No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Stepwise Electric Field Induced Charging Current and Its Correlation with Space Charge Formation in LDPE/ZnO Nanocomposite
Abstract
This study intends to establish the correlation between charging current under stepwise increased electric field and space charge formation in low density polyethylene (LDPE) and LDPE/ZnO nanocomposites. By applying stepwise increased electric field, the charging current and space charge distribution were measured as a function of time under 10, 30 and 50 kV/mm, respectively. The results show that homopolar space charge formation induces increasing charging current with time, while heteropolar space charge formation induces decaying charging current and excellent space charge suppression is accompanied by constant charging current with time. The abundant deep trapping states introduced by nanofillers dominate both the lower steady charging current and space charge suppression in the nanocomposite. The results indicate that charging current behavior induced by stepwise increased electric field is extremely informative and is feasible to be a direct method for rough estimation of space charge formation and suppression in polymer dielectrics. Addition of ZnO nanofillers suppresses space charge accumulation in LDPE, indicating LDPE/ZnO nanocomposite is a potential insulation material for high voltage direction current cables.
... Insulating materials in high voltage cables exhibit space charge accumulation, which can distort the electric field distribution within the insulation and possibly lead to breakdown [95]. Many studies have investigated the space charge profile in polymer nanocomposites used in cable insulation [96][97][98][99][100][101][102][103][104][105]. ...
... The measurements were made with the electric field applied. Hetero-charge concentration, especially at the cathode, increases with increasing applied field [42,96,99] and with increasing temperature gradient [101,111]. As stated above there are two main sources of space charge, namely injection from the electrodes and ionization of impurities, usually present throughout the bulk of the sample [42,99]. ...
... Hetero-charge concentration, especially at the cathode, increases with increasing applied field [42,96,99] and with increasing temperature gradient [101,111]. As stated above there are two main sources of space charge, namely injection from the electrodes and ionization of impurities, usually present throughout the bulk of the sample [42,99]. Injection from the electrodes usually leads to homo-charge accumulation near the electrodes, while ionization of impurities throughout the bulk and transport of charge carriers across the sample to the electrodes usually leads to hetero-charge accumulation near the electrodes. ...
Polymer nanocomposites used in underground cables have been of great interest to researchers over the past 10 years. Their preparation and the dispersion of the nanoparticles through the polymer host matrix are the key factors leading to their enhanced dielectric properties. Their important dielectric properties are breakdown strength, permittivity, conductivity, dielectric loss, space charge accumulation, tracking, and erosion, and partial discharge. An overview of recent advances in polymer nanocomposites based on LDPE, HDPE, XLPE, and PVC is presented, focusing on their preparation and electrical properties.
... Space charge carrier mobility in polymers is facilitated by ionic conduction (impurity ionization) and electronic condition (charge injection from conductor). [13][14][15][16] Recently metal-oxide nanoparticles in low weight fractions (< 5wt. %) have been used in the LDPE in order to supress space charge accumulation, which resulted in a conductivity reduction of the insulation . ...
... %) have been used in the LDPE in order to supress space charge accumulation, which resulted in a conductivity reduction of the insulation . The used metal-oxide nanoparticles being either insulating such as Al 2 O 3 , 17-18 MgO 19-26 and SiO 2 27 or semiconducting such as TiO 2 28 and ZnO 14,[29][30][31][32][33] ; have higher conductivity than that of LDPE matrix. These nanoparticles reduce the charge mobility in LDPE nanocomposites presumably by trapping/adsorbing charge carriers including electrons, holes and polar/ionic species, so the overall nanocomposite conductivity is lowered. ...
... Stress-strain data for unfilled LDPE and its composites were obtained using an Instron 5944 tensile testing machine equipped with a 50 N load cell. The tests were performed at 14 23 ± 1 °C and 50% RH using a strain rate of 50 % min -1 . The cuboid specimens tested were 5 mm wide and 80 μm thick. ...
Polyethylene composites which contain low concentrations of metal-oxide nanoparticles e.g. ZnO and MgO are emerging materials for the use in insulations of extruded high-voltage direct-current (HVDC) cables. The challenge in the development of the composites with ultra-low electrical conductivity is to synthesize uniform and high-purity metal-oxide nanoparticles, which are functionalized with hydrophobic groups in order to make them compatible with polyethylene. The thesis reports different approaches to prepare this new generation of insulation materials.
Different reaction parameters/conditions – zinc salt precursor, precursor concentrations and reaction temperature – were varied in order to tailor the size and morphology of the ZnO nanoparticles. It was shown that different particle sizes and particle morphologies could be obtained by using different zinc salt precursors (acetate, nitrate, chloride or sulphate). It was shown that 60 °C was a suitable reaction temperature in order to yield particles with different morphologies ranging from nano-prisms to flower-shaped superstructures. For removal of reaction residuals from the particles surfaces, a novel cleaning method based on ultrasonication was developed, which was more efficient than traditional water-replacement cleaning. After cleaning, the presence of one atomic layer of zinc-hydroxy-salt complex (ZHS) on the nanoparticle surfaces was suggested by thermogravimetry and infrared spectroscopy. A method involving three steps – silane coating, heat treatment and silica layer etching – was used to remove the last trace of the ZHS species from the nanoparticle surface while preserving its clean and active hydroxylated surface. The surface chemistry of these nanoparticles was further tailored from hydroxyl groups to hydrophobic alkyl groups with different lengths by reactions involving methyltrimethoxysilane (C1), octyltriethoxysilane (C8) and octadecyltrimethoxysilane (C18).
MgO nanoparticles were prepared by aqueous precipitation of Mg(OH)2 followed by a partial transformation to MgO nanoparticles via heat treatment at 400 °C. The surface regions of the MgO nanoparticles convert into a hydroxide phase in humid media. A novel method to obtain large surface area MgO nanoparticles with a remarkable inertness to humidity was also presented. The method involved three steps: (a) thermal decomposition of Mg(OH)2 at 400 °C; (b) silicone oxide coating of the nanoparticles to prevent inter-particle sintering and (c) a high temperature heat treatment at 1000 °C. These MgO nanoparticles showed essentially no sign of formed hydroxide phase even after extended exposure to humid air.
The functionalized metal-oxide nanoparticles showed only a minor adsorption of phenolic antioxidant, which is important in order to obtain nanocomposites with an adequate long-term stability. Tensile testing and scanning electron microscopy revealed that the surface-modified metal-oxide nanoparticles showed improved dispersion and interfacial adhesion in the polyethylene matrix with reference to that of unmodified metal-oxide nanoparticles. The highly “efficient” interfacial surface area induced by these modified nanoparticles created the traps for charge carriers at the polymer/particle interface thus reducing the DC conductivity by more than 1 order of magnitude than that of the pristine polyethylene.
... Cable insulation contains impurities and defects, fonning discretely distributed across the whole energy level. In the polarization process, the charge filled in the defect will come to transitions [8]. When the environment temperature is kept constant, the transition charge in the medium will form a short-circuit current in the extemal circuit. ...
... The activation energy of sampIes can be calculated by (8), and the results are shown in Table V and Fig.3. Based on the distribution characteristic of the insulation breakdown [11], the derived formula of distribution function can be expressed as: ...
To evaluate the aging condition of 110kV XLPE cables accurately, this paper employs the isothermal relaxation current (IRC) method and the activation energy (AE) method, discussing the relationship among the aging factor, the activation energy and the aging condition of cable insulation. The results show that compared with unaged cables, the cables under accelerated thermal aging have 0.348 more on the aging factor and their activation energy is generally lower. Besides, compared with the cables running for 14 years, the cables running for 25 years have 0.327 more on the aging factor, 15.05kJ/mol less on the activation energy and 30.6 years less on the remaining life. All these show that increasing of the internal trap's number, deepening of the internal trap's depth and the disintegration of chemical bonds are major microeconomic performances on cable aging. The differences in batching system and production technology system of cables can change the chemical structure in XLPE molecules. And assessment results are in line with expectations. Microscopically, evaluating the aging condition by IRC and AE has the accuracy and validity, which provides an effective method for the evaluation of XLPE cable's aging condition from electrical and chemical aspects, and guides the maintenance and planning of cables.
... Methods to suppress the accumulation of space charges in HVDC cables have, therefore, attracted significant attention in recent years [10][11][12][13][14][15][16]. Among these methods, doping of pristine insulating materials with nanoparticles forms numerous deep traps that hinder the movement of charge carriers, thereby inhibiting the accumulation of space charges [17][18][19][20][21][22]. Nanoparticles are typically incompatible with polymers, owing to their high surface activity, 2 of 12 which facilitates the agglomeration of nanoparticles inside the polymer, thereby generating defects; this results in the accumulation of high numbers of space charges inside the nanocomposite, which degrades the electrical properties of the composite [23]. ...
Owing to its lack of crosslinking, polypropylene (PP) is considered an environmentally friendly alternative to crosslinked polyethylene as high-voltage direct current (HVDC) cable insulation. However, pure PP can accumulate space charges under a HVDC, and thus must be modified for use as an insulating material for HVDC cables. In this study, 4-methylstyrene is grafted onto PP using an aqueous suspension grafting method to improve its properties. The effects of the swelling time, reaction time, and 4-methylphenylene concentration on the reaction were investigated. The optimum process conditions were determined, including an optimum grafting ratio of 0.97%. The volume resistivity, ability to suppress space-charge accumulation, and DC breakdown strength of modified PP were also studied. Modified PP with a grafting ratio of 0.88% showed optimal space-charge suppression and the highest volume resistivity and breakdown strength. The work will facilitate the design and development of more efficient insulation materials for HVDC cables.
