Conductivity Of Low-density Polyethylene Research Articles

Explanation of the Compensation Law and the Isokinetic Point in the Electrical Conduction of Crosslinked Polyethylene

Thermally activated direct current (DC) electrical conductivity in low-density polyethylene (LDPE) is known to be subject to the compensation law. Accordingly, the preexponential factor follows a specific relation with activation energy, reducing overall changes in conductivity. This relationship is governed by the Meyer-Neldel temperature. However, there is no published evidence for a corresponding isokinetic point, a temperature where the conductivity of all LDPE samples is the same. Here, it is determined that the compensation law applies to both DC and alternating current (AC) conduction for LDPE and for crosslinked polyethylene (XLPE) without an observed isokinetic point. The potential origins of compensation in polyethylene are discussed as well as reasons for similarity between LDPE and XLPE. It is observed that prolonged water exposure removed the compensation behavior. Meanwhile, preheating samples in the oven prior to measurements modifies the compensation behavior and reduced the spread around the isokinetic point. It is thus deduced that an isokinetic point can be observed in polyethylene but is obscured by contributions from water and other impurities.

DC Conductivity Measurements of LDPE: Influence of Specimen Preparation Method and Polymer Morphology

<p>DC conductivity measurements are important for gaining fundamental understanding of conduction mechanisms of insulation materials, as well as in the development of HVDC power system components, such as extruded cable systems. In this study, the influence of sample processing on the morphology and DC conductivity of low-density polyethylene (LDPE) has been studied. Direct compression moulding of LDPE pellets is commonly used in research laboratories for obtaining plaque samples, whereas extrusion is an additional commonly used technique for dispersion of particles in nanocomposites prior to the compression moulding process. In this study LDPE plaques have been obtained by either compression moulding directly from pellets, or by extrusion followed by compression moulding. The morphology obtained in the first case consisted of banded spherulites, whereas the latter method yielded a morphology of small axialites. The difference in sample processing had also an impact on the DC conductivity. The DC conductivity at 22 ­°C and 3.3 kV/mm was of the order of 4E-18 S/m for the plaques obtained by extrusion and compression moulding whereas the plaques obtained by direct compression moulding exhibited a conductivity of 1E-16 S/m. In addition, the reproducibility of the performed DC conductivity measurements was also verified in a round robin test performed between the Royal Institute of Technology and <br />Chalmers Technical University.</p>

DC Conductivity Measurements of LDPE: Influence of Specimen Preparation Method and Polymer Morphology

DC conductivity measurements are important for gaining fundamental understanding of conduction mechanisms of insulation materials, as well as in the development of HVDC power system components, such as extruded cable systems. In this study, the influence of sample processing on the morphology and DC conductivity of low-density polyethylene (LDPE) has been studied. Direct compression moulding of LDPE pellets is commonly used in research laboratories for obtaining plaque samples, whereas extrusion is an additional commonly used technique for dispersion of particles in nanocomposites prior to the compression moulding process. In this study LDPE plaques have been obtained by either compression moulding directly from pellets, or by extrusion followed by compression moulding. The morphology obtained in the first case consisted of banded spherulites, whereas the latter method yielded a morphology of small axialites. The difference in sample processing had also an impact on the DC conductivity. The DC conductivity at 22 ­ºC and 3.3 kV/mm was of the order of 4xE-18 S/m for the plaques obtained by extrusion and compression moulding whereas the plaques obtained by direct compression moulding exhibited a conductivity of 1xE-16 S/m. In addition, the reproducibility of the performed DC conductivity measurements was also verified in a round robin test performed between the Royal Institute of Technology and Chalmers Technical University.

