Surface Potential Decay Research Articles

Understanding Trap Effects on Electrical Treeing Phenomena in EPDM/POSS Composites

POSS (polyhedral oligomeric silsesquioxane) provides an interesting alternative nano-silica and has the potential of superior dielectric properties to restrain electrical degradation. By incorporating POSS into EPDM to suppress electrical tree, one of precursors to dielectric failure, is promising to improve the lifetime of insulation materials. This paper focuses on the electrical treeing phenomena in EPDM/OVPOSS (ethylene propylene diene monomer/octavinyl-POSS) composites based on their physicochemical properties and trap distributions. ATR-IR and SEM characteristics are investigated to observe the chemical structure and physical dispersion of EPDM/OVPOSS composites. Electrical treeing characteristics are studied by the needle-plane electrode, and the trap level distributions are characterized by surface potential decay (SPD) tests. The results show that the 3 wt% EPDM/OVPOSS is more effective to restrain the electrical tree growth than the neat EPDM in this paper. It is indicated that the EPDM/OVPOSS with a filler content of 3 wt% introduces the largest energy level and trap density of deep trapped charges, which suppress the transportation of charge carriers injected from the needle tip and further prevent the degradation of polymer molecules. The polarity effects are obvious during the electrical treeing process, which is dependent on the trap level differences between positive and negative voltage.

Broad bandwidth vibration energy harvester based on thermally stable wavy fluorinated ethylene propylene electret films with negative charges

Wavy fluorinated ethylene propylene (FEP) electret films with negative charges were prepared by a patterning method followed by a corona charging process. The thermal stability of these films was characterized by the surface potential decay with annealing time at elevated temperatures. The results show that thermally stable electret films can be made by corona charging followed by pre-aging treatment. Vibration energy harvesters having a very simple sandwich structure, consisting of a central wavy FEP electret film and two outside metal plates, were designed and their performance, including the resonance frequency, output power, half power bandwidth, and device stability, was investigated. These harvesters show a broad bandwidth as well as high output power. Their performance can be further improved by using a wavy-shaped counter electrode. For an energy harvester with an area of 4 cm2 and a seismic mass of 80 g, the output power referred to 1 g (g is the gravity of the earth), the resonance frequency, and the 3 dB bandwidth are 1.85 mW, 90 Hz, and 24 Hz, respectively. The output power is sufficient to power some electronic devices. Such devices may be embedded in shoe soles, carpets or seat cushions where the flexibility is required and large force is available.

Temperature effect on electrical characteristics of negative DC corona charged polyimide films

The study of the effect of sample temperature on the electric properties of PI films has been investigated. Surface potential decay (SPD) measurements on PI (Kapton HN, thickness: 50 μm) films were measured, after negative corona discharge, as a function of sample temperature varying from 27 to 80 °C. The measurements showed significant increase in the SPD rate with increasing sample temperature. Surface potential profile, return potential appearing after charge neutralization and charging current characteristics have been experimentally recorded for different sample temperatures in order to interpret SPD evolution at high sample temperature. The SPD curves were analyzed by fitting a bi-exponential decay Equation according to the presence of two different decay mechanisms related to shallow and deep traps. The distributions of trapped charges are obtained for different sample temperatures.

Temperature dependent surface potential decay and flashover characteristics of epoxy/SiC composites

