Francesco Rizzo

Electrical Characterization of ENEA High Temperature Superconducting Cable

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance(Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA.

MOD Derived Pyrochlore Films as Buffer Layer for All-Chemical YBCO Coated Conductors

We report a detailed study performed on La2Zr2O7 pyrochlore material grown by the metal-organic decomposition method as buffer layers for YBa2Cu3O7-x (YBCO) coated conductors. High-quality epitaxial LZO thin films have been obtained on single crystal (SC) and Ni-5 %at.W substrates. In order to evaluate structural and morphological properties, films have been characterized by means of X-ray diffraction analyses, atomic force microscopy, and scanning electron microscopy. Precursor solutions and heat treatments have been studied by thermogravimetric analyses and infrared spectra with the aim of optimizing the annealing process. Thin films of YBCO have been deposited by pulsed laser ablation on this buffer layer. The best results obtained on SC showed YBCO films with critical temperature values above 90 K, high self-field critical current density values (Jc >; 1 MA/cm2) and high irreversibility field values (8.3 T) at 77 K together with a rather high depinning frequency νp (0.5 T, 77 K) >; 44 GHz as determined at microwaves. The best results on Ni-5% at.W has been obtained introducing in the heat treatment a pyrolysis process at low temperature in air in order to remove the residual organic part of the precursor solution.

Quench Behavior of a Conduction Cooled

Extensive applications of YBa2Cu3O7-x (YBCO) tapes require a comprehensive knowledge of their behavior in every possible operating condition. In this paper, the thermal stability behavior of a conduction-cooled pancake coil wound with commercial YBCO coated conductors is reported from both the experimental and numerical point of view. The coil stability against heat disturbances is studied in terms of minimum quench energy and normal zone propagation velocity (NZPV) for different values of the coil currents at different temperatures. The quench dynamics of the tape is compared between experiment and the numerical analysis, obtaining a reasonable agreement for voltage traces, NZPV, and temperatures. The model allows a wide range of case studies that can be used to suggest appropriate solutions for the design of detection and protection systems.

\hbox{YBa}_{2} \hbox{Cu}_{3}\hbox{O}_{7 - {\rm x}}

A YBa2Cu3O7-δ (YBCO) film grown by metal organic decomposition (MOD) is a promising candidate in the realization of high-performance superconducting wires with high critical current density. Moreover, artificial pinning centers (APCs) inclusions improve the current transport performance of the films in both high magnetic fields and temperatures. YBCO films with and without artificial pinning centers were successfully obtained on single-crystal SrTiO3 (STO) by an environmental friendly low-fluorine coating solution approach. Percentages of 5% and 10% of BaZrO3 (BZO) are used as APCs. X-ray diffraction analyses have been performed to characterize the structural properties of the as-obtained structure. Current-voltage (I-V) characteristics have been collected at different magnetic fields to investigate the superconducting properties.

Tape Pancake Coil

Extensive applications of YBa2Cu3O7-x (YBCO) tapes require a comprehensive knowledge of their behavior in every possible operating condition. In this paper, the thermal stability behavior of a conduction-cooled pancake coil wound with commercial YBCO coated conductors is reported from both the experimental and numerical point of view. The coil stability against heat disturbances is studied in terms of minimum quench energy and normal zone propagation velocity (NZPV) for different values of the coil currents at different temperatures. The quench dynamics of the tape is compared between experiment and the numerical analysis, obtaining a reasonable agreement for voltage traces, NZPV, and temperatures. The model allows a wide range of case studies that can be used to suggest appropriate solutions for the design of detection and protection systems.

Analysis of the Growth Process and Pinning Mechanism of Low-Fluorine MOD YBa 2 Cu 3 O 7-δ Films With and Without BaZrO 3 Artificial Pinning Centers

The transport properties of nanocomposite YBa2Cu3O7-x (YBCO) films grown by metal-organic decomposition (MOD) and prepared with a low-fluorine-coating solution on SrTiO3 single-crystal substrates were studied with dc and microwave measurement techniques. As demonstrated in previous studies, the combination of these two techniques is a powerful tool for vortex pinning investigation. The BaZrO3 inclusions (BZO) as artificial pinning centers were introduced into the YBCO matrix by the excess of corresponding Ba and Zr precursor salts to the initial YBCO precursor solution up to 5 mol%. The effect of BZO was clearly observed in magnetic field dependence relations of critical current density (Jc) and the complex surface impedance, i.e., ΔZ, at fixed temperature and magnetic field parallel to the c-axis. The result shows that BZO particles mainly act as isotropic pinning centers. The short-range vortex dynamics was probed by the ~47.7-GHz microwave surface impedance technique. The derived vortex parameters r and kp were sensitive to the presence of nanoparticles, indicating the effect of BZO on pinning. The joint field dependence relations of Jc and kp help in elucidating the pinning regime in our nanocomposite MOD YBCO/BZO films.

