Normal zone in YBa2Cu3O6+xYBa_2Cu_3O_{6+x}-coated conductors

We consider the distribution of an electric field in YBCO-coated conductors for a situation in which the DC transport current is forced into the copper stabilizer due to a weak link -- a section of the superconducting film with a critical current less than the transport current. The electric field in the metal substrate is also discussed. The results are compared with recent experiments on normal zone propagation in coated conductors for which the substrate and stabilizer are insulated from each other. The potential difference between the substrate and stabilizer, and the electric field in the substrate outside the normal zone can be accounted for by a large screening length in the substrate, comparable to the length of the sample. During a quench, the electric field inside the interface between YBCO and stabilizer, as well as in the buffer layer, can be several orders of magnitude greater than the longitudinal macroscopic electric field inside the normal zone. We speculate on the possibility of using possible microscopic electric discharges caused by this large ($\sim$kV/cm) electric field as a means to detect a quench.Comment: 8 pages, 4 figure

Experimental and numerical analysis of quench propagation on MgB 2 tapes and pancake coils

In the design and feasibility of electric power applications with MgB2 conductors, thermal conductivity plays an important role. In composite wires and tapes it is mainly determined by the amount of stabilizer (usually copper), while for coils the electric insulation and epoxy binder characteristics are added. In this paper we present results on quench development and propagation on isolated superconducting tapes and small single pancake coils cooled by thermal conduction, for three types of MgB2 tape with different stabilizations. The quench parameters at different temperatures, such as minimum quench energy (MQE), minimum propagation zone (MPZ) and quench propagation velocity (vp), are reported. A complete electrical and thermal characterization of the superconducting tapes and of non-superconducting materials used in the coil manufacture has been made. With these data, numerical computational models using finite element simulations have been performed for isolated wires and single pancake coils, and the results of the relevant quench parameters have been compared with the measured values

UV femtosecond laser cleaning of encrusted historical stained-glasses

Laser irradiation enables the removal of unwanted surface deposits from different materials in a safe and controllable manner. Laser parameters should be carefully selected to achieve the removal of the target contaminants without inducing damage to the substrate. Ultra-short pulse lasers have opened new opportunities for safe and controlled decontamination of cultural heritage materials because the thickness of material that is affected by the laser is limited. In this study, an ultraviolet femtosecond pulsed laser was used for the removal of unwanted encrustation formed on the surface of an historical colourless stained-glass sample from the Cuenca Cathedral in Spain. One of the sides of this glass exhibits a reddish-brown grisaille that also has to be preserved. A laser cleaning process has been designed to avoid heat accumulation while controlling the thickness of ablated material. In this context, a multi-step process was selected in order to be able to eliminate, in a controlled way, the crust layer without damaging the grisaille layer, or the glass substrate. In this case, laser irradiation in beam scanning mode with a pulse repetition frequency of 10 kHz proved to be effective for the safe cleaning of the glass. The latter was analysed before and after laser cleaning by optical and confocal microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray fluorescence, and Raman spectroscopy, confirming that the crust layer was effectively eliminated without damaging the surface

A study of supercooling of the disordered vortex phase via minor hysteresis loops in 2H-NbSe_2

We report on the observation of novel features in the minor hysteresis loops in a clean crystal of NbSe_2 which displays a peak effect. The observed behavior can be explained in terms of a supercooling of the disordered vortex phase while cooling the superconductor in a field. Also, the extent of spatial order in a flux line lattice formed in ascending fields is different from (and larger than) that in the descending fields below the peak position of the peak effect; this is attributed to unequal degree of annealing of the state induced by a change of field in the two cases.Comment: 5 pages of text + 6 figures, submitted to Phys. Rev.