On the mechanism of charge transport in low density polyethylene

Polyethylene based polymeric insulators, are being increasingly used in the power industry for their inherent advantages over conventional insulation materials. Specifically, modern power cables are almost made with these materials, replacing the mass-impregnated oil-paper cable technology. However, for ultra-high dc voltage applications, the use of these polymeric cables is hindered by ununderstood charge transport and accumulation. The conventional conduction mechanisms (Pool-Frenkel, Schottky, etc.) fail to track high-field charge transport in low density polyethylene, which is semi-crystalline in nature. Until now, attention was devoted mainly to the amorphous region of the material. In this paper, authors propose a novel mechanism for conduction in low density polyethylene, which could successfully track experimental results. As an implication, a novel, substantial relationship is established for electrical conductivity that could be effectively used for understanding conduction and breakdown in polyethylene, which is vital for successful development of ultra-high voltage dc cables.

Research on DC dielectric properties of polyaniline nanofibers/LDPE composites

In this article, polyaniline (PANI) nanofibers were prepared and added to low-density polyethylene (LDPE) to produce PANI nanofibers/LDPE composites. LDPE and the composites were tested for direct current (DC) conductivity, breakdown strength, and space charge characteristics. The results suggested that DC breakdown strength of PANI nanofibers/LDPE composites significantly declined once PANI was added, and the decline was more evident with the increase of PANI nanofibers. Meanwhile, the addition of PANI nanofibers contributed to a decrease in the conductivity of LDPE. As the content of PANI nanofibers increased, the conductivity of the composites declined first and then raised. DC conductivity properties of LDPE could be improved by adding an appropriate amount of PANI nanofibers. Compared with LDPE, the space charge distribution was changed in LDPE due to the addition of PANI nanofibers. With the increase of content of PANI nanofibers, the amount of space charges close to the electrodes decreased gradually.

Steady-state DC conductivity of low density polyethylene with deep chemical impurity

This paper presents the effect of deep chemical impurities on the conductivity of insulating polymers. Although plenty of literature available to give conductivity model for insulating polymers but still there is a need of more accurate model. The authors try to propose a new theoretical model for mobility of electrons in order to understand the behaviour of electron towards deep chemical impurities at atomic level using quantum mechanical approach. Our model shows that chemical impurity plays an important role on conductivity of polyethylene.

Conductivity and space charge in LDPE/BaSrTiO3nanocomposites

Nano-sized BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles and a dispersant were incorporated in samples of low density polyethylene (LDPE). The nanoparticle loading was 2% or 10% by weight, and the dispersant loading was 4 parts per hundred relative to BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . dc conductivity measurements were made in the temperature range 30-70°C in vacuum, and in air at 30 °C. The vacuum dc conductivity in LDPE with 0.4% dispersant was approximately five times larger than in LDPE, but in LDPE containing 0.4% dispersant and 10% nanoparticles it was approximately a factor of 6-7 smaller than in LDPE. Since the conductivity of the latter composite was smaller than that of its least conductive component, it may be that the interface regions between the nanoparticles and the LDPE control the dc conductivity. The dispersant and the nanoparticles both increased ε', the real part of the relative permittivity. The ε' values for the nanoparticles, implied by the Lichtenecker-Rother equation for a three-part composite, were of order several hundred, much smaller than the value of at least 10,000 quoted by the nanoparticle suppliers. Space charge measurements were made in air using the Laser Intensity Modulation Method (LIMM), and the profiles calculated using the LIMM Monte Carlo method. The maximum space charge density adjacent to the negative poling electrode was 600 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , for the pure LDPE and the LDPE with dispersant, and 200 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> near the positive electrode. The corresponding maxima in samples with dispersant and nanoparticles were approximately an order of magnitude smaller. The Monte Carlo method is more accurate than other methods of analyzing LIMM data, and this may account for the very large densities close to the electrodes.

Temperature and Electric Field Dependence of Conduction in Low-Density Polyethylene

A traditional constant voltage conductivity test method was used to measure how the conductivity of highly insulating low-density polyethylene (LDPE) polymer films depends on applied electric field, repeated and prolonged electric field exposure, and sample temperature. The strength of the applied voltage was varied to determine the electric field dependence. At low electric field, the resistivity was measured from cryogenic temperatures to well above the glass transition temperature. Comparisons were made with a variety of models of the conduction mechanisms common in insulators, including transient polarization and diffusion and steady-state thermally activated hopping conductivity and variable range hopping conductivity, to determine which mechanisms were active for LDPE and to provide a better picture of its electrical behavior.

