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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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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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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, 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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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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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." Materiais2011. Materials Science Forum. 2012. 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." Materials Science Forum. 2012. 395-400. 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.

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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. Ed. P. M. Vilarinho. Vol. 514-516. Materials Science Forum, 514-516. 2006. 935-939. 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.

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. 353 (2007): 4462-4466. 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.

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. 11 (2004): 25-34. 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., J. Domingues, and I. Franco Study of fractal properties in Lichtenberg figures., 1995. AbstractWebsite
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Lanca, Carmo M., Stefan Peuckert, Eugen R. Neagu, Luis Gil, Paulo C. Silva, and Jose Marat-Mendes. "Electrical Properties Studies of a Cork/TetraPak (R)/Paraffin Wax Composite." Advanced Materials Forum Iv. Eds. AT Marques, AF Silva, APM Baptista, C. Sa, FJLA Alves, LF Malheiros, and M. Vieira. Vol. 587-588. Materials Science Forum, 587-588. 2008. 613-617. Abstract

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.

Lança, Carmo M., E. R. Neagu, C. Dias, and J. Marat-Mendes. "Dielectric spectra of natural cork and derivatives." Vol. 356. International Conference on Dielectric Spectroscopy and Its Applications, 356. 2010. 763-767. Abstract
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