Microstructure And Properties Of Polyimide Composites Modulated Via Two-Dimensional Boron Nitride Nanosheets Synergistically Multi-Dimensional Fillers

It is common known that the selection and structure design of dielectric materialsare of great significance to promote the development of miniaturization,high integration and multi-function of modern electronic devices.Recently,ternary hybrid polymer-based composites have shown their potential in improving the electrical insulation,mechanical and thermal conductivity of polymers.By properly selecting inorganic fillers with different dimensions and characteristics for hybridization and structure design,it can meet different work requirements in specific environment,becoming the research hotsport in the field of polymer science.In this paper,each of zero-dimensional(0-D)nano-sized aluminum nitride(AlN),one-dimensional(1-D)micro-sized titanate nanotubes(TNs),and two-dimensional(2-D)micro-sized expanded layer molybdenum disulfide(E-MoS₂)was mixed with 2-D nano-sized boron nitride nanosheets(BN)to use as hybrid fillers in the matrix of polyimide(PI).Based on that,a series of novel ternary composite films with excellent electrical,mechanical and thermal properties were prepared by in-situ polymerization.The effects of the orientation distribution,interface structure,trap energy level and space charge distribution of 2-D BN nanosheets synergistically multi-dimensional fillers on the microstructure and properties of the composite films were investigated,contributing to the improving of the resistance to electricity.It was found that 0-D AlN and 2-D BN were well dispersed in the PI matrix and formed a“sphere sheet surrounding structure”.Appropriate doping of BN-AlN hybrid filler(3 wt%-5 wt%)can restrain interface state destruction,enhance interface polarization,limite dipoles turning in high frequency range,increase dielectric constant and reduce dielectric loss.When doping 3 wt%,the AC breakdown field strength of PI/BN-AlN composite films can reach 160 k V/mm,which is 45%and 13%higher than pure PI and PI/AlN respectively.The corona aging life and thermal stability of PI/BN-AlN composite films increased greatly with the increases of doping component.There are a lot of hydroxyl groups on the surface of 1-D TNs in the PI/BN-TNs ternary composite films,in which existing many hydrogen bonds,resulting in the BN was attached to the surface of TNs to form a“spatial network bridging structure”,and some TNs were oriented horizontally in the PI matrix due to the inducing of BN on TNs.Small-angle X-ray scattering(SAXS)results show that with the increase of BN-TNs doping,the thickness of interface layer decreases,the mass fractal dimension increases,and the scattering ring has anisotropic and oriented structure.The properties of the uniformity,orientation and good interfacial compatibility of PI/BN-TNs composite films can effectively inhibit the deterioration of electrical and mechanical properties of the composite film.Furthermore,the composite film represents many excellent properties in dielectric,breakdown,tensile and dynamic thermo-machinery.The ultrathin E-MoS₂ nanosheets have larger lattice spacing,along the c-axis(002)crystal surface is expanded to 0.98 nm.The transverse dimension is about 1.5?m and the thickness is about 2 nm.The nanosheets are composed of only two single layers,which form a“layered separation structure”with 2-D BN.A novel PI/BN-E-MoS₂composite film was prepared by polymer in-situ intercalation,which greatly improved the dispersion of the filler in the PI matrix.SAXS results show that the thickness and mass fractal dimension of PI/BN-E-MoS₂ composite film interface layer gradually increase with the increase of doping component,leading to denser structure.Doping BN-E-MoS₂ significantly enhanced the internal interface polarization of the composite film,decreased the limiting effect of the bonding zone on the dipole orientation polarization,and kept the high dielectric constant and restrained the dielectric loss.The breakdown field strength of PI/BN-E-MoS₂composite film is 196 k V/mm at the doping component of 0.25 wt%,and its thermal conductivity is 0.22 Wm^-1K^-1 when the doping component is 3 wt%.The BN-E-MoS₂hybrid filler forms more and more compact heat conduction path in the composite film,which can prolong the phonon mean free path and reduce the phonon scattering at the interface and improve the thermal conductivity of the composite film.Based on the multi-core model,the electrical double-layer model,the energybarrier and the interface trap theory,the long-term breakdown(corona aging)and short-term breakdown failure behaviors of the composite film were analyzed by using the quasi-in-situ technique and the trap nenergy level distribution,the space charge injection and the recombination tested combined with finite element simulation.In addition,through analyzing the mechanism of multi-dimensional hybrid filler system enhancing the electricity resistance of composite films systematically,several models of the corona aging,the short-time breakdown mechanism and the bipolar charge transfer are established respectively,in order to illustrate the action of 2-D BN nanosheets of the multi-dimensional hybrid packing system in the regulation of the resistance to electricity of the composite films doping.The study provides a theoretical basis for expanding the application of 2-D materials in the field of high-insulation.