Preparation And Properties Of Polyimide Films For High Thermal Conductivity Graphite Films

New types of 3C electronic products,the investment of 5G high-frequency commercial base stations and the rapid development of power batteries have put forward higher requirements for thermal conductive materials.Copper,silver and other traditional metal materials due to high density and brittleness limit its use in flexible electronic components.Graphite films with conjugated carbon double bond planar structure are considered as ideal thermal management materials.The main precursor material for preparing graphite film is polyimide（PI）film.At present,the thermal imidization method is mainly used to prepare electrical grade polyimide films in China.The linear expansion coefficient,thermal stability and mechanical properties of polyimide films need to be further improved,which cannot be applied to aerospace,electrical insulation,precision instruments and other high-precision fields.In addition,the graphite film prepared by domestic polyimide film has many defects and defects in appearance,and the thermal conductivity is also insufficient,which seriously restricts its use in heating components.The raw material（PI film）for preparing high-quality flexible and high thermal conductivity graphite film basically depends on imports.This paper is devoted to the preparation of high-performance polyimide films and flexible and thermal conductive graphite films.High performance polyimide films were prepared by chemical imidization.The effects of dehydrating agent content,catalyst content and molar ratio of diamine PDA/ODA on the degree of imidization,mechanical properties and aggregation structure of polyimide films were studied.On this basis,it was proposed for the first time that boron nitride was added as a foaming agent to change the structure of graphite films,so as to improve the flexibility of high thermal conductivity graphite films.The influences of morphologies,particle size and amounts of boron nitride on the structure and properties of polyimide-based graphite films were systematically discussed.Subsequently,the mechanism of boron nitride on improving the flexibility of graphite film was expounded,and the influence of boron nitride content on thermal conductivity was revealed.Finally,the thermal conductivity of the self-made graphite film was compared with that of the graphite film studied at home and abroad.The main conclusions of this paper are as follow:（1）High-quality polyimide films can be prepared by chemical imidization.The molecular chain order and crystallinity of polyimide films prepared by chemical imidization method（molar ratio PAA:AA:CA=1:2:0.4）were greatly improved,and the mechanical properties were also significantly improved.The tensile strength was increased from 110.05 MPa to 205.43 MPa,which was increased by nearly 86.7%,and the elongation at break of the film was increased from 55.3%to 95.2%.In terms of energy consumption,the preparation time of chemical PI films had been greatly reduced,from 4～5 hours of thermal imidization to 41 minutes,and the time in the high temperature region was very short,which saved energy consumption and reduces costs.（2）On this basis,the influence of the molar ratio of PDA and ODA on the comprehensiveness of PI film was discussed.It was found that the thermal stability of the PI film increased with the increase of the proportion of PDA,the temperature of 5wt.%thermal weight loss increased from 570.70°C to 578.02°C,and the temperature of 10 wt.%thermal weight loss increased from 582.84°C to 593.05°C.During the carbonization of the film,the residual carbon content first decreased and then increased,but remained above 55%.Secondly,the higher the PDA content,the stronger the molecular weight rigidity,the better in-plane orientation and crystallinity of the PI film,and the enhanced tensile strength of the PI film,but the elongation at break decreased.When PDA/ODA=2:8,the comprehensive performance of polyimide film is the best,its tensile strength is 205.43 MPa,and the elongation at break is 95.2%.（3）The addition of a trace amount of boron nitride changes the structure of the polyimide-based graphite film and effectively improves the flexibility of the graphite film.The surface of the pure PI-derived graphite film showed bright metallic luster and greater brittleness,while after adding a trace amount of h-BN,the graphite film exhibited a uniform"foamed"structure and the flexibility was significantly improved.The morphology and particle size of boron nitride were studied.It was found that compared with the graphite films derived from boron nitride nanosheets and 0.7μm boron nitride powder,the graphite film derived from the BN/PI composite film doped with 1.5μm boron nitride powder has a finer surface,less foaming and uniform distribution,and a lower resistivity of only 7.2 mΩ·cm.（4）The doping amount of boron nitride（h-BN）was studied and found that with the increase of h-BN content,the crystallinity,the degree of graphitization and thermal conductivity of the rolled graphite film decreased slightly,but the graphite film could bend repeatedly without macroscopic cracks,and the flexibility was greatly improved.The obtained graphite film had comprehensive properties.The h-BN/PI-derived graphite film containing 0.06 wt.%h-BN exhibits the best comprehensive performance,with high graphitization degree（91%）,large thermal diffusivity（818 mm²/s）,excellent thermal conductivity(1522 W·m^-1·K^-1)and excellent flexibility.The comprehensive performance of self-made graphite film is better than the reported value.