermal boundary resistance between graphene and water accurately,hindering the characterization of heat transfer across the interfaces.The 3D graphene structure with covalent-bonding nanofins is prepared by one-step plasma enhanced chemical vapor deposition（PECVD）method.In this structure,the VG nanofins and the substrate are firmly connected by covalent bonds,which not only ensures the stability,but also maintain the heat transfer efficiency inside the graphene skeleton,and also greatly enlarges the heat transfer area.Under the effect of nanofins,the interface heat transfer efficiency between graphene and water is improved.Through molecular dynamics simulation,it is calculated that the thermal boundary resistance between the edge of VG and water is significantly smaller than the value in cross-plane direction between graphene and water.Under the help of the stability of the three-dimensional structure,the thermal conductivity of the aqueous medium can be characterized by the laser flash method.It can be seen that under the same volume filling ratio,the thermal conductivity of the system with vertically oriented graphene nanofins enriched skeleton is about 5%higher.Under the filling ratio of 0.26 vol%,the thermal conductivity of such medium is increased to 2.61 W m^-1 K^-1.The effect of VG nanofins in reducing the thermal boundary resistance and improving the thermal conductivity of the overall mixed medium can be obtained qualitatively through this testing method.On the basis of taking full advantages of the comprehensive properties of VG in interfacial heat transfer,large specific surface area and high electrical conductivity,a VG based thermo-induced electric double-layer capacitor（EDLC）is designed.Such EDLC can be used to recycle the waste heat spread in tiny spaces into electrochemical energy stored in the devices.After surface ozone treatment of VG,the ionic Seebeck coefficient of VG based thermo-induced EDLC under temperature gradient is improved by～4.6 times from 0.18 m V K^-1 to 1.0 m V K^-1,and this value is much higher than that of those carbon-based materials.Density functional theory calculations are conducted to attempt to reveal its inner mechanism.The simulation results show that the functional groups on the graphene surface increase the polarity of the electrode and the efficiency of charge transfer during the ion adsorption process.Meanwhile,the energy storage performance of the EDLC after ozone treatment is improved by～1.29 times.Therefore,the energy recovery performance of VG based thermo-induced EDLC after ozone treatment increases by～34 times within a single thermal charge period.This result demonstrates the importance of the electrode-electrolyte interaction for thermal energy recovery in thermo-induced EDLC and the promising potential of carbon-based materials in thermo-induced EDLC devices.