Magnetical Response and Mechanical Properties of High Temperature Superconductors, YBaCu3O7-X Materials

Since the discovery of the second generation High Superconductor Materials (2G-HTSC) in 1986, their magnetic properties have been widely studied by different research groups around the world. During the last years, the mechanical properties at micro-/ and nanometric scale are starting to be studied in order to know and improve the durability of conventional devices. In this book chapter, we would like to focus our attention on the magnetical response and also the mechanical properties of 2G-HTSC. In relation to the magnetic response, the Meissner effect is one important signature of the superconductivity. In this case, a diamagnetic response is observed due to exclusion of the magnetic flux of the interior of the superconducting material when the temperature is below the critical temperature. This important property allows to distinguish a superconducting material from a conducting one. But in several cases the superconducting materials exhibit a paramagnetic response instead of the conventional diamagnetism. This effect is frequently called paramagnetic Meissner effect (PME). In this case, the magnetic flux is not expelled, and a paramagnetic state can be originated. This effect is observed in several magnetic field regimes, and in some cases the paramagnetic response increases with the applied magnetic field, but in others the paramagnetic response decreases when the magnetic field increases..

Microestructura y propiedades mecánicas del material masivo superconductor YBCO a 300 Y 77 K.

En este trabajo se realiza una caracterización mecánica y microestructural del material masivo superconductor YBCO. El material ha sido procesado mediante dos técnicas distintas, Top-Seeding Melt Growth (TSMG) y Bridgman, y este estudio profundiza en el efecto de la microestructura, el método de procesado y la temperatura de ensayo en el comportamiento mecánico de material. Con el fin de conseguir un amplio conocimiento de sus propiedades mecánicas se han realizado ensayos de resistencia a flexión, tenacidad de fractura y dureza Vickers a 300 y 77 K. Asimismo, se llevaron a cabo ensayos de nanoindentación y el tamaño crítico del defecto semielíptico. Los resultados obtenidos muestran que el comportamiento mecánico de los dos materiales está controlado por defectos y grietas, introducidas durante el procesado. También se ha encontrado un buen acuerdo entre el tamaño del defecto critico detectado experimentalmente con los valores obtenidos mediante de análisis de mecánica de fractura

Comportamiento mecánico de materiales masivos superconductores de segunda generación en función de la temperatura

En este trabajo se han analizado dos materiales masivos superconductores de base YBaCuO, con el objetivo de analizar la influencia del método de procesado (método de Bridgman y método Top-Seeding melt growth) y de la temperatura de ensayo en su comportamiento mecánico. Ambos materiales se ensayaron a temperatura ambiente (300 K) y a baja temperatura (77 K), realizándose ensayos rotura y de tenacidad de fractura en flexión en tres puntos. Además, en uno de los materiales, que presentaba anisotropía microestructural, se realizaron ensayos en las dos direcciones microestructuralmente más relevantes. Los resultados obtenidos muestran que el comportamiento mecánico del material está controlado por los defectos y grietas introducidas durante el procesado y, por lo tanto, si se quiere mejorar las propiedades, debería reducirse la cantidad y el tamaño de estas imperfeccione

Accessing the phase transformation and deformation behavior of metastable stainless steels through cyclic nanoindentation

Austenitic metastable stainless steels are a materials group distinguished by their excellent mechanical properties, offering high potential for further improvement by thermo-mechanical treatments. Under deformation, these steels undergo a complex deformation and phase transformation. Their mechanical properties at macroscale, such as strength, ductility or fatigue behavior, have been largely investigated, yet they are not always predictable, as they highly depend on the microstructural characteristics of the material. In order to achieve a better understanding at the microstructural level, this work aims at the investigation of the deformation mechanisms in metastable stainless steels at sub-grain level and the interaction between grains. Therefore, monotonic and cyclic nanoindentation tests were performed in order to increase the cumulative deformation in a controlled way with the number of cycles. The emerging deformation mechanisms under the indents and on the surface, as well as the resulting morphology and mechanical and magnetic properties of the different phases, were characterized through different advanced microscopy techniques. It was found that, even after a high number of nanoindentation cycles, a loading-unloading hysteresis is present, indicating a reversible plastic behavior (which is believed to be due to formation of unstable dislocations at maximum load). The apparent hardness of the material drops with increasing cycles due to the high plasticity of austenitic stainless steels. Gradual phase transformation was triggered and the load–displacement curves exhibited features, such as pop-ins and changes in the slope and hysteresis size, probably related to the propagation of the induced martensite to the neighboring grains and the resulting stress relaxation. This behavior was found to be highly dependent on the crystalline orientation of the respective indented austenite grains. Nucleation of martensite at shear band intersections was detected by TEM investigation of a horizontal lamella, as well as by MFM, while a FIB tomography revealed the shape and location of the nucleated martensitic zones

Nanoindentation of Bridgman YBCO samples

In this study, the mechanical properties of YBa2Cu3O7−x, obtained by the Bridgman technique, were examined using a Berkovich tip indenter on the basal plane (0 0 1). Intrinsic hardness was measured by nanoindentation tests and corrected using the Nix and Gao model for this material. Furthermore, Vickers hardness tests were performed, in order to determine the possible size effect on these measurements. The results showed an underestimation of the hardness value when the tests were performed with large loads. Moreover, the elastic modulus of the Bridgman samples was 128 ± 5 GPa. Different residual imprints were visualised by atomic force microscopy and a focused ion beam, in order to observe superficial and internal fracturing. Mechanical properties presented a considerable reduction at the interface. This effect could be attributed to internal stress generated during the texturing process. In order to corroborate this hypothesis, an observation using transmission electron microscopy was performed

Dynamic deformation of metastable austenitic stainless steels at the nanometric length scale

Cyclic indentation was used to evaluate the dynamic deformation on metastable steels, particularly in an austenitic stainless steel, AISI 301LN. In this work, cyclic nanoindentation experiments were carried out and the obtained loading-unloading (or P-h) curves were analyzed in order to get a deeper knowledge on the time-dependent behavior, as well as the main deformation mechanisms. It was found that the cyclic P-h curves present a softening effect due to several repeatable features (pop-in events, ratcheting effect, etc.) mainly related to dynamic deformation. Also, observation by transmission electron microscopy highlighted that dislocation pile-up is the main responsible of the secondary pop-ins produced after certain cycles.Peer ReviewedPostprint (author's final draft

Measurement of the Bs0→J/ψKS0B_s^0\to J/\psi K_S^0 branching fraction

The $B_s^0\to J/\psi K_S^0$ branching fraction is measured in a data sample corresponding to 0.41$fb^{-1}$ of integrated luminosity collected with the LHCb detector at the LHC. This channel is sensitive to the penguin contributions affecting the sin2$\beta$ measurement from $B^0\to J/\psi K_S^0$ The time-integrated branching fraction is measured to be $BF(B_s^0\to J/\psi K_S^0)=(1.83\pm0.28)\times10^{-5}$. This is the most precise measurement to date

Measurement of the CP-violating phase \phi s in Bs->J/\psi\pi+\pi- decays

Measurement of the mixing-induced CP-violating phase phi_s in Bs decays is of prime importance in probing new physics. Here 7421 +/- 105 signal events from the dominantly CP-odd final state J/\psi pi+ pi- are selected in 1/fb of pp collision data collected at sqrt{s} = 7 TeV with the LHCb detector. A time-dependent fit to the data yields a value of phi_s=-0.019^{+0.173+0.004}_{-0.174-0.003} rad, consistent with the Standard Model expectation. No evidence of direct CP violation is found.Comment: 15 pages, 10 figures; minor revisions on May 23, 201