Niobium metal is the pure element with the highest superconducting critical temperature (T_c = 9.2 K), which is present in many applications. Particularly, in superconducting radio frequency (SRF) cavities of particle accelerators, the control of the surface characteristics of pure Nb is crucial, as the presence of defects may generate magnetic flux pinning that can increase by more than two orders of magnitude the surface critical current, ic. Several procedures such as chemical- or electro-polishing have been used aiming at cleaning surface contamination and decreasing its roughness. Sub-nanosecond lasers can be applied to generate a broad range of micro and nanostructures (e.g. Laser-Induced Periodic Surface Structures, LIPSS) that strongly modify the materials properties - as wettability, color, oxidation resistance or antibacterial behavior. In this work, we analyze a variety of surface structures generated on pure Nb sheets with different laser systems (UV, Vis and n-IR, fs and ps) by exploring a range of processing parameters. These include pulse overlap, irradiance or the effective number of pulses, under different atmospheres (air, N2, Ar, vacuum). The effects on Tc, critical currents and critical fields (Bc1, Bc2 and Bc3) have been obtained from magnetization, ac susceptibility and heat capacity measurements, revealing their dependence with the different surface nanostructures and the chemical changes generated with these laser treatments.

Trabajo presentado al 2nd International Symposium on Applied Sciences and Engineering (ISASE), celebrado online del 7 al 9 de abril de 2021.

The electromagnetic and thermal properties of a double pancake coil made of second generation high temperature superconductor, 2G-HTS, have been studied. The coil was wound with no-insulation between turns (NI coil) and was later impregnated with epoxy resin and glued to a copper support plate. The coil was thermally anchored to the cryocooler cold finger and cooled by conduction. After several thermal cycles no degradation of its superconducting properties was observed. The coil was operated under high vacuum and high currents (up to 400 A in steady conditions) at different temperatures in the range between 5 K and 77 K, with special focus on the analysis above 30 K. The charge and discharge characteristics, and the experimentally measured and numerically estimated critical currents, have been studied. The different loss contributions during current ramp and the thermal contact conductance between different parts of the double pancake coil have been measured. The implications of these two factors on the thermal stability and the behaviour of the whole cryogenic system are discussed.

It is well known that the use of ultrashort (fs) pulsed lasers can induce the generation of (quasi-) periodic nanostructures (LIPSS, ripples) on the surface of many materials. Such nanostructures have also been observed in sample's surfaces irradiated with UV lasers with a pulse duration of 300 ps. In this work, we compare the characteristics of these nanostructures on 1-mm and on 25-mu m thick niobium sheets induced by 30 fs n-IR and 300 ps UV pulsed lasers. In addition to conventional continuous or burst mode processing configurations, two-dimensional laser beam and line scanning modes have been investigated in this work. The latter allows the processing of large areas with a more uniform distribution of nanostructures at the surface. The influence of the generated nanostructures on the superconducting properties of niobium has also been explored. For this aim, magnetic hysteresis loops have been measured at different cryogenic temperatures to analyse how these laser treatments affect the flux pinning behaviour and, in consequence, the superconductor's critical current values. It was observed that laser treatments are able to modify the superconducting properties of niobium samples.

Resumen del trabajo presentado al European Congress and Exhibition on Advanced Materials and Processes (EUROMAT), celebrado en Estocolmo (Suecia) del 1 al 5 de septiembre de 2019.

Resumen del trabajo presentado al International Symposium Fundamentals of Laser Assisted Micro- & Nanotechnologies (FLAMN), celebrado en San Petersburgo (Rusia) del 30 de junio al 4 de julio de 2019.

Trabajo presentado al Spring Meeting of the European Materials Research Society (E-MRS), celebrado del 27 al 31 de mayo de 2019 en Niza (Francia).

Trabajo presentado a la XXXVII Reunion Bienal de la Real Sociedad Espanola de Fisica celebrada en Zaragoza del 15 al 19 de julio de 2019.

The generation and propagation of quench induced by a local heat disturbance or by overcurrents in MgB2 Rutherford cables have been studied experimentally. The analysed cable is composed of 12 strands of monocore MgB2/Nb/Cu10Ni wire and has a transposition length of about 27 mm. Measurements of intra-and inter-strand voltages have been performed to analyse the superconducting-to-normal transition behaviour of these cables during quench. In case of external hot-spots, two different time-dynamic regimes have been observed, a slow stage for the formation of the minimum propagation zone (MPZ), and a fast dynamics once the quench is triggered and propagates to the rest of the cable. Significant local variations of the quench propagation velocity across the strands around the MPZ have been observed, but with average quench propagation velocities closely correlated with the predictions given by one-dimensional-geometry models. For quench induced by overcurrents (i.e. with applied currents higher than the critical current) the nucleation of many normal zones distributed within the cable, which overlap during quench propagation, gives a distinctive and faster quench dynamics.

Trabajo presentado a la 6th International Conference on Superconductivity and Magnetism (ICSM), celebrada en Antalya (Turquia) del 29 de abril al 4 de mayo de 2018.

Trabajo presentado a la 13th European Conference on Applied Superconductivity (EUCAS), celebrada en Geneva (Suiza) del 17 al 21 de septiembre de 2017.

A cermet composed of a metallic component (nickel) and a ceramic matrix (yttria stabilized zirconia) is commonly used as the anode for solid oxide fuel cells (SOFC). In the present work we intend to improve the performance of Ni-YSZ anodes by surface laser melting. Symmetrical cells, consisting of two NiO-YSZ anodes (similar to 20 pin thickness) separated by a relatively thin YSZ electrolyte (similar to 500 pm) were fabricated by convectional ceramic techniques. Subsequently, laser melting treatments of both anodes were performed using a CO2 laser system, producing a NiO-YSZ eutectic lamellar microstructure. Laser power of 100W and processing rates of 1 mm s(-1) were determined as the optimum processing conditions. Symmetrical processed plates (eutectic sample) were electrically characterized by impedance spectroscopy (EIS), and the results were compared with non-processed plates (ceramic sample). Preliminary EIS results showed that the polarization resistance at higher temperatures (in the range of 900 degrees C) is about 0.5 Omega cm(2) for both the eutectic and the ceramic sample. However, at lower temperatures (in the range of 800 degrees C) the polarization resistance for both samples differs considerably (2.9 and 1.6 Omega cm(2) for the ceramic and eutectic sample, respectively). These experiments confirmed that optimization of the microstructure by laser surface treatment plays a crucial role in the electrochemical properties of the anode cermets. (C) 2015 Elsevier B.V. All rights reserved.

Trabajo presentado a la 14th European Conference on Applied Superconductivity (EUCAS), celebrada en Glasgow (Escocia) del 1 al 5 de septiembre de 2019.