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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. Abstract

Low 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. 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., 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, C. M., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, J. Marat-Mendes, A. T. Marques, A. F. Silva, A. P. M. Baptista, C. Sa, F. J. L. A. Alves, L. F. Malheiros, and M. Vieira, "Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite", Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008. Abstract
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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. Abstract

Low 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., S. Peuckert, E. R. Neagu, L. Gil, P. C. Silva, and J. Marat-Mendes, "Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite", Advanced Materials Forum Iv, vol. 587-588, pp. 613-617, 2008. AbstractWebsite

Lately the electrical and dielectric properties of cork and some cork-based materials (commercial and non-commercial) have been studied in order to understand their ability to store electrical charge. The main problem found so far is related to the water content in cork, only of a few % weight. but large enough to influence greatly the conductivity of cork and, consequently, the charge storage capability. To overcome this problem cork has been combined with hydrophobic materials. In this work a commercial wax (paraffin wax) was used to produce a cork/paraffin composite by hot pressing. After milled and mixed natural cork. TetraPak (R) containers waste and paraffin were pressed to make plaques of a new composite. Different concentrations of cork. TetraPak (R) and paraffin, different granules sire, different temperature and pressure were used to produce the samples. The electrical properties of the new composite were measured by the isothermal charging and discharging current method and the results compared to previously ones obtained for natural cork and other derivative products. The new composite has shown to have lower conductivity than the commercial agglomerate. which makes it a better material for charge storage.

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, 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.;Dias,;D. C. J. Gupta, D. K.;Marat-Mendes, and J., Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene, , pp. 161-164, Jan, 2003. 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, vol. 455-456, pp. 602-605, 2004. 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.

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M.C., L. A. N. Ç. A., N. E. A. G. U. E.R., D. I. A. S. C.J., G. I. L. L., and M. A. R. A. T. - M. E. N. D. E. S. J.N., Electrical properties of cork and derivatives, , vol. 23, pp. 64-70, Jan, 2011. Abstract
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M.C., L., C. I., M. J. Paulo, G. I. L. L., N. E. A. G. U. E.R., D. I. A. S. C.J., and M. - M. J. N., Water Content Control to Improve Space Charge Storage in a Cork Derivative, , vol. 730-732, pp. 395-400, 2012. Abstract
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M.C., L. A. N. Ç. A., N. E. A. G. U. E.R., D. I. A. S. C.J., G. I. L. L., and M. A. R. A. T. - M. E. N. D. E. S. J.N., Electrical properties of cork and derivatives, , vol. 23, issue 3/4, 2011. Abstract
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Madeira, R. M. D., T. Vieira, J. C. Silva, I. R. Oliveira, J. P. Borges, M. M. R. A. Lima, and C. M. Lança, "Piezoelectric Calcium Modified Barium Titanate for Bone Regeneration", Materials Proceedings 2022, Vol. 8, Page 121, vol. 8, no. 1, Basel Switzerland, Multidisciplinary Digital Publishing Institute, pp. 121, jul, 2022. Abstract

Solid state reaction was used to produced barium titanate modified with calcium (BCT) showing the presence of the piezoelectric tetragonal phase after sintering at 1350 °C. Bioglass 45S5 (BG) was synthetized by sol-gel route. From these two materials and commercial hydroxyapatite (HAp) were obtained composites. The BG produced showed some cytotoxic character that was weakened by passivation. All other materials were non-cytotoxic. Contact polarization at constant temperature was chosen composites polarization. Electric/dielectric properties were evaluated by thermally stimulated depolarization currents (TSDC). The material showed bioactivity with the composite with BCT/BG/HAp 90/5/5 (wt%) showing increased bioactivity. In vitro test showed high proliferation rates for the composites.

