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.
Neagu, R. M., E. R. Neagu, M. C. 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.
AbstractThe 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, C. J. Dias, C. M. Lança, and J. N. Marat-Mendes,
The determination of the pull-in voltage from the condition of bridge stability,
, vol. 5, pp. 139-151, Jan, 2010.
Abstractn/a
Neagu, E. R., C. J. Dias, M. C. Lanca, R. Igreja, and J. N. Marat-Mendes,
"Medium Electric Field Electron Injection/Extraction at Metal-Dielectric Interface",
Advanced Materials Forum V, Pt 1 and 2, vol. 636-637, pp. 437-443, 2010.
AbstractThe isothermal charging current and the isothermal discharging current in low mobility materials are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current there is a current related to charge injection or extraction at the metal-dielectric interface and a reverse current related to the charge trapped into the shallow superficial or near superficial states of the dielectric and which can move at the interface in the opposite way that occurring during injection. Two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms which are involved into the transient current and (ii) the height of the potential barrier W-0 at the metal-dielectric interface. The experimental data demonstrate that there is no threshold field for electron injection/extraction at a metal-dielectric interface.
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.
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.
AbstractThe 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, C. J. Dias, C. M. Lança, and J. N. Marat-Mendes,
The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact,
, vol. 356, pp. 833-837, Jan, 2010.
Abstractn/a
Neagu, E. R., R. M. Neagu, C. J. Dias, M. C. Lanca, and J. N. Marat-Mendes,
"The analysis of isothermal current in terms of charge injection or extraction at the metal-dielectric contact",
Journal of Non-Crystalline Solids, vol. 356, no. 11-17, pp. 833-837, 2010.
AbstractThe measured isothermal charging and discharging currents are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms through the dielectric materials. Besides a polarization current, there is a current related with charge injection or extraction at the metal-dielectric contact and a reverse current related to the charge trapped into the superficial trap states of the dielectric and that can jump at the interface in a reverse way. By fitting the experimental data, two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms still involved into the transient current and (ii) the height W-0 of the potential barrier at the metal-dielectric interface immediately after the step voltage is applied. Only the initial part of the measured isothermal charging or discharging current can be used to obtain information about the polarization processes. By transforming the time-domain data into the frequency domain, a maximum for the imaginary part of the dielectric permittivity is obtained, in good agreement with the data obtained from AC dielectric measurements and the finally thermally stimulated discharge current measurements. (C) 2009 Elsevier B.V. All rights reserved.
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, vol. 22, no. 3, pp. 1419-1426, 2015.
Abstractn/a
Neagu, R. M., E. R. Neagu, C. M. Lanca, J. N. Marat-Mendes, A. T. Marques, A. F. Silva, A. P. M. Baptista, C. Sa, F. J. L. A. Alves, L. F. Malheiros, and M. Vieira,
"New Experimental Facts Concerning the Thermally Stimulated Discharge Current in Dielectric Materials",
Advanced Materials Forum Iv, vol. 587-588, pp. 328-332, 2008.
Abstractn/a
Neagu, E. R.;Neagu, R. M.;Lanca, M. C.;Vassilikou-Dova, A.;Marat-Mendes, and J. N.,
Identification of an apparent peak by use of the final thermally stimulated discharge current technique,
, pp. 296-299, Jan, 2005.
Abstractn/a
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.
Abstractn/a
Neagu, E. R., R. M. Neagu, C. J. Dias, M. C. Lanca, P. Inacio, and J. N. Marat-Mendes,
"Electrical Method to Study the Weak Molecular Movements at Nanometric Scale in Low Mobility Materials",
Advanced Materials Forum V, Pt 1 and 2, vol. 636-637, pp. 430-436, 2010.
AbstractFor the characterization of the new materials and for a better understanding of the connection between structure and properties it is necessary to use more and more sensible methods to study molecular movement at nanometric scale. This paper presents the experimental basis for a new electrical method to study the fine molecular movements at nanometric scale in dielectric materials. The method will be applied for polar and non-polar materials characterization. Traditionally, the electrical methods used to study the molecular movements are based on the movements of the dipoles that are parts of the molecules. We have proposed recently a combined protocol to analyze charge injection/extraction, transport, trapping and detrapping in low mobility materials. The experimental results demonstrate that the method can be used to obtain a complex thermogram which contains information about all molecular movements, even at nanoscopic level. Actually during the charging process we are decorating the structure with space charge and during the subsequent heating we are observing an apparent peak and the genuine peaks that are related to charge de-trapping determined by the molecular movement. The method is very sensitive, very selective and allows to determinate the parameters for local and collective molecular movements, including the temperature dependence of the activation energy and the relaxation time.