| Perfluoropolymers are polar materials with excellent charges storage capability.Among them,polytetrafluoroethylene(PTFE)and fluorinated ethylene propylene copolymer(FEP)are the most widely used space-charged electrets.And the storage and stability of the space charge is determined by the TFE primitive.Polyvinylidene fluoride(PVDF)and its copolymers are the most widely used dipolar charge electret materials.These materials have piezoelectric ferroelectric properties,and their piezoelectric ferroelectric properties are determined by VDF motifs.Different primitive and component ratios determine the type of charge storage and its application areas.In addition to the influence of the primitives,the electret charge is also affected by the interfacial structure.In the heterogeneous interface formed by the superposition of multiple dielectrics,the main factors affecting the charge accumulation in the electret polarization process with electrical gradient structure.At present,in the study of electret charge trapping mechanism,the relationship between the element and charge storage,crystal phase structure and dielectric response is still unclear.It is impossible to fully reveal the difference by studying single element or double-element electrets.And the study of the mechanism of the accumulation of interfacial charges is only demonstrated by the classical model,and an effective interface charge accumulation mechanism cannot be obtained.In this paper,polytetrafluoroethylene(TFE)-vinylidene fluoride(VDF)-hexafluoropropylene(HFP)terpolymers THV815,THV500 and THV220 with different ratios of components were studied and used as model materials to study the effects of functional elements VDF,HFP and TFE on the electret charge.Through PP and FEP electrets,the interface with heterogeneous structure was formed to study the effect of the interface electrical gradient on charge accumulation.The stability of charges was studied by measuring surface potential attenuation and thermal stimulation discharge(TSD)spectrum,and the phase structure and dielectric properties were studied by X-ray diffraction and dielectric spectrum.Heterogeneous interface structure is constructed through PP and FEP,electrical(conductivity and dielectric constant)gradients were formed at the interface to study the process and mechanism of heterogeneous interfacial charges accumulation in the polarized electric field.Detailed conclusions were reached as follows:(1)To study the effects of different primitives on charges storage.The surface potential decay at normal temperature after polarization was measured.The initial surface potentials of the THV220,THV500,and THV815 electrets were-458 V,-530 V,and-611 V,respectively.The stable potentials were-6V,-105 V and-198 V,respectively.The surface potential of the polar body increased with the decrease of the polar component VDF,and increased with the TFE.(2)Thermal stimulation discharges were used to study the effect of components at high temperatures on charges storage.There were two types of charges,dipole charge and space charge in THV.The dipole charge was formed by the VDF component.The relaxation peak of the directed dipole in the TSD spectrum appeared in the low temperature region of the spectrum.The space charges de-trapping peak appeared in the high temperature region of the TSD spectrum.The thermal stability of the space charges was higher than the dipole charges.The charges storage stability will be increased with TFE and increased with the decrease of VDF,and in the TSD spectrum,the charges peak shifted toward the high temperature region.(3)The crystal structure of THV was investigated using X-ray diffraction.THV showed a broader diffraction peak than PTFE composed of a single component,which drifts toward a higher angle than the PTFE diffraction peak.HFP and VDF in THV can be regarded as defects introduced into PTFE polymers,and a highly ordered PTFE molecular chain with a helical conformation must be unraveled to accommodate these defects.The increase of VDF components will increased the peak width of the X-ray diffraction spectrum,the molecular structure becomes more chaotic,and the increase of TFE components increased the order of the molecular structure.This explained the effect of different primitives on charge storage from the crystal structure.(4)Dielectric spectra were used to study the dielectric constant and dielectric loss of THV at room temperature.The dielectric constant value gradually increased from THV815 to THV500 to THV220,VDF is a polar component,and the dipole molecule C-F bond is oriented under a strong electric field.HFP molecules increase the disorder of molecular chains and increase the free space for molecular chain motion.The higher the content of the VDF component and the HFP component,the stronger the dielectric response.The dielectric spectrum also provided information on dielectric relaxation,and intermediate frequency relaxation caused by local molecular motion(such as VDF molecular chain torsion and link motion)can be observed in thethree THVs.It was proved from the dielectric spectrum information that the VDF and HFP components make the electret with high polarity and improve processability compared with PTFE,but are not conducive to the stability of space charges.(5)The effect of heterogeneous interface electrical properties on charge accumulation was investigated by measuring the charge characteristics of the electret heterointerface.Studies have shown that by changing the direction of the gradient of electrical properties,it is possible to accumulate positive or negative charges.The charge characteristics are consistent with the Maxwell-Wagner effect,i.e.,as the temperature and the intensity of the polarized electric field increase,the accumulated charge density increases.The conductivity direction and the gradient direction of the dielectric constant at the hetero interface can control the type of accumulated charges.(6)By studying the changes in conductivity and dielectric constant of PP and FEP with temperature and electric field,the effects of conductivity and dielectric constant on charge accumulation during polarization were investigated.Conductivity and dielectric constant are affected by the polarization temperature and electric field,and an increase in conductivity accelerates the charges injection process.Especially with temperature changes,the change in conductivity is significantly higher than the change in dielectric constant.In the process of polarization,the influence of conductivity on charge accumulation dominates.(7)The charge accumulation at the heterogeneous interface is controlled by the Maxwell-Wagner effect and conductance.With the extension of the polarization time,the interface charge density tends to be saturated,and the time required for saturation is shorter than the time calculated by the Maxwell-Wagner effect.The process of actual polarization is a complex process of transitioning from a capacitive distribution field to a resistive distribution field.The measured potential is formed by the interaction of the electrical charges accumulated by the interface and the trap charges captured in the medium.In summary,THVs were used as a model material to study the effects of different primitives on charge storage,and to explore the effect of heterogeneous interface structure on charge accumulation during polarization.The charges properties of the electrets can be adjusted by adding different motifs and adjusting the ratio of the components.The heterogeneous interface charges accumulation process is affected by the electrical gradient structure,and the interface charges accumulation is regulated by the Maxwell–Wagner effect and conductance.The research results in this paperhave a deep understanding of the influence of different primitives and charge decay,crystal phase and dielectric properties,and provide guidance for the development of new materials through different primitives and the regulation of the charge trapping ability of electrets.The mechanism of interface charges accumulation in heterogeneous interface structures are studied,which provides theoretical support for the electret accumulation mechanism of electrets. |
PDF Full Text Request