Fathi Jomni
Tunis University
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Publication
Featured researches published by Fathi Jomni.
Applied Physics Letters | 2009
Abdelkader Kahouli; Alain Sylvestre; L. Ortega; Fathi Jomni; Béchir Yangui; M. Maillard; B. Berge; J.C. Robert; J. Legrand
α, β, and γ relaxation mechanisms have been identified in semicrystalline (45% of crystallinity) parylene-C (–H2C–C6H3Cl–CH2–)n films. C–Cl bonds induce the β-relaxation and explain increase in the dielectric constant as the frequency decreases in usual temperatures of operation for devices incorporating parylene-C. At cryogenic temperature (<−20 °C), γ-relaxation is assigned to the local motions of phenyl groups. Both β and γ relaxation processes obey an Arrhenius law with activation energy Ea(β)=91.7 kJ/mole and Ea(γ)=8.68 kJ/mole. α-relaxation associated with cooperative segmental motions of the (–H2C–∅–CH2–)n chains is observed with a peak at 10−2 Hz for T=80 °C and follows a Vogel–Fulcher–Tamman–Hesse law.
Journal of Physics D | 2011
Abdelkader Kahouli; Fathi Jomni; Alain Sylvestre; Béchir Yangui; J. Legrand
Measurements under both transient and steady-state conditions on parylene C (?H2C???C6H3Cl???CH2?)n, also called PPX C, were made for different electric fields ranging from 8.33 to 33.33?MV?m?1. The transient current behaviour is hyperbolic in nature up to 125??C. Above, the current is transient free and becomes constant reflecting the presence of the steady state. The decay rate of the transient current increases with increasing temperature and field. The transient current is attributed mainly to the dipolar relaxation due to the polarization of the C?Cl dipole. The J?1/T characteristic reflects the change in the conduction regime occurring at a critical temperature associated with the glass transition temperature of the materials. The J?E measurements show that hopping conduction is the possible mechanism below and above Tg of parylene C. The activation energy is determined to be 0.13?eV, independent of the electric fields below Tg and varies from 0.65 to 0.94?eV above Tg, indicating the presence of more than one type of trapping centres in parylene C. The ionic jump distance ?a? is estimated to be 5.60?6.68?? below Tg and 8.36?26.58?? above Tg.
Journal of Physics: Conference Series | 2015
Achraf Kachroudi; S. Basrour; Libor Rufer; Fathi Jomni
This paper reports a novel low-cost fabrication process of a charged cellular microstructured polydimethylsiloxane (PDMS) material referred as piezo-electret or ferro-electret for micro-sensors applications. The dielectric spectra reached on these structures exhibit a high piezoelectric longitudinal coefficient d33 of 350pC/N. A mechanical characterization method proves the reliability of this material for low-frequencies applications around 100Hz.
Journal of Physical Chemistry A | 2015
M. Mokni; Abdelkader Kahouli; Fathi Jomni; J.-L. Garden; E. André; Alain Sylvestre
Parylene is a generic name indicating a family of polymers with the basic chemical structure of poly-p-xylylene. Parylene N and Parylene C are the most popular for applications. Curiously, Parylene D (poly( dichloro-p-xylylene), (C8H6Cl2)) was forgotten for applications. This report is the consequence of a later availability of a commercial dimer of Parylene D and also to the recent advent of fluorinated Parylenes allowing extending applications at higher temperatures. In our work, from a dielectric analysis, we present the potentialities of Parylene D for applications particularly interesting for integration in organic field-effect transistors. Dielectric and electrical properties, macromolecular structures, and dynamics interaction with electric field as a function of frequency and temperature are studied in 5.8 μm thick Parylene D grown by chemical vapor deposition. More exactly, the dielectric permittivity, the dissipation factor, the electrical conductivity, and the electric modulus of Parylene D were investigated in a wide temperature and frequency ranges from -140 to +350 °C and from 0.1 Hz to 1 MHz, respectively. According to the temperature dependence of the dielectric permittivity, Parylene D has two different dielectric responses. It is retained as a nonpolar material at very low temperature (like Parylene N) and as a polar material at high temperature (like parylene C). The dissipation factor shows the manifestation of two relaxations mechanisms: γ and β at very low and high temperatures, respectively. The γ relaxation is assigned to the local motions of the C-H end of the chains when the cryogenic temperature range is approached. A broad peak in tan δ is assigned to the β relaxation. It corresponds to rotational motion of some polar C-Cl groups. For temperature above 260 °C a mechanism of Maxwell-Wagner-Sillars polarization at the amorphous/crystalline interfaces was identified with two activation energies of Ea1 = 2.12 eV and Ea2 = 3.8 eV. Moreover, the conductivity and the dielectric permittivity relaxation processes have been discussed in terms of nearly constant loss (NCL) and universal dynamic regime (UDR). Finally, ionic conduction and electrode polarization effects are identified at very high temperatures and their physical origins are discussed.
