Photo-stimulated Discharge Research Articles

Electronic properties of polyethylene naphthalate as derived from photo-stimulated discharge, luminescence experiments and quantum chemical calculation

The electronic properties of thin films of poly(ethylene 2,6-naphthalate)—PEN, are investigated based on their photo-physical (optical absorption, photoluminescence) and electrical (space charge distribution, photo-stimulated discharge) behavior. Photo-stimulated currents are associated with optical absorption of the material leading to space charge dissipation as demonstrated by space charge distribution measurement. Based on this set of experimental results and quantum chemical calculation performed on PEN macromolecular system, we propose a new scheme for the electronic levels of PEN. This scheme allows understanding the mechanisms at play in photo-stimulated discharge. One of the main conclusions of our work is that photo-stimulated current measurements do not probe the energy level of traps. Detrapping of charges results from a two-step process where the photon energy is absorbed by chromophores that restitute a part of this energy to trapped charges through various mechanisms. Moreover, the new scheme allows discussing the components of the luminescence excited under different stresses, being electric field, electronic and UV irradiation, charge recombination and thermal activation.

Investigation of wide-range trap energy spectra of typical aerospace polymer dielectric materials by using photo-stimulated discharge method

The presence of space charge in polymer insulation materials has emerged as a critical constraint in the advancement of polymer materials for high-voltage electrical devices. Trap properties play a pivotal role in governing the storage and transport of space charges. Thus, precise characterization of the distribution of charge trap energy spectra in polymer dielectric materials holds immense scientific significance for enhancing the properties and applications of such materials. In response, the author develops a platform utilizing the photo-stimulated discharge (PSD) method to measure the trap energy spectra of polymer dielectric materials. This platform allows for accurate, intuitive, and effective characterization of trap parameters across a broader range while preserving the original features of trap energy levels. Experimental results have shown that the laser light source employed in the platform does not induce photodegradation or photodamage to the tested samples. The validity of the PSD measurement system has been confirmed under diverse experimental conditions, yielding trap spectra and pertinent parameters for four representative polymer materials: polyimide, polyester, polyvinylidene fluoride, and polyethylene. This work presents a scientifically effective approach to characterize trap energy spectra in polymer dielectrics, providing valuable insights into the characterization of such spectra.

Measurement setup to simultaneously explore the location and energy of trapped charges in thin polymer films

This research proposes a unique system that combines charge density measurements by the laser intensity modulation method with optically excited current acquisitions using the photo-stimulated discharge technique (PSD). The purpose of this setup is to investigate the relationship between space charge properties (such as density, spatial depth, and time evolution) and the photocurrent-associated energies in order to gain new insights into the trap population and detrapping mechanisms in thin polymer films. This paper presents a description of the technical principles of both methods as well as the whole combined system. The results on a 12 μm-thick polyethylene naphthalate film show pyroelectric currents, which after processing indicate the formation of homocharges whose magnitude and depth decrease after light irradiation. The PSD currents allow the identification of two major energy bands at 3.4 and 5.9 eV (360 and 207 nm, respectively), possibly related to charge detrapping. In addition, current transients during constant wavelength irradiation show that incident photons can interact differently with trapped charges depending on the applied field.

Effect of maleic anhydride on trap levels of alumina/low-density polyethylene by photo-stimulated discharge

ABSTRACTIn this paper, the distribution of trap levels of alumina/polyethylene (Al2O3/LDPE) modified by maleic anhydride (MAH) was studied by photo-stimulated discharge (PSD). The PSD results show that the energy level of charge traps in the 0.5% and 1% Al2O3/LDPE modified by MAH is mainly distributed in 4.68–5.64 eV, and the trap density maximum is at 235 nm and 230 nm, respectively. According to the experimental result, the maleic anhydride can provide pure polyethylene with abundant deep traps. However, it can effectively reduce trap density of the Al2O3/LDPE system, especially, this kind of mechanism of action is more obvious for the 1%-doped Al2O3/LDPE system.

