The Investigation Of All Organic High-energy-density Dielectric Materials

In order to improve the energy density of polymer dielectric materials, dielectric performance should be optimized, that is to say, by means of enhancing the dielectric constant and breakdown strengthen and reducing the dielectric loss especially conduction loss at high electric field. As the commonly used polymer dielectric material, polyvinylidene fluoride(PVDF) showed higher dielectric constant 8.4(100Hz) but lower breakdown field 350MV/m(100Hz), furthermore, it exhibited higher conduction loss at high electric field, therefore, resulting in lower discharged efficiency 30% and lower energy density 2J/cm3. In this paper, the research was divided into two parts.On the one hand, to make clear the effect of thickness and layer number on the dielectric property of polyvinylidene fluoride(PVDF), the different multilayer films were prepared by spin coating. It is found that the dielectric constant of multilayer increase linearly with the number of layers but decrease with the thickness. The relation between them has been explained by both experimental results and theoretical model. It is helpful to optimize the material structure and size parameters of polymer films for high energy capacitors.On the other hand, with PVDF as substrate, a small amount of aromatic polythiourea(ArPTU) added increases the energy density. The blend films based on the PVDF and aromatic polythiourea(ArPTU) were prepared by solution casting method. Because of chemical crosslinking effect between PVDF/ArPTU molecular structure, the blend films can also inhibit the early polarization saturation at low field and result in the significant enhancement of dielectric strength and increase of the energy density and reduction of loss at high field. For the PVDF/ArPTU(90/10) film, the dielectric constant is 9.2(100Hz) and the maximum operating electric filed is about 700MV/m and the released energy density can be up to 10.8 J/cm3 with the efficiency of above 83%. The data of X-ray diffraction and atomic force microscopy images show that these blend films exhibit obvious structure changes with the increasing percent of ArPTU. It is proved that the increase of β phase content and decrease of crystalline grains in blend films are helpful to reduce dielectric loss and enhance the dielectric strength.Most interestingly, when the field is at about 400MV/m, PVDF/ArPTU(95/5) blend film shows irreversible phase transition. Specially, it is found that the energy loss at high field has been reduced 2~4 times along with an interesting irreversible transition under high electric field. The loss at 700MV/m can be kept below 20%. The data of X-ray diffraction reveal that the irreversible phase transition blend film exhibits higher content of polar β phase. Moreover, the highly polar thiourea groups of ArPTU can act as traps for the electrons and reduce conduction loss greatly at high electric field.