V. Borkar and S.K. Mitter. / Caption title. / Includes bibliographical references (p. 7). / Supported by the US Army Research Office. ARO DAAL03-92-G-0115

[pt] Sintetizar Bi-2223 por decomposição peritética com posterior recristalização para estudar a cristalinidade e o efeito da fase líquida na formação de amorfo é de suma importância em pesquisas de fitas supercondutoras 1G (1ª geração) de Bi-2223. Tal rota de tratamento de fitas 1G de fase Bi-2223 já foi demonstrada, mas ainda há desafios devido à sua estreita região de estabilidade, lenta formação e a volatilização de chumbo durante a decomposição. No presente trabalho, amostras de (Bi, Pb)-2223 com 6 porcento de fração de PbO (adicionados aos reagentes) foram aquecidas a temperaturas acima da decomposição peritética e resfriadas lentamente. Empregaram-se análises de DTA/TGA, MEV/EDS, DRX e Análise quantitativa (com uso do método de Rietveld, V(I) e R(T). Os resultados mostram que o balanço de fases e a microestrutura dependem da temperatura de decomposição para fita comercial e caseira e da fração de PbO somente para a fita caseira. Uma investigação sistemática por DRX foi realizada, a fim de elucidar tal processo. Foi observada uma recuperação parcial de Bi-2223 nas fitas estudadas e os resultados indicaram que a decomposição e a recristalização da fase Bi-2223, fase amorfa e fases secundárias geraram mudanças na microestrutura e nas propriedades elétricas da fita 1G seja monofilamentar ou multifilamentar. / [en] Synthesize Bi-2223 by peritectic decomposition with subsequent recrystallization to study the crystallinity and the effect of the liquid phase in the formation of amorphous is very important in research of superconducting tapes 1G Bi-2223. Such a route of treatment tapes 1G phase Bi-2223 has been demonstrated, but there are still challenges due to its narrow region of stability, and slow formation of lead volatilization during decomposition. In this work, samples of (Bi, Pb) -2223 with 6 percent PbO fraction (added to the reactants) were heated to temperatures above the peritectic decomposition and cooled slowly. Analyzes were employed DTA / TGA, SEM / EDS and DRX Quantitative analysis (using the Rietveld method, V (I) and R (T). Results show that the balance of phases and microstructure depend on the decomposition temperature for commercial and homemade ribbon and the fraction of PbO only homemade tape. a systematic investigation by DRX was performed in order to elucidate this process. was observed a partial recovery of Bi-2223 and studied the tapes results indicate that the decomposition and recrystallization of the phase Bi-2223 phase amorphous and secondary phases generate changes in microstructure and electrical properties of the tape 1G be monofilament or multifilament.

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.

The thesis is focused on high-temperature superconducting (HTS) ceramics series of bismuth. Specifically, there is studied Bi2Sr2CaCu2O8+x phase, known as the 2212 phase. The theoretical part describes the basic properties of superconductivity and superconductors, the properties of cuprate ceramics and description of LBCO, YBCO and BSCCO structures. The experimental part deals with the preparation of the precursor own Bi-2212 phase. It is synthesized by a process called sol-gel. Feedstock Bi2O3, CaCO3, SrCO3 and CuSO4 was dissolved in HNO3 and transferred to a complex with ethylenediaminetetraacetic acid (EDTA, Chelaton II). With NH3 pH > 9 was maintained due to the stability of complex. The obtained gel was concentrated, calcinated in a furnace at 860 °C and crushed into powder. Pure powder was subjected to analysis dipping microscope, SEM, XRD, FT-IR, TG-DTA at different atmospheres argon, oxygen and air. Then 1 wt.% of the oxide (La, Y, Sc, Sm) was added to part of the powder precursor and the samples were examined using TG-DTA.

We have acquired Hubble Space Telescope (HST) and Very Large Telescope near-infrared spectra and images of supernova (SN) Refsdal after its discovery as an Einstein cross in fall 2014. The HST light curve of SN Refsdal has a shape consistent with the distinctive, slowly rising light curves of SN. 1987A-like SNe, and we find strong evidence for a broad H alpha P-Cygni profile and Na I D absorption in the HST grism spectrum at the redshift (z = 1.49) of the spiral host galaxy. SNe. IIn, largely powered by circumstellar interaction, could provide a good match to the light curve of SN Refsdal, but the spectrum of a SN IIn would not show broad and strong H alpha and Na I D absorption. From the grism spectrum, we measure an H alpha expansion velocity consistent with those of SN. 1987A-like SNe at a similar phase. The luminosity, evolution, and Gaussian profile of the H alpha emission of the WFC3 and X-shooter spectra, separated by similar to 2.5 months in the rest frame, provide additional evidence that supports the SN. 1987A-like classification. In comparison with other examples of SN. 1987A-like SNe, photometry of SN Refsdal favors bluer B - V and V - R colors and one of the largest luminosities for the assumed range of potential magnifications. The evolution of the light curve at late times will provide additional evidence about the potential existence of any substantial circumstellar material. Using MOSFIRE and X-shooter spectra, we estimate a subsolar host-galaxy metallicity (8.3 +/- 0.1 dex and <8.4 dex, respectively) near the explosion site.

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.