Quench behavior of a conduction cooled YBa2Cu3O 7-x tape pancake coil

The proposed study investigates the aging of two precursor solutions used for the chemical solution deposition of YBa2Cu3O7-x (YBCO) and composite YBCO films with nanostructured inclusions of BaZrO3 (YBCO-BZO). The stability of precursor solutions plays a crucial role in determining the crystalline quality and the superconducting properties of YBCO films. Although low-or free-fluorine solutions are promising in order to improve the synthesis route and to reduce the environmental impact, the poor stability of the solution is the major drawback of the low-fluorine precursors since they degrade faster (4-6 weeks) than other precursor solutions for superconducting layers (e.g., trifluoroacetates solutions are stable up to 6 months) or buffer layers (stable for several months). Therefore, a deeper comprehension of the degradation mechanisms is fundamental to properly optimize the composition of the precursor solution. The current work studies two precursor solutions (for YBCO and YBCO-BZO) that are complex systems, in which different interactions need to be considered: most of all, the nature of ligands (acetate, propionate, ammonia) that complex each metal and influence not only the solution stability but also the quality of the deposited film. Thus, information obtained from different analyses is correlated with morphological, structural, and superconducting properties of the deposited film in order to understand the underlying chemical mechanisms of aging (salts precipitation, oxidation, polymerization) and its effects.

Analysis of Transport Properties of MOD YBCO Films With BaZrO3 as Artificial Vortex Pinning Centers

In order to study the effect of pinning in coated conductors, we have used pulsed laser ablation to grow YBa2Cu3O7-x (YBCO) and YBa2Cu3O7-x/BaZrO3 (YBCO/ BZO) films on metallic templates. Despite the complexity of the structure, after the optimization of the deposition procedure, the YBCO/BZO showed only a moderate reduction of Tc with respect to the pure YBCO sample. We use a very high frequency (~47.7 GHz) dielectric resonator technique in order to induce very short distance vortex shaking, thus probing the steepness of the potential well. We report on measurements of the microwave response of the superconducting structures in terms of the quality factor Q and the frequency resonance ν0 as a function of a moderate dc magnetic field H, at different temperatures T. To keep as much contact as possible to the raw data (Q and ν0), in agreement with a generalized model for vortex dynamics, we determine the pinning parameter r and the upper limit for the creep factor xM. Based on the field dependence of the pinning parameters, the vortex pinning regimes were discussed. We find that the improvement of pinning by BZO is much reduced with respect to the effect observed in films grown on single crystals. In particular, the reduction of the critical temperature must be taken into account in order to appreciate the results.

Aging of Precursor Solutions Used for YBCO Films Chemical Solution Deposition: Study of Mechanisms and Effects on Film Properties

Pulsed laser deposited thin Y Ba2Cu3O7−x (YBCO) films with pinning additions of 5 at. % Ba2Y TaO6 (BYTO) were compared to films with 2.5 at. % Ba2Y TaO6 + 2.5 at. % Ba2Y NbO6 (BYNTO) additions. Excellent magnetic flux-pinning at 77 K was obtained with remarkably high irreversibility fields greater than 10 T (YBCO-BYTO) and 11 T (YBCO-BYNTO), representing the highest ever achieved values in YBCO films.

Measurement of Vortex Pinning in YBCO and YBCO/BZO Coated Conductors Using a Microwave Technique

It is well known that the recrystallization texture of heavily cold-rolled pure copper is almost completely cubic. However, one of the main drawbacks concerning the use of pure copper cube-textured substrates for YBCO coated conductor is the reduced secondary recrystallization temperature. The onset of secondary recrystallization (i.e., the occurrence of abnormal grains with unpredictable orientation) in pure copper substrate was observed within the typical temperature range required for buffer layer and YBCO processing (600–850 °C). To avoid the formation of abnormal grains the effect of both grain size adjustment (GSA) and recrystallization annealing was analyzed. The combined use of a small initial grain size and a recrystallization two-step annealing (TSA) drastically reduced the presence of abnormal grains in pure copper tapes. Another way to overcome the limitation imposed by the formation of abnormal grains is to deposit a buffer layer at temperatures where secondary recrystallization does not occur. For example, La2Zr2O7 (LZO) film with a high degree of epitaxy was grown by metal-organic decomposition (MOD) at 1000 °C on pure copper substrate. In several samples the substrate underwent secondary recrystallization. Our experiments indicate that the motion of grain boundaries occurring during secondary recrystallization process does not affect the quality of LZO film.