Investigations On The Design,Fabrication And Characterization Of Thermal Switch With A High Switch Ratio Based On The High-Thermal-Conductivity Graphene Paper

Graphene,as a two-dimensional material,has a quite high thermal conductivity,high mechanical strength and high conductivity,and has been widely paid attention by researchers since it was discovered.In recent years,with the development of research,there have been various graphene composites emerged to use the high thermal conductivity of graphene.For example,graphene metal composite,graphene polymer composite,graphene nanofluids,etc.,the graphene was used as a matrix or filler to improve the comprehensive thermal conductivity of the composite materials.However,due to the contact interface,agglomeration and uniformity of various components,the thermal conductivity of the composite materials can not achieve ideal performance.Under this circumstance,it is very important to find new methods,new technologies and new processes to improve the comprehensive thermal conductivity of the composite materials and make fully use of the high thermal conductivity of graphene.In this thesis,the tin/graphene paper composite material was prepared and its thermal conductivity was characterized.Then a heat switch was fabricated based on the tin/graphene paper composite material,and the static force was used to drive the heat switch.The main contents and results of this thesis are summarized as follows.The shape memory alloy heat switch was made using the graphene paper with a high thermal conductivity.The structure and working principle of the heat switch are introduced.The thermal conductivity of the switch is simulated and studied experimentally.In the experiment,the temperatures of the inlet and outlet of the heat switch were measured under different linear heat flux densities,and the relationship between the heat flux density and the temperature difference between the inlet and outlet was obtained.The switch ratio of the heat switch obtained in the experiment is 44.3±4.2,and the better thermal adjustment performance was procured.The tin/graphene was made on copper foil substrate by a melt casting approach.The composition,morphology and structure of the composite were studied using the XRD,SEM,Raman and FTIR spectroscopy.The results showed that the thermal conductivities of three samples with thickness of 1.5-3 mm were in the range of 600 to 800 W/m-K.The composition and structure did not change significantly,which reveals that the high thermal conductivity of the composite is come from the graphene paper.The compression and stripping tests showed that the compressive strength of the composite was slightly lower than that of the pure tin metal.A simulation based on the molecular dynamics showed that it is consistent with the experimental results.Selection and manufacture of the heat switch components driven by an electrostatic force.The tin/graphene composite made in Chapter 4 was embedded in the silica gel,and the tin/graphene composite material was used as the main thermal conducting material to make the base of electrostatic driven(ESD)heat switch.The heat per unit time transmitted through the substrate was increased.In terms of a radiator,PDMS was used as the insulation layer to make radiator by sticking PDMS on copper foil.When the state of ESD heat switch is "off",the heat can be transmitted as much as possible,but not emitted;when the switch is "on",the heat can be transferred up and quickly lost to the environment,which overcomes the disadvantages of the low thermal conductivity in the traditional ESD heat switch using polymer as a substrate.A gold coating was sputtered on the lower surface to reduce the emissivity.The upper surface of copper electrode of radiator was coated with discoloration ink.In the "off ' state,the temperature of the discoloration ink is lower than the temperature at which the color changes,thus the surface emissivity is low,reducing the radiation of heat.In the "on" state,however,the temperature of the ink is higher than the temperature at which the color changes,thus the surface emissivity is high,enhancing the radiation of heat.Hence,the switch ratio of the heat switch is improved.In terms of comparison of different materials,better materials were selected as the components of the ESD heat switch,which were further assembled to form a workable switch for test.Comprehensive test of the performance of the ESD heat switch.(1)Measurement of the temperatures of the key components and the environment temperature in the "on"and "off"states of the heat switch.Without considering the convection heat transfer,the heat transfer of the heat switch was calculated in the "on" and "off"states respectively,and then the switch ratio of the heat switch;(2)Calculation of the heat dissipation in the "on" and "off" states considering the convection and radiation;(3)In optimization of the structure of the heat switch for the specific boundary conditions,and alalysis of the system error and measurement error.The experimental results show that the switch ratio is 234.9±25.5 when the distance between the up and down plates is 5 mm.At the end of the thesis,the future work and the improvement measures are also proposed.