Surface Potential Decay Research Articles

Effect of annealing on charging properties of electret fibers

In the present work, effect of annealing on charging and surface morphology of isotactic polypropylene (PP) filaments was reported. To enhance the surface crystallinity of the filaments, samples were annealed at 70 and 110 °C for 12 h. As expected, fiber surface crystallinity increased upon annealing which led more stable electret fibers. However, the change in the initial surface potential was not so remarkable. The same trend was also observed in filaments containing a commercial nucleating agent, NA11 (sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)-phosphate). Surface crystallinity was analyzed using the intensity of specific peaks obtained from attenuated total reflection infrared spectroscopy analysis. The surface charge characteristics of fibers were determined using a modified surface potential decay test.

Moisture effect on surface fluorinated epoxy resin for high-voltage DC applications

Charge accumulation under high-voltage DC is a major concern in the transmission system as its presence distorts the local electric field. By performing chemical treatment on the polymeric insulation via fluorination process, the charge dynamics of the treated material can be modified. Surface fluorination treatment has been proven to be an effective tool to enhance the dielectric properties. The treatment slightly improves the surface conductivity value and enhances the surface potential decay rate, which helps to prevent the accumulation of surface charge and consequently improve the breakdown strength of epoxy samples. However, the authors suspected the introduction of a fluorinated layer on top of the epoxy sample may have the capacity to absorb moisture from the environment. The increase in the surface conductivity by the fluorination treatment may not come from the fluorine layer itself, but rather from the absorbed moisture in the surface layer. The loss of moisture from the surface may lead to a dip in dielectric performance of the treated materials. Hence, a study on the moisture effect on the surface fluorinated epoxy resin was designed. The fluorinated samples were characterised by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) to analyse the morphology as well as the concentration of elements in the treated materials. The samples then were dried in vacuum oven at 105°C and/or inside nitrogen gas chamber at room temperature for 24 hours to forcefully dry out the absorbed surface moisture. The measurements of surface DC conductivity and surface potential decay were taken before and after the drying process. It has been found that the fluorination treatment on epoxy resins did give rise to high moisture content on the surface which plays an important contribution towards the dielectric enhancement properties.

Characterizing traps distribution in LDPE and HDPE through isothermal surface potential decay method

The energy distribution of trapped charges and traps in low density polyethylene (LDPE) and high density polyethylene (HDPE) were investigated based on isothermal surface potential decay (ISPD) method in this paper. Corona charging measurement was conducted by a non-contact setup, which was specially designed to obtain the decay characteristics of thin polymer films with varied charging polarity implying the dynamics of electrons or holes. Considering space charge profile of polymers under high dc electric field and detrapping mechanism after removing the field, a model based on ISPD was proposed to obtain the trap distribution of polymers. By assuming different charge spatial distribution features, two kinds of trap energy distributions of LDPE and HDPE were achieved respectively, which reveal similar distribution characteristics. It is shown that there are mainly deep traps in LDPE and HDPE both for electron-type and hole-type traps. The mean trap depth of hole-type traps is lower than that of electron-type ones in LDPE, whereas the situation in HDPE is reversed. Additionally, more trapped charges are accumulated in HDPE with deeper trap depth than that of LDPE. It is indicated that the obtained results may imply essentially nature of hole/electron traps, i.e., the electron-type traps show an inter-chain character whereas the character of hole-type traps is intra-chain. Furthermore, the energy distribution of traps is further related to the unique aggregation structure of LDPE and HDPE.

Charge Transport in LDPE Nanocomposites Part I-Experimental Approach.

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

Characteristics of trapped charge distribution in LDPE and XLPE used in DC cables based on isothermal surface potential decay method

Characteristics of trapped charge distribution in LDPE and XLPE used in DC cables based on isothermal surface potential decay method

Research on surface potential decay characteristics of epoxy resin charged by direct current corona