Mateo, J., M. C. Lanca, and J. Marat-Mendes, "Infrared spectroscopy studies of aged polymeric insulators", Advanced Materials Forum I, vol. 230-2, pp. 384-387, 2002. Abstract

Thin films of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) were aged under an AC electric field while kept in sodium chloride aqueous solution. After aging the samples showed water trees (localized damaged with the appearance of hydrophilic ramified structures whose size ranges from a few microns to I mm). Some of the samples suffered dielectric breakdown showing small channels (1-2 mm. diameter) crossing the film and sometimes also signs of carbonization. In order to identify the oxidation mechanisms contributing to aging, FTIR was used to analyze both unaged and aged specimens. Comparing between unaged and aged LDPE an increase in the FTIR spectrum for bands at 1720 cm(-1), 1640 cm(-1) and 1590 cm(-1) was visible for the aged samples. The first region corresponds to carbonyl groups (C=O bonds) resulting from oxidation (most probably ketones). While the second one is related to carbon double bonds formed due to chain scission. Finally the third one is due to carboxylates. For the XLPE the analysis is more difficult. Besides aging it needs to be taken into account the by-products of crosslinking that will tend also to diffuse out with time. The main effect of aging is an increase in the concentration of 1640 cm(-1) band (C=C bonds). For the water treed regions dry and wet samples were compared. In the wet ones the absorbance is larger for the 3380 cm(-1) exhibiting, as expected, water absorption in the water treed regions (hydrophilic characteristics were increased).

Mateo, J., M. C. Lanca, and J. Marat-Mendes, "Infrared spectroscopy studies of aged polymeric insulators", Advanced Materials Forum I, vol. 230-2, pp. 384-387, 2002. Abstract

Thin films of low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) were aged under an AC electric field while kept in sodium chloride aqueous solution. After aging the samples showed water trees (localized damaged with the appearance of hydrophilic ramified structures whose size ranges from a few microns to I mm). Some of the samples suffered dielectric breakdown showing small channels (1-2 mm. diameter) crossing the film and sometimes also signs of carbonization. In order to identify the oxidation mechanisms contributing to aging, FTIR was used to analyze both unaged and aged specimens. Comparing between unaged and aged LDPE an increase in the FTIR spectrum for bands at 1720 cm(-1), 1640 cm(-1) and 1590 cm(-1) was visible for the aged samples. The first region corresponds to carbonyl groups (C=O bonds) resulting from oxidation (most probably ketones). While the second one is related to carbon double bonds formed due to chain scission. Finally the third one is due to carboxylates. For the XLPE the analysis is more difficult. Besides aging it needs to be taken into account the by-products of crosslinking that will tend also to diffuse out with time. The main effect of aging is an increase in the concentration of 1640 cm(-1) band (C=C bonds). For the water treed regions dry and wet samples were compared. In the wet ones the absorbance is larger for the 3380 cm(-1) exhibiting, as expected, water absorption in the water treed regions (hydrophilic characteristics were increased).

MC, L., W. W, N. ER, G. R, and M. - M. S. J, "Influence of humidity on the electrical charging properties of cork agglomerates", Journal of Non-Crystalline Solids, vol. 353, issue 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)).

MC, L., P. S, N. ER, G. L, S. PC, and M. - M. S. J, "Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite", Advanced Materials Forum Iv, vol. 587-588, no. 587-588, pp. 613-617, Jan, 2008. Abstract
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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, pp. 4462-4466, Jan, 2007. AbstractWebsite
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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. Abstract
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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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MC, L., and M. - M. S. J, "Dielectric breakdown statistics of polyethylene for progressively-censored data", Advanced Materials Forum Ii, vol. 455-456, no. 455-456, pp. 602-605, Jan, 2004. Abstract
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MC, L., W. W, N. ER, G. R, and M. - M. S. J, "Influence of humidity on the electrical charging properties of cork agglomerates", Journal of Non-Crystalline Solids, vol. 353, pp. 4501-4505, Jan, 2007. AbstractWebsite
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MC, L., M. - M. JN, and D. LA, "The fractal analysis of water trees - An estimate of the fractal dimension", IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION, vol. 8: Univ Nova Lisboa, Univ Leicester, pp. 838-844, Jan, 2001. Abstract
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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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