Study On Preparation And Energy Storage Performance Of Polymer-based Composites With BaTiO₃ Nanowires

Energy storage dielectric capacitor with a very high released power density（MW level）,high operating voltage and fast charge and discharge capability has a very broad application prospects in modern power electronic devices and weapons.However,due to its relatively low energy storage density,the dielectric capacitor has to enlarge its volume to obtain the certain energy in practical application,which is not suitable as the high energy density dielectric capacitor with lightweight and small size required for the hybrid electric vehicle and aerospace.Among the various dielectric capacitor materials,flexible polymer-based nanocomposites are most promising to achieve this goal.The study show that ferroelectric polymer with a small amount of high aspect ratio nanowires can effectively improve the energy density of the composites.However,the nanowire-filled composites remain several problems,such as the the problem of lack of dielectric theoretical research,the difficulty on synthesis of high aspect ratio nanowires,and absence of relative research on the mechanism,which directly restrict the improvement of the energy storage density of the nanowire-filled composites.In order to obtain the nanocomposites with high energy density,our work focused on the BTnws/P（VDF-CTFE）composites,and the following works were as below.Firstly,for the absence of the dielectric theory on the nanowire-filled composites,we used the Maxwell-Garnet dielectric model to discuss the relationship between the dielectric constant of the composites and the aspect ratio of anisotropic BaTiO₃ nanowires.Meanwhile,the software Maple was employed to quantitatively investigate the relationship between the aspect ratio of anisotropic BaTiO₃ nanowires and the dielectric constant of the composites.The result suggested that the dielectric constant could increase along with increase the aspect ratio of BaTiO₃ nanowires in Random（BTnws random distribution）and Z-aligned（BTnws along the direction of the electric field）composites,when the content of the BaTiO₃ nanowires in the composites was relatively low.Among them,the nanowires with the aspect ratio more than 50 shown the most obvious improvement.Secondly,for the difficulty in the growth of high-aspect-ratio BaTiO₃ nanowires due the agglomeration and precipitation in traditional static hydrothermal reaction,we designed a hydrothermal reactor with stirring function,which can effectively control the synthesis of BaTiO₃ nanowires with different aspect ratio by using different stirring rate.The results suggested that BaTiO₃ nanowires with an aspect ratio more than 50 can be obtained at a stirring rate of 500 r/min.BaTiO₃ nanowires with an aspect ratio up to 95 are obtained at a high stirring rate of 1000 r/min.The mechanism of morphology evolution on BaTiO₃ nanowires in the process of agitation hydrothermal was studied,which evidenced that the mechanical force-driven generated during the process of high speed hydrothermal stirring was beneficial to the growth of BaTiO₃ nanowires with high aspect ratio.Subsequently,BaTiO₃ nanowires with high aspect ratio were treated with dopamine initially,and were used for the fabrication of the nanocomposites with randomly distributed BaTiO₃ nanowires（Random BTnws）via a soluton casting method.The effect of dopamine surface modification and annealing process on the properties of the composites was systematically investigated.With an investigation on property regulation of energy storage with various nanowire content,for the Random nanocomposites with 3vol.%BaTiO₃ nanowires,the result shows the energy density can reach 8.2 J/cm3 at an electric field of 3000 kV/cm.What’s more,based on the principle of the templated grain growth method of textured piezoceramics and the principle of minimum energy,the composites with 3vol.%high aspect ratio BaTiO₃ nanowires perpendicular to the electric field（X-Y-aligned）and paralleled to the electric field（Z-aligned）direction were fabricated via new physical-assisted casting method.It was found that:（1）For X-Y-aligned composite film,the highest breakdown strength can be obtained.A（Dmax-Pr）value of 6.86 N.C/cm2,a high energy density of 10.1 J/cm3,and a discharge efficiency of 56.8%can be obtained at an enhanced electric field of 3400 kV/cm in X-Y-aligned composite.（2）For Random composite film,a（Dmax-Pr）value of 6.38 μC/cm2,an energy storage density of 8.2 J/cm3,and a discharge efficiency of 44%can be obtained at an electric field of 3000 kV/cm.（3）Z-aligned composite film has the lowest breakdown field strength.However,the highest（Dmax-Pr）value of 9.93 μC/cm2,high energy density of 10.8 J/cm3,and discharge efficiency of 61.4%can be obtained at a low electric field of 2400 kV/cm.To the best of our knowledge,among ferroelectric nanocomposites,this is the highest energy density ever obtained at such a low electric field.The results show that the excellent energy storage performance can be achieved by controling the orientation of nanowires（anisotropy）.Finally,novel xdBP-8PI composite films with double layer heterostructure were fabricated by using a thin linear PI film as the breakdown layer with a thickness of～8 μm and a breakdown strength of 3787 kV/cm.And the dopamine modified BaTiO₃ nanowires-filled P（VDF-CTFE）was employed as the ferroelectric Layer（dBP）with controllable thickness.The composite films were fabricated via the preparing method of X-Y-aligned composite films,which can easily control the size and grand fabricate.With the control of the thickness x of the ferroelectric layer,the "barrier effect" and "interfacial polarization" existed between the linear medium and the ferroelectric medium were successfully engineered.Thus,the regulation over the breakdown strength and the electric displacement can be realized accordingly,and even,the optimized performances on the energy storage can be finally obtained.When the x was 8,the highest energy storage density of 14.3 J/cm3,and discharge efficiency of 57.3%can be obtained at an enhanced electric field of 3800 kV/cm.When the x was 4,the excellent energy storage performances（Ue=12～12.9 J/cm3 and η=72%～80%）can be achieved under the electric field in the range of 3800～4000 kV/cm,the composite has huge potential application.