GIS spacers made of epoxy resin always suffer from the flashover due to surface charge accumulation. Therefore, it is important to seek a new method to improve the performance of epoxy resin. The epoxy/SiC composites have been proven to have nonlinear conductivity and the potential of being used in the spacer manufacture because of their field grading function under DC voltage. A high temperature rise on the central conductor of GIS can cause a large temperature gradient along the spacers. Exploring the effects of temperature on the electrical properties of the epoxy/SiC composites is helpful in the design the space dependent SiC filler content in GIS spacers. In this paper, three temperatures 20, 50 and 70 °C were studed on the temperature dependent surface potential decay (SPD) and flashover characteristics of the epoxy/SiC composites. At 20 °C, the epoxy/SiC composites, especially the 14 vol % samples, have much better SPD performances and higher flashover voltages than the pure epoxy. As the temperature increases, the SPD performances of all samples are greatly improved. The SPD rate of the 0 vol % samples approaches that of the 6 vol % samples at 50 °C and the 14 vol % samples at 70 °C. When the temperature is over 50 °C, flashover of the epoxy/SiC composites occur at lower voltages compared with the pure epoxy resin. With a further increase of temperature from 50 to 70°C, the flashover voltages of all samples have a significant drop, indicating that the temperature effects on flashover fault must be included in the spacer design and manufacture.

Trap energy distribution in polymeric insulating materials through surface potential decay method

A model based on surface potential decay (SPD) measurement was developed to study the trap energy distribution of hole-type and electron-type traps in materials by considering two distinct energy levels (i.e., shallow and deep traps) and the detrapping process. An approximated Dirac function δ((E-Em)/kT) was also introduced to obtain a consistent unit of trap density. Besides, the space charge distributions (such as uniform, linear and exponent distribution) in positively and negatively charged materials were also taken into account. In order to verify our proposed model, the different SPD curves of low density polyethylene (LDPE), LDPE/zinc oxide (ZnO) microcomposite and nanocomposite were analyzed. The results show that the proposed model is effective to obtain the trap energy distribution in materials. The SPD curves in positively and negatively charged samples show a fast decay followed by a slow decay, which is agreed with our assumption of shallow and deep traps. Two peaks are clearly observed from the trap energy distribution. The trap energy distributions of hole-type and electron-type traps in these materials are distinct from each other. This implies some essential nature of hole-type and electron-type traps (i.e., intra-chain character for hole-type traps and inter-chain character for electron-type traps). In addition, compared to LDPE and LDPE/ZnO microcompoiste, the decreasing energy level and the increasing trap density of shallow traps as well as the decreasing trap density of deep traps in LDPE/ZnO nanocomposite may be attributed to the effect of the interaction zone surrounding nanoparticles.

Charge transport in a double-layered enamel insulation during surface potential decay

This work presents a numerical model for studying charge dynamics during decay of pre-deposited surface charges on an enamel insulation comprising two layers of different materials. It is shown that the considered surface charge decay is dominated by charge injection from insulation surface into the bulk and the drift of injected carriers along the direction of induced electric field. These features allow for estimating charge carriers' mobilities in the studied insulation. The developed model incorporates experimental outcomes as well as the proposed effect of an internal interface between insulating layers on hindering charge transport through the bulk. The transient process of charge accumulation at the internal interface is exhibited and correlated with the well-known Maxwell-Wagner-Sillars polarization model. Contribution of charge injection at each insulation surface into the bulk to the measured potential decay is evaluated and compared.

Charge trapping characteristics of alumina based ceramics

The space charge dynamics is very important for electrical breakdown of alumina based ceramics. In this paper, the charge trapping/detrapping characteristics of alumina based ceramics were studied by means of isothermal surface potential decay (ISPD) method. For alumina and zirconia toughened alumina (ZTA) ceramic samples, the ISPD curves charged by corona discharge as well as microstructure characterization were carried out. For the first time, crossover phenomenon and hollow shaped potential profile were observed and reported in alumina based ceramics, indicating a surface potential decay process dominated by charge injection and volume conduction affected by the trap states in materials. In addition, the comparative trapping characteristics were evaluated based on a charge detrapping controlled decay model. The correlation between trap distribution and microstructure of alumina based ceramics was investigated. It was proposed that different charge trapping characteristics of alumina based ceramic samples was caused by varied shallow trap density of grain boundary.

