| The products in the fields of aerospace,automobile,electronics and the like are constantly being upgraded,which promotes the development of advanced electronic products to high performance,high power density,miniaturization and even micromation.The resulting challenge is how to effectively spread the high heat flux and maintain the electronics in the proper operating temperature range.It is an effective method to use high efficient heat dissipation materials to assist in passive thermal control of the equipment.Carbon materials are ideal choices for heat dissipation due to their excellent mechanical properties,superior thermal conductivity,high temperature resistance,low coefficient of thermal expansion,outstanding chemical stability and low density.Graphene is the basic building block of many carbon materials,and its theoretical thermal conductivity can be as high as 5300 W?m-1?K-1.Although its thermal conductivity is much higher than that of known materials,it is difficult to use it directly in heat dissipation equipment.It is necessary for graphene engineering application to explore how to assemble graphene into macroscopically three-dimensional carbon materials in an orderly manner and perform their excellent thermal conductivity.In addition,graphite films take the lead in the field of engineering electronics heat dissipation because of its large-scale preparation and good thermal conductivity.However,the measurement of in-plane thermal conductivity of carbon films also faces some problems due to their unique lattice orientation,anisotropy and thin thickness.Moreover,without changing the external environment,the heat exchange capacity can be enhanced by the heat conduction and heat radiation relying on the heat-dissipating materials,but the radiation characteristics of the carbon film are still unknown.Therefore,in order to strengthen the heat dissipation performance of carbon film materials,the paper has carried out research on the preparation method of graphene macroscopically materials,the measurement of thermal conductivity of carbon film and the regulation of carbon film radiation characteristics.The specific contents are as follows:1.Preparation and characterization of reduced graphene oxide filmIn this paper,a highly dispersed aqueous graphene oxide solution was prepared by modified Hummers method,and the self-assembled graphene oxide(GO)film can be obtained at the gas-liquid interface.Subsequently,the graphene oxide film was thermal reduced at different temperature and the maximum reduction temperature was up to 2800°C.Combined with AFM,SEM,XRD,Raman and XPS and other characterization methods,the interlayer distribution and chemical composition of the self-assembled GO film were analyzed,and the elemental composition,interlayer structure and the degree of lattice repair of the reduced graphene oxide(r GO)films were investigated.Meanwhile,their mechanical properties and thermal conductivity were qualitative analyzed.The results show that the high temperature thermal reduction can remove the oxygen-containing functional groups in GO,restore the sp2 hybrid carbon lattice structure and improve the thermal conductivity.The higher the temperature,the higher the degree of reduction.Furthermore,the microbubbles formed after graphitization at 2800°C provide the required expansion area for the outer surface when bending the r GO films,endowing the films with a higher flexibility.2.Measurement of thermal conductivity of carbon filmsIn this paper,the in-plane thermal conductivity of carbon film was measured by T-type steady-state method.The test principle of T-type steady-state method was deduced in detail,and the influence of error in the test process was analyzed.On this basis,the sample copper frame was designed and processed and the test system was established.The thermoelectric parameters of the platinum wire were corrected by the hot-wire method,and the platinum wire with the known thermal conductivity was measured as the fiber to be tested.The results confirm the reliability of the test method and the system.Subsequently,the thermal conductivity varying with temperature of GO films?r GO films and graphite films was measured.The results show that the thermal conductivity of r GO increases with the increase of the reduction temperature.When GO was reduced at 2800°C and subjected to the extrusion process,its thermal conductivity can be increased to 336.9W?m-1?K-1?436.0W?m-1?K-1,showing a trend of increasing first and then decreasing within the test temperature range.And the thermal conductivity of graphite film with the same thickness increases first and then decreases in the temperature range of 125K?375K,and the thermal conductivity decreases with the increase of thickness.The graphite film of 17?m has the highest thermal conductivity(1600W?m-1?K-1)at approximately 175K,and the thermal conductivity is 900W?m-1?K-1 when the temperature is increased to 375K.3.Radiation characteristics of carbon film and its improvementsIn this paper,the spectral reflectivity in different bands of r GO and graphite films was measured,and the respective solar absorptivity and infrared emissivity were calculated.The results show that the high ratio of solar absorptivity and infrared emissivity(?s/?)up to 1.62?2.03 is not suitable for heat radiation applications.So,the graphite film was selected as the representative,and it is skillfully combined with silver-PI to obtain a composite film structure with the high thermal conductivity and excellent radiation performance.Through the spectral reflectance test,the solar simulator irradiation experiment and the experiment and simulation of the concentrated heat flux diffusion,the results show that the?s/?of G-Ag-PI is effectively reduced.Under the same solar irradiation,the G-Ag-PI surface can reduce the absorption of solar radiation by 37%compared with the pure graphite film.Both experiments and simulations have confirmed that G-Ag-PI not only has superior radiation performance,but also maintains high thermal conductivity of the underlying graphite film,providing a solution for enhancing the heat dissipation performance of the carbon films. |
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