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A
Neagu, E. R., R. M. Neagu, C. J. Dias, Carmo M. Lança, and J. N. Marat-Mendes. "The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact." 356 (2010): 833-837. Abstract
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Neagu, E. R., R. M. Neagu, C. J. Dias, M. C. Lanca, and J. N. Marat-Mendes. "The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact." Journal of Non-Crystalline Solids. 356 (2010): 833-837. AbstractWebsite

The measured isothermal charging and discharging currents are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms through the dielectric materials. Besides a polarization current, there is a current related with charge injection or extraction at the metal-dielectric contact and a reverse current related to the charge trapped into the superficial trap states of the dielectric and that can jump at the interface in a reverse way. By fitting the experimental data, two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms still involved into the transient current and (ii) the height W-0 of the potential barrier at the metal-dielectric interface immediately after the step voltage is applied. Only the initial part of the measured isothermal charging or discharging current can be used to obtain information about the polarization processes. By transforming the time-domain data into the frequency domain, a maximum for the imaginary part of the dielectric permittivity is obtained, in good agreement with the data obtained from AC dielectric measurements and the finally thermally stimulated discharge current measurements. (C) 2009 Elsevier B.V. All rights reserved.

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ER, Neagu, Dias CJ, Lanca MC, Igreja R, Inacio P, and Marat-Mendes JN. "Charge Carriers Injection/Extraction at the Metal-Polymer Interface and Its Influence in the Capacitive Microelectromechanical Systems-Switches Actuation Voltage." JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY. Vol. 10. JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10. Univ Nova Lisboa, 2010. 2503-2511. Abstract
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Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, and J. N. Marat-Mendes. "Charge Carriers Injection/Extraction at the Metal-Polymer Interface and Its Influence in the Capacitive Microelectromechanical Systems-Switches Actuation Voltage." Journal of Nanoscience and Nanotechnology. 10 (2010): 2503-2511. AbstractWebsite

Opposite results concerning the sign of the parasitic charge accumulated at the metal dielectric contact in RF microelectromechanical systems (MEMS) capacitive switches are found in the literature. The mechanism concerning charge injection/extraction at the metal-dielectric contact and its influence on the pull-in voltage needs to be further clarified. A model-switch, for which only one dimension is in the microns range, is used to study the behaviour of a capacitive RF MEMS switch. The aim is to analyze how the electric charge is injected/extracted into or from the dielectric material under the applied field and to obtain realistic data to understand how this parasitic charge influences the pull-in voltage V-pi and the pull-off voltage V-po. A triangle voltage is employed to measure V-pi and V-po by measuring the isothermal charging/discharging currents. Our results demonstrate that V-pi is strongly dependent on the injected/extracted charge on the free surface of the dielectric. The charge injected/extracted at the bottom side of the dielectric has no influence on the actuation voltage. The charge injected/extracted on the free surface of the dielectric determines an increase of the modulus of V-pi and, eventually, the switch can fail to actuate. An estimation of the charge stored into the material was obtained (i) by measuring the charging current and the discharging current and (ii) from the value of the V-pi. The parasitic charge necessary to keep the bridge stick to the insulator is 5.3 x 10(-4) cm(-2) for our experimental conditions. The modification of the V-pi determined by the stored charge in the dielectric is analyzed. An increase of the relative dielectric permittivity by a factor of 2 produces a decrease of the actuation voltage of 10%. A variation of 30% in the elastic constant determines a variation of about 20% in the V-pi. A voltage threshold for charge injection/extraction was not observed.

Freitas, M. C., M. C. Lanca, A. M. Carvalho, and F. Decorte. "CODES TO COMPUTE RELEVANT GAMMA-GAMMA AND GAMMA-X TRUE-COINCIDENCE LINES IN ABSOLUTE COUNTING OF GAMMA-RAYS WITH A LEPD." Biological Trace Element Research. 26-7 (1990): 33-41. AbstractWebsite
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Freitas, M. C., M. C. Lanca, A. M. Carvalho, and F. Decorte. "CODES TO COMPUTE RELEVANT GAMMA-GAMMA AND GAMMA-X TRUE-COINCIDENCE LINES IN ABSOLUTE COUNTING OF GAMMA-RAYS WITH A LEPD." Biological Trace Element Research. 26-7 (1990): 33-41. AbstractWebsite
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MC, Lanca, Neagu ER, and Marat-Mendes JN. "Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene." Journal of Physics D-Applied Physics. 35 (2002): L29-L32. Abstract
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Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes. "Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene." Journal of Physics D-Applied Physics. 35 (2002): L29-L32. AbstractWebsite

A new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.

Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes. "Combined isothermal and non-isothermal current measurements applied to space charge studies in low-density polyethylene." Journal of Physics D-Applied Physics. 35 (2002): L29-L32. AbstractWebsite

A new experimental procedure combining usual isothermal DC charging and discharging with non-isothermal current measurements has been recently proposed. It is mainly suitable for very high insulating polymers and it was successfully applied to the study of space charge trapping and transport in low-density polyethylene. The analysis of the isothermal currents revealed the presence of different traps whose characteristic (de)trapping times can be deduced. The isothermal procedures allowed the selective charging of the sample. By choosing the charging field and the ratio of charge/discharge times, non-isothermal analysis permitted the differentiation of three or four peaks (at approximate to50degreesC, approximate to65degreesC, approximate to70degreesC and approximate to85degreesC) associated with charge detrapping from surface or near-surface (<20 mum) traps. These traps have activation energies between 0.21 and 1.54 eV. The mobility at 30degreesC is around 5 x 10(-16) m(2) V-1 s(-1). Samples had to be conditioned before each experiment in order to obtain reproducible results.

Lanca, M. C.;Dias,;Das C. J. Gupta, D. K.;Marat-Mendes, and J. "Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene." 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena. 2003. 161-164. Abstract
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Lanca, M. C., C. J. Dias, D. K. Dasgupta, J. Marat-Mendes, and Ieee Ieee Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene., 2003. AbstractWebsite

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30 degreesC in the range of 10(-5) Hz to 10(5) Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used. A lock-in amplifier was used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the gamma and beta transitions. This peak decreases with aging disappearing for the most aged samples.

Lanca, M. C., C. J. Dias, D. K. Dasgupta, J. Marat-Mendes, and Ieee Ieee Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene., 2003. AbstractWebsite

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30 degreesC in the range of 10(-5) Hz to 10(5) Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used. A lock-in amplifier was used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the gamma and beta transitions. This peak decreases with aging disappearing for the most aged samples.

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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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., 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.

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ER, Neagu, Neagu RM, Dias CJ, Lanca MC, and Marat-Mendes JN. "The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current." Journal of Applied Physics. 104 (2008). AbstractWebsite
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Neagu, E. R., R. M. Neagu, C. J. Dias, M. C. Lanca, and J. N. Marat-Mendes. "The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current." Journal of Applied Physics. 104 (2008). AbstractWebsite

There is a sustained interest both from theoretical and from practical points of view to understand the isothermal charging and the isothermal discharging currents in dielectrics. The measured currents are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. As long as we do not know the nature of the origin of the current, it is not clear what information we can get by analyzing the experimental data. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current, there is a current related with charge injection or extraction at the metal-dielectric contact and a reverse current related to the charge trapped into the superficial trap states of the dielectric and that can jump at the interface in a reverse way. An analytical expression for the current is proposed. By fitting the experimental data to this analytical equation, two important parameters can be determined: (i) the highest value of the relaxation time for the polarization mechanisms still involved into the transient current and (ii) the height of the potential barrier W-0 at the metal-dielectric interface at the initial time when the step voltage is applied. The value obtained for Al-polyethylene terephthalate interface is (0.43 +/- 0.02) eV. For a charging voltage of 220 V there are 6x10(14) trapped electrons/m(2). (c) 2008 American Institute of Physics.

Neagu, E. R., R. M. Neagu, C. J. Dias, Carmo M. Lanca, and J. N. Marat-Mendes. "The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current." Journal of Applied Physics. 104 (2008). Abstract
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Neagu, E. R., R. M. Neagu, C. J. Dias, Carmo M. Lança, and J. N. Marat-Mendes. "The determination of the pull-in voltage from the condition of bridge stability." 5 (2010): 139-151. 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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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.

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.

MC, Lanca, Dias CJ, Das Gupta DK, and Marat-MendeS J. "Dielectric properties of electrically aged low density polyethylene." Advanced Materials Forum I. Vol. 230-2. Key Engineering Materials, 230-2. 2002. 396-399. Abstract
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