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A
A., Pedrosa, LANÇA M.C., Borges J.P., NEAGU E.R., DIAS C.J., Marat-Mendes, and J.N. "Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite." 14th International Symposium on Electrets. International Symposium on Electrets. 2011. Abstract
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A., Pedrosa, LANÇA M.C., Borges J.P., NEAGU E.R., DIAS C.J., Marat-Mendes, and J.N. "Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite." International Symposium on Electrets. 2011. 55-56. Abstract
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AS, Pedrosa, Borges JPMR, Neagu E, Marat-Mendes JN, and Lança MCH Electrical characterization of biphasic ceramics used in hard tissue replacement. Coimbra Portugal, 2013. Abstract

Hydroxyapatite [Hap; Ca10(PO4)6(OH)2) and b-tricalcium phosphate [b-TCP; Ca3(PO4)2] are biocompatible calcium phosphates used in skeletal surgery. The natural HAp is one of the main components of bone and, as a synthetic material, has been widely used for bone replacement presenting good bioactivity. Nevertheless synthetic HAp presents a slow in vivo degradation rate which is disadvantageous for bone’s reparative process. b-TCP has also good osteogenic characteristics presenting the ability to form strong bonds with the bone however, its degradation rate is too fast [1]. Therefore, a composite combining these two ceramics is valuable as it exhibits a suitable degradation rate. Because of the piezoelectric properties of bone it is known that electrical polarization of calcium phosphates can enhance the bioactivity and biointegration of implants [2]. Previous studies have already showed that HAp/b-TCP ceramics can be electrically polarized and that electrical polarization enhances osteogenesis in the early stage of the implantation process. However further studies are required to understand, optimize and improve the polarization technique [1]. In this work a commercial biphasic ceramic powders were pressed in a mold at 200 MPa to produce disc shaped samples. Afterwards, the samples were sintered at temperatures from 950ºC to 1150ºC and the influence of the heat treatment in the electrical polarization and subsequent bioactivity was investigated. The samples were polarized under a high DC electric field at relatively lower temperature (200oC) compared to previous studies and the stability of polarization was tested using TSDC (thermally depolarization currents) measurements. It was studied the influence of the water, initially present in the material, in the total charge deposited during polarization, its stability and its relation with heat treatment after pressing. The influence of the addition of b-TCP on sample’s stored charge was also evaluated. Finally bioactivity tests in a simulated body fluid solution were made taking into account the signal of the charge in each surface of the disc samples so that the results could be compared to previous ones.

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ER, Neagu, Dias CJ, Lanca MC, Igreja R, and Marat-Mendes JN. "Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface." Materials Science Forum. 2010. 437-443. Abstract
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ER, Neagu, Dias CJ, Lança MC, Igreja R, Inacio P, and Marat-Mendes J. N. "The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition." 354 (2011): 385-390. Abstract
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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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ER, Neagu, Dias CJ, Lança MC, Igreja R, and Marat-Mendes JN. "Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface." Vol. 636-637. Materials Science Forum, 636-637. 2010. 437-443. Abstract
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ER, Neagu, Dias CJ, Lanca MC, Igreja R, Inacio P, and Marat-Mendes JN. "On the width of the thermally stimulated discharge current peak." IEEE International Conference on Solid Dielectrics-ICSD. 2010. Abstract
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ER, Neagu, Neagu RM, Dias CJ, Lanca MC, Inacio P, and Marat-Mendes JN. "Electrical Method to Study the Weak Molecular Movements at Nanometric Scale in Low Mobility Materials." Vol. 636-637. Materials Science Forum, 636-637. 2010. 430-436. Abstract
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ER, Neagu, Dias CJ, Lanca MC, Igreja R, Inacio P, and Marat-Mendes JN. "The study of the molecular movements in the range of glass transition by the final thermally stimulated discharge current technique." IEEE International Conference on Solid Dielectrics-ICSD. 2010. Abstract
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ER, Neagu, Dias CJ, Lança MC, Igreja R, Inacio P, and Marat-Mendes J. N. "The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition." 354.2 (2011). Abstract

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623K. Two weak relaxations have been observed around 337K and around 402K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W=(0.84±0.03) eV ) and above ( W=(0.82±0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β1 sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β2 sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573K, is high than 8895s.

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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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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Inês, Cunha, Marques João Paulo, Gil Luís, Neagu E. R., Dias C. J., Marat-Mendes J. N., and Lança M. C. "Water Content Control to Improve Space Charge Storage in a Cork Derivative." Materials Science Forum. 2012. 395-400. Abstract
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Inês, Cunha, Marques João Paulo, Gil Luís, Neagu E. R., Dias C. J., Marat-Mendes J. N., and Lança M. C. "Water Content Control to Improve Space Charge Storage in a Cork Derivative." Materiais2011. Materials Science Forum. 2012. Abstract
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J, Mateo, Lanca MC, and Marat-MendeS J. "Infrared spectroscopy studies of aged polymeric insulators." Advanced Materials Forum I. Vol. 230-2. 2003 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 230-2. 2002. 384-387. Abstract
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Judeinstein, P., M. C. Lanca, J. Marat-Mendes, and J. Rault. "Pore dimension of water trees in PE: NMR studies." Polymer. 41 (2000): 8151-8154. AbstractWebsite

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

Judeinstein, P., M. C. Lanca, J. Marat-Mendes, and J. Rault. "Pore dimension of water trees in PE: NMR studies." Polymer. 41 (2000): 8151-8154. AbstractWebsite

In PE films aged under electric field the crystallisation of water (and melting of ice) has been studied by quadrupolar NMR, this technique allows one to determine the concentration of water as low as 10(-4). It is shown that the pore dimensions of the tracks forming the water trees of the order of 2.5 nm, are independent of the ageing time. The mobility of water in these water trees and in porous glass, of similar pore dimensions, are compared. (C) 2000 Elsevier Science Ltd. All rights reserved.

L
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., I. Franco, J. M. Mendes, and Ieee SIMULATION OF THE FERROELECTRIC SWITCHING OF PVDF AND ITS COPOLYMERS., 1992. AbstractWebsite
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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., C. J. Dias, D. K. Dasgupta, and J. Marat-Mendes. "Dielectric properties of electrically aged low density polyethylene." Advanced Materials Forum I. Ed. T. Vieira. Vol. 230-2. Key Engineering Materials, 230-2. 2002. 396-399. 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., 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., 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.