Photo-stimulated discharge current in polyimide films

Photo-stimulated discharge (PSD) method is one of the ways to measure the trap level of space charge in the dielectric materials. Compared with the thermally stimulated discharge (TSD) method, it has no observable destroy to the sample's microstructure during the procedure of the light scanning and can therefore be considered to be a more accurate measuring method. However, the light scanning procedure may possibly lead to the occurrence of the external photoelectric effect between the electrode-dielectric interfaces or the internal photoelectric effect in the sample bulk, where the extra current due to the two photoelectric effects can seriously distort the PSD current. In this paper, the total current excited by the scanning light on the polarized or being polarized polyimide films are studied. The influence of the applied field intensity, the electrode structures, the evaporated electrode thickness and the isothermal decaying current of the polarized films on the stimulated current is evaluated, which is comprehensively analyzed to extract the space charge dependent PSD current from the distorted current. The charge trap level based on the PSD method is finally obtained.

Research on the electrical conduction characteristic of polyimide/Al2O3 nanocomposite films with low threshold field

Polyimide/Al2O3 (PI/Al2O3) composite films with different doping concentrations were fabricated by in-situ polymerization. Based on the SEM images of surface and fracture for PI composite films, it shows that the Al2O3 nanoparticles were well dispersed in the PI matrix. The electrical conduction characteristic was carefully investigated via current-voltage characteristics, photo-stimulated discharge and isothermal discharge current, respectively. The experimental results show that a remarkable nonlinear electrical conduction characteristic of PI/Al2O3 composites was observed under external electric field. The conduction current density of composites was increased by doping Al2O3 nanoparticles, while the threshold for the accumulation of space charge is reduced distinctly. The trap energy level density of PI composites doped 4wt% Al2O3 nanoparticles (~8.85×1020/eV·m3) was higher than that of pure PI (~4.6×1020/eV·m3). Furthermore, the trapped charge layer was formed because a large number of shallow traps were introduced into PI matrix, which results in the change of electrical conduction characteristic. Besides, the appearance of trapped charge layer also leads to the improvement of corona-resistance property for PI nanocomposite films.

Influence of photoconductivity on the study of trap levels in polyimide films by photo-stimulated discharge

In this paper, the distribution of the charge trap levels in polyimide (PI) films was investigated by using photo-stimulated discharge (PSD) technique, which was to explore the effect of photoconductivity on photo-stimulated discharge current. The PSD spectra show that the photo-stimulated discharge current of the PI films is mainly distributed in the range of 649–320 nm, but there are two significant current peaks when the wavelength is 300 nm and 244 nm, respectively. By studying the reason of generating two current peaks, it is concluded that the two current peaks are not generated by trapped charge de-trapping but generated by photoconductivity of charge-transfer complex in the PI films. According to the research, it is concluded that the trap levels in PI films are mainly distributed in the range of 1.92 eV–3.88 eV.

Analysis and interpretation of photo-stimulated discharge spectrum for polypropylene films under different electric and geometrical conditions

Experiments on polypropylene films under different electric and geometrical conditions are carried out to investigate various causes that would affect the interpretation of photo-stimulated discharge spectrum. The samples used in the experiment are metalized with finger-like shape or circular shape electrodes. The results show that the occurrence of photoemissions from electrode or polymer at wavelengths less than about 260 nm is possible, depending on the polarity and amplitude of applied voltage. The photocurrent spectra for samples with circular electrodes characterize two charge de-trapping peaks and the peak at 273 nm is also partially contributed by the photoelectrons from electrode.

Effect of Anhydride Grafting Agent on Trap Levels of Low-Density Polyethylene

Trap distribution of the low-density polyethylene(LDPE) modified by anhydride grafting agent was studied and the effect of grafting agent on the trap levels was analyzed by the method of Photo-stimulated Discharge in this paper. Different concentrations and types of same concentration of LDPE modified by anhydride grafting agent were measured by using continuous UV scanning. The results showed that different concentrations and types of grafting agent had effect on trap levels distribution of LDPE. The trap depth would decrease with the anhydride grafting agent content increasing and increase with its damaged condition increasing when anhydride grafting agent was destroyed. Anhydride grafting agent are widely applied to the production and research of high pressure insulating polyethylene materials. The purpose of this paper is to elaborate the effect of anhydride grafting agent on the insulating properties of polyethylene material and explain the strongest binding capacity of anhydride grafting agent to space charge.

