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Imagerie Magnéto Optique pour les matériaux supraconducteurs

Abstract : This work was devoted to the realization and the use of a device of imagery Magnéto-Optique (MO) for the study of superconductors. The principal originality of this device is to allow measurements under current of transport (30A), under magnetic field (until 100mT), and/or under axial stress (80kg maximum).
The MO studies are related to high temperature superconductors taht are elaborated or studied at the CNRS-CRETA and laboratory of Crystallography. In particular, a device for the rolled assisted texturation under magnetic field of Bi2212 ribbons was developed. Since we precisely know the conditions of texturation, it possible to better interpret the data resulting from the characterizations, and in particular of the MO images. We could highlight by MO obervations a texture amelioration under magnetic field : the trapped flux is higher, and more important for the filaments located at the center of the ribbons. In addition the measured critical current at 4,2K is increased by 30 to 40 % thanks to the texturation under magnetic field.
In melt textured YBCO, we highlighted thanks to the MO images that the adaptation of the oxygen rate in the atmosphere of the furnace at the time of the phases of rise and descent in temperature is crucial for obtaining a homogeneous oxygen doping. The study carried out thanks to the MO device revealed that the drilling of a network of holes allows creates a significant improvement of the quality of the texture and of the local current density.
Coated Conductors provided by Theva also were the subject of MO studies. The MO observations under axial stress revealed that the critical current was limited by the appearance of contraction bands (Lüders Bands) in the substrate, inducing cracks in the plug and superconductive layers. These cracks were observed for the first time with MO during this thesis. The MO imagery also makes it possible to highlight the presence of horizontal cracks due to the side compression of the MgO layer.
Finally self field measurements in thin films were made in parallel with MO observations. It appears that the defects created artificially by lithography do not involve a measurable difference on the critical current density (although the local density of current exceeds Jc), but rather an increase in the flux flow or flux creep resistivity.
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Contributor : Alain Villaume <>
Submitted on : Friday, September 15, 2006 - 4:58:17 PM
Last modification on : Friday, July 10, 2020 - 7:58:34 AM
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  • HAL Id : tel-00087773, version 2

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Alain Villaume. Imagerie Magnéto Optique pour les matériaux supraconducteurs. Supraconductivité [cond-mat.supr-con]. Université Joseph-Fourier - Grenoble I, 2006. Français. ⟨tel-00087773v2⟩

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