Smart Materials and Structures | 2016
Achraf Kachroudi; S. Basrour; Libor Rufer; Alain Sylvestre; Fathi Jomni
Micro-structured cellular polydimethylsiloxane (PDMS) materials were prepared by a low-cost molding process allowing us to control geometry and sample size. Cellular structures are charged with a triangular quasi-static voltage with amplitudes between 1 kV and 4 kV and a frequency of 0.5 Hz fixed after having evaluated the conditions enhancing the piezoelectric response of the cellular PDMS. The piezo-electret PDMS material charged at room temperature has a piezoelectric coefficient d 33 of 350 pC/N, which is ten times larger than that of polyvinylidene fluoride. The high piezoelectric coefficient with a very low elastic modulus of 300 kPa makes these materials very useful for wearable device applications. The piezoelectric coefficient d 33 of the samples poled at high temperatures improves thermal stability but reduces PDMS piezo-electret piezoelectricity, which is explained by the structures stiffness. These results are useful and allow us to set the conditions for the preparation of the piezo-electret materials according to desired applications.
Smart Materials and Structures | 2015
Achraf Kachroudi; S. Basrour; Libor Rufer; Alain Sylvestre; Fathi Jomni
Electro-active polymers are emerging in the fields of actuators and micro-sensors because their good dielectric and mechanical properties makes them suitable for such applications. In this work, we focus on micro-structured (cellular) polymer materials (referred as piezoelectrets or ferroelectrets) that need prior charging to attain piezoelectric behaviour. The development of such applications requires an in-depth knowledge of the intrinsic dielectric properties of such structures and models to enable the accurate prediction of a given micro-structured material’s dielectric properties. Various polymers including polypropylene, polytetrafluoroethylene, fluoroethylenepropylene, cyclo-olefines and poly(ethylene terephthalate) in a cellular form have been studied by researchers over the last fifteen years. However, there is still a lack of information on the intrinsic dielectric properties of the most recently used dielectric polymer (polydimethylsiloxane, PDMS) over wide frequency and temperature ranges. In this work, we shall propose an exhaustive equivalent electrical circuit model and explain how it can be used to predict the micro-structured PDMS complex permittivity versus frequency and temperature. The results obtained from the model were found to be in good agreement with experimental data for various micro-structured PDMS materials. Typically, for micro-sensor applications, the dielectric constant and dielectric losses are key factors which need to be minimized. We have developed a configuration which enables both to be strongly reduced with a reduction of 16% in the dielectric constant of a micro-structured PDMS compared with the bulk material. In addition, the phenomena responsible for dielectric losses variations with frequency and temperature are discussed and correlated with the theoretical model. Our model is thus proved to be a powerful tool for the control of the dielectric properties of micro-structured PDMS material for micro-sensor applications.
