L'étude théorique du fonctionnement d'un dispositif SHT nécessite la résolution d'équations couplées magnétothermiques. Etant donné le couplage fort qui existe pour ces matériaux, l'utilisation d'outils numériques est quasiment indispensable. Un code de calcul basé sur la Méthode des Différences Finies, permettant de résoudre des problèmes 1D et 2D, a été développé dans ce sens. Il est alors possible de simuler numériquement le comportement des SHT.<br />L'étude des pertes dans une amenée de courant en Bi-2223, parcourue par du courant alternatif 50 Hz, soumise un champ magnétique continu et plongée dans un bain azote liquide, est ensuite réalisée de manière théorique et expérimentale. Des instabilités thermiques ont été observées expérimentalement. Ce phénomène a été étudié à partir de la recherche des solutions d'équilibre stable et instable. On a pu ainsi définir, pour un courant et un champ magnétique donné, une température maximale au-dessus de laquelle la récupération n'est plus possible.<br />Les pastilles YBCO peuvent piéger de forts champs magnétiques et ainsi réaliser des cryoaimants très performants. La réponse dynamique de ses pastilles, soumises à des variations de champ magnétique, est abordée de manière détaillée (distributions de la densité de courant, du champ magnétique et de la température). Les résultats de simulations montrent des différences significatives lorsque les influences du champ magnétique et de la température sont prises en compte dans la loi de comportement E(J). Un optimum du champ magnétique maximal à appliquer a pu être déterminé. Cette information est intéressante car elle permet un dimensionnement efficace du dispositif d'aimantation pulsé.

The Bi2Sr2CaCu2O7+d, i.e. Bi2212 phase of bismuth derived high temperature superconductors (HTS), powder precursor were synthesized via sol – gel technique using ethylendiamintetraacetic acid (Chelaton II) as a chelating agent. Metal nitrate’s solutions were mixed with EDTA suspension. The pH value was adjusted to 9 by NH4OH by reason forming of stable metal’s complexes. The mixture was heated to 80 °C for gelation. Solution taken before solid gel was form is used for preparation of Bi2212 layer on to a-Al2O3 surface via spin coating deposition’s technique. Reactive powder used for bulk sample preparation was made by calcination (800 °C) of pyrolyzed xerogel (500 °C). Sintering in oxygen atmosphere was proceeding at temperature from 850 to 880 °C. Bulk sample properties were compared with sample prepared by common ceramic method. Simultaneous TG-DTA, IR spectroscopy and heating microscopy were used for sample characterization. Furthermore, construction of furnace for sintering in O2 atmosphere was described.

Effects of Nb2O5 addition on the formation and properties of the Bi-based superconductors were studied. The superconducting samples were synthesized by two different methods, the solid state reaction (SSR) and ammonium nitrate melt route (ANM) to obtain information about the growth techniques. Ten samples (five with each method) with the chemical compositions of Bi1.6Pb0.4Sr2Ca2Cu3NbxOy, with x= 0.00, 0.05, 0.10, 0.20, and 0.30 were prepared. Phase identification, structural and microstructural analysis were carried out using powder X-ray diffraction method, scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The transition temperature and the critical current measurements were carried out using the electromagnetic measurement system. The critical parameters of the samples were also determined by AC susceptibility and magnetization measurements. Improvements in the formation of the Bi-2223 phase with Nb addition have been observed by both methods. The ANM method leads mixtures of Bi-2223 and Bi-2212 phases, but with the SSR method pure Bi-2223 phase was obtained. With small amounts of Nb addition the critical temperature, Tc, remains the same for ANM samples and it increases for the SSR samples. For both series of samples Tc decreases with further increase of Nb content. With Nb addition the critical currents of the ANM sample almost remain the same and those of the SSR samples increases. Observations based on AC susceptibility measurements showed that Nb addition improve inter-grain coupling. Inductive critical current densities obtained from magnetization measurements agree with the critical currents stated above and show that Jc increase with increase Nb content.