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Lanca, M. C., I. Franco, J. M. Mendes, and Ieee, SIMULATION OF THE FERROELECTRIC SWITCHING OF PVDF AND ITS COPOLYMERS, , pp. 62-66, 1992. AbstractWebsite
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Lanca, M. C., I. Franco, J. M. Mendes, and Ieee, SIMULATION OF THE FERROELECTRIC SWITCHING OF PVDF AND ITS COPOLYMERS, , pp. 62-66, 1992. AbstractWebsite
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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, vol. 353, no. 47-51, pp. 4462-4466, 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. (c) 2007 Elsevier B.V. All rights reserved.

MC, L., F. M, N. E, D. LA, M. - M. S. J, T. A, and Z. S, "Space charge analysis of electrothermally aged XLPE cable insulation", Journal of Non-Crystalline Solids, vol. 353, pp. 4462-4466, Jan, 2007. AbstractWebsite
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MC, L., F. M, N. E, D. LA, M. - M. S. J, T. A, and Z. S, "Space charge analysis of electrothermally aged XLPE cable insulation", Journal of Non-Crystalline Solids, vol. 353, issue 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.

Neagu, E. R., M. C. Lanca, C. J. Dias, and J. N. Marat-Mendes, "Space Charge and Dipolar Charge Contribution at Polar Polymers Polarization", Ieee Transactions on Dielectrics and Electrical Insulation, vol. 22, no. 3, pp. 1419-1426, 2015. AbstractWebsite
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MC, L., N. ER, N. RM, D. CJ, M. - M. JN, and D. - G. DK, "Space charge studies in LDPE using combined isothermal and non-isothermal current measurements", IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, vol. 11: Univ Wales, Univ Nova Lisboa, pp. 25-34, Jan, 2004. Abstract
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Lanca, M. C., E. R. Neagu, R. M. Neagu, C. J. Dias, J. N. Marat-Mendes, and D. K. Das-Gupta, "Space charge studies in LDPE using combined isothermal and non-isothermal current measurements", Ieee Transactions on Dielectrics and Electrical Insulation, vol. 11, no. 1, pp. 25-34, 2004. AbstractWebsite

Using a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

Lanca, M. C., E. R. Neagu, R. M. Neagu, C. J. Dias, J. N. Marat-Mendes, and D. K. Das-Gupta, "Space charge studies in LDPE using combined isothermal and non-isothermal current measurements", Ieee Transactions on Dielectrics and Electrical Insulation, vol. 11, no. 1, pp. 25-34, 2004. AbstractWebsite

Using a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

MC, L., N. ER, D. LA, and M. - M. S. 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, no. 514-516, pp. 935-939, Jan, 2006. 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, vol. 514-516, pp. 935-939, 2006. 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.

MC, L., N. ER, and M. - M. 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, no. 480, pp. 501-505, Jan, 2005. Abstract
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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, vol. 480, pp. 501-505, 2005. 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.

Lanca, M. C.;Neagu, E. R.;Marat-Mendes, and J. N., Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, Jan, 2002. Abstract
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Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes, Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, 2002. AbstractWebsite

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes, Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, 2002. AbstractWebsite

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

MC, L., N. ER, S. P, G. L, and M. - M. S. J, "Study of electrical properties of natural cork and two derivative products", Advanced Materials Forum Iii, Pts 1 and 2, vol. 514-516, no. 514-516, pp. 940-944, Jan, 2006. Abstract
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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, vol. 514-516, pp. 940-944, 2006. 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.

Lanca, M. C., J. Domingues, and I. Franco, Study of fractal properties in Lichtenberg figures, , pp. 133-144, 1995. AbstractWebsite
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Lanca, M. C., J. Domingues, and I. Franco, Study of fractal properties in Lichtenberg figures, , pp. 133-144, 1995. AbstractWebsite
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Neagu, E. R., C. J. Dias, M. C. Lança, and J. N. Marat-Mendes, The study of molecular movements in dielectrics using isothermal and non- isotehermal current measurements, , vol. 183, pp. –-, Jan, 2009. Abstract
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Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, J. N. Marat-Mendes, and Ieee, "The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique", Proceedings of the 2010 Ieee International Conference on Solid Dielectrics (Icsd 2010), 2010. Abstract
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Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, J. N. Marat-Mendes, and Ieee, "The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique", Proceedings of the 2010 Ieee International Conference on Solid Dielectrics, 2010. Abstract

The electrical methods used to study the molecular movements are based on the movement of the dipoles under DC or AC electric field. We have proposed recently a combined measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in polar or non-polar dielectric materials. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 572 K. A strong relaxation was observed around 402 K and a very weak relaxation around 345 K. This is the beta relaxation which is quite complex. As concern the behavior at high temperatures, above the beta relaxation, a high peak was observed that shifts continuously to higher temperatures as the charging temperature and/or the charging field increase. The maximum current of the peak increases and the temperature corresponding to the maximum current increases as the charging temperature and/or the charging field increase, given a direct observation of the so called cross-over effect related to current decay for sample charged at high fields and/or high temperatures.