Production Of High-concentration Boron Nitride Nanosheets Dispersions And Its Application In P(VDF-HFP) Composites

With high power density and long service life,high-energy storage impulse capacitors have been widely used in various fields,including daily electronic consumption,particle accelerators,military products and others.It is of significance for the development of novel high impulse capacitors to obtain high-performance dielectric composites having high dielectric constant,high energy storage density and high breakdown electric field.Meanwhile,the enhancement on the maximum operating temperature of dielectric materials is beneficial to the enlargement of application scope of polymer dielectric impulse capacitors.Hexagonal boron nitride nanosheets(BNNSs),as a structural analog to graphene of two-dimensional layered structure,have exceptional physiochemical properties.However,the “lip-lip” interactions between individual BNNSs layers made the exfoliation process more difficult than graphene.Therefore,the efficient,controllable,defect-free production and surface functional modification of BNNSs is the key to realize the application of BNNSs.Here,we firstly studied the exfoliation conditions of hexagonal boron nitride powder(h-BN)and the mechanism.Then we used the solution casting method to prepare BNNSs/P(VDF-HFP)composites.Finally,we used the stretching method and designed the multilayer structure to study the influence of dielectric properties of preparing conditions and the structure of P(VDF-HFP)composites.The contents are as follows:(1)Preparation of BNNSs assisted with hyperbranched polyethylene(HBPE).One pot synthesized the different molecular weight HBPE via ethylene “chain walking” polymerization mechanism with a Pd-diimine catalyst.Then production of stable high-concentration BNNSs dispersions in conventional organic solvents with low-boiling-point and low-polarity through liquid-phase noncovalent exfoliation of h-BN powder assisted with HBPE as the stabilizer.Moreover,transmission electron microscopy(TEM),atomic force microscopy(AFM),wide-angle X-ray diffraction(XRD),X-ray photoelectron(XPS)and Raman spectroscopy were used to characterize the morphology and structure of BNNSs.The results reveal that the obtained BNNSs are mainly few-layered with a thickness of 3-5 nm,the lateral dimension in the range of 0.2-0.4 μm and almost no oxidation and no hexagonal lattice change in the exfoliation process.Meanwhile,we systematically investigated the effects of different molecular weight of HBPE,sonication time,the amounts of h-BN and the mass ratio of HBPE to h-BN in the exfoliation of h-BN.In addition,in the exfoliation process,there exist noncovalent CH-π interactions between BNNSs and HBPE,while HBPE adsorbs onto the surface of BNNSs and keeps them from the restacking.(2)BNNSs/P(VDF-HFP)composites were prepared by solution casting method.Polarized optical microscopy(POM),Fourier-transform infrared spectra(FTIR),XRD,differential scanning calorimetry(DSC)analysis,Scanning electron microscopy(SEM),thermal conductivity and dielectric property measurements were used to analysize the structure,crystallinity,dielectric,thermal and energy storage properties of the composite films.The results show that the BNNSs exhibit heterogeneous nucleation effect in the composite films and effectively improve the crystallinity,which up to 39.95% and the β phase content can be as high as 92%.When the content of BNNSs is 0.5 wt%,the dielectric constant of BNNSs/P(VDF-HFP)composite is 35.5 at 100 Hz with loss tangent of 0.058.The maximum polarization is found to be 4.05 μC/cm2 and the charge-discharge efficiency is 79%.The composite film exhibit good discharge energy density of 5.63J/cm3 at 325 MV/m,which was 1.34 times than the neat film.In addition,the composite film thermal conductivity is obviously improved.The thermal conductivity increased to 0.91 Wm-1K-1,which was 4.13 times than the neat film,when the content of BNNSs is 1 wt%.(3)The effect of stretching process on the structure and dielectric properties of the 0.5 wt% BNNSs/P(VDF-HFP)composite films were studied by using different stretching temperature,rate and ratio.The results show that the composite films β phase content can be as high as 99.84%,the dielectric constant is 36 at 100 Hz when the tensile rate is 50 mm/min,temperature is 80°C and the stretching ratio is 2.Preparation of multilayer P(VDF-HFP)and its composite films are according to hot pressing method.We studied the influence of the hot pressing condition containing temperature,time and layers on the structure and dielectric and energy storage properties of multilayer neat P(VDF-HFP)films.The results reveal that when the hot pressing temperature is 165°C,time is 10 min and the layer is 10 the dielectric constant of the multilayer neat P(VDF-HFP)is 16.5 at 100 Hz,which is 2.06 times than the neat single layer film.Moreover,the maximum polarization as high as 5.72 μC/cm2 and the charge-discharge efficiency is 70%.The multilayer neat film exhibit good discharge energy density of 4.7 J/cm3,which was 2.33 times than the neat single layer film at the same electric field.In addition,a preliminary study was carried out on multilayer BNNSs/P(VDF-HFP)composite films.The preliminary results are as follows: When the stretching rate is 20 mm/min,temperature is 80°C and the ratio is 2,the dielectric constant of multilayer composite films 51015 is up to 49.6 at 100 Hz,which is increased by about 521% compared with the single layer neat film.