Trapped Space Charge Research Articles

Space charge behaviors of PP/POE/ZnO nanocomposites for HVDC cables

Polypropylene (PP) has been paid much attention due to its high melting point, excellent electrical insulting performance and thermoplastic property, which is potential to replace the XLPE as HVDC cable insulating material. Blending PP with polyolefin elastomer (POE) is an effective way to modify its stiffness and brittleness at room temperature. However, space charge behavior of PP/POE blend, as a great concern under dc stress, is not clear and needs further investigation. To research the space charge behaviors, pure PP, PP/POE blend and its nanocomposites with different contents were prepared. Then, mechanical properties, permittivity constant, breakdown strength, volume resistivity, space charge behaviors and trap level distribution were investigated. The results indicate the addition of POE enhances the mechanical flexibility of PP greatly, and nano-sized ZnO doping has little effect on the mechanical flexibility of PP/POE blend. The nanocomposites show lower permittivity constant, higher breakdown strength and higher volume resistivity than the PP/POE blend. Compared to pure PP, the space charge accumulation and electric field distortion get severe in PP/POE blend. However, by nanoparticles doping, space charges are remarkably suppressed, which is related to the increased trap level density in nanocomposites. It indicates the PP/POE/ZnO nanocomposites have much potential for HVDC cable application, which show high mechanical flexibility as well as excellent electrical performance.

Effect of Fe3O4 nanoparticles on positive streamer propagation in transformer oil

Fe3O4 nanoparticles with an average diameter of 10 nm were prepared and used to modify streamer characteristic of transformer oil. It was found that positive streamer propagation velocity in transformer oil-based Fe3O4 nanofluid is greatly reduced by 51% in comparison with that in pure oil. The evolution of streamer shape is also dramatically affected by the presence of nanoparticles, changing from a tree-like shape with sharp branches in pure oil to a bush-like structure with thicker and denser branches in nanofluid. The TSC results reveal that the modification of Fe3O4 nanoparticle can greatly increase the density of shallow trap and change space charge distribution in nanofluid by converting fast electrons into slow electrons via trapping and de-trapping process in shallow traps. These negative space charges induced by nanoparticles greatly alleviate the electric field distortion in front of the positive streamer tip and significantly hinder the propagation of positive streamer.

Large improvement in trap level and space charge distribution of polypropylene by enhancing the crystalline − amorphous interface effect in blends

The accumulation of space charge in high voltage direct current cable will bring the insulation to failure; the addition of nanoparticles can markedly improve the space charge distribution characteristics inside the cable insulation, but particle agglomeration and cavitation lead to difficulty in controlling the properties of nanocomposites. In this paper, polypropylene (PP)/propylene-ethylene copolymer (PEC) and PP/ethylene-octene copolymer (EOC) blends were prepared by mechanical blending in order to improve both mechanical properties and space charge distribution. Dynamic mechanical thermal analysis shows that both blends have excellent mechanical properties for recyclable power cable. Pulsed electro-acoustic and thermally stimulated depolarization current tests illustrate that PP/EOC blends significantly decrease space charge accumulation and remarkably increase the trap density in the bulk compared with PP and PP/PEC blends. The increase of the trap density in PP/EOC blends can be explained as the result of the shallow traps introduced by the crystalline − amorphous interface existing in the boundaries of spherulites. The shallow traps can act as hopping sites to improve the transportation of space charges. © 2016 Society of Chemical Industry

Solution-processable triarylamine-based high-performance polymers for resistive switching memory devices

This review summarizes the most widely used mechanisms in high-performance polymeric resistive memory devices, such as charge transfer, space charge trapping and filament conduction. In addition, recent studies of functional high-performance polymers for memory device applications are reviewed, compared and differentiated based on the mechanisms and structural design methods used. The reported memory properties show extremely high endurance during long-term operation, making high-performance polymers very suitable materials for memory applications.

Surface flashover characteristics in polyimide/ZnO nanocomposite under DC voltage in vacuum

It is well known that the solid materials flashover phenomenon is very complicated and influenced by many factors, such as surface conditions, electrodes and traps, etc. In this paper, dc flashover experiments of polyimide (PI) and PI/ZnO nano-composite are studied in vacuum to reveal the surface flashover mechanism. The trap levels and space charge distributions were obtained by thermally stimulated current (TSC) and pulsed electro-acoustic (PEA) methods respectively. Further researches on surface states of flashover tested sample, such as observation by scanning electron microscopy (SEM), and measurement of secondary electron emission (SEE) coefficient, were carried out as well. Experimental results showed that the dc surface flashover voltage of PI/ZnO nano-composite is significantly enhanced compared to that of pure PI. The experiments results can be explained as that the doped ZnO nano-particles introduced deeper traps, which can reduce the level of charge carrier mobility, secondary electron emission and space charge accumulation. Meanwhile, the gas desorption efficiency of nano-composite is much lower than that of pure PI. It suggests that the doped ZnO nano-particles plays a critical role in affecting the surface flashover performance under dc voltage in vacuum.

