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2007
MC, Lanca, Wirges W, Neagu ER, Gerhard R, and Marat-MendeS J. "Influence of humidity on the electrical charging properties of cork agglomerates." Journal of Non-Crystalline Solids. 353 (2007): 4501-4505. AbstractWebsite
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MC, Lanca, Fu M, Neagu E, Dissado LA, Marat-MendeS J, Tzimas A, and Zadeh S. "Space charge analysis of electrothermally aged XLPE cable insulation." Journal of Non-Crystalline Solids. 353 (2007): 4462-4466. AbstractWebsite
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MC, Lanca, Wirges W, Neagu ER, Gerhard R, and Marat-MendeS J. "Influence of humidity on the electrical charging properties of cork agglomerates." Journal of Non-Crystalline Solids. 353.47-51 (2007). AbstractWebsite

Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)).

Lanca, M. C., W. Wirges, E. R. Neagu, R. Gerhard, and J. Marat-Mendes. "Influence of humidity on the electrical charging properties of cork agglomerates." Journal of Non-Crystalline Solids. 353 (2007): 4501-4505. AbstractWebsite

Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.

Lanca, M. C., M. Fu, E. Neagu, L. A. Dissado, J. Marat-Mendes, A. Tzimas, and S. Zadeh. "Space charge analysis of electrotherinally aged XLPE cable insulation." Journal of Non-Crystalline Solids. 353 (2007): 4462-4466. AbstractWebsite

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing. (c) 2007 Elsevier B.V. All rights reserved.

MC, Lanca, Fu M, Neagu E, Dissado LA, Marat-MendeS J, Tzimas A, and Zadeh S. "Space charge analysis of electrothermally aged XLPE cable insulation." Journal of Non-Crystalline Solids. 353.47-51 (2007). AbstractWebsite

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.

2006
MC, Lanca, Neagu ER, Dissado LA, and Marat-MendeS J. "Space charge studies in XLPE from power cables using combined isothermal and thermostimulated current measurements." Advanced Materials Forum Iii, Pts 1 and 2. Vol. 514-516. Materials Science Forum, 514-516. 2006. 935-939. Abstract
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MC, Lanca, Neagu ER, Silva P, Gil L, and Marat-MendeS J. "Study of electrical properties of natural cork and two derivative products." Advanced Materials Forum Iii, Pts 1 and 2. Vol. 514-516. Materials Science Forum, 514-516. 2006. 940-944. Abstract
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Lanca, M. C., E. R. Neagu, L. A. Dissado, and J. Marat-Mendes. "Space charge studies in XLPE from power cables using combined isothermal ans thermostimulated current measurements." Advanced Materials Forum Iii, Pts 1 and 2. Ed. P. M. Vilarinho. Vol. 514-516. Materials Science Forum, 514-516. 2006. 935-939. Abstract

Cross-linked polyethylene (XLPE) peelings from aged power cables from three different sources were studied using a combined procedure of isothermal and thermo-stimulated current measurements. Different parameters, such as electric field, temperature, charging/discharging times, can be selected in order to make an analysis of the space charge characteristics (such as, relaxation times and activation energies). Three different cables peelings were analyzed: A - electrically aged in the laboratory at high temperature, B - service aged for 18 years and C - thermally aged in the laboratory at high temperature. The results were compared for the different types of samples and also with previous results on laboratory aged and produced films of low-density polyethylene (LDPE) and XLPE.

Lanca, M. C., E. R. Neagu, P. Silva, L. Gil, and J. Marat-Mendes. "Study of electrical properties of natural cork and two derivative products." Advanced Materials Forum Iii, Pts 1 and 2. Ed. P. M. Vilarinho. Vol. 514-516. Materials Science Forum, 514-516. 2006. 940-944. Abstract

Electrical properties of natural cork, commercial cork agglomerates (for floor and wall coverings) and a recently developed composite of cork/TetraPak (R) were studied. Measurements of isothermal charge and discharge currents were made for natural cork samples in different directions (axial, radial and tangential cuts). The isothermal current characteristics and the samples conductivity were investigated under different conditions (electric field, temperature and environmental conditions: in air at ambient relative humidity (RH), dry air and vacuum), also the samples could be or not conditioned (dried in vacuum or in a P2O5 atmosphere at room temperature). From these results the influence of water on the electrical properties of natural cork could be seen. In order to compare the three different cork materials a preliminary study was made. Isothermal charge and discharge currents and conductivity after 1h charging were measured and compared for different electric fields and temperature in air at ambient RH.

