R., N. E., C. Dias, L. M. Carmo, I. R., I. P., and M. - M. J. N.,
Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers,
, pp. 145-146, Jan, 2011.
Abstractn/a
Lança, C. M., E. R. Neagu, C. Dias, and J. Marat-Mendes,
Dielectric spectra of natural cork and derivatives,
, vol. 356, pp. 763-767, Jan, 2010.
Abstractn/a
Lanca, M. C., M. Brandt, E. R. Neagu, C. J. Dias, and J. N. Marat-Mendes,
"Dielectric spectra of natural cork and derivatives",
Journal of Non-Crystalline Solids, vol. 356, no. 11-17, pp. 763-767, 2010.
AbstractCork is a cellular biomaterial that has unique characteristics that make it suitable for many types of applications. Since it is also an electrical insulator, the study of its electrical and dielectric properties can lead to new interesting applications. The moisture present in cork and derivatives has a very important role on the dielectric properties. In this work a composite made of both recycled cork and TetraPak (R) used containers was studied and compared with other cork products. The dielectric relaxation spectra of natural cork (as received), commercial cork agglomerate and of a composite cork/Tetrapak (R) was investigated in the temperature range of -50 to 120 degrees C and in the frequency range of 10(-1) Hz-2 MHz. For some samples of the composite a small amount of paraffin was added. The highest values for the imaginary part of the dielectric permittivity were found for the commercial material and the composite without paraffin. The lowest was found for the cork/TetraPak (R)/paraffin composite. The influence of humidity content was investigated for the composite with wax. Natural cork shows a peak around 80 degrees C (not seen in the derivative materials). The commercial agglomerate and the cork/TetraPak (R)/paraffin composite show a peak around 40-50 degrees C. In the composite this peak becomes smaller as humidity is removed. (C) 2009 Elsevier B.V. All rights reserved.
MC, L., D. CJ, D. G. DK, and M. - M. S. J,
"Dielectric properties of electrically aged low density polyethylene",
Advanced Materials Forum I, vol. 230-2, no. 230-232, pp. 396-399, Jan, 2002.
Abstractn/a
Lanca, M. C., C. J. Dias, D. K. Dasgupta, and J. Marat-Mendes,
"Dielectric properties of electrically aged low density polyethylene",
Advanced Materials Forum I, vol. 230-2, pp. 396-399, 2002.
AbstractLow density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 degreesC in the range of 10(-5) Hz to 105 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to gamma and beta transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.
Lanca, M. C., C. J. Dias, D. K. Dasgupta, and J. Marat-Mendes,
"Dielectric properties of electrically aged low density polyethylene",
Advanced Materials Forum I, vol. 230-2, pp. 396-399, 2002.
AbstractLow density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 degreesC in the range of 10(-5) Hz to 105 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10(-5) Hz to 10(-1) Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10(-1) Hz to 10(1) Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10(-1) Hz to 10(5) Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to gamma and beta transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.
Lanca, M. C., and J. Marat-Mendes,
"Dielectric breakdown statistics of polyethylene for progressively-censored data",
Advanced Materials Forum Ii, vol. 455-456, pp. 602-605, 2004.
AbstractThe 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, vol. 455-456, pp. 602-605, 2004.
AbstractThe 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.
Neagu, E. R., R. M. Neagu, C. J. Dias, C. M. Lança, and J. N. Marat-Mendes,
The determination of the pull-in voltage from the condition of bridge stability,
, vol. 5, pp. 139-151, Jan, 2010.
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