Journal of Physics D | 2016
M. Kassmi; Fathi Jomni; P Gonon; O Khaldi; Laurence Latu-Romain; C Mannequin; Ahmad Bsiesy; S. Basrour; B Yangui
The electrical reliability of HfO2 based metal–insulator–metal capacitors is investigated under AC stress voltage. The capacitance–time (C–t) and conductance–time (G–t) responses are studied for different stress amplitudes and frequencies. Time-to-breakdown is observed to strongly depend on the electrode nature. Electrical degradation is discussed via a model based on oxygen vacancy/oxygen ions generation. Defect generation is controlled by the injecting nature of electrodes. Partial recovery, and so time-to-breakdown, are controlled by the ability of electrodes to store oxygen.
Proceedings of SPIE | 2015
S. Hammami; Claire Jean-Mistral; Fathi Jomni; O. Gallot-Lavallée; P. Rain; B. Yangui; Alain Sylvestre
Dielectric elastomers such as 3M VHB4910 acrylate film have been widely used for electromechanical energy conversion such as actuators, sensors and generators, due to their lightweight, high efficiency, low cost and high energy density. Mechanical and electric properties of such materials have been deeply investigated according to various parameters (temperature, frequency, pre-stress, nature of the compliant electrodes…). Models integrating analytic laws deduced from experiments increase their accuracy. Nevertheless, leakage current and electrical breakdown reduce the efficiency and the lifetime of devices made with these polymers. These two major phenomena are not deeply investigated in the literature. Thus, this paper describes the current-voltage characteristics of acrylate 3M VHB4910 and investigates the stability of the current under high electric field (kV) for various temperatures (from 20°C to 80°C) and over short (300 s) and long (12h) periods. Experimental results show that, with gold electrodes at ambient temperature, the current decreases with time to a stable value corresponding to the conduction current. This decrease occurs during 6 hours, whereas in the literature values of current at short time (less than 1 hour) are generally reported. This decrease can be explained by relaxations mechanisms in the polymer. Schottky emission and Poole-Frenkel emission are both evaluated to explain the leakage current. It emerges from this study that the Schottky effect constitutes the main mechanism of electric current in the 3M VHB4910. For high temperatures, the steady state is reached quickly. To end, first results on the leakage current changes for pre-stretch VHB4910 complete this study.
Proceedings of SPIE | 2016
S. Hammami; Claire Jean-Mistral; Fathi Jomni; Alain Sylvestre
Dielectric elastomers generators (DEGs) constitute promising systems due to their high energy density. This latter is influenced by viscoelasticity and the leakage current. An understanding of this leakage current and how it can be influenced by the stretch state of material is required to predict or optimize DEGs. In this context, our work consisted in studying the evolution of the leakage current in commercial electroactive polymer (3 M VHB4910) using silver grease as electrodes. This analysis has been performed in order to evaluate the influence of three different factors: the biaxial prestretch (λ2 = 4, 9 and 16), the temperature (from 20°C to 80°C) and the high electric field (from 1MVm-1 to 20MVm-1). Main results are (i) the increase in the leakage current at higher pre-stretch due to the increase of the electric field, (ii) a predominant Schottky conduction mechanism (iii) a lower current at room temperature for asymmetric pre-stretch compared to an equivalent area surface ratio with symmetric pre-stretch, (iv) the point iii fails when the material works at temperatures higher than room temperature. Probable changes in the molecular chains with strain explain these results.
Journal of Physics: Conference Series | 2016
Achraf Kachroudi; S. Basrour; Libor Rufer; Fathi Jomni
This paper reports a design of a new prototype of air-spaced cantilevers made from a micro-structured PDMS piezo-electret material for accelerometer and energy harvesting applications. The test performed on these cantilevers in a sensor mode exhibits a stable sensitivity of 385 mV/g for a frequency ranging from 5 Hz to 200 Hz that encompass most macro-scale vibrations. In the energy harvesting mode, the cantilever generates a power of 103 nW with a load resistance of 217 MΩ.