Surface potential decay on silicon rubber samples at reduced gas pressure

Accumulation of interfacial charges is an inherent feature of HVDC insulation based on solid and gaseous media. The collected surface charges can alter the geometrical electric field leading to undesirable phenomena such as partial discharges and even unexpected flashovers. In the present paper, surface potential decay on silicone rubber samples is analyzed at reduced pressures of ambient air that allows for elimination of surface charge neutralization by gas ions. Thus, influences imposed by bulk and surface conduction in the solid material are studied by means of computer simulations and experimental measurements. The results allow for identifying levels of bulk and surface conductivities above which the corresponding charge decay mechanism becomes dominant. It is shown that with a negligible space charge effect and significant surface leakage, there exists a notable spread of charge along gas-solid interface yielding visible crossover phenomenon in charge decay characteristics. It is also demonstrated that the effect of space charge in the material bulk on surface potential decay can only be significant within layers of material finer than ca. 100 μm.

Unusual effect of temperature on direct fluorination of high temperature vulcanized silicone rubber and properties of the fluorinated surface layers

In order to investigate the effects of fluorination temperature on surface layer characteristics (chemical composition and structure, morphology, and thickness) and surface layer properties, high temperature vulcanized silicone rubber samples were fluorinated in a laboratory vessel using a F 2 /N 2 mixture with 12.5% F 2 by volume, at 0.1 MPa and the different temperatures of 25, 55, and 85 °C, for the same time of 30 min. ATR-IR analysis indicates that the substitution of fluorine atoms for hydrogen atoms of methyl groups is dominant for the fluorination at the different temperatures, while a substitution for methyl groups cannot be excluded. SEM cross-section and surface images show the fluorinated layers with nanostructured surfaces. The surface layer fluorinated at 55 °C appears to have the lowest degree of fluorination, but it clearly has the largest thickness and surface nanostructure size among the fluorinated layers. This unusual phenomenon is caused by two competing effects, the thermal activation of the substitution reaction and fluorine diffusion through the surface layer and the steric hindrance by the fluorinated methyl groups to the fluorination and fluorine diffusion. Contact angle measurements reveal that the fluorinated surfaces have high hydrophobicity and oleophobicity and thus an extremely low surface energy. Surface potential decay measurements show a much more rapid decay of potential on the fluorinated samples, compared to the unfluorinated sample. There is no significant difference in surface hydrophobicity or surface conduction between the fluorinated samples, due to the combined effect of compositional and structural changes. The difference in surface oleophobicity between the fluorination at 55 °C and the fluorination at 25 or 85 °C provides further evidence for the difference in their surface chemistry, because surface geometry of the fluorinated samples has little effect on the oleophobicity.

Temperature dependent trap level characteristics of graphene/LDPE nanocomposites

The trap level characteristics are attracting much attentions in electrical and electronic engineering, which are closely associated with electrical properties of insulating materials. This paper try to modify the trap level characteristics of Low-density Polyethylene (LDPE), by incorporating graphene nanofillers into polymer with a filler content of 0.001, 0.003, 0.005, 0.007 and 0.01 wt%. The effects of filler content and ambient temperature on the trap level distributions are investigated, based on the isothermal surface potential decay (ISPD) method under 30, 50 and 70 °C. The experimental results show that the deep trap level and density increase with the filler content from 0 to 0.005 wt%, and then decrease with the further increase to 0.01 wt%. The increases of deep trap level and density affect the detrapping processes and suppress the electron injection into polymer bulks and cause a much slower downward trend in ISPD curves, especially at ambient temperature of 70 °C. Additionally, the apparent deep trap levels increase as temperature increases, since deeper and deeper traps are involved in the charge-release mechanism. It is concluded that the graphene nanofiller loading with a content of 0.005 wt% introduces large quantities of deep charge traps in the polymer-filler interfacial regions, which reduce the detrapping rate of charges and suppress the charge injection into polymer.