Surface charge accumulation and decay behaviors of dielectric materials are the key factors restricting the development of high voltage direct current power equipment. For flat samples, the density of surface charges deposited by corona can be regarded as a linear change with the surface potential. For this reason, the behavior of surface charge decay can be directly related to that of surface potential. According to the corona charging process, the surface charge deposition and detrapping process, as well as the charge transport process in the bulk, we may establish a physical model dynamic response to the surface potential. Influences of grid voltage, relative permittivity, and bulk conductivity on the surface potential decay process can be obtained through calculating the surface potential decay behaviors of epoxy resin. The higher the grid voltage, the faster the surface potential decays. At the typical parameter value of epoxy resin (relative permittivity 3.93, bulk conductivity 10-14 S m-1), the normalized decay rate can be fitted by two straight lines in a log-log plot; moreover, the calculated results show a linear variation of power factors with the grid voltage, while the power function shows a relationship between the characteristic time and the grid voltage. The bigger the relative permittivity, the slower the surface potential decays. In the typical parameter area of epoxy resin (relative permittivity 3-4), the surface potential decay time constant increases from 1720 s to 2540 s, showing a linear variation. Also the bigger the bulk conductivity, the faster the surface potential decays. In the typical parameter area of epoxy resin (bulk conductivity 10-15-10-13 S m-1), the surface potential decay time constant decreases from 24760 s to 260 s, showing a power function relationship.

Modeling on Effects of Charging Time and Recombination on Surface Potential Decay Crossover of LDPE

Modeling on Effects of Charging Time and Recombination on Surface Potential Decay Crossover of LDPE

Mechanisms of surface potential decay on enamel wire coatings

Results of measurements of surface potential decay on coating layer of enameled wires are presented. In experiments, surfaces of winding samples were charged by dc corona of both polarities and decay of surface potential induced by charges deposited on the materials was recorded utilizing non-contact technique. The power-law type of time dependences of surface potential decay rates was obtained that indicated a dominant role of charge injection and/or dipolar polarization in the decay process. Furthermore, several decay mechanisms were employed for examining the experimental data, which revealed that potential decay was initially governed by the injection of charges due to electron transfer between the deposited charges and the material. Later on, the contribution of polarization to decay process was significant, while intrinsic conduction became the prevailing decay mechanism only at the latest stage as polarization was stabilized.

Highly hydrophobic and partially conductive polydimethylsiloxane surface produced by direct fluorination and subsequent annealing

Highly hydrophobic materials are of interest in many scientific and technological areas. To improve surface properties of polydimethylsiloxane (PDMS), PDMS sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at about 55 °C under 0.1 MPa for different times of 15, 30, and 60min. Surface wettability and surface energy were evaluated by contact angle measurements and surface energy calculations, and surface electrical properties were investigated by measurements of surface conductivity and surface potential decay. These results show that direct fluorination produced a highly hydrophobic and partially conductive PDMS surface with low surface energy, which had little correlation with fluorination time in the investigated range of 15 to 60 min. Moreover, annealing of the surface fluorinated PDMS sheets at 150 °C for 60 min caused a further increase in surface hydrophobicity, while reduced surface conduction of the fluorinated PDMS sheets. Attenuated total reflection infrared analyses and scanning electron microscope surface and cross-section observations reveal substantial changes in physicochemical characteristics of the sheet surface layers due to direct fluorination. The high hydrophobicity or low surface energy is attributed to the changes in chemical composition and structure of the surface layers and in their surface roughness. The partial conductivity of the fluorinated PDMS surfaces is a result of the competition between the compositional change and the structural change.

Influence of thermal degradation on surface discharge characteristics for oil-polypropylene insulation

Oil-polypropylene (PP) composite insulation has been widely used in power capacitors, and also being extended to other equipment such as specific transformers, etc. As discharge can readily creep along PP surface, the interface between oil and PP is a weak point in the insulation structure. The oil-PP insulation degrades under the impact of thermal and electrical stress during normal operation. In this study, the influence of thermal degradation on surface discharge characteristics for oil-PP insulation is investigated. Degraded samples are obtained by thermally accelerated aging of virgin oil-PP samples. The microscopic morphology of PP samples are observed with SEM, and the physical and electrical properties of degraded oil are measured. In addition, the isothermal surface potential decay (ISPD) measurement is employed to investigate the charge keeping capability of PP samples. After that, surface discharge detection experiments under AC voltage are performed for the virgin and degraded samples. The ISPD results indicate that charge decay rate of degraded PP sample is faster than virgin sample, which imply that space charge memory effect of virgin PP samples is more remarkable. For this reason, the PD inception phase of virgin sample is smaller than those of degraded samples. It is found that thermal degradation of oil-PP insulation enhances the surface discharge activity. It is considered that rough surface profile and higher bubble conservation capability of degraded PP sample, and impurities in degraded oil are responsible for the enhancement effect of thermal degradation on surface discharge.