... In contrast to the phenomenon that space charge accumulation becomes more remarkable with an increase in the trap depth or trap density in 320 kV HVDC cable, it has been reported in many researches that space charge accumulation can be effectively suppressed with increasing trap depth or density for plate samples [15,[20][21][22][23][24]. This means that space charge behavior in cylindric cable is significantly different from that in the plate samples and much more complex. ...
Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m-3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3,) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000×1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.
... Therefore, the probability of charge trapping increases with the presence of nanoparticles. On the other hand, it was experimentally observed in [36] that shallow traps have larger cross-section compared to deeper traps. The rate of charge trapping is proportional to the capture cross-section of traps. ...
In this study, the relationship between thermal ageing and charge trapping properties of epoxy-based nanocomposites has been investigated. With ageing, any dielectric material undergoes thorough degradation. This degradation significantly affects the space charge accumulation and charge trapping behaviour of the dielectric, which are very important parameters for insulation health under high-voltage direct current (HVDC) environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of pure epoxy and epoxy alumina (Al(2)O(3)) nano-composites at different ageing conditions. A methodology based on polarisation–depolarisation current (PDC) measurements has been proposed to identify the current component due to a dipolar relaxation in measured total IRC. This will help to identify the trap distribution characteristics more accurately compared to conventional IRC measurements. It was experimentally observed that the addition of nanoparticles significantly reduces trapped charge formation and reduces thermal degradation. It is observed that aging leads to the generation of deeper traps, while the addition of Al(2)O(3) nanoparticles mainly enhances the density of shallow traps. Results presented in this work indicate that epoxy–alumina nanocomposites are very much suitable in HVDC applications from the perspective of trapped charge accumulation.
... The formation and accumulation of space charges has become one of the major problems that urgently needs to be solved for the development of high voltage direct current (HVDC) cable polyethylene (PE) insulation materials. The space charges are formed by the transfer and storage of the charge carriers injection by the electrodes and the ionization of the impurities or additives such as antioxidants, etc., in the bulk of PE insulation materials, which could lead to local electric field distortion and the local field may be eight times more than the applied electric field especially when the polarity of the line is inverted [1][2][3][4]. The distortion of the electric field caused by space charge accumulation could affect the conduction, breakdown and ageing phenomena of PE insulation materials, and reduce the long-term reliability and service life of direct current (DC) cables [5]. ...
To suppress space charge accumulation and improve direct current (DC) electrical properties of insulation materials, crosslinked polyethylene modified with 2-(4-benzoyl-3-hydroxyphenoxy) ethyl acrylate (XLPE/BHEA) containing polar functional groups was prepared by melt blending. The gel content, thermal elongation, tensile strength, elongation at break, elasticity modulus, differential scanning calorimetry (DSC) and X-ray photoelectron spectra (XPS) measurement results demonstrated that the BHEA could slightly enhance the crosslinking of polyethylene (PE) and affect the mechanical properties and crystallization of XLPE, and the BHEA molecule was not easy to precipitate from XLPE after the crosslinking process. XLPE modified with 3.0 phr (parts per hundreds by weight) BHEA could effectively suppress space charge accumulation, reduce DC conduction and improve DC breakdown strength of XLPE at a higher temperature. Deeper traps were introduced in XLPE/BHEA composites due to the polar functional groups in BHEA, which could raise the potential charge injection barrier and reduce the charge carrier number and mobility to suppress space charge accumulation and reduce the conduction current density.
... Space charge generates dissipative loss, reduces the withstand voltage, intensifies the local electric field and can produce insulation failure [37]. The addition of small volume fractions of nanofillers can reduce the space charge extent [8], [39], [40] this effect being especially notorious at low filler concentration [41], thus enhancing the volume resistivity. The tendency to suppress charge injection is believed to be due to deep trap mechanisms [42] that capture the charge injected by the electrodes, thus, reducing the electrical conduction [43] by inhibiting charge injection and avoiding space charge buildup. ...
Natural rubber (NR) products are being applied in many protective tools for life-line applications, including insulating boots, blankets, sleeves, insulating gloves, or flexible coverings. Due to the inherent risk of such works it is imperative ensuring the quality of the safety products involved. The dielectric properties of NR-based products rely heavily on the compounding formulation and manufacturing processes involved. Considerable efforts are being applied to improve the dielectric performance of NR formulations to ensure that maintenance personnel work in the safer and most comfortable conditions. This work studies the addition of different weight fractions of surface-modified BaTiO
3
-OH nanoparticles to NR formulations to enhance dielectric properties such as breakdown strength and leakage current. The results presented in this work show that the addition of a low weight fraction of surface-modified BaTiO
3
-OH nanofillers (≤ 1 wt.%) enhances the surface and volume resistivity, crystallinity, breakdown strength under ac conditions and lowers the leakage current under dc stress.
... This is interesting considering that the conduction current is always found to show monotonous decay with time. Furthermore, it has been confirmed that the gradual increase, decay and changeless of the conduction current with time at high electric field are closely related to the accumulation of homopolar space charges, heteropolar space charges, and suppression of space charge formation, respectively [35] . The abnormal characteristics of conduction current and space charge in the nanocomposite are dominated by the introduced abundant deep trapping states. ...
This paper intends to reveal the interface trapping effect on the charge transport characteristics of LDPE/ZnO nanocomposite by combined study of the nanocomposite structures, trap level distributions, space charge distributions, conduction current and charge mobility. Experimental results show that nanofillers in the nanocomposites significantly altered the microstructure of the polymer matrix by introducing lots of smaller spherulites and increasing the crystallinity, leading to a significant increase in the interface areas between amorphous and crystalline regions. The trap density in the nanocomposites is about 2–6 times that in LDPE. The traps are essentially cavities, located at the interface areas between nanofillers and polymer matrix and interface areas between amorphous and crystalline regions. A new method was proposed to evaluate the carrier mobility as a function of discharge time and trap depth from the IDC results directly. The carrier mobility of LDPE is about 4–20 times higher than that of the nanocomposites. Compared with LDPE, bulk space charge accumulation is significantly suppressed in the nanocomposites. An interesting discovery is that there is a clear correspondence between the space charge accumulation and current increase in I-t characteristics under high electric field. The study indicates that the low charge carrier mobility and space charge suppression in LDPE/ZnO nanocomposites and the I-t behavior are all closely related to the interface trapping effects in the nanocomposites.
... Therefore, the suppression of space charge accumulation under high electric field has become one key issue in HVDC cable development [7]. To overcome the space charge injection and accumulation in HVDC polymer cables, the semi-conductive or insulating nanoparticles have been widely doped into polyolefin, such as MgO, ZnO, Al 2 O 3 [8][9][10]. In this way, the space charge accumulation can alleviate under the electric stress through doping a small amount of nanoparticles. ...
High-performance polypropylene (PP) plays an important role in electrical/electronic engineering fields. Especially in high voltage direct current (HVDC) cables it compares well to cross linked polyethylene (XLPE) for its potential application as an eco-friendly material without cross-linking. The space charge injection under high electric stress is main obstacle for the development of HVDC cables. Here we adopted chemical modification on PP with polar functional group to improve the electrical properties, whereby the molecular structure has been designed to obtain excellent insulating material. The space charge suppression, dielectric properties and crystal characteristics of PP with and without grafting with maleic anhydride (MAH) were investigated. Results demonstrated that the MAH was successfully grafted onto PP macromolecular chain. Compared to pure PP, the grafting with 2 wt% MAH can effectively suppress space charge injection and provide better stability in volume resistivity as temperature increases. Besides, their dielectric properties were studied, and the mechanism of space charge suppression was proposed. This provides a useful method to prepare the HVDC cable insulating materials.
... The electron trap characteristic was tested based on the TSC method. The TSC method is a significant method to study important parameters of electrets such as thermal charge mobility and trapped charge density, etc. [34][35][36]. In this work, the sample was firstly charged under dc electric field (40 kV/mm) at 323 K. ...
The doping effect of graphene nanoplatelets (GNPs) on electrical insulation properties of polyethylene (PE) was studied by combining experimental and theoretical methods. The electric conduction properties and trap characteristics were tested for pure PE and PE/GNPs composites by using a direct measurement method and a thermal stimulated current (TSC) method. It was found that doping smaller GNPs is more beneficial to decrease the conductivity of PE/GNPs. The PE/GNPs composite with smaller size GNPs mainly introduces deep energy traps, while with increasing GNPs size, besides deep energy traps, shallow energy traps are also introduced. These results were also confirmed by density functional theory (DFT) and the non-equilibrium Green’s function (NEGF) method calculations. Therefore, doping small size GNPs is favorable for trapping charge carriers and enhancing insulation ability, which is suggested as an effective strategy in exploring powerful insulation materials.
... Apart from mechanical properties, dielectric characteristics of HDPE composites have shown improvement on addition of nanoparticles such as Zinc oxide, fumed silica and organoclay and are reported to be useful in the high voltage cable systems and core steel sheets of high voltage transformers [11][12][13]. Grigoriadou et al. [14] have observed enhancement of UV stability and mechanical properties for similar HDPE nanocomposites. Dielectric properties like permittivity, loss factor and tan d show improvement by addition of carbon nano-fibres to polyethylene [15]. ...
