A. Vannozzi

Metal propionate synthesis of epitaxial YBa2Cu3O7-x films

A modified TFA-MOD method, using only barium trifluoroacetate, is presented. The yttrium and copper triflouroacetates were replaced by the alcoholic solutions of Cu and Y acetates dispersed in propionic acid. Fourier transformed infrared spectroscopy (FT-IR), thermal analyses (DTA/TG) coupled with mass spectrometry (MS) and X-ray diffraction analyses were used to study the decomposition of the precursor. The method permits the shortening of the pyrolysis time by a factor 4, with respect to conventional TFA-MOD method, due to the smaller amount of evolved hydrofluoric acid. Using this method 600 nm thick YBCO films were grown both on (100)SrTiO3 and on CeO2/YSZ/CeO2/Pd buffered Ni-5at.%W substrates. The as obtained films exhibit good morphological, structural and superconducting properties with Tc (R=0) greater than 91K and with an out-of-plain texture of 0.240 and 1.90, respectively.

Biaxial texturing of Ni alloy substrates for YBCO coated conductors

Abstract Biaxial cube texturing and surface quality of the metallic substrate are key parameters for high J c coated conductors. Experiments have demonstrated that the sharp cube texture can be developed up to 11 at.% V, 12 at.% Cr and 5 at.% W alloys; the respective cube textured area fractions result to be 96%, 97% and 99%. For all the studied alloys, FWHM values of ω - and φ -scans are in the range of 5–6° and 6–8°, respectively. Microstructural analyses reveal smooth surfaces with grooves localized at the grain boundaries of twinned cube textured and misoriented grains.

Pulsed laser deposition of high critical current density YBa2Cu3O7−x/CeO2/Ni–W architecture for coated conductors applications

Epitaxial YBa2Cu3O7−x (YBCO) films with critical current densities exceeding 1 MA cm−2 (at 77 K in self magnetic field) were epitaxially grown on biaxially textured Ni 5 at% W (Ni–W) substrate using only a CeO2 buffer layer. Both YBCO and CeO2 films were deposited in situ by pulsed laser ablation. The oxidation behaviour of the Ni–W substrate enables the epitaxial growth of a CeO2 oxide buffer layer without using the forming gas (Ar 4%H2). SEM investigations of YBCO and CeO2 films reveal a dense microstructure without cracks or porosity. X-ray pole figures show a cube-on-cube epitaxy with a [100]YBCO//[110]CeO2//[100]Ni–W epitaxial relationship. The YBCO superconducting film has a good out-of-plane and in-plane texture with a full-width-half-maximum of 5.8° and 6.6°, respectively. No severe degradation of JC with film thickness, up to 1 μm thick films, was observed. The use of a single buffer layer on Ni–W substrates represents an important simplification in the scaling-up for long tape fabrication.

Transport Property Improvement by Means of BZO Inclusions in PLD Grown YBCO Thin Films

A detailed study on YBa₂Cu₃O_7-delta (YBCO) films with BaZrO₃ (BZO) inclusions (YBCO-BZO) deposited on SrTiO₃ substrate with different BZO contents is presented. X ray diffraction, SEM imaging and superconductive critical temperature (<i>T</i> _c) measurements were performed to evaluate film growth quality. Independently from the BZO content, all YBCO-BZO films showed <i>T</i> _c values close to that of pure YBCO indicating that the <i>T</i> _c reduction which is usually observed on YBCO-BZO films can be successfully recovered by changing the deposition conditions. All YBCO-BZO films showed an improved critical current density (<i>J</i> _c) dependence on magnetic field if compared to the typical <i>J</i> _c(<i>H</i>) of YBCO, resulting in higher irreversibility fields up to 9 T at 77 K. SEM observation on etched samples suggests a direct link between the defect density measured in YBCO-BZO samples and that calculated from the observed in field performances. The analysis of <i>J</i> _c(<i>B</i>) as a function of applied magnetic field direction revealed that the pinning due to isotropic pinning centers typical for YBCO is coupled with a <i>c</i>-axis correlated pinning in YBCO-BZO which can be ascribed to BZO induced columnar defects. <i>J</i> _c (<i>B</i>) recorded at several temperatures down to 10 K revealed matching effect features.

Chemically processed BaZrO3 nanopowders as artificial pinning centres

This work reports on the preparation of a BaZrO3 (BZO) nanopowder by the chemical decomposition of an oxalate precursor starting from barium acetate and zirconium oxychloride. The X-ray peak profile analyses and the scanning electron microscopy (SEM) have indicated that the mean powder size ranges from 30nm to 340 nm for the thermal treatment temperature varying from 700°C to 1200°C, respectivelly. The as prepared powder has been used to manufacture YBCO-5 mol. % BZO composite targets from which epitaxial YBCO thin films on (100)SrTiO3 substrate were ablated. The X-ray analysis have revealed that the BZO nanoislands are epitaxially grown throughout the YBCO film, having the same epitaxial relationship as YBCO. Moreover, as revealed by SEM, the presence of BZO considerably improves the morphology of the YBCO film. The critical temperature (Tc) registered a slight decrease with the BZO concentration. Nevertheless, the BZO addition resulted in an improvement of the critical current density, Jc. The JcvsB curve exihibits a large plateau extended up to about 2.5 Tesla. The YBCO-BZO composite films showed pinning forces with a c-axis correlated character, as revealed by the angular behaviour of Jc.

