Study On Carbonization And Graphitization Process Control Of High Thermal Conductivity Graphite Film Derived From Polyimide

To improve the heat dissipation of high-power and high-integration devices,and satisfy people’s demands of equipments getting light,long-battery-life and other high performance,it is particularly urgent to find heat dissipation materials with low thermal expansion rate,high thermal conductivity and high flexibility.Polyimide（Polyimide,abbreviated as PI）refers to a class of polymers containing imide rings（-CO-N-CO-）on the main chain.PI film with specific structure is easily transformed into a graphite structure with high thermal conductivity after high temperature treatment.The graphite film derived from PI has excellent thermal conductivity and electrical conductivity,and the production process is easy to control,considered to be one of the most promising precursors for the preparation of graphite films derived from PI.In this paper,commercial Kapton-type polyimide is selected as the research object to prepare a series of carbon films and graphite films by adopting pieces-heating and roll-heating as two methods of heat treatment with multiple temperature gradients between 450℃and 2800℃with a specific heating process.Focusing on the effects of carbonization and graphitization process on the quality of graphite film,discussing the structural evolution and property change rules of PI during heat treatment,furthermore,the thermal decomposition kinetics of PI is studied.The pyrolysis process of PI has been better understood.According to XRD analysis,Du Pont PI has the characteristics of partial crystallization,and the degree of crystallization is relatively low;According to FTIR and TG analysis,the size,mass and molecular structure of Du Pont PI film hardly changed when the heat treatment temperature reached 450℃,it starts to decompose after 500℃,the ranger of main decomposition temperature is 550～700℃,and the released gases are mainly CO,N₂,CO₂,H₂O,etc.A series of carbon films were prepared by heat treatment of Du Pont polyimide films cut into sheets under vacuum conditions.It was found that the carbon film prepared between 500 and 700℃had the most obvious thermal shrinkage and the fastest mass loss,and the resistance of carbon film was only 0.75Ωat 1200℃;According to FTIR and Raman tests,it was found that the chemical structure of Kapton type polyimide was destroyed between 500℃and700℃,D peak and G peak begin to appear on the Raman spectrum of carbon film at600℃;The R value decreases at 800-1200℃,the molecular structure begins to reorganize,and the structure gradually changes from disorder to order,but at this time it is still an amorphous turbostratic carbon structure.A series of graphite films were prepared by heat treatment of Du Pont polyimide in an argon atmosphere by rolling.It was found that the size change of the graphite film showed a trend of first increase and then decrease;The appearance of all samples showed a bright black brittle carbon film before 2200℃,the appearance transition occurred between 2200 and 2300℃,the appearance of the samples prepared at 2300℃changed from a bright black brittle carbon film to a matte gray graphite film appearance.In this temperature range,the theoretical calculation value of the graphitization degree increases from 53.49%to 90.70%;When the temperature is lower than 2200℃,the microscopic surface of the graphite film is smooth and has no obvious defects.At 2200℃,a large number of small pores are observed on the surface.The surface of samples begins to foam at 2300℃,and the bubbles are getting bigger as the temperature increases;According to XRD and Raman spectroscopy,the degree of graphitization,crystallinity and order of the film increased with the increase of temperature,the G’peak,which represents the interaction between carbon layers,appears at 1800℃.When the temperature increases from 2200℃to 2300℃the intensity of the G’peak increases,and the peak shape becomes sharper;At 2800℃,the theoretical value of the graphitization degree of the sample reaches 99%,the thermal conductivity reaches 1246.36W/mk.The apparent pyrolysis activation energies of Du Pont PI in air and nitrogen atmospheres were calculated by Kissinger method and Flynn-Wall-Ozawa method.the average activation energies calculated by the Kissinger method and the Flynn-Wall-Ozawa method are 176.51k J/mol and 195.57k J/mol in the air atmosphere,respectively.The average activation energies calculated by Kissinger method and Flynn-Wall-Ozawa method are 288.83k J/mol and 238.22k J/mol under nitrogen atmosphere,respectively.The pyrolysis reaction order was calculated by using Coats-Redfern method,n=2;The upper limit temperatures of Du Pont PI for long-term using in air and nitrogen atmospheres were estimated to be 315℃and 481℃,respectively.