Study On Polyimide Resin-based Thermally Conductive Composites

Polyimide（PI）is one of the most comprehensive polymer materials found to date,and is often used as a special engineering plastic in electronic packaging,new energy,semiconductor and aerospace applications.However,as traditional polymer materials,most PIs have the inherent defect of poor intrinsic thermal conductivity,which cannot meet the growing demand for rapid heat dissipation in thermal management applications and seriously affects the operational stability and service life of equipment,greatly limiting the expansion of applications in more new fields.Therefore,the design and preparation of high thermal conductivity PI materials with good overall performance is one of the hot spots of current research.In terms of measures to improve the thermal conductivity of polymers,the preparation of filled thermally conductive polymers is a greener,simpler and more effective method than the molecular structure design engineering,which is a complex process and more difficult to prepare,and is the most common means to develop new PI thermally conductive materials.The process conditions during PI synthesis can have an impact on the final product quality.In this project,PI was firstly prepared by a two-step method,and the optimal process conditions for the preparation of PI and the structural transformation history of molecules during imidization were investigated by means of infrared,gel permeation chromatography and thermogravimetric analysis.The results show that the best reaction time for the polycondensation was 6h.At this reaction time,the molecular weight of the precursor polyamide acid（PAA）reached the highest and the molecular weight distribution was narrow.The imidization reaction mainly occurred at120～250°C,and the cyclization degree could reach 98%after high temperature treatment at 300°C.In order to improve the thermal conductivity of PI,a series of inorganic particle/polyimide thermally conductive composites filled with different particle sizes of two-dimensional lamellar hexagonal boron nitride（h-BN）and alumina（Al₂O₃）high thermal conductivity fillers were introduced into the PI matrix for filling modification,and a series of inorganic particle/polyimide thermally conductive composites filled with fillers alone or in combination were prepared.The effects of particle size,filling amount and particle dispersion state in the matrix on the thermal stability,mechanical properties and thermal conductivity of PI were investigated.The results show that the addition of thermally conductive particles well maintains the excellent thermal stability of PI,but also introduces more defects to reduce the mechanical properties of the material.due to its larger aspect ratio and smaller specific surface area,the large particle size flake h-BN filler is more likely to undergo forced orientation or self-orientation along the plane direction in the matrix during processing and molding,forming a more efficient thermal propagation path.The in-plane thermal conductivity（λ_∥）of the 30wt%10μm h-BN filled composite is 11.73W/（m?K）,an increase of 174%over the pure PI.The compounding of fillers of different shapes and particle sizes can play a good synergistic role to form more heat transfer pathways,thus further improving the thermal conductivity of the material.Theλ_∥of the composites reached 7.02W/（m?K）and 12.73W/（m?K）when the micron-scale h-BN and nanoscale Al₂O₃compounding ratios were 6:4（10wt%filling）and 9:1（30wt%filling）,respectively,both of which were higher than that of the single filler-filled system with the same filling amount.Finally,in this paper,boron nitride nanosheets（BNNS）were prepared by liquid-phase exfoliation of h-BN and compounded into PI matrix.The effects of filling amount,dispersion state and orientation degree of thermally conductive particles on the mechanical properties and thermal conductivity of PI were investigated.The results show that compared with h-BN/PI composites,the exfoliated BNNS has higher dispersion uniformity in PI,and the tensile strength of BNNS/PI composites is improved,but the elongation at break is decreased.BNNS with single or few layers have greater aspect ratio and thermal conductivity anisotropy,higher degree of orientation in the matrix,easier to form heat conduction paths at low filling amounts,and betterλ_∥enhancement for PI.Theλ_∥of h-BN/PI is 7.06W/（m?K）under30wt%dosage filling,while theλ_∥of BNNS/PI is 7.78W/（m?K）,which is an10%improvement over the former.