Thermally stimulated depolarization current and dielectric spectroscopy used to study dipolar relaxations and trap level distribution in PMMA polymer

In this work, thermally stimulated depolarization current (TSDC) and dielectric relaxation spectroscopy (DRS) techniques were performed in complementarities to characterize the dielectric relaxation processes in poly(methylmethacrylate) (PMMA) polymer. Three relaxations were highlighted. Based on correlation between TSDC and DRS data, they were attributed respectively to dipolar β and α relaxations as well as interfacial ρ relaxation. New experimental procedure of fractional polarization was used to investigate space charge trap depth distribution within the band-gap of PMMA. Then the individual peaks were fitted to Bucci–Fieschi model, to evaluate the relaxation parameters. The obtained values were discussed in correlation to those evaluated from DRS data fitted to Havriliak–Negami and Arrhenius models. The various observed peaks in the thermograms are discussed on the basis of space charge polarization. The trap energies were evaluated from the fit to the Bucci–Fuschi model. Similarly other parameters such as relaxation time and charge release are evaluated.

Low voltage resistive memory devices based on graphene oxide–iron oxide hybrid

Low cost resistive switching memory devices using graphene oxide–iron oxide (GF) hybrid thin films, sandwiched between platinum (Pt) and indium-tin-oxide (ITO) electrodes, were demonstrated. The fabricated devices with Pt/GF/ITO structure exhibited reliable and reproducible bipolar resistive switching performance, with an ON/OFF current ratio of 5×103, excellent retention time longer than 105s, SET voltage of 0.9V, and good endurance properties. In all aspects of the device characteristics, the GF based devices outperformed graphene oxide (GO) based devices. Ohmic conduction was found to be dominant current conduction mechanism in all switching regions except for the high voltage regime where space charge limited conduction and trap charge limited conduction were found to be the main current conduction mechanism. X-ray photoelectron spectroscopy and transmission electron microscopy/selected area diffraction analysis revealed γ-Fe2O3 and Fe3O4 iron oxide phases coexist in the hybrid films. While the desorption/adsorption of oxygen-related functional groups on the GO sheets is the dominant resistive switching mechanism in Pt/GO/ITO devices, the formation/rupture of multiple highly conducting Fe3O4 filaments at the iron oxide/GO interface additionally facilitate the switching in the present Pt/GF/ITO devices. Thereby, excellent electrical switching performance was achieved.

Electro-optical modulation with immunity to optical damage by bipolar operation in potassium lithium tantalate niobate.

A method for suppressing the formation of optical damage in quadratic electrooptic devices operated at short wavelengths is presented. Formation of optical damage is attributed to the generation of a trapped space charge induced by photoionization of impurity ions by the propagating beam. It is shown that in potassium lithium tantalate niobate where the electrooptic effect is quadratic, operating the electrooptic device by a bipolar driving voltage prevents the space charge from accumulating, which inhibits the formation of the optical damage. A 6 hours continuous operation of electrooptic modulator for a 30 W/cm(2) at λ = 445 nm input beam is demonstrated.

Low-Temperature TSDC Studies of Molecular Dynamics in Pristine and O7+ Ion–Modified Liquid Crystalline Polyurethane

The molecular dynamics of pristine and 100 MeV O7+ ion–modified liquid crystalline polyurethane (LCPU) was carried out using the thermally stimulated depolarization currents (TSDC) method. The TSDC spectra of pristine LCPU samples consist of current peaks appearing around 110,160, 210, and 260 K and are called the γ, β, α, and δ peaks respectively. The γ-peak is attributed to the crankshaft motion of the spacer group (but-2-ene group). The β-peak is due to the localized rotational fluctuations of C˭O groups. The α-peak is associated with the dipolar orientation of imines group present in the urethane linkage, and the δ-peak has its origin in the space charge trapping mechanism. In the TSDC spectra of O7+ ion–irradiated LCPU samples the γ-peak almost disappears, which shows that the ion induced modification in the methylene linkage. A relaxation process owing to new sub-polar groups formed due to dehydrogenation of CH2 groups and occurring in the vicinity of β-relaxation is discussed. The radiation-induced chain scissioning at NH groups in LCPU is also confirmed from the fluence effect on the α-peak. The irradiation results in the production of a large number of defect sites and an increase in the same results in the enhancement of δ-relaxation.