Surface potential decay of DC-corona-charged PET films on humid electrodes

Surface potential decay measurement is a widely used tool to test the electrical properties of insulation materials. The aim of the present paper is to evaluate the effect of a humid electrode on the surface potential decay process. The experiments are performed on 1-mm thick samples of PET films (50 mm × 50 mm) in ambient air (temperature: 25 °C–29 °C; relative humidity: 42%–48%). The samples are placed on a grounded humid electrode (aluminum plate covered by a humid textile: 52 mm × 52 mm), and are charged by exposing them for 10 s to the negative DC corona discharge generated by a high-voltage wire-type dual electrode.The results show that the humid electrode has a significant influence on the surface potential evolution during the first moments after corona charging. Bipolar charge injection is the main physical mechanism that explains this potential decay.

Experimental modeling of the electric potential decay at the surface of polypropylene films (PP)

Surface potential decay (SPD) measurements have been considered as the most appropriate technique for the investigation of the corona charging of dielectrics. The aim of the present paper is to point out the interactions between three factors namely: grid potential, grid current and sample size (length), by taking advantage of the experimental design methodology. The experiments were performed on 0.08 mm – thick samples of PP films, in ambient air (temperature: 20 °C–22 °C; relative humidity: 56 %–62 %), The domains of variation of the three factors considered in the experiments were as follows: – 8 to – 12 kV, for grid potential – 50 to – 90 μA for grid current and 70–150 mm, for sample length. The relative SPD after 300 s and 900 s were considered as output variables of the process. The models that express these variables as quadratic polynomial functions of the three factors were obtained using the Design of Experiments methodology and the commercial software MODDE 5.0. They point out that the surface potential decay is faster at higher applied voltages, lower grid currents and smaller sample areas. The grid voltage has a stronger effect at lower values of the grid current. Both the grid voltage and the corona current have a stronger effect of the SPD in the case of the samples of smaller areas. These observations suggest the optimal operating conditions of the corona charging devices for this kind of dielectric materials.

Electret properties of PP/ZnO and PP/CuO composite films

In the present paper the influence of both time and storage temperature on the surface potential decay of polypropylene (PP) composite films was investigated. We have investigated the 200 μm PP composite films with different weight concentrations of the particles (ZnO or CuO) - 0 wt.%, 2 wt.% and 4 wt.%. The samples were charged in a corona discharge by means of a corona triode system for 1 minute under room conditions. Positive or negative 5 kV voltage was applied to the corona electrode and 1 kV voltage of the same polarity as that of the corona electrode was applied to the grid. After charging, the surface potential was measured using the method of the vibrating electrode with compensation. The results obtained show a significant change in the electret behavior of the composite films after the inclusion of ZnO or CuO particles with different concentration into the PP matrix. The possible surface potential decay mechanisms responsible for the electret’s behavior were discussed. It was established that the surface potential decay depended on the corona polarity, the particles concentration and the type of the particles. It has been experimentally demonstrated that the normalized surface potential of PP composite films with different concentrations of ZnO or CuO particles decays faster than the one of the pure PP films.

The energy distribution of trapped charges in polymers based on isothermal surface potential decay model

Space charge formation in polymeric materials can cause some serious concern in real operation, because it has significant influence on the performance of polymers. For example, space charge in some insulating materials can severely distort the electric field, even lead to materials degradation. On the contrary, in the case of its applications, space charge stored in electrets can greatly improve their properties. It is therefore important to understand trapped charge distribution in materials as it is considered to be a novel indicator for effective evaluation of aging status and electric withstanding strength of insulating materials. In this paper, a model based on isothermal surface potential decay (ISPD) is proposed to study the distribution of trapped charges by considering the physical mechanism of the detrapping process. By measuring the ISPD characteristics of polymeric materials and fitting the data according to the assumption of shallow and deep traps, the distribution of trapped charges is obtained, which may be related to the change of aggregation structure of polymers. In order to verify the model, it is used to analyze different ISPD decay curves of polypropylene (PP) and low density polyethylene (LDPE), as well as the ISPD data of PP electrets with/without pressure expanding treatment. The results show that the proposed ISPD model is effective and convenient. Two peaks are observed on the curve of the trapped charge density versus the trap level. The obtained distribution of the trapped charges in polymers can reveal the different nature of electron/hole traps and the different transportation behavior of hole/electron carriers, i.e., the electron-type traps show an inter-chain character while the character of hole-type traps is intra-chain. In addition, the distribution of trapped charge is further related to aggregation structure of PP and LDPE, as well as PP electrets with/without pressure expanding treatment.

