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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, vol. 357, no. 2, pp. 385-390, 2011. 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.

Neagu, R. M., E. R. Neagu, C. M. Lanca, and J. N. Marat-Mendes, "New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials", Advanced Materials Forum Iv, vol. 587-588, pp. 328-332, 2008. Abstract

The thermally stimulated discharge current (TSDC.) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. flee experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature T-ma shifted with about 10 to 30 K above the charging temperature T-c. The shift of T-ma with respect to T-c depends on the experimental conditions. The peak width at the half maximum intensity decreases as T-c increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

Neagu, E. R., R. M. Neagu, M. C. Lanca, and J. N. Marat-Mendes, "The time as a parameter to investigate the landscape of the apparent activation energies in the final thermally stimulated discharge current measurements", 12th International Symposium on Electrets (ISE 12), Proceedings, pp. 292-295, 2005. AbstractWebsite

The experimental results obtained in a wide range of temperatures, for polyethylene terephthalate, demonstrate that the apparent activation energy changes when the charging (polarization) time or the isothermal discharging time, prior to the final thermally stimulated discharge current measurement, are used as variable parameters. Consequently, the charging and/or discharging time can be used as a variable parameter to investigate the landscape of the apparent thermal activation energies. A continuous distribution of the traps in the range from 0.4 to 3 eV was observed. The experimental results demonstrate that there is a range of experimental conditions for which the thermal activation energy is independent of the experimental parameter values. This is the activation energy value which should be used to characterize a certain mechanism.

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Oliveira, I. R., A. M. Barbosa, K. W. Santos, M. C. Lança, M. M. R. A. Lima, T. Vieira, J. C. Silva, and J. P. Borges, "Properties of strontium-containing BG 58S produced by alkali-mediated sol-gel process", Ceramics International: Elsevier, jan, 2022. AbstractWebsite

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de Oliveira, I. R., I. dos Santos Gonçalves, K. Wallace dos Santos, M. C. Lança, T. Vieira, J. C. Silva, I. F. Cengiz, R. L. Reis, J. M. Oliveira, and J. P. Miranda Ribeiro Borges, "Biocomposite Macrospheres Based on Strontium-Bioactive Glass for Application as Bone Fillers", ACS Materials Au, vol. 3, no. 6: American Chemical Society, pp. 646–658, 2023. AbstractWebsite

Traditional bioactive glass powders are typically composed of irregular particles that can be packed into dense configurations presenting low interconnectivity, which can limit bone ingrowth. The use of novel biocomposite sphere formulations comprising bioactive factors as bone fillers are most advantageous, as it simultaneously allows for packing the particles in a 3-dimensional manner to achieve an adequate interconnected porosity, enhanced biological performance, and ultimately a superior new bone formation. In this work, we develop and characterize novel biocomposite macrospheres of Sr-bioactive glass using sodium alginate, polylactic acid (PLA), and chitosan (CH) as encapsulating materials for finding applications as bone fillers. The biocomposite macrospheres that were obtained using PLA have a larger size distribution and higher porosity and an interconnectivity of 99.7%. Loose apatite particles were observed on the surface of macrospheres prepared with alginate and CH by means of soaking into a simulated body fluid (SBF) for 7 days. A dense apatite layer was formed on the biocomposite macrospheres' surface produced with PLA, which served to protect PLA from degradation. In vitro investigations demonstrated that biocomposite macrospheres had minimal cytotoxic effects on a human osteosarcoma cell line (SaOS-2 cells). However, the accelerated degradation of PLA due to the degradation of bioactive glass may account for the observed decrease in SaOS-2 cells viability. Among the biocomposite macrospheres, those composed of PLA exhibited the most promising characteristics for their potential use as fillers in bone tissue repair applications.

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P, J., L. MC, M. - M. S. J, and R. J, "Pore dimension of water trees in PE: NMR studies", POLYMER, vol. 41: Univ Paris 11, Univ Nova Lisboa, Univ Paris 11, Univ Paris 11, pp. 8151-8154, Jan, 2000. 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), pp. 55-56, 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, , pp. 55-56, 2011. AbstractWebsite
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Prezas, P. R., B. M. G. Melo, L. C. Costa, M. A. Valente, M. C. Lança, J. M. G. Ventura, L. F. V. 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, vol. 424, issue 1, pp. 28-38, 2017. 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.