Multiwalled carbon nanotubes (MWCNT’s) and Nano-Silicon dioxide (SiO2) reinforced HDPE composites have been developed. The MWCNT and Nano- SiO2 filler content was varied up to 5 wt % in HDPE with the addition of Polyethylene grafted glycidyl methacrylate compatibilizer and 3-aminopropyl triethoxy silane coupling agent. The dielectric and mechanical properties of the HPDE nanocomposites have been investigated in this study. Addition of MWCNT and Nano-SiO2 are observed to enhance the tensile strength and modulus of the nano-composites. The electrical properties like breakdown voltage and volume resistivity showed improvement with the addition of Nano-SiO2.
... The samples were put between a thick aluminum electrode in contact with the piezoelectric sensor and semiconductive (SC) electrode with the diameter of 12 cm and 2 cm, respectively. 18 All the samples were applied a DC electric field of approximately 30 kV/mm for 3600 s at room temperature. Voltage-off measurements were carried out during the depolarization time. ...
The sandwich-structured Al2O3/low density polyethylene (Al2O3/LDPE) nanocomposite dielectrics consisting of layer-by-layer with different concentration Al2O3 loading were prepared by melt-blending and following hot pressing method. The space charge distribution from pulsed electro-acoustic method and breakdown strength of the nanocomposites were investigated. Compared with the single-layer Al2O3/LDPE nanocomposites, the sandwich-structured nanocomposites remarkably suppressed the space charge accumulation and presented higher breakdown strength. The charges in the sandwich-structured nanocomposites decayed much faster than that in the single-layer nanocomposites, which was attributed to an effective electric field caused by the formation of the interfacial space charges. The energy depth of shallow and deep traps was estimated as 0.73 eV and 1.17 eV in the sandwich-structured nanocomposites, respectively, according to the thermal excitation theoretical model we proposed. This work provides an attractive strategy of design and fabrication of polymer nanocomposites with excellent space charge suppression.
The interface is one of the most important factors affecting the macroscopic properties of polymer nanocomposite dielectrics. However, the study of the interface still faces many challenges, such as the evolution mechanism of the interface microstructure, interface compatibility, interface polarization, and the internal mechanism of crystallization. Due to the lack of direct observation and characterization of the interface, the theoretical research is seriously hindered. The influence of the nanoparticles embedded in the nanodielectric and the interfacial region on charge transport and accumulation is still unclear. Since the nanoscale interface is beyond the spatial resolution of traditional analytical techniques, the understanding of the interfacial charge behavior of polymer nanocomposites is largely dependent on speculation and indirect experimental results. Atomic force microscopy (AFM), as a nanometer high-resolution measuring instrument, has become an important means to study the interfacial microregions of polymer nanocomposite dielectrics. In this paper, the latest research progress of various interface models and functional AFM in interface structure, charge transport, and interface polarization are reviewed. The existing problems and possible development directions in the future are also discussed.
Nanodielectrics show excellent dielectric properties, and many scholars try to explain this phenomenon from different angles, among which, the interface effect between nanoparticles and polymer matrix has aroused great interest of researchers. However, due to the complexity of the interface region, it is impossible to intuitively obtain the microstructure and interaction mechanism between the polymer chain and the nanoparticles in the interface region, so researchers have proposed different interface models to speculate and explain the macroscopic properties of the nanocomposites. Such as diffusion electric double layer model, multi-core model, interphase volume model, double layer model, water shell model, multi-zone structure model, penetration theory model, three-dimensional electrostatic model, deep trap model, etc.
In this paper, the conduction current characteristics of PP films treated by different DC electrical prestress are studied through experiments. The results show that the conduction current through the film is reduced by applying a strong homopolar DC prestress for 30 minutes, and the conduction current of the film decreases with the increase of homopolar prestress field. In contrast, the conduction current of the film increases with the application of heteropolar DC electrical prestress, and the conduction current increases with the increase of heteropolar prestress field. The relationship between conduction current and prestress is attributed to the effect of space charges. Under the action of homopolar DC prestress, homopolar space charges accumulate on the surface of the film, which reduce the electric field around the interface between the film surface and the electrode, reducing charge injection and the conduction current. In contrast, heteropolar DC prestress and heteropolar space charges have exactly the reverse effect. The research indicates that electrical prestress can adjust the injection and distribution of space charge, thereby regulating the conduction current characteristics of materials and improving the electrical properties of materials in practical use.
Inorganic metal oxide nanoparticles of different structure can be combined with the polymers, giving nanocomposites of diverse structure with interesting physicochemical properties and application potentials. This review covers the fundamental science, synthesis, surface functionalization, characterization, and physicochemical properties various metal oxide nanostructures and the strategies for their incorporation into the polymer matrices. An overview of the recent developments and applications of metal oxide nanoparticle/polymer nanocomposite have been presented. The review typically summarizes the significant developments over the past half‒decade and discusses them critically, presenting selected examples, and bringing together many important references of primary literature.
Polyethylene (PE) is widely used in the electrical insulation systems because of its excellent mechanical and electrical properties. However, space charge will accumulate in insulating materials and cause electrical field distortion under high voltage. Linear low density polyethylene/linear low density polyethylene grafted polystyrene/polystyrene coated magnesium oxide (LLDPE/LLDPE-g-PS/[email protected]) nanocomposites were prepared by melt blending. The morphology, thermal, mechanical, and electrical properties of LLDPE/LLDPE-g-PS/[email protected] nanocomposites were investigated. Scanning electron microscopy indicated that the [email protected] nanoparticles were well dispersed in the matrix, and the direct current breakdown strength was increased about 51.7% when 2.0 wt % [email protected] added.
In recent years, growing awareness of global environmental conservation has increased the demand for power leveling through inter-grid connections between nations or islands, and the introduction of renewable energy raises more necessity of energy leveling and remote connections between power-generation facilities and power demand area. There is a lot of market interest in high-voltage direct current (HVDC) technology, which is advantageous for long-distance power transmission. HVDC power cables are playing an important role in this to connect HVDC-generation facilities and the existing power network efficiently. Solid insulation power cables, in particular, are attracting attention worldwide and various insulation materials have been proposed. Though cross-linked polyethylene (XLPE) cable is widely used for alternative current (AC) transmission, if attempts are made to adapt AC-XLPE cables for DC transmission as significant electric field enhancement by space-charge accumulation may occur, then breakdown would take place especially during surge voltage in opposite polarity and polarity reversal. Therefore, it is desired to innovate a special insulating material for DC transmission to suppress the accumulation of space charge.
In this chapter about such insulating material for DC transmission cable, nanocomposite that is composed from polymer materials as a matrix and nanofiller, polymer blend, and copolymer as kinds of composites in a broad sense are described. And for the real production of the nanocomposite materials in the near future, to increase the performance and reliability of the production and a deeper understanding of insulation and dielectric phenomena, some issues and perspectives are shown.
This study outlines a sustainable development of Mn-ZnO reinforced low-density polyethylene (LDPE) based feedstock filament, using twin-screw extrusion (TSE) process,
for fused filament fabrication (FFF) based additive manufacturing (AM) technology. MnZnO based reinforcement, known for its functional properties, has been utilized to tune the thermo-mechanical, morphological, and piezoelectric characteristics of LDPE. Functional feedstock filaments have been manufactured by varying twin screw compounder processing parameters such as barrel temperature (120, 130, and 140℃), screw torque (0.15, 0.20, and 0.20N.m) and forced loading (5, 10 and 15kg). The results of the study suggested that the better mechanical properties of feedstock filament were attained at 140℃ barrel temperature, 0.25N.m screw torque, and 10kg forced loading in TSE. The result of the tensile properties is supported by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive energy-dispersive X-ray spectroscopy (EDS), piezoelectric property (D33 measurement), shape memory effect and differential scanning calorimetric (DSC) analysis.
The development of insulating materials is a crucial knob for HVDC cables. The space charge effect and positive conductivity-temperature relationship of cable insulating materials render electric field distortion, which is detrimental to the reliability of HVDC cable insulation. To address this issue, polymer nanodielectrics have been used to improve the insulation performance. However, great challenges lie ahead including controllable large-scale fabrication and performance regulation of polymer nanodielectrics, mainly because of the complexity, uncertainty, and variability of the polymer matrix-particle interface. In recent years, surface ligand engineering has emerged as an effective way to tune the particle dispersion by controlling enthalpic and entropic interactions among nanoparticles, ligands, and polymer matrices, so as to introduce tunable functionalities and optimize the performance of polymer nanodielectrics. Herein, we outline recent advances of surface ligand engineering to achieve various exciting properties of polymer nanodielectrics for HVDC cables, leading to inhibited space charge, reduced DC conductivity and temperature sensitivity, improved breakdown strength, inhibited electrical tree aging, and improved thermal conductivity, etc. We aim to summarize surface ligand strategies for advanced polymer nanodielectrics and how they function, point out the knowledge gaps between lab results and industrial applications, and offer suggestions to guide the rational design of more effective and efficient polymer nanodielectrics for future HVDC transmission engineering.
Space charge accumulation is the main factor in accelerating the degradation of polymeric insulation in high-voltage direct current (HVDC) cables. It is essential for the development of HVDC cables to suppress the space charge in the insulating layer. In this paper, an approach is presented to decrease carrier injection from the inner semiconductive layer to the insulating layer, using a magnetic semiconductive compound. The semiconductive shielding compound was prepared by adding strontium ferrite to a carbon black/ethylene-vinyl acetate/low-density polyethylene (LDPE) matrix. The addition of strontium ferrite led to an increase in the residual magnetic induction of the semiconductor shielding layer. Unmagnetized and magnetized semiconductive compounds were used as electrodes to test the injection of carriers into the LDPE insulation layer. When SrFe12O19 had been added, the charge injected into the LDPE by the magnetized semiconductive layer was less than with an unmagnetized semiconductive layer. When the content of SrFe12O19 was 5 wt. %, 10 wt. %, 30 wt. %, and 50 wt. % in the semiconductive compound, the charge in the LDPE was reduced by 4.2%, 8.1%, 12.5%, and 27.1%, respectively.