Effect of temperature on space charge trapping and conduction in cross-linked polyethylene

This paper presented the measurement results of space charge distribution and high field conduction in cross-linked polyethylene (XLPE) plate sample at various electric fields for a broad temperature range from 30 to 90 °C. The temperature effect on charge trapping and transport mechanism is discussed based on estimation of threshold characteristic and apparent mobility. It is shown that the threshold field for space charge accumulation apparently decreased with the temperature, probably associated with acceleration of charge injection and ionic dissociation. The apparent mobility of charges showed an exponential dependence on temperature, which indicates enhancement of charge detrapping and transfer rate at higher temperature. The field dependence of the space charge decreases with the temperature, results in more space charge accumulate at room temperature than at high temperature at certain high field. This behavior indicates that the space charge formation rate increases slower than the charge detrapping rate as the temperature increases. Besides, negative charge is always dominant in the sample at the measuring field and temperature range. This is probably due to stronger electron injection at XLPE-aluminum interface or more negative ion formed by dissociation of chemical species.

Effect of Cu doping on the resistive switching of NiO thin films

Bipolar resistive switching is observed in the GaIn/Cu:NiO film/ITO device with active layer deposited by sol-gel spin-coating. The first-principles calculations indicate that Cu dopants with valence of +1 are located at the substitutional Ni sites rather than the interstitial ones. Cu doping introduces more oxygen vacancies in the film and increases the carrier mobility, however, excessive Cu dopants may assemble at the grain boundary resulting in larger set voltage. Current–voltage measurements indicate that the trap charge limited and space charge limited conduction dominate the high resistance state, while the low resistance state follows the Ohmic mechanism. The switching is attributed to the formation/rupture of oxygen vacancy filaments.

Interpretation of the thermal activation energy of conduction for molecular semiconductor thin films with blocking contacts

We report the in situ electric properties of lead phthalocyanine two-terminal devices. The application of a strong bias voltage induced current rectification and shifted the activation energy of conduction ±0.1 eV from the equilibrium value. The rectification and activation energy splitting could be removed by annealing the device at 175 °C. The results are explained by postulating that the device current is injection limited. In the proposed model the measured activation energy represents an energy barrier height for charge injection which may increase or decrease according to the amount of trapped space charge at the electrode interface. Further consideration of this model suggests that the trap filling level in the phthalocyanine film electrode must be in thermal equilibrium with, and effectively pinned to, the Fermi level of the metal electrodes.

Charge trapping and transport phenomenon in aged and unaged epoxy resin and polyethylene

Summary Polymeric dielectrics are prone to space charge trapping within their volume. This in turn affects many of their dielectric properties including breakdown strength and charge transport. In the current paper, we attempt to characterise charge accumulation and transport in polymeric dielectrics under the application of DC field. Epoxy (EP) resin and polyethylene (PE) are chosen as the dielectric materials to be investigated on account of their prevalence in the high voltage industry. Pulsed electro-acoustic measurements are performed on EP resin and PE specimens. Further, effect of ageing on charge trapping is studied for both EP resin and PE, by thermally ageing the materials under controlled conditions. In addition, the dielectric materials are exposed to a humid environment over long periods and the effect of prolonged exposure to moisture on charge trapping studied. From the experimental results, density of trapped charge is obtained for the different cases. Fourier transform infrared spectroscopy is carried out to identify chemical changes in dielectric materials due to ageing, with an aim to relating physicochemical changes in the material to charge trapping. Copyright © 2014 John Wiley & Sons, Ltd.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Fundamental Study on Diagnostic Method of Water Tree Degraded XLPE Cables using Residual Charge Detection with Pulse Voltages

One of the degradation diagnostic methods for water tree degradation of XLPE cables is the residual charge method. In this method, a DC voltage is applied to a water tree degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages. In this study, we first applied pulse voltages with a pulse width of about 1ms to an artificial water tree degraded XLPE cable, and found that the residual charge increased with the increase in the degradation degree. Second, we confirmed that residual charges were successfully detected in the removed 6.6kV XLPE cables that had water tree degradation. Finally, we investigated the relationship between the residual charge signal and the AC breakdown voltage of the removed 6.6kV XLPE cables. The results clarified that the removed cables with larger residual charge and longer duration of current signal related to water tree degradation had a lower AC breakdown voltage.