Cube-textured substrates for YBCO-coated conductors: microstructure evolution and stability

The realization of YBCO-based coated conductors with high critical current density involves the deposition of highly biaxially textured YBCO films. The use of epitaxial growth shifts this requirement to the template used for YBCO deposition. In the rolling-assisted biaxially textured substrate (RABiTS) approach, an appropriately oriented template is provided by a cube-textured substrate. The development of a cube texture is the result of recrystallization occurring in a heavily deformed tape, which is activated by high-temperature annealing. In the case of Ni-based alloys, thermal treatment at temperatures ranging from 900 to 1150??C for at least 30?min is commonly used. The determination of the minimum conditions for thermal treatments in terms of temperature?time involved in the recrystallization process is therefore of practical interest. In this work, Ni?5 at.% W alloy has been studied as a substrate for YBCO-coated conductors. 100??m thick tapes have been obtained through heavy cold rolling, followed by annealing in high vacuum. Different thermal treatments with rates of 20??C?min?1 have been performed in order to study the formation and the evolution of the cube texture. Moreover, the annealing time has been varied in order to inspect the thermal stability of the substrate microstructure at the relatively high deposition temperature of YBCO films. It is found that the substrate begins recrystallizing at 700??C and that the cube texture is fully developed at temperatures higher than 800??C, while annealing at 900??C for 1?h produces a stable microstructure at the typical YBCO deposition conditions.

Critical current density of YBa2Cu3O7-x films with BaZrO3 inclusions on SrTiO3 and Al2O3 substrates

Recently, many efforts have been dedicated to the development of a reliable technology for the introduction of artificial pinning sites in YBa2Cu3O7-x (YBCO) films with the aim of improving the in-field Jc performances. One of the most effective technique resulted to be the inclusion of BaZrO3 (BZO) second phase embedded in the YBCO films. In this contribution we present Jc measurements on BZO-added YBCO films deposited on SrTiO3 (STO) and CeO2-buffered-Al2O3 (ALO) substrates. Samples were deposited by pulsed laser ablation technique using a composite YBCO + 5mol.% BZO target at the optimum conditions for fully oxygenated c-axis oriented YBCO films. Despite of a slight Tc reduction, BZO addition in YBCO-STO films resulted in an improvement of in-field performances with the appearance of a Jc plateau in the low field region which extends up to about 2.5 Tesla irrespective of the temperature at least in the investigated range (down to 65K). On the other hand, samples deposited on ALO did not exhibit any remarkable difference neither in the Jc value nor in the magnetic field dependences as compared with pure YBCO. The presence of 0° (magnetic field parallel to the c-axis) peaks in the Jc. angular behaviour revealed a c-axis correlated character of the pinning forces in BZO added YBCO films grown on both STO and ALO substrates. X-ray diffraction measurements and AFM investigations were carried out in order to determine the influence of BZO addition on films crystalline quality and microstructure.

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.

YBa 2 Cu 3 O 7−δ Films With BaZrO 3 Inclusions for Strong-Pinning in Superconducting Films on Single Crystal Substrate

The addition of artificially induced defects improves the pinning efficiency of magnetic flux vortices in YBa2Cu3O7-delta (YBCO) films. In YBCO films by pulsed laser ablation (PLD) technique this goal can be pursued by the introduction of epitaxial second phase nanoinclusions in the YBCO matrix. In the present work, the effect of BaZrO3 (BZO) inclusions in stoichiometric YBCO films on transport properties has been studied. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared with BZO mean grain size of about 1 mum. Structural and morphological characterizations of the films obtained by PLD from these targets have been carried out through X-ray diffractometry and Scanning Electron Microscopy (SEM) analysis. Dc electrical transport measurements at 77 K have revealed a better magnetic field retention of the critical current density, Jc, being at 3 T the 14.6% and the 16.4% of the zero field values for YBCO-BZO 5% and 7%, respectively. The complex surface impedance has been measured by means of a dielectric resonator at a high microwave frequency (50 GHz). In order to identify the potentially strong pinning induced by the inclusions, the field-dependent complex resistivity has been determined as a function of temperature in a moderate magnetic field (B < 1T). Very high depinning frequency are estimated in YBCO films with BZO inclusions indicating extremely high vortex pinning and steep potential wells.

An Extended Characterization of European Advanced

Within the framework of ITER-related projects, new tasks have been recently launched by EFDA CSU Garching (European Fusion Development Agreement Close Support Unit Garching), for the definition and production on industrial scale of advanced Nb3Sn strands, to be used in the manufacturing of the ITER high field CS and TF magnets. We performed an extended characterization of the advanced Nb3Sn strands coming from different European companies, in terms of strand layout (diameter, thickness of Cr coating, Cu:non-Cu ratio), critical transport current, RRR, and hysteresis losses. The results of the measurement campaign show that the upgraded strands meet the latest ITER requirements, with an overall critical transport current of at least 200 A (at 12 T, 4.2 K), equivalent to a non-Cu Jc of about 800 A/mm2, a Cu:non-Cu ratio of about 1, a strand diameter of 0.81 mm, and with non-Cu hysteresis losses limited to less than 1000 kJ/m3 on a plusmn3 T field cycle at 4.2 K