Investigation of surface and bulk charge traps of polyimide film by using the photo-stimulated discharge method

The spatial distribution of the charge trap depths in polyimide films was investigated by the photo-stimulated discharge (PSD) method. The charges were implanted by corona charging or electron beam irradiation. The characterizations of the PSD spectra regularly varied with the corona charging durations and the electron beam irradiation with different energy was adopted to find out the cause of the change. The experimental results show that the trap depths have a non-uniform spatial distribution along the thickness direction in polyimide films.

Influence of interface on the electrical properties of polyimide/TiO2composite films

Polyimide/TiO 2 (PI/TiO 2 ) composite films were fabricated by in-situ polymerization and are reported in this paper. The effect of interface (PI-TiO 2 ) on dielectric and breakdown properties was investigated via conductivity, photo-stimulated discharge (PSD), permittivity, thermo-gravimetric analysis (TGA) and breakdown characteristic. The results show that interface between PI and TiO 2 in composites contains deep charge traps with density proportional to TiO 2 doping concentration. Interface enhances space-charge polarization to increase conductivity and loss tangent at low frequency. The pinning influence of interface on PI molecular chain motion is reflected in the downtrend of permittivity (0%-4% nano-TiO 2 doping concentration) and increasing decomposition temperature (T PI <;T 3% <;T 7% ). When exposed to corona ageing, the PI matrix rather than interface or nano-TiO 2 is initially damaged, and the remaining mixture of interface and nano-TiO 2 are accumulated gradually on the PI/TiO 2 surface forming a “protective” layer which slowing down further PI/TiO 2 erosion in corona ageing.

The Research on the Effects of Electrical Aging on Trap Distribution in Polyethylene Doped with Nanoparticies

The photo-stimulated discharge (PSD) technology was used to characterize the trap distribution of polyethylene (PE) and the PE that doped with 5% MgO before and after electrical aging. For the PE with the aging time increasing, the internal shallow trap of the material gradually reduced and deep trap gradually increased. But for the PE that doped with 5% MgO with the aging time increasing, shallow trap gradually increased and the deep trap gradually reduced. Results show that: during the electrical aging process the PE produces a large amount of free radicals which can introduce deep traps. Adding impurity of MgO can effectively restrain charge moving that injected from electrode, thus to improve the internal electric field distortion of PE.

Study on evolution of deep charge traps in polyimide irradiated by low-energy protons using photo-stimulated discharge technique

In this paper, low-energy proton irradiation effects on the evolution of deep charge traps in proton pre-irradiated polyimide are investigated using the photo-stimulated discharge (PSD) technique. The results indicate that in the PSD spectrum there appear three discharge bands with some sub-peaks in the wavelength range from 250 to 300 nm, 300 to 325 nm and 325 to 350 nm, respectively. After proton irradiation, the positions of the PSD bands show almost no change, while the integral intensity of the main PSD band (300–325 nm) decreases drastically from the original 175 pC in the pristine polyimide to the lowest one at about 10 pC in the irradiated sample, when 60 keV proton fluence is 1 × 1014 p cm−2. However, the intensity shows a certain recovery to about 50 pC as 60 keV proton fluence increases further to 1 × 1016 p cm−2. Further analyses indicate that the evolution of the PSD intensity is dependent on the displacement effect induced by proton irradiation. Structural analyses show that the degradation of carbonyl group in polyimide is the main reason for the decrease in PSD intensity due to proton pre-irradiation.

