Structure Control And Dielectric Energy Storage Properties Of Polymer Nanocomposites Based On Boron Nitride Nanosheets

Dielectric film capacitors have become the non-substitutable units in electronic and electrical systems because of their ultra-high power density,fast charge/discharge rate and good cyclability.However,the existing dielectric materials fall shorts of increasing demand in pulse power and electrical energy systems because of their limited energy density.And there exists paradox in dielectric materials for achieving high dielectric constant and high breakdown strength simultaneously.It’s thus of primary importance to develop new classes of high-energy-density dielectrics.On the other hand,take the inverter in electrical vehicle as an example,capacitors in pulse power system usually work at elevated temperatures.It’s of crucial importance to develop dielectrics that possess high energy density,low loss and are being able to function under elevated temperatures.This thesis focuses on designing polymer nanocomposite dielectrics from the following two aspects:room temperature and high temperature.High energy density at room temperature and high energy density low loss at elevated temperatures are expected in polymer nanocomposites incorporating with boron nitride nanosheet（BNNS）by resonably selecting polymer matrixes,preparating high K inorganic fillers and structural design.The main contents and results of this thesis are listed as follow:1.Ternary nanocomposites were prepared by mixing high dielctric constant polymer matrix P（VDF-TrFE-CFE）（poly（vinylidene fluoride-trifluoroethylene-chlorofluoroethylene））,boron nitride nanosheets（BNNS）and barium strontium titanate nanowire（BST NWs）.BNNS were prepared through tip-type sonication method and BST NWs was synthesized from a two-step hydrothermal process.Scanning electron microscope（SEM）analyses indicate that BNNS and BST NWs disperse well in the polymer matrix.Dielectric properties and breakdown strength analyses indicate that the dielectric constant and breakdown strength of the obtained ternarynanocompositescanbeincreasedsimultaneously.Electrical displacement-electrical field（D-E loop）analyses indicate that discharged energy density of 24.4 J cm^-3 is achieved in the ternary nanocomposite,which is 295%that of the pristine polymer.2.Sandwich structured nanocomposites using PVDF as the polymer matrix were prepared through hot press method followed by quenching in iced water,in which the outer layer are incorporated with BNNS and the middle layer contains BST NWs.SEM image shows that the sandwich structured nanocomposite exhibits clear layered structure with BNNS and BST NWs well dispersed in each individual layer.X-ray diffraction analysys indicates the major phase of PVDF in the nanocomposites is a mixture of nonpolarαandγphase,which is favor for the switching of dipoles.Electric field distribution and numerical simulation show the electric field in the nanocomposites is redistributed for the different dielectric properties of each individual layer and the development of electric tree can be restrained in the middle.Breakdown strength test and D-E loop analysys indicate that a breakdown strength of588 MV m^-1 as well as an energy density of 20.5 J m^-3 are achieved in the nanocomposite.3.Poly（methyl methacrylate）/boron nitride（PMMA/BNNS）nanocomposites were prepared through solution cast method.SEM image shows BNNS disperses well in PMMA.Thermal conductivity results show the incorporation of BNNS in the polymer matrix increases the thermal conductivity,which is 1.21 W m^-1 K^-1 for PMMA/BNNS.Current density measurement indicates that BNNS in the nanocomposite decreases the high-temperature high-field current density.D-E loop analyses indicate charge-discharge efficiency over 93%as well as discharged energy density that is 300%that of pristine PMMA is achieved in the PMMA/BNNS nanocomposite at optimized filler content at 70℃.At 70℃ and under an electric field of 200 MV m^-1,the nanocomposite discharges an energy density that is 210%that of commercial available BOPP with a power density of 0.39 MW cm^-3,which is1.77 times that of BOPP.The superior energy storage performance of the nanocomposite at high temperature is ascribed to the decreased electrical conductivity and increased thermal conductivity.4.Different kinds of single-layer or trilayered nanocomposites were prepared by solutioncastmethodusingcross-linkeddivinyltetramethyldisiloxane-bis（benzocyclobutene）（c-BCB）as polymer matrix.SEM images indicate good dispersion properties of the fillers in the nanocomposites.Dielectric spectroscopy and D-E loop show multiple structure is beneficial for the reduction of conduction loss at high temperatures compared with single-layer nanocomposites such as c-BCB/BT or c-BCB/BNNS/BT nanocomposites.At 150℃ and under an electric field of 200 MV m^-1,the conduction loss is less than 7%in the sandwich structured nanocomposites with c-BCB/BNNS as outer layer and c-BCB/BT as central layer.The sandwich structured nanocomposite is able to discharge an energy density of 1.1 J cm^-3,which is 280%that of BOPP,with a high charge-discharge efficiency over 93%.Cyclability measurement indicates a good cyclic performance for the sandwich structured nanocomposite with less than 3%fluctuation of the discharged energy density during30000 test cycles.Phase-field simulation of current density shows that more sophisticated multilayered structures might not present additional advantages for dielectrics operating at elevated temperature compared with the sandwich structures as long as the layer composition remains the same.The reversed sandwich structured nanocomposite with c-BCB/BT as outer layer and c-BCB/BNNS as central layer shows worse energy storage and efficiency performance than the sandwich structured nanocomposites with c-BCB/BNNS as outer layer and c-BCB/BT as central layer.Analyses from the fitting curves using Schottky charge injection model and Poole-Frenkel（P-F）emission model show that c-BCB/BNNS as outer layer can effectively increase barrier height at the electrode/dielectric interface and deepen the trap depth.As a result,it’s difficult for charges to be injected into the dielctric materials from electrodes and escape from the trap depth at elevated temperatures,which will reduce current density and conduction loss of the dielectrics.