Preparation And Properties Of PVDF-based Dielectric Materials With High Dielectric Constant And Low Loss

Film capacitors with microsecond discharge speeds and ultra-high power densities up to 10⁷-10⁸W/kg are widely used in various advanced electronic and power systems,such as electrical energy storage,filtering,and power conditioning.In recent years,with the urgent demand for miniaturization and integration of functional devices in power systems,the discharge energy density of film capacitors urgently needs to be further improved.Benefiting from the advantages of light weight,easy processing,high breakdown strength and high reliability,polymers are the preferred dielectric materials for film capacitors.However,compared with ceramics,the dielectric constant of polymers is generally low,which greatly limits the energy density of polymer-based film capacitors.Although methods such as introducing high dielectric constant ceramic materials into polymers to increase their dielectric constant are widely used,the inverse relationship between dielectric constant and dielectric loss,as well as dielectric constant and breakdown strength hinders polymer-based nanometers.The energy storage density and charge-discharge efficiency of composite materials are effectively improved.In view of this,different from the traditional method of adding high dielectric constant ceramic fillers,this thesis uses high dielectric constant ferroelectric polymer as the matrix,by adding ultra-small size,low dielectric constant nanoparticles and combining to construct a fluorinated interface With the new technology,the dielectric constant and breakdown strength can be improved without changing or even reducing the dielectric loss,and a class of polymer-based nanocomposite dielectric materials with high energy storage density and high discharge efficiency have been developed;At the same time,combined with experiments and theoretical simulations,the mechanism of small size and low content fillers inducing high dielectric properties was explained,and a new type of polymer nanocomposite dielectric mathematical model was established.Aiming at the problem of balancing the dielectric constant and dielectric loss in polymer nanocomposites,ultra-small nano-SiO₂particles（10 nm）were synthesized by the silane hydrolysis method and fluorinated modified,and then combined with ferroelectric polymer P（VDF-HFP）was used for solution compounding to prepare a class of P（VDF-HFP）/fluorinated nano-SiO₂composite dielectric materials.The effect of nano-SiO₂content on the crystalline structure,dielectric and mechanical properties of the polymer was systematically studied..The results show that the ultra-small nano-Si O2 promotes the crystallization of the polymer,induces the transformation of the crystal phase from the non-polarαphase to the polarβphase,reduces the polymer crystal grain size,and P（VDF-HFP）/fluorinated nano-SiO₂composite The transparency and flexibility of the film are improved.At the same time,with 10 wt%nano-SiO₂filling,the dielectric constant of the P（VDF-HFP）/fluorinated nano-SiO₂composite film is increased from about 11 to 27,and the dielectric loss is reduced from 0.042 to 0.03,which overcomes the dielectric constant.The inverse relationship with the dielectric loss,the discharge energy density is 12.1 J/cm³,and the charge-discharge efficiency is 71.2%.In order to further study the regulating effect of small size nanoparticles on the dielectric properties of polymers,Cd_1-xZn_xSe_1-yS_ycore-shell quantum dots（QDs,7.6nm）were synthesized by high-temperature injection method,and they were combined with ferroelectric polymer P（VDF-HFP）was used for solution compounding to prepare P（VDF-HFP）/quantum dot nanocomposite dielectric material.The influence of quantum dot content on the crystalline structure,dielectric and mechanical properties of the polymer was systematically analyzed.The results show that QDs can promote the crystallization of the polymer and improve the mechanical properties of the composite film.At the same time,it can significantly enhance the dielectric properties of the composite when filled with a low content.Under the filling of QDs with a low dielectric constant of less than 1 vol%,The dielectric constant of the polymer is greatly increased from 9.3 to 18.6,while the dielectric loss remains at about 0.04,while the breakdown strength is increased from453.8 MV/m to 621.1 MV/m,which overcomes the inversion of dielectric constant and breakdown strength relation.Therefore,the composite membrane finally achieved an ultra-high energy density of 27.4 J/cm³and a charge-discharge efficiency of 70.2%.With the aid of crystal structure analysis and density functional theory simulation,it is found that the dielectric enhancement mainly comes from the polar phase transition and interface polarization induced at the interface.A new type of"polymer-filler-interface"three-phase dielectric mathematical model is established,which is effective Explains the regulating effect of the interface structure on the dielectric properties.In order to further control the polymer-quantum dot interface structure to reduce the dielectric loss and improve the charge and discharge efficiency,the carboxyl-fluoride dual ligand quantum dots were synthesized by the ligand modification method and combined with the ferroelectric polymer P（VDF-HFP）.Was compounded to prepare P（VDF-HFP）/dual ligand quantum dot nanocomposite dielectric material.The influence of quantum dot content and ligand structure on the crystalline structure,dielectric and mechanical properties of the composite was systematically studied.The results show that the dual-ligand quantum dots promote the crystallization of the polymer and induce the dielectric enhancement of the polymer.At the same time,the dual-ligand quantum dots have a significant inhibitory effect on the dielectric loss of the polymer,and the dielectric loss is reduced from 0.045 to 0.029.The penetration strength is increased to 531.1 MV/m,which overcomes the inverse relationship between dielectric constant,dielectric loss and breakdown strength.The composite film finally achieved a high energy density of 21.4 J/cm³and a high charge and discharge efficiency of 78.3%.Based on structural analysis and in-situ electric field-luminescence spectroscopy,the loss suppression principle of the nanocomposite film is analyzed.The double-ligand quantum well structure with high energy barrier limits the electrons from quantum dots to adjacent quantum dots or polymer matrix.Long-distance transport,thereby suppressing conductance and energy loss.In order to further optimize the dielectric properties of the polymer and obtain high breakdown strength and high charge and discharge efficiency,a perfluorosilsesquioxane（F-POSS）was synthesized using perfluorinated compounds as monomers.Diffraction analyzed the chemical composition and crystal structure of F-POSS and characterized its dielectric properties.It was found that it has a wide band gap of 4.65 eV,a large dielectric constant（4.1）,and a significantly low dielectric loss（0.006）.And high insulation performance.After mixing it with P（VDF-HFP）to form a film,F-POSS significantly suppresses the energy loss of the ferroelectric polymer（dielectric loss is reduced from 0.045 to 0.016）,which is due to the fluorinated group’s effect on the current The capture and movement of electrons are restricted;at the same time,the breakdown field strength is greatly increased to 700 MV/m,and the charging and discharging efficiency is as high as82%.In order to combine the dielectric enhancement effect of QDs with the loss suppression and breakdown resistance characteristics of F-POSS,a"sandwich"structure was used to prepare a P（VDF-HFP）/QDs/F-POSS three-layer composite film by hot pressing.The P（VDF-HFP）/F-POSS film is used as the upper and lower layers to bear the high electric field,and the P（VDF-HFP）/QDs film is used as the intermediate polarization layer to achieve high breakdown and high polarization,As a result,an ultra-high discharge energy density of 31.1 J/cm³and ultra-high charge-discharge efficiency of 78.5%are obtained under the high electric field of650 MV/m,which provides a new way for the development of high-energy density dielectric materials.