Effects of non‐linear conductivity on charge trapping and de‐trapping behaviours in epoxy/SiC composites under DC stress

Gas-insulated switchgear (GIS) spacers are made of epoxy resin. However, the surface charge accumulation has been a great concern to the safe operation of GIS, which causes the frequent flashover faults on spacers. In this study, micro-silicon carbide (SiC) particles with non-linear conductivity were added into epoxy matrix and the filler content varied from 0 to 14 vol%. Then, the bulk conductivity and surface potential decay (SPD) tests were conducted. The obtained results showed that the epoxy/SiC composites have obvious non-linear conductivities and the non-linear-conductivity threshold decreases with the increasing filler content. The addition of SiC can effectively resist the rise of surface potential and enhance the surface charge dissipation process. From the trap energy distributions, it can be inferred that the deep traps of ~0.9 eV should be the intrinsic traps of epoxy and the shallow traps of ~0.8 eV are considered to be introduced by SiC. Furthermore, the simulation results confirmed that the sharp increase of carrier mobility in non-linear region significantly reduces the remaining time and possibility of a de-trapped charge being recaptured by traps before reaching the grounded electrode. Therefore, the high conductivity in non-linear region contributes a lot to the increase of SPD rate.

Temperature-Dependent Surface Charge and Flashover Behaviors of Oil-Paper Insulation Under Impulse With Superimposed DC Voltage

Oil-paper insulation will inevitably endure impulse voltage with a superimposed dc voltage (superimposed voltage) because of introduced lightning or switching, which can induce charge accumulation and trigger flashover during the operation of a converter transformer. Temperature rise is also a nonnegligible factor that can significantly influence the performances of oil-paper insulation. This paper aims to reveal the effects of ambient temperature and superimposed voltage on surface charge and flashover behaviors in an oil-paper insulation system. Surface potential decay (SPD) experiments were performed under superimposed voltage with various polarity and amplitude combinations and at the temperature ranging from 20 °C to 80 °C. The results show that the initial surface potential and decay rate increase as the amplitude of the impulse voltage increases at 20 °C. The initial surface potential declines, and the process of SPD is accelerated at higher temperatures because of the increased concentration of shallow traps. Subsequently, flashover tests were conducted under superimposed voltage. It is found that flashover occurs after applying superimposed voltages even with a rather low dc voltage. In addition, the dc voltage component is primarily responsible for flashover. Weibull results also indicate that it is prone to trigger flashover at higher temperatures, because thermal excitation helps more carriers to escape from deep traps and participate in the process of flashover. Accordingly, the insulation characteristics of oil-paper system can be significantly influenced by superimposed voltage and temperature, which should draw great concern among the power grid.

Trap Distribution and Dielectric Breakdown of Isotactic Polypropylene/Propylene Based Elastomer With Improved Flexibility for DC Cable Insulation

In this paper, we report on electrical and mechanical properties of isotactic polypropylene (PP) blended with polyolefin elastomer (POE) and propylene-based elastomer (PBE). Carrier trap distribution of the samples was estimated by isothermal surface potential decay measurement, while dc breakdown strength was measured through a pair of semicircle electrodes. Elongation at break and tensile strength were obtained to examine the variation in mechanical property of PP caused by the addition of elastomers. Furthermore, scanning electron microscope (SEM), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) have been employed to assist the understanding of morphology of the blends, thermal properties, and mechanical properties. Obtained results have indicated that with the increase of the elastomer content from 0 to 30 wt%, the trap depth appeared to be shallower and the dc breakdown strength tended to be reduced for both PP/PBE and PP/POE samples. Compared with PP/POE blend, PP/PBE blend had deeper trap depth, which should be responsible for its higher dc breakdown strength. In addition, PP/PBE blend has presented a better performance in elongation at break and tensile strength measurement. With the growth of the elastomer content, the crystallinity of the blends appeared to decrease, whereas the melting and the crystallization temperatures did not change remarkably. The SEM inspections and DMA results revealed better compatibility between PP and PBE compared with that between PP and POE, which should be the reason for the better electrical and mechanical properties of PP/PBE blend. The blend of PBE with low content could result in remarkably improved flexibility of PP with acceptable electrical strength for dc cable insulation.