Potential decay on silicone rubber surfaces affected by bulk and surface conductivities

A decay of electric surface potential on a pre-charged gas-solid interface involves several charge transport processes and each of them may become dominant under certain environmental conditions (temperature, pressure, humidity). In the present paper, surface potential decay on flat samples of different kinds of silicone rubber used in HVDC applications is analyzed at reduced pressures of ambient air that allows for minimizing the involvement of the gas phase, i.e. surface charge neutralization by gas ions. Effects imposed solely by bulk and surface conduction in the solid material are studied experimentally and by means of computer simulations. The results allow for evaluating threshold values of volume and surface electric conductivities at which these mechanisms become most essential. Field dependent bulk conductivities are deduced from the surface potential decay characteristics obtained for the studied materials.

Evolutions of surface characteristics and electrical properties of the fluorinated epoxy resin during ultraviolet irradiation

The photoinduced evolutions of surface physicochemical characteristics and electrical properties of the fluorinated epoxy resin have been investigated. Cured epoxy resin sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 0.1 MPa and 95 °C for 30 min. The fluorinated epoxy sample together with the unfluorinated (original) one for a comparison were exposed to ultraviolet (UV) radiation with wavelengths of 320 to 390 nm. During UV exposure, the evolution of surface physicochemical characteristics was investigated by attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and the sessile drop technique. The corresponding evolution of surface electrical properties was evaluated by measurements of surface potential decay and surface conductivity. These results have shown that the crosslinking reaction rather than photodegradation occurred for the fluorinated sample, compared to a continuous degradation of the original sample during the UV exposure. These results also indicate that surface conduction and its sensitivity to humidity increased very significantly with UV exposure time for the original sample, while the opposite changes in surface conduction and its moisture sensitivity with the exposure time were found for the fluorinated sample. A relationship between the evolutions of surface electrical properties and surface physicochemical characteristics has been established.

Dynamic surface potential decay tendency of cellular polypropylene after temperature control corona process

In order to evaluate the temperature influence on the charge storage property of cellular polypropylene, especially for its surface charge deposition, the tested polymers were charged through a special corona triode system. A temperature controlling corona point-to-plane unipolar poling system was used in the material treatment process of the cellular polymer. In this paper, the dynamic behaviour of surface potential has been recorded and analysed. In addition, the physical mechanism of unipolar corona has been used to analyse the surface modification of the tested polymers specifically, which is closely related to surface charge deposit and chemical bonds generated in the poling procedure. The results of this paper play a significant role in the understanding of the charge storage mechanism of the foam polymers and their future application in the fields of energy harvester, sensors and actuators, etc.

Surface potential decay of PMMA and POM in air

Perspex (polymethyl methacrylate (PMMA)) and delrin (polyoxymethylene (POM)) are subjected to high voltages and then the surface potential decay is measured using an electrostatic voltmeter. The surface trapping parameters of PMMA are calculated using isothermal current theory and a plot between trap density and energy level is obtained. It is observed that the maximum electron and the hole traps in the surface layer of PMMA are ∼1.5 × 1017 eV−1m3 and 0.8 × 1017 eV−1 m3, respectively, and the energy level of its electron and hole traps are in the range of 0.60–0.90 eV and 0.78–0.9 eV, respectively. The introduction of roughness on POM surface leads to an increase in the energy level from 0.55–0.87 eV to 0.55–0.89 eV. It is also observed that there is an increase, both in peak surface potential and time to decay in PMMA as compared to POM. This holds true for positive as well as negative dc voltages and is caused by the charge retentivity in the material resulting from the length of the polymeric chain.

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon.

Surface fluorinated epoxy resin for high voltage DC application

Charge accumulation under high voltage DC is a major concern in the transmission system as its presence distorts the local electric field. By performing chemical treatment on polymeric insulation via fluorination process, the charge transport characteristics of the material can be modified. In doing so, excellent surface properties of fluoropolymers can be attained without compromising the bulk properties of the original polymeric insulation. The change in chemical components at the surface of polymeric insulation should lead to a corresponding change in electrical properties at the surface and so suppress charge accumulation. In this paper, epoxy resin samples were formulated and treated with various surface fluorinating conditions. The samples then were characterised by scanning electron microscope (SEM), Raman spectroscopy and DC surface conductivity measurements. The surface potential decay measurement was performed and the result shows that there is a significant change in surface potential decay measurement with the introduction of surface fluorinated layer. The pulsed electroacoustic (PEA) measurement was used to further explain the decay mechanisms responsible for the observed phenomena. Surface DC flashover test using a pair of finger electrodes had also been conducted. It has been found that the introduction of fluorinated surface layer on epoxy resins plays an important role in improving the surface dielectric properties as apparent from the experimental results.