Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, P. Inacio, and J. N. Marat-Mendes,
"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, no. 4, pp. 2503-2511, 2010.
AbstractOpposite results concerning the sign of the parasitic charge accumulated at the metal dielectric contact in RF microelectromechanical systems (MEMS) capacitive switches are found in the literature. The mechanism concerning charge injection/extraction at the metal-dielectric contact and its influence on the pull-in voltage needs to be further clarified. A model-switch, for which only one dimension is in the microns range, is used to study the behaviour of a capacitive RF MEMS switch. The aim is to analyze how the electric charge is injected/extracted into or from the dielectric material under the applied field and to obtain realistic data to understand how this parasitic charge influences the pull-in voltage V-pi and the pull-off voltage V-po. A triangle voltage is employed to measure V-pi and V-po by measuring the isothermal charging/discharging currents. Our results demonstrate that V-pi is strongly dependent on the injected/extracted charge on the free surface of the dielectric. The charge injected/extracted at the bottom side of the dielectric has no influence on the actuation voltage. The charge injected/extracted on the free surface of the dielectric determines an increase of the modulus of V-pi and, eventually, the switch can fail to actuate. An estimation of the charge stored into the material was obtained (i) by measuring the charging current and the discharging current and (ii) from the value of the V-pi. The parasitic charge necessary to keep the bridge stick to the insulator is 5.3 x 10(-4) cm(-2) for our experimental conditions. The modification of the V-pi determined by the stored charge in the dielectric is analyzed. An increase of the relative dielectric permittivity by a factor of 2 produces a decrease of the actuation voltage of 10%. A variation of 30% in the elastic constant determines a variation of about 20% in the V-pi. A voltage threshold for charge injection/extraction was not observed.
MC, L., F. M, N. E, D. LA, M. - M. S. J, T. A, and Z. S,
"Space charge analysis of electrothermally aged XLPE cable insulation",
Journal of Non-Crystalline Solids, vol. 353, issue 47-51, 2007.
AbstractCross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.
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.
AbstractDuring 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.
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.
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