Study On Glass-assisted Open-circuit Thermal Stimulation Discharge Technology

The study of electret charge storage and decay law by Thermal Stimulation Discharge(TSD)technique has greatly deepened the understanding of electret formation mechanism and has become the most important research tool in electret research.There are generally two kinds of experimental methods,short-circuit TSD and open-circuit TSD.Because the upper and lower electrodes of short-circuit TSD are close to the material,the description of the dipole charge in the material is very accurate,but the measurement of electret with surface charge will cause charge attenuation,and the discharge current efficiency is very low.In the open-circuit TSD experiment,there is an air gap between the sample and the upper electrode.With the increase of temperature,the electret will be deformed,which will affect the results.Combined with the comprehensive analysis of short-circuit TSD and open-circuit TSD,a glass-assisted open-circuit thermal stimulation discharge experiment method is proposed in this paper.Using this method,it is not only possible to distinguish dipole charge from space charge,surface charge from internal charge,but also to prevent material deformation.In the course of this paper,a model for the flow of charge within the electret in glass-assisted open-circuit thermal stimulation discharge experiments was proposed,and the analytical formula for the TSD current was derived.The feasibility of this method was demonstrated by applying it to test the discharge currents of tetrafluoroethylene(TFE)-vinylidene fluoride(HFP)-hexafluoropropylene(VDF)copolymer(THV),fluorinated ethylene propylene copolymer(Fluorinated ethylene propylene,FEP),and melt-blown polypropylene(Melt-blown polypropylene,MBPP)electret.Finally,the influence factors of glass-assisted open-circuit thermal stimulation discharge spectrum are studied.The main conclusions are as follows:(1)Based on the open-circuit thermal stimulation discharge technology,a high-resistance insulator,namely glass material,was inserted between the upper electrode and the electret,and the glass-assisted open-circuit thermal stimulation discharge technology was proposed.The analytical formula of glass-assisted open-circuit thermal stimulation current was derived,and the feasibility of the technology was verified.(2)Glass-assisted open-circuit thermal stimulated discharge experiments on THV500 and THV815 polar electrets show that the technique can distinguish between dipole and space charges.The results show that both materials have the same dipole peak at about 40℃.The peak temperature of the discharge peak of THV815 is higher than that of THV500,which has better charge storage stability,and the area enclosed by the discharge peak and the coordinate axis is larger,indicating that THV815 has a larger charge space density.(3)The glass-assisted open-circuit thermal stimulation discharge experiments on FEP non-polar electret demonstrate that the technique can distinguish between surface charge and body charge.By changing the initial surface potential and heat treatment time of the FEP electret,the discharge spectrum results show that the higher the surface potential,the higher the discharge current.The longer the heat treatment time,the surface charge decreases and the deep trap charge increases slowly.(4)The glass-assisted open-circuit thermal stimulation discharge technique can also distinguish electret bodies with different charge storage mechanisms.Using the glass-assisted open-circuit thermal stimulation discharge spectra of corona injector MBPP and water injector MBPP,combined with the surface potential and filtration efficiency test,it is found that the peak-peak discharge value of water injector is smaller than that of corona injector,the peak-peak temperature is slightly increased,the surface potential is smaller,but the distribution is more uniform,and the filtration efficiency is higher,which proves that the two charge storage mechanisms are different.(5)By studying the effects of the glass thickness,and the glass-assisted open-circuit thermal stimulation discharge spectrum with electrodes plated on top of the glass,it is found that the larger the glass thickness is,the smaller the discharge current peak becomes.The electrode plating on the top of the glass can indirectly increase the discharge current by increasing the area of the upper electrode.(6)Based on the effect of glass permittivity on the results of discharge spectra,it is concluded that for electrets with charge stored in the electrets or deeper traps,glass-assisted open-circuit thermal stimulation discharge experiment does not require high requirements on glass,and all three kinds of glass selected in this paper can be measured;For electrets containing surface charges,quartz glass with a smaller dielectric constant is needed.