Trap levels in low density polyethylene doped with nanoparticles by photo-stimulated discharge

Trap levels in nanoparticle doped low density polyethylene (LDPE) are investigated by photo-stimulated discharge (PSD). The PSD spectra in MgO/LDPE and Al2O3/LDPE nanocomposites with the different mass percentages and different nanocomposites with the same mass percentage are obtained by continuous scanning mode, which can be qualitatively estimate the trap energy distribution. The trap levels in Al2O3/LDPE nanocomposite are quantitatively described by step scanning mode. The experimental results indicate that the trap levels in Al2O3/LDPE nanocomposites with the Al2O3 content of more than 0.2% and MgO/LDPE nanocomposites with the MgO content of more than 0.5% are deeper than those of the virgin LDPE. According to the relevant reports on the effect of nanoparticle doping on space charge injection, it is considered that the suppression of space charge is probably correlated with the deeper trap levels in nanocomposites.

Methodological investigation on photo-stimulated discharge to obtain accurate trap information in polymer dielectrics

Based on photo-stimulated discharge (PSD) theory about release of charges from the mono- and multi-levels of traps, two methods on the energy distribution of trapped charges (charge distribution)—step scanning method and combined calibration–scanning method are proposed. The typical polymeric dielectric, low-density polyethylene (LDPE), is selected to be investigated in this paper. The linear relation between PSD current and time at selected wavelengths in LDPE samples indicates that the effective interaction cross sections between the photons and trapped charges depend on the photon energy. It is indicated that both of the largest ratios in charge distributions based on the two methods are around the energy level of 4.97 eV for LDPE. Compared with the step scanning method, the experimental time is extremely shortened and thermal release of trapped charges is decreased in the combined calibration–scanning method, and charge distribution based on combined calibration–scanning method is more reliable.

STUDY ON DEEP TRAP STATES IN POLYMERS BY PHOTO-STIMULATED DISCHARGE BASED ON THE OPO LASER

A photo-stimulated discharge (PSD) method is more effective than thermal methods to characterize the deep space charge traps in dielectric materials. In this paper, a new PSD measuring system was first built up based on the optical parametric oscillator laser. The laser can generate continuously tunable wavelengths whose range are 210–420 nm and with average output pulse energy 2.3 mJ. The system overcomes the shortcoming of weak output energy in past experiments, especially in the range of ultraviolet band. The PSD current spectra of PET were measured by this system. The results show that the energy level of charge traps in the PET film is mainly distributed in 4.08~5.18 eV, and the trap density maximum is at 4.22 eV. These data can provide the experimental basis for further research and improvement on electrical properties of materials.

Experimental study on the distribution of trap levels in dielectric by photo-stimulated discharge

The distribution of trap levels in the low density polyethylene (LDPE) was studied by means of photo-stimulated discharge (PSD). And two scanning modes, continuous scanning and step scanning, were discussed. It was pointed out that there may be some problems because of the incomplete detrapping of the trapping charges for the continuous scanning. The principle and the experimental process for the step scanning were presented. It is found that there is a linear relationship between the logarithm of PSD current produced under the irradiation of the monochromatic light and the time, which is in accordance with the theoretical analysis and indicates that the retrapping effect of the carriers can be neglected. The charges are trapped in the levels of 4.36—6.22 eV in LDPE, the main part of which were trapped in the energy levels of 4.78—5.18 eV.

Photostimulated discharge in electret polymers: an alternative approach for investigating deep traps

The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm/sup 2/ were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d/sub 33/ coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 nm could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-)piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets.

Ultraviolet-induced discharge currents and reduction of the piezoelectric coefficient in cellular polypropylene films

Photostimulated discharge spectroscopy of cellular polypropylene films between 200 and 400 nm showed the existence of at least three distinct trapping levels at 4.6, 5.6, and 6.3 eV. The effects of UV irradiation on the piezoelectric d33 coefficient was studied by monitoring thickness-extension resonances in the dielectric spectrum. Prolonged irradiation at wavelengths below 210 nm led to a reduction of the piezoelectric coefficient, caused by partial discharge of the polymer foam.