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A., P., L. A. N. Ç. A. M.C., B. J.P., N. E. A. G. U. E.R., D. I. A. S. C.J., Marat-Mendes, and J.N., Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite, , pp. 55-56, Jan, 2011. Abstract
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A., P., L. A. N. Ç. A. M.C., B. J.P., N. E. A. G. U. E.R., D. I. A. S. C.J., Marat-Mendes, and J.N., "Influence of Polarization on the Bioactivity of Nanopowders of Hydroxyapatite", 14th International Symposium on Electrets, 2011. Abstract
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AS, P., B. JPMR, N. E, M. - M. JN, and L. MCH, Electrical characterization of biphasic ceramics used in hard tissue replacement, , Coimbra Portugal, pp. 217, 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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Dias, I. J. G. J. G., A. S. S. Pádua, E. Pires, J. P. M. R. Borges, J. C. Silva, and M. C. Lança, "TSDC and surface potential measurements of charged hydroxyapatite/BaTiO 3 biocoatings deposited by CoBlast", ISE19-19th Int Symp on Electrets, 18-22 Sept., Linz, Austria, Johannes Kepler Univ., pp. 77, 2023. Abstract

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Dias, I. J. G., S. A. Pádua, E. A. Pires, J. P. M. R. Borges, J. C. Silva, and C. M. Lança, "Hydroxyapatite-Barium Titanate Biocoatings Using Room Temperature Coblasting", Crystals 2023, Vol. 13, Page 579, vol. 13, no. 4: Multidisciplinary Digital Publishing Institute, pp. 579, mar, 2023. AbstractWebsite

The use of orthopaedic and dental implants is expanding as a consequence of an ageing population and also due to illness or trauma in younger age groups. The implant must be biocompatible, bioactive and interact favourably with the recipient's bone, as rapid osseointegration is key to success. In this work, Ti-6Al-4V plates were coated using the CoBlastTM technique, with hydroxyapatite (HAp) and HAp/BaTiO3 (barium titanate, BT) non-piezoelectric cubic nanopowders (HAp/cBT) and piezoelectric tetragonal micropowders (HAp/tBT). The addition of BT, a piezoelectric ceramic, is a strategy to accelerate osseointegration by using surface electric charges as cues for cells. For comparison with commercial coatings, plates were coated with HAp using the plasma spray technique. Using XRD and FTIR, both plasma spray and CoBlastTM coatings showed crystalline HAp and no presence of by-products. However, the XRD of the plasma-sprayed coatings revealed the presence of amorphous HAp. The average surface roughness was close to the coatings' thickness (≈5 $μ$m for CoBlastTM and ≈13 $μ$m for plasma spray). Cytotoxicity assays proved that the coatings are biocompatible. Therefore, it can be concluded that for HAp-based coatings, CoBlastTM is a viable alternative to plasma spray, with the advantage of facilitating room temperature addition of other ceramics, like piezoelectric BaTiO3.

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ER, N., N. RM, D. CJ, L. MC, I. P, and M. - M. JN, Electrical Method to Study the Weak Molecular Movements at Nanometric Scale in Low Mobility Materials, , vol. 636-637, pp. 430-436, Jan, 2010. Abstract
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ER, N., D. CJ, L. MC, I. R, I. P, and M. - M. J. N., The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition, , vol. 354, issue 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, N., N. RM, D. CJ, L. MC, and M. - M. JN, "The determination of the metal-dielectric interface barrier height from the open-circuit isothermal charging current", Journal of Applied Physics, vol. 104, Jan, 2008. AbstractWebsite
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ER, N., D. CJ, L. MC, I. R, and M. - M. JN, Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface, , no. 636-637, pp. 437-443, Jan, 2010. Abstract
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ER, N., D. CJ, L. MC, I. R, I. P, and M. - M. J. N., The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition, , vol. 354, pp. 385-390, Jan, 2011. Abstract
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ER, N., D. CJ, L. MC, I. R, I. P, and M. - M. 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: Univ Nova Lisboa, pp. 2503-2511, Jan, 2010. Abstract
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ER, N., D. CJ, L. MC, I. R, and M. - M. JN, Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface, , vol. 636-637, pp. 437-443, Jan, 2010. Abstract
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ER, N., D. CJ, L. MC, I. R, I. P, and M. - M. JN, On the width of the thermally stimulated discharge current peak, , Jan, 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, vol. 26-7, pp. 33-41, 1990. 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, vol. 26-7, pp. 33-41, 1990. AbstractWebsite
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Gavinho, S. R., P. R. Prezas, D. J. Ramos, I. Sá‐Nogueira, J. P. Borges, C. M. Lança, J. C. Silva, C. M. R. Henriques, E. Pires, J. S. Kumar, and M. P. F. Graça, "{Nontoxic glasses: Preparation, structural, electrical and biological properties}", International Journal of Applied Ceramic Technology: John Wiley {&} Sons, Ltd (10.1111), pp. ijac.13243, apr, 2019. AbstractWebsite
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Hammami, I., S. R. Gavinho, A. S. Pádua, M. C. do Lança, J. P. Borges, and J. C. Silva, "Extensive Investigation on the Effect of Niobium Insertion on the Physical and Biological Properties of 45S5 Bioactive Glass for Dental Implant", International Journal of Molecular Sciences, vol. 24, issue 6, pp. 5244-5263, 2023.
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Inês, C., M. J. Paulo, G. Luís, N. E. R., D. C. J., M. - M. J. N., and L. M. C., "Water Content Control to Improve Space Charge Storage in a Cork Derivative", Materiais2011, 2012. Abstract
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Inês, C., M. J. Paulo, G. Luís, N. E. R., D. C. J., M. - M. J. N., and L. M. C., Water Content Control to Improve Space Charge Storage in a Cork Derivative, , pp. 395-400, Jan, 2012. Abstract
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J, M., L. MC, and M. - M. S. J, "Infrared spectroscopy studies of aged polymeric insulators", Advanced Materials Forum I, vol. 230-2, pp. 384-387, Jan, 2002. 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, vol. 41, no. 22, pp. 8151-8154, 2000. 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, vol. 41, no. 22, pp. 8151-8154, 2000. 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, J. N. Marat-Mendes, and Ieee, Comparative study of space charge in aged low-density polyethylene and crosslinked polyethylene, , pp. 209-212, 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 I. Ieee, Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene, , pp. 161-164, 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.