Correlation between I (current)-V (voltage) characteristics and thermally stimulated depolarization current of Mn-doped BaTiO3 multilayer ceramic capacitor

I (Current)-V (voltage) characteristics of Mn-doped BaTiO3 multilayer ceramic capacitor were investigated. The increase of Mn concentration reduced the slope of current increase with increasing dc field and enhanced the dc field to the abrupt increase of current and thereby breakdown. Ohmic, Schottky, and Poole-Frenkel type conduction was successively observed with the increase of dc field in all specimens. The region of Child's law did not appear before breakdown, which suggests space charge limited (SCL) conduction with field enhanced trap barrier lowering. A peak of thermally stimulated depolarization current (TSDC) beyond 300 °C associated with trapped space charges appeared, which increased by the increase of Mn concentration. In addition, the peak intensity increased to a maximum value and then decreased again with increasing polarization dc field in TSDC measurement. Its activation energies were around 1.9 eV for all specimens, which is very close to the energy level for the ionization of Mn3+/Mn2+. These results experimentally demonstrate the role of the variable valence acceptor Mn as trapping center for injected charges, and the occurrence of field enhanced trap charge release, and therefore, the conduction mechanism can be described by the SCL model incorporating the Poole-Frenkel effect.

A dielectric study on colloidal silica nanoparticle Layer-by-Layer assemblies on polycarbonate

This study focuses on the dielectrical characterization of polycarbonate films coated with silica nanoparticle bilayers assembled through the Layer-by-Layer (LbL) technique. This is the first attempt of dielectric characterization performed on LbL-treated plastic substrates. To this aim, LbL coatings consisting of oppositely charged colloidal silica have been built on a polymeric substrate (polycarbonate). Then, dielectric features such as space charge accumulation, electrical voltage breakdown, and resistance to corona discharge (through laser ablation) have demonstrated that the colloidal silica nanoparticle assemblies can influence the nature of the trapped space charges and affect the resistance of polycarbonate to corona discharge, changing the distribution of the laser energy on impact.

A serially coupled stationary phase method for the determination of urinary 8-oxo-7,8-dihydro-2′-deoxyguanosine by liquid chromatography ion trap tandem mass spectrometry

Oxidative attack to DNA is of particular interest since DNA modifications can lead to heritable mutations. The most studied product of DNA oxidation is 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG). While 8-oxodG determination in blood and tissue cells is prone to artifacts, its measurement in urine employing liquid chromatography tandem mass spectrometry (LC-MS/MS) has gained more and more interest for increased reliability. LC-MS/MS can be affected by matrix effects and this is particularly true when ion trap is used as MS analyzer, due to ion accumulation in the trap and related space charge effect. In the present work, we have developed a LC-MS/MS method where the combination of cation exchange and reverse phase solid phases resulted in LC separation optimization. This together with the employment of an isotopically labeled internal standard, allowed the usage of ion trap LC-MS/MS, typically not employed for quantitative measurement in biological samples, for the measurement of 8-oxodG in urine samples from control populations.Four different urine matrices were employed for method validation. Limit of quantitation was set at least at 0.5ng/ml. While analyzing urine samples from healthy volunteers, 8-oxodG levels reported as ng/ml were statistically different comparing males with females (p<0.05, Mann Whitney test); while comparing results normalized for creatinine no statistical significant difference was found. Mean urinary 8-oxodG level found in healthy volunteers was 1.16±0.46nmol/mmol creatinine.The present method by enhancing at best the chromatographic performances allows the usage of ion trap LC-MS/MS for the measurement of 8-oxodG in urine samples from control populations.

Poole Frenkel current and Schottky emission in SiN gate dielectric in AlGaN/GaN metal insulator semiconductor heterostructure field effect transistors

We analyze the anomalous I-V behavior in SiN prepared by plasma enhanced chemical vapor deposition for use as a gate insulator in AlGaN/GaN metal insulator semiconductor heterostructure filed effect transistors (HFETs). We observe leakage current across the dielectric with opposite polarity with respect to the applied electric field once the voltage sweep reaches a level below a determined threshold. This is observed as the absolute minimum of the leakage current does not occur at minimum voltage level (0 V) but occurs earlier in the sweep interval. Curve-fitting analysis suggests that the charge-transport mechanism in this region is Poole-Frenkel current, followed by Schottky emission due to band bending. Despite the current anomaly, the sample devices have shown a notable reduction of leakage current of over 2 to 6 order of magnitudes compared to the standard Schottky HFET. We show that higher pressures and higher silane concentrations produce better films manifesting less trapping. This conforms to our results that we reported in earlier publications. We found that higher chamber pressure achieves higher sheet carrier concentration that was found to be strongly dependent on the trapped space charge at the SiN/GaN interface. This would suggest that a lower chamber pressure induces more trap states into the SiN/GaN interface.