Magnétostriction | EPFL Graph Search

La magnétostriction désigne la propriété que possèdent les matériaux ferromagnétiques de se déformer en fonction de l'orientation de leur aimantation, par exemple sous l'influence d'un champ magnétique. Les matériaux ferromagnétiques présentent aussi un effet magnétostrictif inverse, appelé effet magnéto-mécanique, qui se caractérise par la modification de la susceptibilité magnétique, voire de l'aimantation, en présence de contraintes mécaniques dans le matériau. Mise en évidence James Prescott Joule démontra son existence en 1847 en mesurant l'allongement d'un échantillon de fer soumis à un champ magnétique. La magnétostriction contribue au ronronnement qu'on peut entendre près des transformateurs et des appareils électriques fonctionnant sous haute tension, d'une fréquence fondamentale égale à deux fois celle du courant, c'est-à-dire 2× = en Europe. La force électromécanique I×B, produit du courant I par le champ B, participe aussi à ce bruit en agissant sur les cond

Key-features in processing and microstructure for achieving giant electrostriction in gadolinium doped ceria thin films

Husnain Ashraf, Mahmoud Hadad, Paul Muralt, Silviu Cosmin Sandu

Gadolinium doped ceria is a well-known oxygen ion conduction material for solid fuel cell electrolytes. With its centrosymmetric average lattice and relatively low bulk dielectric constant it does not look interesting for electromechanical applications. However, a giant electrostriction (ES) effect was recently found in Ce0.8Gd0.2O2-X thin films. It was explained by the dynamic response of oxygen vacancies to an external electric field. In this work, the giant ES response has been reproduced in sputter deposited thin films. The proper transverse bulk ES coefficient has been derived from the measured clamped value. For this purpose, the Young's modulus was measured by nanoindentation. The highest ES coefficient was found as 9.0 x 10(-19) (m/V)(2) for the strain coefficient, and 2.3 x 10(-7) N/V-2 for the effective stress coefficient. Specific growth conditions must be chosen in order to obtain a microstructure exhibiting the giant ES effect. There is evidence for a higher oxygen deficiency than needed to compensate the gadolinium dopants (Gd'). It was observed that the nature of the bottom electrode impacts on the size of the effect. The highest response was obtained at films grown on Al bottom electrodes. To learn more on the mechanisms of the giant ES effect, a bipolar cycling was performed to test the delay time for the ionic reorientation in changing the sign of polarization, as observed in the stress loop. The maximal response was observed below 100 Hz in this bipolar mode, showing that the time for 180 degree reorientation amounts to several milliseconds. (C) 2016 Acta MateriaIia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2016

Modélisation et réglage d'un entraînement à haute performance par un moteur réluctant

Michel Girardin

Usually, each phase of the direct reluctance motor is fed separately by its own inverter producing a squarewave phase current depending on the rotor position. In the present work, a star-connection of the three stator phases is used and the direct reluctance motor is fed by a three-phase switched inverter. The space phasor theory shows that three-phase sinewave currents can produce a constant electromagnetic torque. In this case, the rotating reference frame rotates at half of the rotor speed and in the opposite direction. This command strategy allows to reduce the stator deformation due to the electromagnetic radial force which is very high in this type of motor and which is mainly responsible for the acoustic noise. The electromagnetic torque control is realised by a sliding mode control of the three phase currents. In the rotating reference frame, it is possible to show that the eight commutation states of the switched inverter are placed around an ellipse in the complex plane of the derivatives of the space phasor of the phase current. A study of this ellipse has allowed to determine the maximal torque as a function of the speed for a given continuous voltage. It is important to insure that the torque reference will never exceed this limitation in order to avoid a pullout phenomenon, highly unwanted in the speed control. Despite the fact that the resolver inside the casing of the direct reluctance motor produces a small periodic error, it is accurate enough for the position measurement and to produce the three current references. However, the derivative of the position measurement provides a speed evaluation with such a ripple that the speed control cannot be outstanding. So, for the speed measurement, an additional laser rotary encoder is used. The experimental study in the steady-state mode of the speed control shows the presence of an electromagnetic torque ripple. This is a residual ripple caused by the dissimilarity between the stator poles teeth-shapes and the magnetic saturation effect. Although this torque ripple is rather low compared with the usual command strategy, two methods are proposed in order to compensate it. The first method is based on the use of a correction factor depending on the tooth angle. First, in steady-state conditions, the correction factor is stored off-line in a table. Then it is used in-line as a feedforward disturbance compensation. The second method of reducing the residual torque ripple consists in implementing an observer of variable disturbance. This is a disturbance observer able to determine the frequency content of the torque ripple. For instance, in order to observe the continuous component, the fundamental wave as well as the harmonics 2, 3, and 6, the observer must have ten poles. The main difficulty to deal with such an observer is the fact that each pole moves quickly as a function of the motor speed. So, in order to guarantee the observer stability over a large range of speed, the poles are located dynamically regarding to the motor speed by the way of a discontinuous adaptation of the feedforward coefficients. The theoretical study and the numerical simulation have allowed to formulate some criteria about the way of locating the poles. Experimental results confirm the efficiency of the observer of variable disturbance in order to reduce the electromagnetic torque ripple.

