Publications

Export 114 results:
Sort by: Author [ Title  (Desc)] Type Year
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 
{
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
n/a
W
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
n/a
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
n/a
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, vol. 730-732, pp. 395-400, 2013. Abstract
n/a
M.C., L., C. I., M. J. Paulo, G. I. L. L., N. E. A. G. U. E.R., D. I. A. S. C.J., and M. - M. J. N., Water Content Control to Improve Space Charge Storage in a Cork Derivative, , vol. 730-732, pp. 395-400, 2012. Abstract
n/a
U
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
n/a
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.

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.

T
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

n/a

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.

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, , 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.

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.

S
Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, J. N. Marat-Mendes, and Ieee, "The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique", Proceedings of the 2010 Ieee International Conference on Solid Dielectrics (Icsd 2010), 2010. Abstract
n/a
Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, J. N. Marat-Mendes, and Ieee, "The Study of the Molecular Movements in the Range of Glass Transition by the Final Thermally Stimulated Discharge Current Technique", Proceedings of the 2010 Ieee International Conference on Solid Dielectrics, 2010. Abstract

The electrical methods used to study the molecular movements are based on the movement of the dipoles under DC or AC electric field. We have proposed recently a combined measuring protocol to analyze charge injection/extraction, transport, trapping and de-trapping in polar or non-polar dielectric materials. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 572 K. A strong relaxation was observed around 402 K and a very weak relaxation around 345 K. This is the beta relaxation which is quite complex. As concern the behavior at high temperatures, above the beta relaxation, a high peak was observed that shifts continuously to higher temperatures as the charging temperature and/or the charging field increase. The maximum current of the peak increases and the temperature corresponding to the maximum current increases as the charging temperature and/or the charging field increase, given a direct observation of the so called cross-over effect related to current decay for sample charged at high fields and/or high temperatures.

Neagu, E. R., C. J. Dias, M. C. Lança, and J. N. Marat-Mendes, The study of molecular movements in dielectrics using isothermal and non- isotehermal current measurements, , vol. 183, pp. –-, Jan, 2009. Abstract
n/a
Lanca, M. C., J. Domingues, and I. Franco, Study of fractal properties in Lichtenberg figures, , pp. 133-144, 1995. AbstractWebsite
n/a
Lanca, M. C., J. Domingues, and I. Franco, Study of fractal properties in Lichtenberg figures, , pp. 133-144, 1995. AbstractWebsite
n/a
MC, L., N. ER, S. P, G. L, and M. - M. S. J, "Study of electrical properties of natural cork and two derivative products", Advanced Materials Forum Iii, Pts 1 and 2, vol. 514-516, no. 514-516, pp. 940-944, Jan, 2006. Abstract
n/a
Lanca, M. C., E. R. Neagu, P. Silva, L. Gil, and J. Marat-Mendes, "Study of electrical properties of natural cork and two derivative products", Advanced Materials Forum Iii, Pts 1 and 2, vol. 514-516, pp. 940-944, 2006. Abstract

Electrical properties of natural cork, commercial cork agglomerates (for floor and wall coverings) and a recently developed composite of cork/TetraPak (R) were studied. Measurements of isothermal charge and discharge currents were made for natural cork samples in different directions (axial, radial and tangential cuts). The isothermal current characteristics and the samples conductivity were investigated under different conditions (electric field, temperature and environmental conditions: in air at ambient relative humidity (RH), dry air and vacuum), also the samples could be or not conditioned (dried in vacuum or in a P2O5 atmosphere at room temperature). From these results the influence of water on the electrical properties of natural cork could be seen. In order to compare the three different cork materials a preliminary study was made. Isothermal charge and discharge currents and conductivity after 1h charging were measured and compared for different electric fields and temperature in air at ambient RH.

Lanca, M. C.;Neagu, E. R.;Marat-Mendes, and J. N., Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, Jan, 2002. Abstract
n/a
Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes, Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, 2002. AbstractWebsite

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

Lanca, M. C., E. R. Neagu, and J. N. Marat-Mendes, Studies of space charge in electrically aged low density polyethylene, , pp. 19-22, 2002. AbstractWebsite

Space charge in electrically aged LDPE was studied using a recently developed technique combining isothermal charging and discharging with non-isothermal measurements. Samples were aged in a NaCl aqueous solution at 40degreesC for 1500h under an AC field of 6MV/m (50Hz). The samples were then isothermally DC charged and discharged (both currents recorded). Next a non-isothermal experiment with constant heating rate was performed. Finally the sample was kept at the highest temperature and the final isothermal discharge current registered. The last step has to be carried on for long time to ensure an almost complete discharge of the remnant charge so that results become reproducible and possible to analyze. Selective charging (careful choice of the field, temperature and the ratio of charging/discharging times) revealed the presence of different trapping sites. From the analysis of the isothermal and non-isothermal data the relaxation times and activation energies could be obtained.

MC, L., N. ER, and M. - M. JN, "Space charge studies of aged XLPE using combined isothermal and thermostimulated current measurements", Cross-Disciplinary Applied Research in Materials Science and Technology, vol. 480-481, no. 480, pp. 501-505, Jan, 2005. Abstract
n/a