Cable insulation made of polypropylene is of great prospective since it is recyclable to some degree. There are many problem the cable insulation need to deal with in HV system. The tree properties of polypropylene with repetitive pulse voltage at low temperature is discussed firstly. At different temperature, the pattern and length of electrical tree are shown in our results. This research analyzed the influence of temperature on the grow speed of electrical tree. The effects of mechanical stretching on space charge behaviors of pp/poe blend is also included in this chapter. This part presents the influence of stretching rate on electric properties. Finally, this chapter also provided some research of PP/POE/ZnO nanocomposites for HVDC cables. This part includes the space charge behavior analysis considering nano fillers.
The effect of filler surface functionalization with 3-aminopropyltriethoxysilane (APTES) on the charge trapping and transport was studied in polypropylene (PP)/(ethylene-octene) copolymer (EOC)/silica nanodielectrics. Different reaction conditions were utilized for silica functionalization to alter the deposited layer morphology. This approach made it possible to engineer the filler-polymer interface to achieve optimized dielectric properties for the nanocomposites.. The successful chemical modification of the silica surface was confirmed via Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) and X-Ray Photoelectron Spectroscopy (XPS). Subsequently, the effect of the engineered filler-polymer interface on the nanocomposites’ crystallinity was analyzed with Differential Scanning Calorimetry (DSC). Scanning Electron Microscopy (SEM) was utilized to observe the morphology of the nanocomposite as well as the silica dispersion. Finally, the effect of the silica functionalization on the dielectric properties of PP/EOC/silica nanocomposites was tested via Thermally Stimulated Depolarization Current (TSDC) and Broadband Dielectric Spectroscopy (BDS). The results suggested that the presence of the amine functionality on the silica reduces interfacial losses in nanocomposites, and hinders further injection of space charge by introducing deep trap states at the filler-polymer interface. Under certain conditions, APTES can form an “island-like” morphology on the silica surface. These islands can facilitate nucleation, inducing transcrystallization at the filler-polymer interface. The “island-like” structures present on the silica would further contribute to the induction of deep traps at the filler-polymer interface resulting in reduction of space charge injection.
Space charge suppression is one of the key goals for the development of HVDC cable insulation. This study intends to reveal space charge suppression mechanism in LDPE based nanocomposites by numerical simulation and experiments in terms of the effect of trap characteristics. The simulation model takes into account bipolar charge injection, transport, charge trapping, recombination and extraction. Both the experiments and simulations clearly indicate that a dramatic increase in the deep trap density can result in remarkable bulk space charge suppression. The large number of deep traps introduced in the LDPE based nanocomposite by nanofillers should be responsible for its excellent space charge suppression effect. The simulation results indicate that the increase in the trap density shows much more remarkable space charge suppression performance as compared to the trap depth, while the increase in both the trap density and trap depth shows the most remarkable space charge suppression performance. A trap regulated space charge suppression model has been proposed indicating that the trap density and depth dominate space charge behavior via regulating the balance between electrode injection current and bulk conduction current due to its control on the rates of charge trapping and detrapping processes, control on the number and location of the trapped charges. The dramatic increase in the trap density and depth results in extremely low bulk conductivity and carrier mobility. A large number of the injected charges will be trapped immediately around electrode interface forming a thin trapped charge layer. The trapped charge layer will produce a strong opposite electric field and enhance the potential for charge injection, leading to a significant decrease in the injection current and finally a balance between the injection current and the bulk conduction current at a lower current level, resulting in remarkable bulk space charge suppression. In addition, high bulk conductivity due to lower trap density and lower trap depth can also contribute to space charge suppression due to the quick relaxation of the bulk space charges, though it will increase the leakage current.
Blends of polyethylene (PE) with nanocomposites of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene grafted maleic anhydride (SEBS-MA) thermoplastic elastomer filled with Zinc Oxide (ZnO) nanoparticles have been studied as potential candidates for applications in HV insulation systems including HVDC cables. In particular, the dielectric and mechanical properties of PE/SEBS-MA/ZnO blend nanocomposites have been evaluated and compared to those of PE/ZnO homopolymer nanocomposites prepared as a reference. PE/ZnO materials were characterized by homogeneous distribution of nanoparticles and presence of agglomerations attributed to insufficient compatibility between the metal oxide nanoparticles and the polyolefin matrix. However, nanoscale dispersion was achieved in SEBS-MA/ZnO and PE/SEBS-MA/ZnO nanocomposites due to improved compatibility between the nanoparticles and SEBS-MA. Besides, in PE/SEBS-MA/ZnO blend nanocomposites, ZnO nanoparticles remained exclusively confined in SEBS-MA or at the interface between PE and SEBS-MA. In terms of dielectric properties, the unfilled blend PE/SEBS-MA featured reduced breakdown strength and resistance to surface erosion by partial discharges in comparison with neat PE. However, upon addition of ZnO the blend PE/SEBS-MA presented higher performance when compared to PE. At 1 wt% ZnO loading, the resistance to surface erosion of PE/SEBS-MA increased by 45% higher than neat PE/SEBS-MA, 38% higher than unfilled PE and 30% higher PE/ZnO nanocomposite containing the same ZnO loading. Besides, blend nanocomposites exhibited dielectric losses lower than PE/ZnO nanocomposites at power frequencies and temperatures up to 80 °C. The breakdown strength of both sets of nanocomposites decreased compared to unfilled materials, at large loadings of nanoparticles. However, smaller reduction was observed in the case of PE/SEBS-MA/ZnO nanocomposites due to improved nanoparticles dispersion. Finally, PE/SEBS-MA/ZnO nanocomposites featured enhanced mechanical flexibility when compared to PE/ZnO nanocomposites.
Space charge accumulation is a critical issue for the deterioration of the elevated temperature insulating property of polymeric materials. We present the influence of surface-modified ZnO nanoparticles on the space charge distribution and direct current (DC) resistivity of polypropylene (PP)/elastomer blends under elevated temperature. Octyltrimethoxysilane, a silane coupling agent, was used for the surface modification of nanoparticles. Morphology characterization results indicated that the elastomer and coated ZnO were well dispersed in the PP matrix. It was observed that the coated ZnO can significantly improve the insulating properties, including a minimized electric field distortion (4.0%) and increased DC volume resistivity (1.41 × 10¹⁸ Ω m) under an electric field of 40 kV/mm and 70 °C. The DC resistivity of 2 ph PP/elastomer/ZnO ternary composites was improved by 13.4 times compared with that of pure PP/elastomer blends. The suppression of space charges may originate from deep traps existing in spherulite boundaries and interfacial zones between polypropylene and ZnO. This work provides an effective method to endow a recyclable insulating material with outstanding elevated temperature insulating performances.
Electric field distortion caused by the space charge accumulation in the insulation is considered to be one of the obstacles for the operation of high voltage DC cables. In this paper, nano-CB/LDPE composites were fabricated by using the melt blending method. Space charge characteristics and field distortions of nanocomposite films were measured under direct current (DC) field by pulsed electro-acoustic (PEA) method. Dielectric properties and DC breakdown strength were also investigated. Experimental results indicate that, by adding appropriate doping content of CB nanoparticles, the space charge injection threshold of the films was increased, which suppressed homo-polarity space charge injection and ameliorated the field distribution. Breakdown strength of nanocomposite decreased whereas the dielectric permittivity and dielectric loss tangent increased slightly. However, too much CB nanoparticles mixed in LDPE could weaken the suppression effect on charge injection and field distortion, and the LDPE with 0.03wt% of CB nano-particle showed a relative better ability of inhibiting field distortion.
Thermoplastic elastomer nanocomposites based on respectively polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene grafted maleic anhydride (SEBS-MA) block copolymers and containing functionalized zinc oxide (ZnO) nanoparticles have been investigated as candidate materials for high voltage (HV) insulation systems. The dispersion of the organically modified ZnO nanoparticles has been successfully tuned through the MA graft and the block copolymer nanostructure. In particular, nanocomposites with signs of rheological percolation, indicating the formation of a network between individually dispersed nanoparticles and polymer chains, have been obtained at ZnO content as low as 5 wt% (0.9 vol%). This behavior resulted in an enhancement of the thermal conductivity and better control of the electrical conductivity while maintaining breakdown strength and dielectric losses in the same range of the unfilled insulating matrices. Furthermore, the resistance to surface erosion by partial discharges was significantly improved: in the presence of 5 wt% of individually dispersed ZnO nanoparticles, the eroded volume was reduced 10 times.
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.
Novel insulating low-density polyethylene (LDPE/ZnO) nanocomposites with ultra-low electrical conductivity for the long-distance transmission of energy are presented. An up-scaled aqueous synthesis was used in order to obtain ultra-pure 20 nm–2 μm ZnO particles with different morphologies of nanoprisms, stars and flower-shapes. The surface chemistry of the nano-sized particles was changed from hydroxyl groups to alkyl groups of different lengths by reacting the nanoparticles with methyltrimethoxysilane (C1), octyltriethoxysilane (C8) or octadecyltrimethoxysilane (C18). The interfacial adhesion between the nanoparticles and the LDPE matrix was assessed by tensile testing and from scanning electron microscopy by evaluation of the void formation at the polymer/nanoparticle interfaces during tensile deformation. The composite based on 3 wt. % of C8-coated nanoparticles, with the highest interfacial adhesion and the most uniform dispersion, exhibited a DC conductivity more than two orders of magnitude lower than that of the pristine LDPE. The LDPE composites with dispersed micrometre-sized hierarchical ZnO particles of different porosities also exhibited a lower DC conductivity than the LDPE. The results obtained indicate that, in addition to proper particle dispersion, the nature of the “effective” interfacial surface plays an important role for the development of traps for charge carriers at the polymer/particle interface.
