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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, 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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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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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., 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.

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, 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., W. Wirges, E. R. Neagu, R. Gerhard, and J. Marat-Mendes. "Influence of humidity on the electrical charging properties of cork agglomerates." Journal of Non-Crystalline Solids. 353 (2007): 4501-4505. 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)). (c) 2007 Elsevier B.V. All rights reserved.

Lanca, M. C., J. N. Marat-Mendes, and L. A. Dissado. "The fractal analysis of water trees - An estimate of the fractal dimension." Ieee Transactions on Dielectrics and Electrical Insulation. 8 (2001): 838-844. AbstractWebsite

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.

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. 356 (2010): 763-767. AbstractWebsite

Cork 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.