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R., N. E., C. Dias, L. M. Carmo, I. R., I. P., and M. - M. J. N., "Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers", 14th 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.

R., N. E., C. Dias, L. M. Carmo, I. R., I. P., and M. - M. J. N., Discrimination between Space Charge and Dipolar Contributions in Ferroelectric Polymers, , pp. 145-146, Jan, 2011. Abstract
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RM, N., N. ER, L. MC, and M. - M. JN, "New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials", Advanced Materials Forum Iv, vol. 587-588, no. 587-588, pp. 328-332, Jan, 2008. Abstract
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Tavares, C., T. Vieira, J. C. Silva, J. P. M. R. Borges, and C. M. Lança, "Bioactive Hydroxyapatite Aerogels with Piezoelectric Particles", Biomimetics 2024, Vol. 9, Page 143, vol. 9, no. 3: Multidisciplinary Digital Publishing Institute, pp. 143, feb, 2024. AbstractWebsite

Open-cell foams based on hydroxyapatite (HAp) can mimic the extracellular matrix (ECM) to better replace damaged hard tissues and assist in their regeneration processes. Aerogels of HAp nanowires (NW) with barium titanate (BT) particles were produced and characterized regarding their physical and chemical properties, bioactivity, and in vitro cytotoxicity. Considering the role of piezoelectricity (mainly due to collagen) and surface charges in bone remodeling, all BT particles, of size 280 nm and 2 and 3 µm, contained BaTiO3 in their piezoelectric tetragonal phase. The synthesized nanowires were verified to be AB-type carbonated hydroxyapatite. The aerogels showed high porosity and relatively homogeneous distribution of the BT particles. Barium titanate proved to be non-cytotoxic while all the aerogels produced were cytotoxic for an extract concentration of 1 mg/mL but became non-cytotoxic at concentrations of 0.5 mg/mL and below. It is possible that these results were affected by the higher surface area and quicker dissolution rate of the aerogels. In the bioactivity assays, SEM/EDS, it was not easy to differentiate between the apatite deposition and the surface of the HAp wires. However, a quantitative EDS analysis shows a possible CaP deposition/dissolution cycle taking place.

Teixeira, S. S., M. P. F. Graça, J. Lucas, M. A. Valente, P. I. P. Soares, M. C. Lança, T. Vieira, J. C. Silva, J. P. Borges, L. - I. Jinga, G. Socol, C. Mello Salgueiro, J. Nunes, and L. C. Costa, "Nanostructured LiFe5O8 by a Biogenic Method for Applications from Electronics to Medicine", Nanomaterials, vol. 11, no. 1: MDPI AG, pp. 193, jan, 2021. AbstractWebsite

The physical properties of the cubic and ferrimagnetic spinel ferrite LiFe5O8 has made it an attractive material for electronic and medical applications. In this work, LiFe5O8 nanosized crystallites were synthesized by a novel and eco-friendly sol-gel process, by using powder coconut water as a mediated reaction medium. The dried powders were heat-treated (HT) at temperatures between 400 and 1000 °C, and their structure, morphology, electrical and magnetic characteristics, cytotoxicity, and magnetic hyperthermia assays were performed. The heat treatment of the LiFe5O8 powder tunes the crystallite sizes between 50 nm and 200 nm. When increasing the temperature of the HT, secondary phases start to form. The dielectric analysis revealed, at 300 K and 10 kHz, an increase of $ε$′ (≈10 up to ≈14) with a tan$δ$ almost constant (≈0.3) with the increase of the HT temperature. The cytotoxicity results reveal, for concentrations below 2.5 mg/mL, that all samples have a non-cytotoxicity property. The sample heat-treated at 1000 °C, which revealed hysteresis and magnetic saturation of 73 emu g−1 at 300 K, showed a heating profile adequate for magnetic hyperthermia applications, showing the potential for biomedical applications.