Study On The Preparation And Properties Of High Thermal Conductivity Polyimide Films

The rapid development of new electronic devices has placed higher demands on heat-dissipation materials.Filled thermally conductive polymers are widely used in the field of electronic thermal management for their low cost and good performance.Polyimide/boron nitride（PI/BN）composite films are valuable in electronic thermal management applications due to the superior thermal conductivity and excellent insulation properties of BN.However,it is difficult to improve the out-of-plane thermal conductivity of the composite film due to the anisotropy of BN thermal conductivity,and the application of PI/BN composite films in electronic devices is severely limited due to the high filling amount and the high price of BN.This paper is devoted to the preparation of PI/BN composite films with high out-of-plane thermal conductivity at low load.In the paper,BN was prepared by a chemical reaction at high temperatures using sodium chloride as a template and boron oxide and ammonia as the sources of boron and nitrogen,respectively.Then,the two-dimensional structure of BNNS-t was obtained by the treatment of ultrasound.On this basis,the improvement of the out-of-plane thermal conductivity of PI/BNNS-t composite films was systematically investigated by the high aspect ratio and angle shape of the prepared two-dimensional structure BNNS-t.Meanwhile,the effect of different mass ratios of boron oxide and sodium chloride on the density of the prepared three-dimensional structure BN was investigated based on the improved BN synthesis process,and the three-dimensional structure of the prepared BN was preserved by modification.In addition,the effect of the template method on the orientation of BN and the great enhancement of the out-of-plane thermal conductivity of PI/BN composite films by this three-dimensional structure of BN were investigated by means of schematic principles and computational simulations.Finally,the practical applications of the prepared PI/BN films for heat transfer were investigated by means of infrared thermography.The main conclusions of this paper are as follows:（1）For the prepared two-dimensional structure of BNNS-t,BNNS-t has a higher aspect ratio and more uniform transverse dimensions relative to conventional BNNS,and a few nanosheets with angular shapes are present in the obtained BNNS-t.The synergistic effect of the above-mentioned properties of BNNS-t facilitates the construction of a three-dimensional thermal conductivity network of PI/BNNS-t composite films,which can significantly improve the out-of-plane thermal conductivity.And then,the out-of-plane thermal conductivity of PI/BNNS-t composite film reaches 0.67 W·m^-1·K^-1 at 40%load,which is nearly 3.5 times higher than that of PI film.（2）For the prepared three-dimensional boron nitride（HCBN）,The as-obtained HCBN powders have a unique hollow cube-like structure with an ultra-low density of2.67×10^-2 g/cm³ and nearly 8 times the volume of the same mass of two-dimensional（2D）BNNS,making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix.In addition,the high out-of-plane thermal conductivity of 4.93 W·m^-1·K^-1 at 23.3 wt%loadings was obtained by doping it into a polyimide（PI）matrix.This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings,17.6times higher than pure PI.Therefore,the prepared thermal conductivity composite film of the polyimide has great potential for application in electronic thermal management.