X. Granados

Growth, nanostructure and vortex pinning in superconducting YBa2Cu3O7 thin films based on trifluoroacetate solutions

Chemical solution deposition (CSD) is a very competitive technique to obtain epitaxial films and multilayers of high quality with controlled nanostructures. Based on the strong attractiveness from the cost point of view, the production of long length coated conductors based on the CSD approach is being extensively developed. The trifluoroacetate route (TFA) is the most widely used route to achieve epitaxial YBa2Cu3O7 (YBCO) layers with high critical currents, however a deep understanding of all the individual consecutive processing steps, as well as their mutual influence and relationship, is required to achieve superconducting materials with high performance. In this work, we review advances in the knowledge of all the steps relevant to the preparation of YBCO thin films based on TFA precursors as a CSD methodology: solution preparation and deposition, pyrolysis processes, intermediate phase evolution, nucleation and growth phenomena, microstructural evolution and its influence on percolating supercurrents, as well as vortex pinning by natural existing defects. Finally, we discuss the open issues still existing in the TFA approach, particularly that of film nanostructuration, and we provide a future outlook for this outstanding methodology.

Extraordinary thermopower in magnetoresistive (La1−xYx)0.67Ca0.33MnO3 oxides

We report that the thermopower Q(T) of the (La1−xYx)0.67Ca0.33MnO3 (x=0–0.25) magnetoresistive oxides, displays a pronounced peak at temperatures Ts close to the ferromagnetic ordering temperature. The temperature Ts decreases monotonously when increasing x whereas the maximal Q(Ts) is found to increase with the Y contents. The huge enhancement of Q at Ts is found to be as high as 460% for x=0.25. It is shown that the extraordinary thermopower observed at Ts results from the reduced charge carrier mobility when approaching Ts and the variation of Q(T) with the Y contents reflects the accompanying band narrowing.

Radial and axial flux superconducting motors in a levitating rotor configuration

The capability of superconducting blocks to be partially magnetized allows both bearing and powering functions in a conventional stator and superconducting rotor configuration. This levitating rotor concept is investigated by means of the basic parameters: torque, levitation and centering forces in two motors with radial and axial excitation respectively. We report on the design and behavior of a high speed motor with a cylindrical HTSC rotor excited by a conventional four poles four phases armature generating a radial field. We also report on a second motor with a disk shaped HTSC rotor excited by a conventional, four poles three phases, double armature which generates an axial field in the gap. Torque, power and centering and levitating forces measurements are also provided.

Low-power superconducting motors

Since the discovery and development of adequate superconducting materials, the development of motors has been a challenge for applications. The basis of design, however, has been the substitution of copper wires by superconducting tapes in coils in order to obtain a higher working field, thus improving power density and efficiency. In the case of high-power motors, the benefit is clear. The cost of the materials, cryogenics and building procedures could be assumed by the clear benefit in size, weight, efficiency, and, in some cases, reliability. Otherwise, low-power motors require a different treatment. Superconducting wires are insufficiently developed to produce small coils for a high field with the adequate shape. Air gaps are more critical than in large motors and cost has a major impact. Only very specific applications, in which standard technology cannot give a reliable and satisfactory solution, could benefit from superconductivity. This paper summarizes our work realized by applying superconducting pellets in low-power motors, thus improving their power density, reliability, dynamics and regularity. Applications to cryogenics, control and high speed have been our focus.

High critical current YBa2Cu3O7 artificial joints using Ag foils as welding agent

A new high quality joining technique of melt processed YBCO has been developed based on interfacial melting induced by a metallic silver foil inserted between two single domain YBCO tiles. This simple technique allows joining large monoliths to form the complex-shaped pieces required for many power applications. Hall probe magnetic flux profile analysis under magnetic field and high field magnetoresistance measurements demonstrate that the superconducting behaviour of the joint is indistinguishable from the bulk.

Hybrid superconducting fault current limiter based on bulk melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ ceramic composites

A hybrid fault current limiter (FCL) based on single domain bars of Y123/211 melt textured composites has been designed to operate at a 1 kV/400 A line. This consists of a conventional Cu primary coil inductively coupled to a secondary coil formed by the paralleling of single Cu turns each one shorted by a superconducting bar. The superconducting material has been prepared by the solidification techniques of Bridgman and top seeding in air which has allowed us to obtain bars with transport critical current densities of 20000 A/cm/sup 2/ at 77 K. Preliminary tests on the quench current, recovery time and AC-losses in normal operation have been performed. The differences and advantages of a hybrid FCL in comparison with a resistive and an inductive FCL is discussed.

Characterization of superconducting rings using an in-field Hall probe magnetic mapping system

A Hall probe magnetic imaging system that works in magnetic fields in the range -1 T to 1 T has been implemented, and it has been used to characterize the superconducting behavior of single domain melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ rings. We show that in addition to the analysis of the evolution of the local magnetic field distribution when the external magnetic field is cycled, the hysteretic behavior of the magnetic moment can also be investigated after integration of the local magnetic field. The critical current density has been determined through the critical state model and it has been compared to that calculated by inversion of the Biot-Savart law. A remarkable agreement is achieved with both methods.

Modified growth mechanism in directionally solidified YBa/sub 2/Cu/sub 3/O/sub 7/

Minute amounts of CeO/sub 2/ additions have a strong effect on the solidification path, physicochemical parameters and final size distribution of Y/sub 2/BaCuO/sub 5/ (211) precipitates of directionally solidified YBa/sub 2/Cu/sub 3/O/sub 7/ (123). Inductive and transport critical currents density (J/sub c/) measurements indicate a clear increase with CeO/sub 2/ addition.<<ETX>>

In-field Hall probe mapping system for characterization of YBCO welds

Artificial welding of melt-textured YBCO blocks opens the door to the fabrication of large, complex-shaped pieces required for applications. In order to evaluate the superconducting quality of the welds, we have developed a Hall probe mapping system, able to record the local magnetization at 77 K under dynamic applied fields in the range of -1 to 1 T. The system was used to characterize welded samples prepared with a new Ag induced surface melting joining technique. The magnetization maps of unwelded and welded samples of various qualities are compared and discussed. The current distributions associated to the Hall maps were calculated using the Caragol software. The magnetization and current distribution maps over the joint show that good quality welds can be reached with this joining method.

Quench behavior of the switching elements of a hybrid HTS current limiter

In the hybrid HTS current limiter, current limitation is due to the transition to the normal resistive state of YBCO bars, which are shorting the secondary of a transformer. The HTS bars act as current driven switches allowing two states, a low impedance state in which the bar is in its superconducting state and a higher impedance state arisen after the quench. From the authors measurements, one can conclude that the triggering current is essentially related to the contact resistance, which induces a hot spot. The resistance developed during the switching time enables the limiter to work. However, the maximum resistance which can be arisen during the quench is limited by the low propagation speed of the transition front, from the contacts to the center of the bars, in the adiabatic approach. In order to stimulate a multi hot spot quench, the authors have adjusted the triggering current by Mg doping the YBCO, thereby homogenizing the quenching current along the bars. They have also developed switching elements based on a quasi isothermal approach of the quench, which diminishes the high thermal gradients developed in the adiabatic quench induced by a hot spot. Measurements of the performances of both switching systems are reported.