2005
Neagu, E. R.;Neagu, R. M.;Lanca, M. C.;Vassilikou-Dova, A.;Marat-Mendes, and J. N. "Identification of an apparent peak by use of the final thermally stimulated discharge current technique." 12th International Symposium on Electrets. 2005. 296-299. Abstract
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MC, Lanca, Neagu ER, and Marat-Mendes JN. "Space charge studies of aged XLPE using combined isothermal and thermostimulated current measurements." Cross-Disciplinary Applied Research in Materials Science and Technology. Vol. 480-481. Materials Science Forum, 480-481. 2005. 501-505. Abstract
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Neagu, E. R.;Neagu, R. M.;Lanca, M. C.;Marat-Mendes, and J. N. "The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements." 12th International Symposium on Electrets. 2005. 292-295. Abstract
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Lanca, M. C., M. Fu, E. Neagu, L. A. Dissado, J. Marat-Mendes, A. Tzimas, and S. Zadeh Comparative study of space charge in the polymeric insulation of power cables using PEA, isothermal and non-isothermal currents measurements., 2005. AbstractWebsite

An understanding of space charge build-up in the polymeric insulation of power cables is important in determining how aging occurs and progresses and, also in predicting cable lifetime. In this investigation electric-field induced space charge in peelings from XLPE (cross-linked polyethylene) cables was measured using two different methods: the pulsed electro-acoustic technique (PEA) and the combined procedure of isothermal and non-isothermal charging/discharging currents (FTSDC). These two methods allow the study of space charge in highly insulating materials. Also, since electric fields of different orders of magnitude are applied to the sample in the two methods, it is possible to analyze different characteristics of the space charge traps. Prior to the measurements the samples were subjected to conditioning to remove volatiles. Cable peelings from various brands aged under different conditions (including field aged and thermally aged samples) were studied as received from the manufacturers. Some of the samples have undergone further ageing in AC electric field (50Hz) for 1000h to see the influence of further ageing on space charge build-up. The results for the different types of samples are compared in an attempt to correlate different ageing parameters.

Neagu, E. R., R. M. Neagu, M. C. Lanca, A. Vassilikou-Dova, and J. N. Marat-Mendes Identification of an apparent peak by use of the final thermally stimulated discharge current technique., 2005. AbstractWebsite

The final thermally stimulated discharge current (FTSDC) technique can be used to analyze charge trapping and transport in insulating materials. The experimental conditions can be selected so that the FTSDC is mainly determined by the space charge detrapping. Measurements of the FTSDC in a wide temperature range including the local (secondary) beta relaxation and the non-local (primary) cc relaxation, for different polymers, demonstrate the existence of an apparent peak. The shift of peak temperature T-m with respect to the charging temperature T-p is analyzed. The interval T-m - T-p decreases from about 25 K to zero, as T-p approaches the glass transition T-g. T-m - T-p is lower for materials of lower conductivity. The peak width at the half maximum intensity decreases as Tp increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and detrapping processes are affected by the strong thermal motion.

Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes. "Space charge studies of aged XLPE using combined isothermal and thermostimulated current measurements." Cross-Disciplinary Applied Research in Materials Science and Technology. Ed. A. MendezVilas. Vol. 480. Materials Science Forum, 480. 2005. 501-505. Abstract

Space charge in electrically aged cross-linked polyethylene (XLPE) was studied using a procedure combining isothermal and non-isothermal measurements of charge and discharge currents. Aging is carried out using an AC field while immersing the disk-shaped samples in an ionic aqueous solution at constant temperature. After aging the samples were isothermally DC charged and discharged. Next a non-isothermal experiment with constant heating rate was performed (FTSDC). Finally the sample was kept at the highest temperature in order to completely discharge the polymer. The space charge introduced in the XLPE during aging can be analyzed from the study of the FTSDC spectra. The thermogram (FTSDC) shows a very broad peak. The peak is attributed to trapped space charge in traps with long relaxation times. It is possible to decompose it into three or four individual peaks and obtain the corresponding activation energies. The results were compared with previous ones obtained for LDPE (low density polyethylene) aged under similar conditions.

Neagu, E. R., R. M. Neagu, M. C. Lanca, and J. N. Marat-Mendes The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements., 2005. AbstractWebsite

The experimental results obtained in a wide range of temperatures, for polyethylene terephthalate, demonstrate that the apparent activation energy changes when the charging (polarization) time or the isothermal discharging time, prior to the final thermally stimulated discharge current measurement, are used as variable parameters. Consequently, the charging and/or discharging time can be used as a variable parameter to investigate the landscape of the apparent thermal activation energies. A continuous distribution of the traps in the range from 0.4 to 3 eV was observed. The experimental results demonstrate that there is a range of experimental conditions for which the thermal activation energy is independent of the experimental parameter values. This is the activation energy value which should be used to characterize a certain mechanism.