The effect on space charge accumulation of the introduction of nano-filler into polyethylene is usually studied using thin 100 μm samples in electric fields reaching 100 kV/mm. However, the insulation thickness in an actual HVDC cable is usually some centimeters. Thus there is a great thickness difference between the experiment sample and the insulation application. Here we investigate the thickness dependence of space charge accumulation in unfilled and LDPE doped with nano-SiO2 over a range of temperatures in a field of 50 kV/mm. It is found that the space charge in LDPE/SiO2 nanocomposites exhibits marked thickness dependence whereas in the unfilled LDPE it shows very little thickness dependence. A simulation based on the bipolar injection model and Takada's trapping potential model has been carried out in order to explain the differences. It is found that when the simulation included the effects of charge injection, extraction, trapping, recombination, mobility and diffusion, it is able to explain the heterocharge formation in the unfilled LDPE, and the thickness dependence in the two kinds of materials.
This letter intends to reveal the mechanism of space charge suppression in low density polyethylene (LDPE)/ZnO nanocomposites. Trap level and space charge distributions were obtained from modified isothermal discharge current method and pulsed electro-acoustic (PEA) method, respectively. The results showed that ZnO nanoparticle doping introduced large amounts of deep trapping states, significantly reduced space charge accumulation and conduction current. The results can be explained in terms of deep trapping states resulted from the interface regions and morphology structure changes by nanoparticles doping, which greatly reduced the charge mobility, raised the charge injection potential at the contact and weakened impurity ionization.
Space charge suppression mechanism in nano-composite polymer material is studied using experimental results and numerical simulation. Recently, many kinds of nano-composite polymeric materials have been reported to have improved their characteristics under high electric field. For example, LDPE/MgO nano-composite, which is made up of low density polyethylene (LDPE) and nano size filler of magnesium oxide (MgO), exhibits high volume resistivity and high dielectric strength under dc electric field. Authors have investigated the space charge behaviour in LDPE/MgO nano-composite under high electric field using pulsed electro-acoustic (PEA) method. It has been found that, compared to LDPE, the space charge formation is also suppressed in the nano-composite material. As a reason for the suppression, we have suggested that the induced dipole polarization around MgO filler formed by dc stress application might play a role of carrier trap sites. From the numerical calculation, distortion of electric potential around MgO is seen to be much larger than that around naturally included dipole. It means that the MgO acts as a deep trap site as different from some defect or ions included in LDPE. Using the numerical calculation based on such electric potential distortion, we have tried to simulate the space charge distribution in LDPE/MgO under high dc electric field. The simulation results are in good agreement with the experimental results.
The incorporation of silica nanoparticles into polyethylene has been shown to increase the breakdown strength significantly
compared to composites with micron scale fillers. Additionally, the voltage endurance of the nanocomposites is two orders
of magnitude higher than that of the base polymer. The most significant difference between micron-scale and nano-scale fillers
is the large interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal
in controlling properties, this paper compares the behavior of nanoscale silica/cross-linked low density polyethylene nanocomposites
with several silica surface treatments. In addition to breakdown strength and voltage endurance, dielectric spectroscopy,
absorption current measurements, and thermally stimulated current determinations (TSC) were performed to elucidate the role
of the interface. It was found that a reduction in the mobility in nanocomposites as well as a change in the defect size may
be key to explaining the improvement in the properties.
Electronic charge transport in insulating polymers is briefly discussed, particularly the measurement of charge carrier mobility. The more frequently used techniques of space charge profile measurent in dielectrics are then described, and data for polyethylene published during the last 5-6 years are reviewed.
The dielectric properties of epoxy nanocomposites with insulating nano-fillers, viz., TiO2, ZnO and AI2O3 were investigated at low filler concentrations by weight. Epoxy nanocomposite samples with a good dispersion of nanoparticles in the epoxy matrix were prepared and experiments were performed to measure the dielectric permittivity and tan delta (400 Hz-1 MHz), dc volume resistivity and ac dielectric strength. At very low nanoparticle loadings, results demonstrate some interesting dielectric behaviors for nanocomposites and some of the electrical properties are found to be unique and advantageous for use in several existing and potential electrical systems. The nanocomposite dielectric properties are analyzed in detail with respect to different experimental parameters like frequency (for permittivity/tan delta), filler size, filler concentration and filler permittivity. In addition, epoxy microcomposites for the same systems were synthesized and their dielectric properties were compared to the results already obtained for nanocomposites. The interesting dielectric characteristics for epoxy based nanodielectric systems are attributed to the large volume fraction of interfaces in the bulk of the material and the ensuing interactions between the charged nanoparticle surface and the epoxy chains.
The incorporation of silica nanoparticles into polyethylene increased the breakdown strength and voltage endurance significantly compared to the incorporation of micron scale fillers. In addition, dielectric spectroscopy showed a decrease in dielectric permittivity for the nanocomposite over the base polymer, and changes in the space charge distribution and dynamics have been documented. The most significant difference between micron scale and nanoscale fillers is the tremendous increase in interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal in controlling properties, the bonding between the silica and polyethylene was characterized using Fourier transformed infrared (FTTR) spectroscopy, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS). The picture which is emerging suggests that the enhanced interfacial zone, in addition to particle-polymer bonding, plays a very important role in determining the dielectric behavior of nanocomposites.
Space charge is a recent hot topic in the field of electrical insulation, since it can be measured nondestructively as standard practice. It has been suspected long that space charge might be critically involved in local dielectric breakdown or tree initiation. It is widely accepted that space charge is formed under dc stress in dielectrics. The author demonstrated a hypothetical space charge-dominant model for tree initiation in 1979 in Japanese and in 1991 in English, and proposed that charge can be accumulated inside polymer dielectrics near the divergent high field region even under ac stress. The model consists of charge injection and extraction, charge trapping and space-charge forming, polymer scission due to high energy injected and extracted electrons, oxidation, electric field enhancement, and finally tree initiation. The author will explain his old hypotheha1 space charge dominant model for tree initiation, various relevant phenomena and data that could prove the validity of the model, try to interpret several recently discovered phenomena with his model, and re-evaluate it as compared with modern concepts for tree initiation.
Dielectric Polymer Nanocomposites provides the first in-depth discussion of nano-dielectrics, an emerging and fast moving topic in electrical insulation. The book provides an overview of the background, principles and promise of nanodielectrics, as well as a discussion of the processing of nanocomposites. Special considerations are also given to clay based processes, mechanical, thermal and electric properties and surface properties, as well as erosion resistance. Carbon nanotubes are discussed as a means of creation of nonlinear conductivity. Editor J. Keith Nelson brings together the leading minds in the field, whose contributions to the book also: •Discusses the background, principles and importance of nano-dielectric composites •Includes complete coverage of nanodielectric composites such as cryogenic applications, high voltage stress grading materials and applications in the capacitor industry •Provides detailed coverage on the processing of nanocomposites Dielectric Polymer Nanocomposites is perfect for academics and researchers working in or interested in learning more about this growing field. © Springer Science+Business Media, LLC 2010. All rights reserved.
There is charge injection/extraction at a metal-dielectric contact. In general, the problem is treated by analogy with that for metal-semiconductor contact. However, the electrical conduction through dielectrics is bulk and electrode limited. Consequently, it is difficult to discriminate between the two effects and to have specific information only about one of the mechanisms. We propose a method that allows to determinate the sign and the value of the electric charge injected/extracted at the metal-dielectric contact. The method is based on the modification of the external electric field of a dielectric when electric charge is injected/extracted onto its surface. The lowest surface charge density measured in the presented experiments was around 1.8 10 -6 cm-2 which corresponds to a medium distance of about 300 nm between two trapped charges. The injection/extraction process takes place no matter if there is a conduction current through the sample.
Polyethylene samples were surface fluorinated by two kinds of F2/N2 mixture, containing 0.04% and 0.2% oxygen by volume, respectively. Infrared analyses do not clearly indicate the presence of oxygen in the surface layer fluorinated by the former, but clearly show that oxygen has been incorporated into the surface layers fluorinated by the latter. Moreover, the amount of fluorine introduced in the fluorinated layer is found to increase with the fluorine partial pressure or fluorination time. Thicknesses of the fluorinated layers are determined by SEM cross section images. Space charge measurements show that the thin fluorinated layer (0.24 μm) without appreciable oxygen can block the charge injection from electrodes very effectively, while the thick fluorinated layer (0.39 μm) with oxygen does not show the effectiveness. In this case, a further thick fluorinated layer is needed to achieve the blocking effect. The permittivity of the fluorinated layers is evaluated by the polar component of surface energy, and the results suggest an increase of the permittivity because of the incorporation of polar groups. Thermally stimulated discharge current measurements reveal different charge trapping properties between the fluorinated layers and a negative influence of oxygen on the decrease in free volume of the surface layer. The decrease in free volume is more important in suppressing the charge injection than the increase in surface layer permittivity and the change in surface trap.
