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2017
Prezas, P. R., B. M. G. Melo, L. C. Costa, M. A. Valente, MC Lança, J. M. G. Ventura, LFV Pinto, and M. P. F. Graça. "TSDC and impedance spectroscopy measurements on hydroxyapatite, β-tricalcium phosphate and hydroxyapatite/β-tricalcium phosphate biphasic bioceramics." Applied Surface Science. 424.1 (2017): 28-38. Abstract

Bone grafting and surgical interventions related with orthopaedic disorders consist in a big business, generating large revenues worldwide every year. There is a need to replace the biomaterials that currently still dominate this market, i.e., autografts and allografts, due to their disadvantages, such as limited availability, need for additional surgeries and diseases transmission possibilities. The most promising replacement materials are biomaterials with bioactive properties, such as the calcium phosphate-based bioceramics group. The bioactivity of these materials, i.e., the rate at which they promote the growth and directly bond with the new host biological bone, can be enhanced through their electrical polarization.In the present work, the electrical polarization features of pure hydroxyapatite (Hap), pure β-tricalcium phosphate (β-TCP) and biphasic hydroxyapatite/β-tricalcium phosphate composites (HTCP) were analyzed by measuring thermally stimulated depolarization currents (TSDC). The samples were thermoelectrically polarized at 500. °C under a DC electric field with a magnitude of 5. kV/cm. The biphasic samples were also polarized under electric fields with different magnitudes: 2, 3, 4 and 5. kV/cm. Additionally, the depolarization processes detected in the TSDC measurements were correlated with dielectric relaxation processes observed in impedance spectroscopy (IS) measurements.The results indicate that the β-TCP crystalline phase has a considerable higher ability to store electrical charge compared with the Hap phase. This indicates that it has a suitable composition and structure for ionic conduction and establishment of a large electric charge density, providing great potential for orthopaedic applications.

2015
Neagu, E. R., M. C. Lanca, C. J. Dias, and J. N. Marat-Mendes. "Space Charge and Dipolar Charge Contribution at Polar Polymers Polarization." Ieee Transactions on Dielectrics and Electrical Insulation. 22 (2015): 1419-1426. AbstractWebsite
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2013
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.

Lanca, M. C., I. Cunha, J. P. Marques, E. R. Neagu, L. Gil, C. J. Dias, and J. N. Marat-Mendes. "Water Content Control To Improve Space Charge Storage in a Cork Derivative." Advanced Materials Forum Vi, Pts 1 and 2. Eds. AMP Pinto, and AS Pouzada. Vol. 730-732. Materials Science Forum, 730-732. 2013. 395-400. Abstract
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2012
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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M.C., Lan, Cunha I., Marques Jo Paulo, GIL L., NEAGU E.R., DIAS C.J., and Marat-Mendes Jose N. "Water Content Control to Improve Space Charge Storage in a Cork Derivative." 730-732 (2012): 395-400. Abstract
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2011
R., Neagu E., C. Dias, Lança M. Carmo, Igreja R., Inacio P., and Marat-Mendes J. N. "Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers." International Symposium on Electrets. 2011. 145-146. Abstract
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M.C., LANÇA, NEAGU E.R., DIAS C.J., GIL L., and MARAT-MENDES J.N. "Electrical properties of cork and derivatives." 23 (2011): 64-70. 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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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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R., Neagu E., C. Dias, Lança M. Carmo, Igreja R., Inacio P., and Marat-Mendes J. N. "Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers." 14th International Symposium on Electrets. International Symposium on Electrets. 2011. Abstract

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 °C and the second associated with the Curie transition around 96 °C.

Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, J. N. Marat-Mendes, and Ieee Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers., 2011. AbstractWebsite

The final thermally stimulated discharge current method allows a better selection of the experimental conditions for sample polarization. By decreasing the ratio between the charging time and the discharging time, the apparent peak is of the same order of magnitude as the genuine peaks and there is only a partial overlap between then. Two peaks have been identified for polyamide 11, one associated with the glass transition around 60 degrees C and the second associated with the Curie transition around 96 degrees C.

M.C., LANÇA, NEAGU E.R., DIAS C.J., GIL L., and MARAT-MENDES J.N. "Electrical properties of cork and derivatives." 23.3/4 (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." 14th International Symposium on Electrets. International Symposium on Electrets. 2011. Abstract
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Pedrosa, A., M. C. Lanca, J. P. Borges, E. R. Neagu, C. J. Dias, J. N. Marat-Mendes, and Ieee. "Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite." 2011 14th International Symposium on Electrets (Ise) (2011): 55-56. Abstract
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Pedrosa, A., M. C. Lanca, J. P. Borges, E. R. Neagu, C. J. Dias, J. N. Marat-Mendes, and Ieee Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite., 2011. AbstractWebsite
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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.

Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, and J. N. Marat-Mendes. "The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition." Journal of Non-Crystalline Solids. 357 (2011): 385-390. AbstractWebsite

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 623 K. Two weak relaxations have been observed around 337 K and around 402 K. 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 beta relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the beta relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the beta(1) sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the beta(2) 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 573 K, is high than 8895 s. (C) 2010 Elsevier B.V. All rights reserved.

2010
Neagu, E. R., R. M. Neagu, C. J. Dias, Carmo M. Lança, and J. N. Marat-Mendes. "The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact." 356 (2010): 833-837. 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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Neagu, E. R., R. M. Neagu, C. J. Dias, Carmo M. Lança, and J. N. Marat-Mendes. "The determination of the pull-in voltage from the condition of bridge stability." 5 (2010): 139-151. Abstract
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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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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, 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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