Peak Effect, Fishtail Effect and Plateau Effect : The Reentrant Amorphization of Vortex Matter in 2H-NbSe_2

The magnetic field dependence of the critical current is studied in single crystal samples of the weak pinning type-II superconductor 2H-NbSe$_2$ in the high temperature and the low field region of the (H,T) phase space, in the vicinity of the reentrant peak effect. The experimental results demonstrate various pinning regimes : a collective pinned quasi-ordered solid in the intermediate field that is destabilized in favor of disordered vortex phases in both high fields near H$_{c2}$ and at low fields near H$_{c1}$. The temperature evolution of the pinning behavior demonstrates how the amorphous limit (where the correlation volume is nearly field independent) is approached around the so-called nose region of the reentrant peak effect boundary. Furthermore, the data show that the rapid approach to the amorphous limit naturally yields a peak effect, i.e., a peak in the critical current, in the high field regime, but yields a ``plateau effect'' instead in the low field regime in an analogous way. With increasing effective disorder the peak effect shifts away from H$_{c2}$ and resembles a ``fishtail'' anomaly.Comment: 6 pages of text and 4 figures. Paper submitted to Phys. Rev.

II Escuela de materiales: una propuesta para introducir la enseñanza sobre materiales en educación secundaria y bachillerato

Trabajo presentado al XI Congreso Nacional de Materiales: "Educación en Materiales", celebrado en Zaragoza del 23 al 25 de junio de 2010.Cuando la Sociedad Española de Materiales encargó al Instituto de Ciencia de Materiales de Aragón la organización de la II Escuela de Materiales le pidió que estuviera especialmente dirigida a profesores de educación secundaria y bachillerato. Por ello, desde un principio se organizó en colaboración con el Programa Ciencia Viva. El objetivo era dedicar la Escuela a un tema que permitiera introducir los materiales a partir de diversos experimentos que se pudieran realizar con facilidad en las aulas. En este contexto se eligió como tema “Aplicaciones de los cristales líquidos”. La estructura de la escuela ha constado de dos fases. En una primera sesión presencial, investigadores explicaron a los profesores participantes los fundamentos necesarios para comprender cómo funcionan las diversas aplicaciones. En una segunda fase, los propios profesores han trabajado en grupo para elaborar diversos materiales didácticos. Algunas empresas que comercializan productos basados en cristales líquidos han aportado materiales para ser utilizados en las aulas.Peer reviewe

Microstructure and Transport Properties of Bi-2212 Prepared by CO2 Laser Line Scanning

3rd International Conference on Superconductivity and Magnetism (ICSM) -- APR 29-MAY 04, 2012 -- Istanbul, TURKEYWOS: 000317014500040A novel Laser Line Scanning method has been applied to process rectangular cross-section Bi-2212 monoliths containing 2.9 % Ag using a CO2 Laser. Although previous work has suggested the use of nIR lasers (a parts per thousand 0.8-1.1 mu m) for melt-processing metal oxide superconductors, the results obtained here demonstrate that mid-IR radiation from a CO2 laser (10.6 mu m) may be just as convenient for such a purpose while it enables processing large surface areas. The samples described here were processed at traverse rates ranging between 15 and 60 mm/h, exhibited a complex textured microstructure and yielded highest I (c) values of 71 A at 77 K

Laser cleaning of archaeological bronze artefacts

The study is devoted to the development of tailored laser cleaning procedures for the selective removal of dangerous corrosion products on archaeological bronze artefacts, without affecting the metallic substrate. In order to optimise the cleaning methodology and assess the real feasibility of the laser treatments as conservation procedure for metallic works of art, a deeper knowledge of the laser-material interaction and an investigation of the effect of the different experimental parameters on the metallic surface are necessary. In this study, a set of artificially-patinated Cu-based reference samples were cleaned by using a near-IR Q-switched Yb:YAG fibre laser. Several laser parameters (e.g. pulse duration and output laser power) were considered. The artificial patinas and the cleaned samples were characterised from the chemical, microstructural and morphological point of view by means of X-Ray micro-diffraction, scanning electron microscopy and confocal microscopy in order to assess the effect of the laser parameters as a function of the composition of the patina. The preliminary experimental findings show that laser treatment is a powerful technique for cleaning ancient metallic artefacts; however, a careful selection of the laser parameters has to be performed in order to avoid any damage to the metallic surface. Further investigations are in progress to better understand the etching mechanism of the patina and to develop a non-invasive cleaning methodology