Characteristics of Trap Level Distribution in Graphene Oxide/Low Density Polyethylene Nanocomposites Based on the Isothermal Surface Potential Decay Method

Characteristics of Trap Level Distribution in Graphene Oxide/Low Density Polyethylene Nanocomposites Based on the Isothermal Surface Potential Decay Method

Nanoscale static voltage generation and its surface potential decay using scanning probe microscopy

This work aims at the manipulation of nanoscopic voltage produced through uniform and non-uniform rubbing in neat unpolarised polymer polyvinylidene fluoride. A metal–insulator configuration is considered for the analysis. The decay of surface potential in such a configuration is also addressed in this work. The polarity of the voltage observed on the film depends on the work function of the metal electrode in contact as well as the electronegativity of polymeric material under study. Scanning probe microscopic techniques such as dynamic contact mode electrostatic force microscopy, scanning tunnelling microscopy are used for the investigation of specific electrostatic potential variation on polymer films. Effect of contact electrification leads to nanoscopic domains of voltage generation on the surface of the tribolayers. Electrostatic potential developed on the surface of unrubbed polymer film using modulated tip is in the range of 20–40 mV. The range of voltage generated increased from 20 to 125 mV in the case of rubbed polymer films. Charge retention is discussed through obtaining surface potential decay trend at various intervals. This also plays an important role in the generation of the voltage as well as the current. The above scenario has been demonstrated in both rubbed and unrubbed scenarios. Charge decay is observed to be gradually decreasing from 40 to 29.5 mV in unrubbed surface and 125 to 14 mV in rubbed surface for various time intervals. The obtained results suggest insignificance of triboelectric series on contact electrification between similar tribolayers.

The role of micro-structure changes on space charge distribution of XLPE during thermo-oxidative ageing

In order to investigate the differences of the space charge distribution of XLPE aged at temperatures below and above the melting point, thermo-oxidative ageing was conducted on 110 kV cross-linked polyethylene (XLPE) cable insulation at 100 °C and much elevated temperature of 160 °C respectively. The XLPE insulation showed moderate changes on micro-structure and space charge distribution until the moment when the Oxidation Induction Time (OIT) dropped to zero. It was deduced by the results of Fourier Transform Infrared (FTIR) Spectroscopy, X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscope (SEM) tests that thermo-oxidative ageing could lead to a significant increase in carbonyl groups and a decrease in crystallinity. At 100 °C, the thermo-oxidative ageing mainly affected the amorphous regions because of the much denser structures of crystalline regions, which might make the oxygen diffusion more difficult. At 160 °C, the thermo-oxidative ageing affected the melted spherulites and leaded to obvious impact on lamellae, resulting in a significant decrease in crystallinity and the destruction of spherulites. The space charge distribution of aged XLPE samples was studied by Pulsed Electroacustic Method (PEA) test. Hetero-charges could be found in aged XLPE samples. These hetero-charges were derived from ionization of polar groups in XLPE generated during ageing. The surface trap parameters of XLPE samples were investigated by Isothermal Surface Potential Decay (ISPD) test. It was deduced that shallow traps with the energy level of about 0.95 eV and deep traps with the energy level of about 1.05 eV on the surface of XLPE were introduced by carbonyl groups and interfaces of crystal/amorphous respectively. Thermo-oxidative ageing at 160 °C dramatically decreased the deep trap density because of the destruction of spherulites, leading to enhanced electrode injection. As a result, compared with those of unaged samples and samples aged at 100 °C, increased amount of homo-charges were found in the XLPE samples aged at 160 °C.