A programme of experiments has been undertaken to examine transient charging/discharging currents and steady state currents in low-density polyethylene (LDPE) under the application of direct fields. This has been undertaken for pristine material and for material which has received doses of radiation between 104 and 106 Gy from either a 60Co γ-source or a research reactor. The material was irradiated in ambient air or dry nitrogen. Measurements were made for applied fields in the range 6.7 × 105-5.3 × 107 V m -1 and temperatures between ambient and 90°C. With pristine material at low fields, transient charging/discharging currents decreased monotonically with time. However, the mechanism changed at higher fields with a peak occurring in the charging transient indicating a space-charge limited process. Substantial charge injection was also in evidence as demonstrated by anomalous discharging currents. This transient response was echoed by the current/voltage characteristics of steady state behaviour. Gamma and neutron irradiation brought about a change in this situation and the charge transport mechanism altered gradually from space-charge-limited conduction to an ohmic process with increasing dose. The role played by charge traps appears to be significant.
Voltage reversal will strongly affect the stability of cable in service, especially under temperature gradient due to the temperature rise of the cable. In this paper, the results of space charge profiles and field distortions in low density polyethylene (LDPE) films after dc pre-stress of 50 kV/mm for 2 h under temperature gradient and then during different voltage reverse polarities or during different voltage reverse periods were presented. The test results show that the charge accumulation and field distortion are different under different voltage polarities due to the dependence of charge injection on electrode materials. In this work, under the Al / SC (Semiconductive) electrode system with a temperature difference of 40°C, the maximal transient field occurs near the low temperature Al electrode when polarity varies from negative to positive while near the high temperature SC electrode when polarity varies from positive to negative. In addition, the faster the voltage reversal at room temperature the higher the maximal transient field, but the faster the voltage reversal the lower the maximal transient field under temperature gradient. Moreover, the maximal field appears during voltage inversion at room temperature while appears at pre-stress under temperature gradient.
In order to investigate the effect of inorganic nanoparticle fillers on the electrical properties of low density polyethylene (LDPE), LDPE/ZnO nanocomposites were prepared with a variety of filler loadings by melt blending method and the corresponding electrical properties are investigated. Experiments show that, the trap density is increased by 3-5 times in the nanocomposite as compared with LDPE. The conduction current of the nanocomposite is decreased to 0.5 - 0.25 of the value of LDPE both at low and high electric fields. The electrical breakdown strength is increased at low filler contents and decreased at high contents (>1 wt%). The electrical treeing life time is elongated 50 times at most in the nanocomposites. Both homo- and hetero-space charges are remarkably suppressed in the nanocomposite with 0.5 and 7 wt% contents. The study indicates that the improvements on the electrical properties, especially the charge transport and space charge suppression in the nanocomposites are closely related to the significant increase of deep trap density. The effect of UV light absorption and electric field homogenization by ZnO can also contribute to the enhancement of electrical breakdown strength and elongation of electrical treeing life.
Transient currents in low-density polyethylene have been investigated at
various temperatures, field strengths and for nonconditioned and
conditioned samples, under vacuum and ambient air conditions. Anomalous
charging and discharging currents have been observed. The anomalous
discharging current is almost the mirror image of the anomalous charging
current. Two patterns have been observed for temperatures below and
above 50°C. Three characteristic zones have been identified, from
which one may obtain information about dielectric polarization, bulk
conduction effects and electrode effects and diffusion.
The incorporation of nanometric size particles in a matrix to form dielectric composites shows promise of materials (nanodielectrics) with new and improved properties. It is argued that the properties of the interfaces between the particles and the matrix, which will themselves be of nanometric dimensions, will have an increasingly dominant role in determining dielectric performance as the particle size decreases. The forces that determine the electrical and dielectric properties of interfaces are considered, with emphasis on the way in which they might influence composite behaviour. A number of examples are given in which interfaces at the nanometric level exercise both passive and active control over dielectric, optical and conductive properties. Electromechanical properties are also considered, and it is shown that interfaces have important electrostrictive and piezoelectric characteristics. It is demonstrated that the process of poling, namely subjecting macroscopic composite materials to electrical stress and raised temperatures to create piezoelectric materials, can be explained in terms of optimizing the collective response of the nanometric interfaces involved. If the electrical and electromechanical features are coupled to the long-established electrochemical properties, interfaces represent highly versatile active elements with considerable potential in nanotechnology.
Recently a number of papers have reported enhanced electrical (ageing and breakdown) performance of nano-composites when compared to the properties of the base resin material. The physical reasons for this improvement are the subject of many current studies and have yet to be fully formulated. In this work, the build-up and decay of space charge in epoxy resin materials, containing nano-particles of alumina of various concentrations, 0.5% to 2% was investigated during the application of DC voltage and after voltage removal. Space charge profiles in thin film samples were investigated using the Pulsed Electro-Acoustic (PEA) technique. The results were compared with the space charge behavior of the unfilled resin and for the 1% nano-filled resin with differing levels of absorbed moisture. The results demonstrate that the space charge behavior is dominated by the properties of the host resin. However, the accumulation of positive charge at the anode is affected by the concentration of filler particles. The results also demonstrate the importance of controlling the amount of absorbed moisture in the samples as this governs the dynamics of the charge build-up and decay in the samples.
The pulsed electroacoustic (PEA) method has been widely used to observe space charge distributions in various solid dielectric materials. The sensitivity of the conventional PEA system is around 1 C/m3. When the charge density is less than 1 C/m3, however, it is difficult to obtain an accurate result because the signal due to the Maxwell stress becomes comparable with the signal to be measured. The Maxwell stress is generated by applying the pulsed electric field to the dielectrics, and independent from the existence of either induced charges by dc bias voltage or internal charges. In order to eliminate the influence of the Maxwell stress, we have developed a new PEA system with a dual polarity pulse generator. The system achieved to measure space charge density around 0.03 C/m3.
There has been speculation in the past about the extent to which space charge formed by the flow of prebreakdown current affects the electric strength of polythene. In the present work, conduction and absorption-current measurements up to about half the breakdown field have given clear evidence of space-charge effects. Further evidence was obtained from measurements of the apparent `intrinsic¿ electric strength under various modes of stressing. It was found that the electric strength of polythene measured with direct voltage below about 30°C exceeded the corresponding value measured with impulse voltages by up to about 25% The impulse electric strength could be increased to equal the direct voltage by prestressing each specimen for 4min with direct voltage of the same polarity as the impulse voltage. However, when the impulse was of opposite polarity to the direct prestressing voltage, the electric strength decreased with increase in the prestressing field. The effect was dramatic, for the impulse electric strength changed from the normal value of about 6MV cm¿1 to less than 2MV cm¿1 as the prestressing field was increased to about 75% of the electric stength measured with direct voltage. Experiments in which the breakdown impulse was applied some time after removal of the prestressing field showed that the impulse electric strength at 20°C recovered in about 10¿2s to the value obtained with impulses without prestressing. Thus the prestressing caused no permanent damage. The observed effects are attributed to space-charge injection from the electrodes. The observations are not consistent with the concept that breakdown occurs when the field at a point within the specimen exceeds a critical value, the intrinsic electric strength of polythene.
An anomalous discharge current flowing in the same direction as the charging current was observed in a low-density polyethylene film with evaporated silver electrodes at room temperature (25°C) after the application of a very high field above 2× 108 V/m. The discharge transient current was computer-analyzed numerically by an explicit difference method. The anomalous current was attributed to the large amount of two types of injected space charge (electrons and holes) with ohmic electrodes, and not to one type of injected space charge (electrons or holes) with blocking electrodes.
Space charges formed in low-density polyethylene (LDPE), to which high voltage is applied, are usually composed of injected homocharges from electrodes and ionized heterocharges from bulk impurities. In this paper, the heterocharges in LDPE were discriminated from homocharges using charge-injection suppression layers. Recent advanced techniques have made it possible to observe directly the space charge distributed in dielectrics. However, it is difficult to study the characteristics of charge injection from electrodes and ionization of impurities individually as the two processes may interfere with each other and become inseparable. Therefore, a layer that suppresses the charge injection from electrodes was used to discriminate between injected and ionized charges. To observe the space-charge distributions, the pulsed electroacoustic (PEA) method, one of the advanced techniques, was employed. In LDPE with suppression layers on both surfaces, hetero-space charges, which are attributed to ionization of impurities, were observed. On the other hand, in LDPE without the suppression layers, small amounts of homo-space charges, which are attributed to net charges from both the electrode injection and the bulk ionization, were observed. Amounts of charge supplied from electrodes were estimated by subtracting the amount of space charge in the sample with suppression layers from that in the sample without suppression layers.
Measurements are reported of the effect of field and temperature on decaying and steady currents in an epoxy polymer. Fields up to 140 MV m−1 were used and the steady current was found to increase as exp (α F½). The decaying currents followed the t−n law and increased much less rapidly with increasing field and temperature than did the steady currents.
Absorption of water by the polymer caused a marked increase in steady-state conduction but the current characteristics were similar to those of dried specimens.
It is suggested that the decaying currents are due to electronic hopping conduction, whereas the steady currents are ionic.
The space charge characteristics in cross-linking polyethylene (LPE) are investigated under various electrical stresses from dc to power frequency. The pulsed electro-acoustic method is improved to meet the needs of measurement under ac voltages. The frequency and phase angle dependences of the space charge are studied by means of phase-locking and frequency-converting techniques. Test results show that space charge can develop and accumulate in LPE under an ultralow frequency voltage. The critical frequency of the applied ac voltage is less than 0.02 Hz. The mechanism of the formation of space charge in LPE under an ac voltage is discussed through degassing of a sample in vacuum.
