Experimental Study On The Effect Of Thermal Annealing Induced By Current On Thermal Conductivity Of Carbon Materials

Electronic industry is developing towards high power,intelligence and integration.A series of problems exist in electronic devices,which makes carbon materials have a good application prospect in the field of thermal management materials.However,the high thermal conductivity of carbon materials is limited to the nanoscale.The thermal conductivity of macroscopically sized carbon products is of varying quality.High temperature heat treatment is considered to be an effective method to improve the thermal conductivity of macroscopic carbon products.Exploring the influence mechanism between high temperature heat treatment and thermal conductivity of carbon materials is very important for the understanding and preparation macroscopic carbon material products with high thermal conductivity.The polyacrylonitrile based carbon fiber?PAN-CF?,carbon nanotube conductive films?CNTB?,carbon nanotube fibers?CNTF?and ultra-thin graphite paper?GP?were measured by transient electro-thermal technology?TET?.The experimental phenomena were analyzed based on numerical simulation method to explore the effect of annealing temperature on thermal conductivity of PAN-CF,CNTB,CNTF and GP.The various phonon scattering mechanisms during current thermal annealing are analyzed and summarized.Raman was used to characterize the carbon materials after current thermal annealing,and the microstructure evolution of carbon materials during current thermal annealing was analyzed.The correlation mechanism between thermal conductivity and electrical conductivity of carbon materials was investigated under current thermal annealing.The main experimental conclusions are as follows:?1?The thermal conductivity of the unannealed PAN-CF,CNTB,CNTF and GP samples were measured.The thermal conductivity of the four carbon materials is much lower than that of carbon materials at the nanoscale.The main reasons for the sharp decrease of the strong thermal conductivity of carbon nanomaterials are as follows: the chemical component defects and configuration defects of materials caused by chemical and physical processing in the preparation process;the great thermal resistance between the carbon nano sheets.?2?The effect of current thermal annealing on thermal conductivity of different carbon materials is different.The temperature distribution along the length of the sample is not uniform in current annealing process.Ultimately,the average thermal diffusivity of PAN-CF and CNTB1 increase by 3.45 and 1.34 times,respectively.The thermal conductivity of PAN-CF and CNTB1 increase with the increases of annealing temperature.There are “jumps” in thermal conductivity of CNTB2 and CNTF.The thermal conductivity of CNTB1 and CNTB2 are anisotropic.The average thermal diffusivity of GP decreases by 1.85 times.The thermal conductivity of GP decreases with the increases of annealing temperature.?3?The impurity atoms are removed by high-temperature heat treatment,and the structure order of graphite microcrystals is improved,the size of microcrystals increases,the number of phonon scattering centers decreases,and the ?_effective mean free path of phonons increases,which improves the thermal conductivity of PAN-CF and CNTB1.The switch-on behavior of CNTB2 and CNTF is attributed to the thermal expansion mismatch between pure carbon nanotubes and impurity-embedded carbon nanotubes.The anisotropy of thermal conductivity of CNTB1 and CNTB2 is mainly caused by the complexity and disorder of multi-wall carbon nanotubes.The large contact thermal resistance and a large number of grain boundaries between graphite flakes,the difference in thermal expansibility between flakes,and the sublimation of C atoms caused by current thermal annealing are the main reasons for the decrease of GP thermal conductivity.Phonon scattering in carbon materials is influenced by many scattering mechanisms.?4?With the enhancement of current annealing effect,the graphitization degree of the four kinds of carbon materials increased in different degrees,the microstructure of the materials increased in order,and the defect density decreased.In the process of graphitization,C atom gains energy and electrons are in an excited state,which induces hybridization effect of C atom and leads to optimization and improvement of the plane structure and spatial configuration of crystal.?5?With the increase of annealing current,the resistance of PAN-CF decreases,the resistance of CNTB increases,and the resistance of CNTF and GP decreases first and then increases.The thermal diffusivity of sample has a good linear relationship with the reciprocal of sample resistance.