Influence of antioxidants on electrical conduction in LDPE and XLPE

In order to study the effect of antioxidants on high-field electrical conduction of cross-linked polyethylene (XLPE), measurements were made of conduction currents flowing in XLPE specimens containing phenolic and sulfur-type antioxidants. Currents flowing in XLPE insulation containing these antioxidants were found to be less than those flowing in XLPE containing no antioxidants. The smallest detected current occurred in XLPE insulation containing the sulfur-type antioxidant dilauryl thiodipropionate, whereas larger currents were detected in XLPE containing phenolic antioxidants. It was also found that the magnitude of conduction current in XLPE samples containing the sulfur-type antioxidant depended on the concentration of the added antioxidant. The degree of measured crystallinity of XLPE was also found to be less than that of low-density polyethylene (LDPE), while the crystallinity was not found to be influenced by added antioxidants.

Electric Charge Trapping and Transport at Medium Fields in Low-Density Polyethylene

The transient conductivity in low-density polyethylene is studied. Isochronal currentvoltage measurements for 1800 s and 1 day time intervals are carried out under dry N2 atmosphere. When after every measurement the sample is fully discharged at high temperature the isochronal current - voltage characteristic reveals an ohmic behavior. When the next field increase is applied without sample discharging the current-voltage characteristic is super-quadratic. We explain this increase of the current assuming that a fraction of the previous injected charge is detrapped by the field and it contributes to current increase. Consequently the current – voltage characteristic is strongly dependent on the time lag between two successive rises in the field. Neither the Poole-Frenkel mechanism nor the Richardson – Schottky mechanism can by used to explain the experimental results. The isothermal charging and discharging currents are explained assuming the movement of injected/ejected charge in the resultant local field. The values obtained for the adjustable parameters of the model are in good agreement with the values in the literature.

Electrical conduction and space charge trapping in highly insulating materials

The electrical conduction in low-density polyethylene (LDPE) was investigated in the temperature range from 20°C to 95°C and for applied fields up to 12 MV m−1. The objective was to study the effects of the space charge (SC) accumulated in the material on the low and medium field quasi-steady-state conductivity in LDPE. Very long time (a week or longer) isothermal current measurements were carried out to focus on the situation when the SC effects are dominant. Complementary results were obtained using the isothermal discharge current, the final thermally stimulated discharge current and the isothermal final discharge current measurements. The results are explained by taking into account the constraint imposed by the trapped SC on charge injection, trapping and transport processes.

Space-charge-controlled conductivity in low-density polyethylene

We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.

Influence of annealing treatment on the electrical conductivity of low density polyethylene

AbstractThe results obtained with films of a low density polyethylene subjected to variations in temperature from ambient to 60 °C and in an applied field from low values of around 1 V µm−1 to high values (about 50 V µm−1) are presented. To identify the probable conduction mechanism in such films, a model based on the proposition of Nath and Perlman is investigated which takes into account the effects of space charge and Poole field‐assisted detrapping. We observe a relatively good agreement in measurements of current as a function of voltage and temperature. This allows us to evaluate a parameter related to trap characteristics (density, energy level, trap‐site separation, etc). Complementary measurements by differential scanning calorimetry show that the structure of the films is modified by temperature variations within the range used to study the conduction mechanism. These structural changes are then correlated with the experimental parameter obtained from Nath's model.© 2001 Society of Chemical Industry

The effect of low-molecular-weight species on space charge and conduction in LDPE

We have investigated the effect of low-molecular mass species on the space charge behavior such as space charge distribution and electrical conduction of LDPE (low density polyethylene). We also attempted to explain the relationship between the space charge distribution and electrical conduction. The heterocharge, conduction current and effective charge mobility decreased when the low-molecular-mass species in LDPE was removed. It was found that the decrease in conduction current was related to the decrease of heterocharge in LDPE.