Neagu, E. R., R. M. Neagu, M. C. Lanca, and J. N. Marat-Mendes. "The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements." 12th International Symposium on Electrets (ISE 12), Proceedings (2005): 292-295. AbstractWebsite

The experimental results obtained in a wide range of temperatures, for polyethylene terephthalate, demonstrate that the apparent activation energy changes when the charging (polarization) time or the isothermal discharging time, prior to the final thermally stimulated discharge current measurement, are used as variable parameters. Consequently, the charging and/or discharging time can be used as a variable parameter to investigate the landscape of the apparent thermal activation energies. A continuous distribution of the traps in the range from 0.4 to 3 eV was observed. The experimental results demonstrate that there is a range of experimental conditions for which the thermal activation energy is independent of the experimental parameter values. This is the activation energy value which should be used to characterize a certain mechanism.

2004
Lanca, M. C.;Neagu, E. R.;Marat-Mendes, and J. N. "Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene." Proceedings of the 2004 Ieee International Conference on Solid Dielectrics, Vols 1 and 2. 2004. 209-212. Abstract
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Lanca, M. C.;Neagu, E. R.;Marat-Mendes, and J. N. "Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene." Proceedings of the 2004 Ieee International Conference on Solid Dielectrics, Vols 1 and 2. 2004. 209-212. Abstract
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MC, Lanca, and Marat-MendeS J. "Dielectric breakdown statistics of polyethylene for progressively-censored data." Advanced Materials Forum Ii. Vol. 455-456. Materials Science Forum, 455-456. 2004. 602-605. Abstract
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MC, Lanca, Neagu ER, Neagu RM, Dias CJ, Marat-Mendes JN, and Das-Gupta DK. "Space charge studies in LDPE using combined isothermal and non-isothermal current measurements." IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION. 11 (2004): 25-34. Abstract
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Lanca, M. C., E. R. Neagu, J. N. Marat-Mendes, and Ieee Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene., 2004. AbstractWebsite

Polyethylene is one of the most widely used polymeric insulators in medium and high voltage power cables. However the importance of space charge distribution and its influence on the electrical aging in this polymer is not fully understood. The very good insulating properties of the material implying very long relaxation times (few days and even longer are usual) and low currents (few pA or below) make individual measurements of isothermal charge/discharge currents and thermostimulated currents difficult to analyze and reproduce. A single type of measurements does not take into account the space charge that remains trapped for long times. A combined procedure of isothermal and non-isothermal current measurements developed for high insulating polymers was used for low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films electrically aged. The press-molded LDPE and XLPE films were electrically aged under similar conditions using an AC electric field while immersed in a sodium chloride aqueous solution at constant temperature (electro-thermal aging). The use of the combined procedure for current measurement allowed obtaining information about space charge traps, activation energies and relaxation times for both LDPE and XLPE. This data was used to compare electrical aging under similar conditions for the two types of polyethylene.

Lanca, M. C., E. R. Neagu, J. N. Marat-Mendes, and Ieee Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene., 2004. AbstractWebsite

Polyethylene is one of the most widely used polymeric insulators in medium and high voltage power cables. However the importance of space charge distribution and its influence on the electrical aging in this polymer is not fully understood. The very good insulating properties of the material implying very long relaxation times (few days and even longer are usual) and low currents (few pA or below) make individual measurements of isothermal charge/discharge currents and thermostimulated currents difficult to analyze and reproduce. A single type of measurements does not take into account the space charge that remains trapped for long times. A combined procedure of isothermal and non-isothermal current measurements developed for high insulating polymers was used for low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films electrically aged. The press-molded LDPE and XLPE films were electrically aged under similar conditions using an AC electric field while immersed in a sodium chloride aqueous solution at constant temperature (electro-thermal aging). The use of the combined procedure for current measurement allowed obtaining information about space charge traps, activation energies and relaxation times for both LDPE and XLPE. This data was used to compare electrical aging under similar conditions for the two types of polyethylene.

Lanca, M. C., and J. Marat-Mendes. "Dielectric breakdown statistics of polyethylene for progressively-censored data." Advanced Materials Forum Ii. Eds. R. Martins, E. Fortunato, I. Ferreira, and C. Dias. Vol. 455-456. Materials Science Forum, 455-456. 2004. 602-605. Abstract

The dielectric breakdown of thin films of low-density polyethylene (LDPE) electrically aged in an aqueous solution of NaCl under an AC electric field was investigated. A two-parameter Weibull function was used for the dielectric breakdown time to failure. The probability of failure for a sample was obtained by the White method for progressively censored data. Samples aged at different temperatures were compared. The results show that initially the samples aged at lower temperature (approximate to25degreesC) are more prone to fail, while those aged at higher temperature (50degreesC) fail at longer times. This was attributed to a competition between oxidation and diffusion.