Preparation Of Graphitized Polyimide Film Based Composites With High Thermal Conductivity

Graphitized polyimide film(GPF)with high thermal conductivity is widely used in heat dissipation of electronic equipment.A large number of byproducts of graphitized polyimide tape(GPT)and its broken powder(GPP)products were produced during the production of GPF.Therefore,how to effectively utilize these byproducts is the main research purpose of this paper.In this paper,GPF and GPT as well as GPP were used as thermal conductive fillers,and their resin-based composites and carbon/carbon composites were prepared by hot pressing under different technological conditions,respectively.The main research contents and results are as follows:1.XRD and Raman spectra of GPF show that GPF possesses a highly graphitized structure.The thermal diffusion coefficient and thermal conductivity of GPF measured by laser flash method are about 900.40 mm~2/s and 1406.42 W/(m·K),respectively,which are good agreement with the reference values provided by manufacturing company.2.In order to explore the possible applications of GPT and GPF as general thermal conductive composites,the GPT and GPF were firstly coated with epoxy resin(EP),then the stacked GPT and laminated GPF were formed by a hot-pressing process to produce composites,respectively.The crystal structure,morphology and optical texture of GPT and GPF as well as their EP-based composites were characterized by X-ray diffraction,scanning electron microscopy and polarized light microscopy.The effects of volume fraction and dimension of GPF and GPT on the thermal conductivity of their composites were further investigated.The thermal conductivity and thermal diffusion coefficient of GPT/EP composites increase with GPT volume fraction.Owing to the gap among GPT,the thermal conductivity perpendicular to the hot-pressing direction of GPT/EP composite stacked with 80 vol.%GPT is changeable from 453.02 W/(m·K)to 615.15 W/(m·K).The GPF/EP composite laminated with80vol.%GPF,showing a highly oriented sandwich structure,possesses the thermal conductivity of 894.36W/(m·K).However,the thermal conductivity along the hot-pressing direction of 80 vol.%GPT/EP and 80 vol.%GPF/EP composites is 1.82W/(m·K)and 1.15 W/(m·K),respectively.The obvious differentia in the thermal conductivity at perpendicular and parallel to hot-pressing directions further confirms that the two composites are highly oriented.3.In order to explore the possible application of GPP based carbon/carbon composite with good thermal conductivity in high temperature environment,the thermoplastic polyimide resin(PI)and mesophase pitch(MP)as binders were mixed with graphitized polyimide sheets having different particle sizes(GPP1-GPP5)by a ball-milling treatment process at low speed,respectively.And the GPP/PI and GPP/MP composites were obtained by vacuum hot-pressing and low-temperature molding processes,respectively,and further high-temperature carbonization as well as graphitization treatments.The structure and thermal properties of the composites were characterized by XRD,SEM and PLM.The research results show that graphitized GPP1/PI and GPP1/MP composites with mass fraction of 18%PI and 14%MP as a binder show the highest thermal conductivity of 217.23 W/(m·K)and 219.22W/(m·K)at the room temperature,respectively.Due to the small particle size of the graphite powder,the GPP1 orientation shows a disordered structure.However,the orientation of MP in composites after carbonization was relatively low,so the GPP1/PI composites with room temperature thermal conductivity of 147.56 W/(m·K)was significantly higher than that of 80.93 W/(m·K)of GPP1/MP composites.With the increase of the graphite particle size from GPP2 to GPP5,and the thermal conductivity of their composites increase significantly owing to the orientation increase with the GPP sizes.Because the orientation of graphitized MP carbon is much higher than that of PI resin carbon,the room temperature thermal conductivity of graphitized GPP5/PI composites is 418.29 W/(m·K),which is lower than that of459.85 W/(m·K)for GPP5/MP composites.But the thermal conductivity of former is326.03 W/(m·K)still significantly higher than 276.26 W/(m·K)of the latter.Raman spectra analyses test showed that joint inducing action of GPP and thermal compressive stress promotes the graphitization of PI and MP binders,leading to the graphitization degree improvement of PI and MP carbon.At the same time,the hot pressing process also promotes the formation of a high-oriented structure in the composites,so the thermal conductivity of the composites parallel to the hot pressing direction at room temperature is only 23.54 W/(m·K)and 28.79 W/(m·K),respectively.