Effects of molecular structure on electrical conduction in low-density polyethylene above its melting point

The electrical conduction of various kinds of low-density polyethylene (LDPE) has been studied above the melting point. LDPEs are characterized by the amount and types of branches, double bonds, and oxygen-containing groups. Two components of conduction currents were found: one obeyed Ohm's law in the low field range and the other was proportional to the square of the field at lower temperatures in the high field range, and was also inversely proportional to sample thickness for a constant field. The conduction mechanism of the latter component is ascribed to space charge limited current (SCLC). Among the features of the LDPE molecular structure, only the oxygen-containing groups were well correlated with the SCLC. In fact, the oxygen-containing groups reduced the SCLC, suggesting that they act as traps even in the molten state. Branches and double bonds are also known to act as traps in the solid state, but they bore no consistent relation to the electrical conduction in the molten state.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal conductivity of low density polyethylene filled with carbon black.

Thermal conductivity of low density polyethylene (LDPE) filled with carbon black (CB) was measured as a function of CB fraction. The thermal conductivity increases with increasing the weight fraction of CB. The particle size and the dispersion state of CB also affected the thermal conductivity. In order to study the thermal conductivity at high CB fraction, liquid paraffin (LPA) was used instead of LDPE. The thermal conductivity of the complex of CB and LPA exhibited a maximum value at the CB weight fraction of 0.5. The increase of the thermal conductivity with the CB weight fraction came from the filling effect of the CB, and the decrease came from mixing of air. In order to interpret these experimental facts, three-layers model consisting of CB, PE and air was proposed. The results calculated from the model agreed well with the exprimental results.

Simultaneous thermally stimulated luminescence and conductivity in low-density polyethylene

Thermally stimulated luminescence (TSL) and conductivity (TSC) in low-density polyethylene samples were measured simultaneously, while the sample was being heated to room temperature after X-ray irradiation at -190 degrees C. The TSC curves of samples from which absorbed air had been removed showed five peaks, labelled C1-C5 in order of increasing temperature, while the TSL glow curves showed only three peaks (L1-L3). The effects on these peaks of (i) immersing the samples in fuming nitric acid, (ii) annealing them in vacuum, and (iii) exposing them to an oxygen/ozone mixture, were investigated. It is concluded that the C1 and L1 peaks are associated with the same traps, these traps being formed by the polymer chain themselves in the chain-fold regions of the sample, and being broken up by the onset of side-chain motion. The same is true for the C2 and L2 peaks, except that a different set of traps and 'crankshaft' motion are involved. There is no obvious correlation between the L3 peak and any of the TSC peaks; the L3 traps are also formed in the chain-fold regions, and are broken up by the glass transition. The C3, C4 and C5 traps are probably structural defects of various kinds in the crystalline regions of the samples, from which electrons escape by thermal excitation. When the samples contain absorbed air, a hitherto unreported peak is observed in the TSC curve, corresponding to the TSL 'gas' peak.

Electric conduction of low density polyethylene induced by high dose rate gamma rays

Radiation induced conduction in low density polyethylene under gamma-ray irradiation at high dose rates, 1×103 to 1×104 Gy/h, was studied. After irradiation was initiated, the induced current quickly plateaus, but then subsequently begins to increase slowly with time until it attains an equilibrium value. The effect of the polarity and magnitude of the electric field and of the temperature upon time dependence of the induced current were investigated. The slowly increasing induced current can be explained by thermal detrapping from relatively deep traps; the trap depth is calculated to be ca. 0.8 eV. The induced current of the sample increases as its crystallinity increases. It was revealed that a sample with high crystallinity has shallow traps and consequently has many charge carriers and increases its induced current.

On the determination of electrical conductivity in polyethylene

There are significant differences in the field dependence of conductivity in low-density polyethylene as reported by various investigators. These differences are attributable partly to the presence of different kinds of impurities in variable concentrations in polyethylene samples of various origin, but are mainly due to the different measurement procedures adopted. The significance of the time which elapses after the application of voltage to the electrodes, as well as the effect of conditioning, e.g. thermal and voltage stressing of the specimen before and during the measurement, is discussed. Whether it is always necessary to obtain a steady-state current in order to determine the true conductivity is somewhat debatable.