EPFL2000

Nanoionics Phenomena and Devices with Oxide Thin Films

Mahmoud Hadad

The number of studies on solid state ionic thin films (Nanoionics) has increased dramatically over last decades due to their high potential in micro and nanoscale technologies. Micro solid oxide fuel cells (micro-SOFC), and resistive switching memories (ReRAMs) are the most promising topics in this field, having impacts on portable energy sources, and future non-volatile memories. One can also add the new discovery of giant electrostriction (GES) in Gd-doped ceria (CGO), proposing ion conducting thin films as a new electromechanical family. While very astonishing results have been obtained, there are still some remaining challenges that are needed to be addressed: a) Mechanical stability in micro-SOFC, b) performance study of classically known fast ion conducting -i.e. yttrium stabilized zirconia (YSZ) and CGO- in ReRAMs, and c) Investigating and understanding of the acting mechanism leading to novel properties in CGO thin films. In micro-SOFC, we have proposed a ring-shaped corrugated structure that can result in more stable membranes up to millimeters wide openings. Stable liberation borders, low mechanical buckling, increased area density in given volume, and high surface utilization are the main characteristics of such membranes. The electrochemical performance of a 500 um wide micro-SOFC cell has been measured at 350 °C and a power density of 12.5 mW/cm2 has been obtained. This value is close to the reported performance in the literature at the same temperature. An open circuit voltage of 0.7 V was achieved. The optimal membrane diameter in a single PEN cell (Positive electrode-Electrolyte-Negative electrode) was identified as 0.5 mm. For upscaling to several Watts of electrical output, it is proposed to fabricate a planar array of such cells. Furthermore, we have studied the potential of CGO and YSZ thin films to be used as switching layer in ReRAMs. Both films have shown outstanding electrical characteristics comparable to high-performance switching materials with high stability and endurance. CGO thin films are able to operate at V(set)=1 V and V(reset)=-0.6 V over multiple cycles. YSZ thin films switch between V(set)=1.1 V and V(reset)=-0.6 V. Both cells have shown stable response over 10000 cycles with ON/OFF ratio of 100 and 5 for CGO and YSZ, respectively. It turned out that the switching mechanism in CGO films is rather of multi-filamentary mechanism as the low resistance state (LRS) and forming voltage decreases with increasing the device size. In contrast, in the YSZ-based memories, less filaments are expected to be formed since the scaling effect was not clearly observed. Finally, we have investigated the GES in CGO thin films and we succeeded to reproduce the giant response, which was found to take place only at low frequency. We have shown that the giant electrostriction is related to the defects which are produced during rather specific deposition parameters such as low deposition temperature, an optimal ion bombardment, and high deposition rate. It has been found that a higher oxygen vacancy concentration than introduced by Gd doping is needed. Additional oxygen vacancies are most likely compensated by the reduction of Ce(4+) to Ce(3+). A large dielectric constant of over 1000 was observed at low frequency reminding at the phenomenon of the colossal dielectric constant. Moreover, the films are characterized by a strong nonlinear I-V behavior with a threshold voltage of about 10 V, very much like a varistor.