Polyethylene (PE) samples were surface fluorinated by the F2/N2 mixture for 30, 60, 120 or 240 min to investigate the influence of the fluorinated layer characteristics on space charge accumulation. After polarization at 50 kV mm−1 dc electrical field and 40 °C for 240 min, the charge amounts of the samples fluorinated for the different times, normalized to the charge amount of the original sample, are 1.17, 0.51, 0.49 and 0.22, respectively, showing significant suppression of the charge accumulation by the longer treatments, especially for the 240 min treatment. Infrared analyses and SEM cross-section images of the fluorinated samples indicate the increases in degree of fluorination and thicknesses of the fluorinated layer with the treatment time, and the fluorinated layer thicknesses were determined to be 0.39, 0.45, 0.65 and 0.80 µm. Surface energy calculations show that the polar component increased from 2.9 to 14.7 mJ m−2 after the 30 min treatment, and subsequently decreased to 11.8, 11.5 and 9.5 mJ m−2 for the longer treatments of 60, 120 and 240 min. This suggests a similar change in surface layer permittivity with the treatment time. The fluorinations led to the shift of thermally stimulated discharge current peak from 156 °C of the original sample to 145, 142, 144 and 149 °C of the fluorinated samples, and thus reduced the trap depth of the surface layer. But, the longer treatments of 60, 120 and 240 min significantly improved the barrier properties of the surface layer to the diffusion of the chemical species from the semi-conductive electrode to the PE by the decrease in free volume of the surface layer. The chemical species diffused into the sample surface layer reduced the depth of surface traps. The decrease in free volume is more important in suppressing the charge accumulation than the increase in surface layer permittivity and the change in surface trap, because it would reduce the charge transport in the surface layer and the charge direct injection at both electrodes.
A study of variations of charging and discharging transients in low-density (0.918 gm cm-3) polyethylene has been made over a wide range of fields and temperature and a limited range of sample thickness and electrode materials. In the temperature range 113-273K two broad isochronal (i.e. current at constant time) peaks were observed at 138K ( gamma -relaxation) and 203K respectively. The observed behaviour of both peaks may be explained by dipolar processes accompanying molecular relaxations in the bulk. Above 273K the 'absorption current' may be due to a charge carrier hopping mechanism.
Modified isothermal discharge current theory (MIDC) was thoroughly investigated in order to obtain the trap level distribution in polymers. In this theory, a new function was defined to weight the contribution of any trap level to the depolarization current at an instant time. The demarcation energy was used to study the trap empty process. Analysis showed that only electrons with trap levels very close to the demarcation energy significantly contribute to the external current. As an application example, this theory was applied to determine the trap level distribution in Kapton 100HN and 100CR (nanodielectrics) PI films before and after corona ageing. Experiments showed that traps in 100CR PI films were about twice as much as that in the 100HN PI films through all trap levels. It's also found that the quantity of traps was increased evidently after corona ageing and lots of deeper traps were produced in 100HN film, whereas for 100CR, changes were not very significant. Since both physical and chemical defects intrinsic or extrinsic can change the trap level distribution and make a significant impact on the dielectric behavior. So modified isothermal current theory may be a useful tool for determining the defect characteristic and investigating the electrical properties of polymer dielectrics.
The space charge distribution in polyethylene blends under direct current electrical field was measured by a pulsed electro-acoustic method. It was found that blending LDPE with 0.5 wt% HDPE decreased the amount of accumulated space charges and improved their distribution. Small-angle light scattering and differential scanning calorimetry showed that crystallization of LDPE/HDPE started at higher temperature than virgin LDPE, and the sizes of LDPE/HDPE spherulites were smaller than that of LDPE. HDPE plays a role on nucleation during the crystallization process. Crystalline form was investigated by wide-angle X-ray diffraction and the results indicate that the crystal form did not change after blending. The reduction of space charges in the blended sample can be explained as the result of the dissipation of charges through boundary regions of smaller spherulites. Copyright © 2004 Society of Chemical Industry
Current transients observed when PE slabs with various electrode arrangements are illuminated are shown to be thermal in origin.
In an externally applied field the current is mainly due to capacitive changes, while in zero external field the current from
a previously polarized sample is due to nonlinear expansion of the dielectric and the resulting change in the profile of the
immobile charge, i.e. field distribution within the sample. The observation of current transients through non-linear expansion
(CTNE) of the dielectric thus allows nondestructive detection of arbitrary space charge profiles.
Extra electrons forced into polycrystalline CdS from ohmic indium electrodes result in space‐charge‐limited current. The transient current resulting from a step voltage is measured and found to decay in a dispersive form of t-(1-α) over several orders of magnitude in time. This dispersive decay is attributed to extensive trapping and detrapping of the electrons and is analyzed in terms of the stochastic transport theory of Scher and Montroll [Phys. Rev. B 12, 2455 (1975)].
Electrical conduction in polymers under a relatively low applied electric field is considered to be ionic and is affected strongly by the structural factors of the polymers. The following equation for the electrical conductivity σ was derived in which free volume Vf, jump energy Ej, and ionic dissociation energy W were taken into consideration: σ=σ0 exp - [γ Vi*/Vf+(Ej+W/2Є)(kT)-1] , where σo is a constant, γ the numerical factor to correct the overlap of free volume, Vi* the critical volume required for transport of an ion, Є the dielectric constant, k Boltzmann's constant, and T the absolute temperature. This equation describes well the conduction phenomena in polymethylmethacrylate, polystyrene, and an unsaturated polyester. Relationships between electrical conduction and free volume are discussed.
Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities such as space charge, partial discharges and conduction current. The latter is, indeed, the scope of this paper, which is taken from the Whitehead Memorial Lecture given at the 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the latest achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute to ageing rate under dc, ac or impulsive voltage, directly through electric field alteration, or indirectly modifying a fundamental ageing factor such as partial discharges.The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/ reliability.
The paper discuss means to reduce the space charge accumulation in polymeric HVDC cables by optimising the composition of the semiconductive material used in the conductor and insulation screens. A semiconductive material contains a polymer, sufficient content of conductive carbon black to attain electrical conductivity, an antioxidant for retardation of the thermo-oxidative degradation and finally a peroxide to allow crosslinking to gain improved mechanical properties. We are reporting that the semiconductive screens will significantly influence the accumulation of space charges in the insulating layer. The effect of the various components of the semiconductive material is carefully studied. The main conclusion is that both the type of polymer and type of carbon black are important for the electrical properties. It is revealed that superior space charge properties are obtained by the use of acetylene carbon black instead of conventional furnace carbon black, and, above all, by the use of a non-polar polymer instead of a conventional copolymer-based polymer. The study has been conducted on press-moulded plaques with semiconductive electrodes on both sides. The space charge profiles have been obtained using a PEA (pulsed-electro-acoustic) system.
Epoxy-based nanocomposites (NCs) were prepared using clay modified by either primary amine or tertiary amine, and the effect of the difference in modifier on the thermal and dielectric properties of the NCs were discussed. The NC with clay fillers modified by the primary amine, 1C, shows a glass transition end temperature (Teg) at a temperature 20°C lower than the neat epoxy (N). This indicates that the resin of 1C is less crosslinked than that of N. On the other hand, the sample 3C, in which the clay was modified by the tertiary amine, shows a DSC spectrum close to that of N. Namely, 3C has a high crosslinking density similar to N. While the three samples show a relaxation peak in their dielectric loss spectra, the peak appears at high frequencies in 1C compared to N and 3C. Moreover, ionic conduction current flows more at high temperatures in 1C than in N or 3C. These facts are ascribable to the difference in their crosslinking densities.
This is the author's final draft of the paper published as Nanotechnology, 2004, 15 (5), pp. 586-595. The final version is available from http://www.iop.org/. Doi: 10.1088/0957-4484/15/5/032. The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated.
Space charge can be measured non-destructively as a standard
practice now. It has been long suspected that space charge might be
critically involved in local dielectric breakdown or tree initiation. It
is widely accepted that space charge is formed under dc stress in
dielectrics. The author demonstrated a hypothetical space
charge-dominant model for tree initiation in 1979 in Japanese and in
1991 in English, and proposed that charge can be accumulated inside
polymer dielectrics near the divergent high field region even under ac
stress. The model consists of a charge injection and extraction process,
a charge trapping and space-charge forming process, a polymer scission
process due to high energy injected and extracted electrons, an
oxidation process, an electric field enhancement process, and a tree
initiation process. The author explains his hypothetical space charge
dominant model for tree initiation, various relevant phenomena and data
that could prove the validity of the model, tries to interpret several
recently discovered phenomena with his model, and reevaluates it as
compared with modern concepts for tree initiation
Applicability of polyethylene polymerized using a metallocene
catalyst to an insulation material for DC power cables has been
investigated. The accumulation properties of space charge in a range
from room temperature to 90°C was examined using the pulse
electroacoustic method. The result indicated that hetero-charge was
observed in the vicinity of the surface of a sample at increased
temperature, and that hetero-space charge produced by activation of an
antioxidant added to the sample was increased or decreased with the
increase of the temperature of the sample. A hetero-charge absorber was
added for controlling the hetero-space charge. As a result,
hetero-charge was not observed on the sample in a high electric field
extending from room temperature to high temperature. It was confirmed
that the volume resistivity of the sample was increased at high
temperature
It is shown that studies of absorption current, TSDC (thermally stimulated discharge current), and TTC (thermal transient current) provide useful information on dielectric polarization in synthetic polymer insulation. Particular results are presented on the time dependence of charging and discharging currents in polypropylene over the temperature range 87-373 K at a constant field of 6.67×107 Vm-1 and thermally stimulated discharge current spectra in polyetherether ketone charged at room temperature in nitrogen gas
Space charge accumulation in low-density polyethylene film containing a small amount of MgO nanoparticles (LDPE/MgO nanocomposite film) subjected to an electric field greater than 100 kV/mm has been studied using an improved pulsed electroacoustic (PEA) system. No marked space charge accumulation was observed in LDPE/MgO nanocomposite films. To determine the mechanism of no space charge accumulation in the LDPE/MgO nanocomposite film, we compared electric potential wells produced by a permanent dipole moment such as that of carbonyl groups (C=0) and an induced dipole consisting of MgO nanoparticles (spherical dielectrics) under a high electric field to create a trapping site for electric charge carriers. The trapping depth created by the permanent dipole moment such as that of the carbonyl groups (C=0) of chemical defects is approximately 0.45 eV. However, the potential well induced by high-permittivity dielectric nanoparticles (MgO) is about 1.5 to 5.0 eV, which is much deeper than that induced by chemical defects. The suppression of space charge formation is explained using the potential well model consisting of a dipole induced by a high-permittivity dielectric nanoparticle. We explained the suppression mechanism of charge accumulation in the LDPE/MgO film that contains deep traps.
DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm.
Polymer nanocomposites possess promising high performances as engineering materials, if they are prepared and fabricated properly. Some work has been recently done on such polymer nanocomposites as dielectrics and electrical insulation. This was reviewed in 2004 based on the literatures published up to 2003. New significant findings have been added since then. Furthermore, a multi-core model with the far-distance effect, which is closely related to an "interaction zones", has been proposed from consideration of mesoscopic analysis of electrical and chemical structures of an existing interface with finite thickness. It is speculatively examined in the paper how the model works for various properties and phenomena already found in nanocomposites as dielectrics focusing on electrical characteristics, resistance to high voltage environment, and thermal properties.
Topics related to space charge phenomenology, particularly the origin of space charge and its interaction with the host insulating polymer, have been widely discussed in recent years. Due to the development of reliable techniques for direct observation of space charge it is now possible to obtain, besides obvious indications on space charge and electric field magnitude and location, quantities that provide additional information on space charge accumulation and depletion rate, apparent mobility and trap depth distribution. Therefore, commonly debated topics such as the nature of charge carriers, the conduction mechanism, the charge dynamics at interfaces and the relation with nanostructure can be re-examined on the basis of space charge observation and this is, indeed, the purpose of this paper.
A multi-core model, i.e. a simplified term of a multi-layered core model, is proposed as a working hypothesis to understand various properties and phenomena that polymer nanocomposites exhibit as dielectrics and electrical insulation. It gives fine structures to what are called "interaction zones". An interfacial layer of several tens nm is multi-layered, which consists of a bonded layer, a bound layer, and a loose layer. In addition, the Gouy-Chapman diffuse layer with the Debye shielding length of several tens to 100 nm is superimposed in the interfacial layer to cause a far-field effect. Nano-particles may interact electrically with the nearest neighbors each other due to this effect, resulting in possible collaborative effect. Such a multi-core model with the far-field effect is discussed, for example, to explain partial discharge (PD) resistance of polyamide layered silicate nanocomposites, and is verified to demonstrate its effectiveness.
It has been known that the electrical breakdown of insulating materials is strongly affected by the formation of space charge in the bulk of the materials. Many researchers have attempted to study the relationship between the space charge and the breakdown; however, it has not been clarified yet. Although the pulsed electroacoustic (PEA) method has been widely used to observe space charge profiles, previous works have not shown clear evidence of the influence of the space charge on the breakdown. Therefore, we have developed a new PEA system with an interval of 0.5 ms to observe the space charge distribution continuously under the ultra-high electric field. Using this system, we observed the space charge dynamics in the low-density polyethylene (LDPE) at and around the breakdown. We also investigated the dependence of the applied electric field on space charge behavior. From the results, it is found that the injected charge packet moved faster and deeper under a relatively lower electric field rather than that under a higher electric field. Furthermore, we found that the maximum electric field in each specimen was almost the same when the breakdown happened in a specimen.
The effects of an antioxidant additive on polymer morphology and space charge formation have been investigated, as a function of temperature, in melt-quenched films of low-density polyethylene. On quenching, the additive-free polymer crystallizes to give extensive spherulitic structures; similar objects are not seen in the system containing the antioxidant. The pulsed electro-acoustic method was then used dynamically to follow both the formation of space charge at high voltage and its subsequent decay under short circuit conditions in both material systems. Data were obtained at room temperature, 50 and 70°C. Comparing the additive free material with identical polymer containing a standard anti-oxidant package indicates that the addition of the antioxidant markedly changes both space charge formation and decay processes. At room temperature, both materials were found to behave similarly, suggesting that the changes in gross morphology described above are of secondary importance. At higher temperatures, the addition of the antioxidant enhances negative charge accumulation in the material, implying its association with relatively deep traps for the negative charges within the bulk.
Measurement of dc charging current and space-charge observations,
performed on specimens made by low-density polyethylene (PE) and
crosslinked PE, are considered for dc electrical threshold
investigation. The threshold is inferred from charging-current
measurements through steady-state voltage-current characteristics, as
well as from the total trapped charge, obtained by space-charge
measurements carried out at 0 V and under short-circuit after poling for
a pre-selected time. It is shown that both techniques provide similar
information regarding the electrical threshold values, which vary for
the two tested materials. Being the threshold associated with mechanisms
of charge storage, its evaluation can help in insulation design and
material characterization when electric field, especially dc, is a
significant stress in service. Other significant information for design
purposes comes from the rate of charge accumulation as function of the
applied field, which is estimated through the charge-field
characteristic derived from space-charge measurements
The packet-like space-charge behaviour in an oxidized crosslinked
polyethylene (XLPE) film doped with antioxidant is discussed through
computer simulation. The model is based on the following assumptions,
(1) both electrons and holes are injected from the electrodes and they
migrate to the opposite electrodes by hopping transport, (2) holes are
also supplied from acceptor sites depending on the field, (3) the holes
are excited to the hopping level only via an intermediate level. In this
model, a space-charge packet is not caused by the excess of mobile
electrons but by the deficiency in mobile holes. The ionized acceptors
left behind by hole migration form an apparent negative space charge
packet and it propagates towards the anode as the high field region
moves
In the last two decades, a significant effort has been directed to
understand the internal charge distributions in dielectrics. This has
resulted in the development of a number of methods which give detailed
information about space charge distributions. This review presents the
evolution of several experimental approaches for the determination of
spatial charge and field distributions in dielectrics, including the
nondestructive methods of direct probing such as the thermal-pulse
methods and the methods using pressure waves that propagate through the
charged sample. Detailed information is also provided on the principles
used in each technique, typical resolution, limitations and advantages
associated with each method. The applications of the methods to several
insulating materials charged with various mechanisms are described. The
paper concludes with a discussion on a number of quite interesting
techniques which seem most promising
An investigation of the effect of trapped space charges on the
aging of polymeric insulating materials subjected to thermo-electrical
stress is reported in this paper. Possible scenarios of degradation
mechanisms, thermally activated, but accelerated by the presence of
space charges, are examined. The model which derives from this approach
has some interesting features: it is characterized by electrical and
thermal thresholds, its parameters have a physical background, it can be
cast into a probabilistic framework. Acceleration of aging due to space
charges is attributed to a reduction of the free-energy barrier to
degradation, seen as a local partially-reversible reaction, which is
caused by energy stored in space-charge centers. The validity of the
model is limited to dc voltage, and to the time of formation of
microcavity-crazes, rather than to breakdown times, since other
mechanisms will occur under electrical field once large enough cavities
are formed in the insulation. The model is applied to the results of
thermo-electrical life tests performed on PET, showing very good
fitting, as well as interesting relationships between parameter
estimates and insulation morphology. It is shown that the model can also
fit well to ac life data, where it takes on a phenomenological meaning
The pulsed electroacoustic (PEA) method is widely known to be an
excellent method for measuring the space charge distribution that
accumulates in thick dielectric materials (1 to 5 mm). We have refined
the PEA measurement system for use on thin dielectric materials. With
FFT deconvolution to compensate for the frequency characteristics of the
piezoelectric sensor, the resolution in the depth direction is improved
to ~3 μm. This improved system is thus expected to be useful for the
quantitative and nondestructive measurement of the space charge
distribution in dielectric films
It is suggested that a major field of study in the future
development of dielectrics will concern their properties when relatively
few molecules are involved. Such smallness arises naturally at
interfaces of nanometric thickness and will occur also when dielectrics
are employed in the nano-technical devices of the future. It already
occurs in living systems where the dielectric and conductive properties
of biomaterials are vital in sustaining activity. The transverse and
lateral properties of interfaces, including the effects of molecular
ordering, are considered and it is suggested that the advent of scanning
tunneling and atomic force microscopies provides a significant
opportunity for nanometric dielectric studies. An important feature,
suggested for future exploitation, is the cross-coupling in interfaces
of force fields arising from electrical, mechanical, chemical and
entropic potential gradients. Application of these concepts to biology
and to the behavior of polymer gels which may lead to development of
muscle-like actuators and transducers are considered. Finally, attention
is drawn to the likely role of nanometric interfacial processes in the
initiation of electrical breakdown in insulating materials