Effect of ground electrode on charge injection and surface potential of corona charged polyethylene film

The surface potential decay measurement is a simple and low cost tool to assess electrical properties of insulation materials; therefore, understanding the physical mechanisms of the surface potential decay becomes necessary. With our recent space charge measurement results on corona charged samples, bipolar charge injection on corona charged samples had been observed. Based on this new fact, it is anticipated that the ground electrode should have significant effect during corona charging and subsequently decay processes. In the paper, low density polyethylene (LDPE) film with gold ground electrode was compared with LDPE film with aluminium ground to study effect of ground electrode on charge injection and surface potential decay processes. Charging current during the corona charging, surface potential decay and space charge dynamics after corona charging in the samples with either gold coated or aluminium ground electrode were measured. Differences have been observed for gold ground electrode when compared with aluminium ground electrode. Higher work function of gold electrode is responsible for the observed differences. A preliminary simulation has also attempted to show that the bipolar injection may take place in corona charged LDPE films.

Dielectric and Carrier Transport Properties of Silicone Rubber Degraded by Gamma Irradiation.

Silicone rubber (SiR) is used as an insulating material for cables installed in a nuclear power plant. Gamma rays irradiated SiR sheets for various periods at temperatures of 145 and 185 °C, and the resultant changes were analyzed by examining complex permittivity spectra and surface potential decay characteristics. Three different processes, namely, instantaneous polarization, electrode polarization due to the accumulation of ions to form double charge layers at dielectric/electrode interfaces, and DC conduction caused by directional hopping of ions, contribute to the complex permittivity. By fitting the spectra to theoretical equations, we can obtain the dielectric constant at high frequencies, concentration and diffusion coefficient of ions and DC conductivity for the pristine and degraded samples. The instantaneous polarization becomes active with an increase of dose and ageing temperature. The thermal expansion coefficient estimated from the temperature dependence of dielectric constant at high frequencies becomes smaller with an increase in dose, which is in good agreement with the experimental results of the swelling ratio. Additionally, trap distributions are calculated from surface potential decay measurements and analyzed to explain the variation in conductivity. Trap energy increases firstly, and then decreases with an increase in dose, leading to a similar change in DC conductivity. It is concluded that generations of both oxidative products and mobile ions, as well as the occurrence of chain scission and crosslinking are simultaneously induced by gamma rays.

Nonlinear conduction and surface potential decay of epoxy/SiC nanocomposites

The nonlinear conduction of polymeric insulation may solve the surface charge accumulation, avoiding the distortion of electric field and the flashover phenomenon. As the silicon carbide (SiC) powder is usually used to improve the nonlinear conduction of polymer composites, the epoxy/SiC nanocomposites are prepared and the dc conductivity, permittivity and isothermal surface potential decay characteristics are investigated. The experimental results show that the conduction of high filler loading is field dependent and is attributed to a new conduction mechanism. The permittivity is closely related to the interaction zone around nanoparticles. The isothermal surface potential decay characteristics are greatly affected by the charging polarity, charging level and filler loading. The investigation indicates that the nonlinear conduction plays an important role in the surface potential decay.

Temperature accelerated discharging processes through the bulk of PECVD silicon nitride films for MEMS capacitive switches

The effect of temperature on discharging processes of PECVD silicon nitride films has been investigated in Metal-Insulator-Metal (MIM) capacitors and RF MEMS capacitive switches with the aid of Kelvin Probe method. The surface potential decay of MIM capacitors during discharge has been monitored using a Single-point Kelvin probe system at various temperatures (300K–400K) while the shift of bias at minimum up-state capacitance during discharge has been employed in MEMS capacitive switches in order to determine the discharge current through the bulk of the film at different temperatures. The results indicate that the discharging process through the bulk of silicon nitride films is thermally activated and the corresponding value of the discharging current increases with temperature, due to the increase of the film conductivity. Moreover, the value of minimum up-state capacitance of the switches is found to be significantly affected by temperature, by the mechanical properties of the moving armature as well as by the variance of charge density distribution. Finally, different thermally activated mechanisms have been observed in MIM capacitors and MEMS capacitive switches, due to different charging processes on these devices.