The fabrication of nanostructured YBCO thin films by the incorporation of nanosized oxide secondary phases has been shown to strongly enhance their pinning performances, ensuing great potential for use in a broad number of power applications. It is now clear that different nanoparticle (NP) characteristics, growth processes and conditions lead to a rich zoology of defects in the film, whose effectiveness depends on the magnitude and orientation of the magnetic field and the temperature. We have demonstrated that the huge isotropic pinning forces obtained in CSD nanocomposites arise from the strains associated to intergrowths emerging from the NPs [1]. Moreover, natural defects of many types such us, vacancies, staking faults or twin boundaries my act as pinning centers, as well.
Here we present our present understanding of an extensive research on CSD nanocomposites prepared following two different approaches. On First, the in-situ approach, where adding metalorganic salts in the precursor solution, different randomly-oriented NP (BZO, BYTO, YO2 and mixed) are segregated. The second approach named ex-situ is a radically new concept consisting on the incorporation of prefabricated NP colloids (CeO2, ZrO2, CoFe2O4, BZO) in the solution to have much better control of the NP characteristics.
Only with the full understanding of the correlation between vortex pinning and nanostructure in this complex materials, will allow to specific nano-engineering the pinning landscape for best superconducting performances. In this contribution, we report on a variety in-situ and ex-situ YBCO nanocomposites where processing conditions has enabled us to control the density, length and stoichiometry of intergrowth emerging from oxide nanoparticles. The synergetic combination of natural and engineered defects in these complex systems will be evaluated by means of in situ angular transport measurements and high resolution STEM.
[1] A. Llordes et al., Nature Materials, 329, 2012

We acknowledge the financial support from the EU-FP7 NMP-LA-2012-280432 EUROTAPES and COST action MP1201, and MAT2014-51778-C2-1R
 

YBa₂Cu₃O₇(YBCO) coated conductors are promising for superconducting magnets, superconducting cables, and ship propulsion motors, and high critical current density(J_c) is required for these applications. Nanorods improved J_c significantly in YBCO films, but further enhancement of J_c has been still desired. Nanorod density, diameter, straightness, interface sharpness, and elastic strain determine vortex pinning, but their influence on J_c is not well understood. Nanorod materials are believed to be the most crucial factor for diameter and elastic strain etc. Therefore, J_c behavior should be systematically investigated by changing nanorod material and content to understand vortex pinning mechanism, but such systematic studies have not yet been reported. In the present study, YBCO+BaSnO₃(BSO), YBCO+BaZrO₃(BZO), and YBCO+BaHfO₃(BHO) films were prepared using pulsed laser deposition, where various content of BSO, BZO, and BHO were introduced into the films. Structure of the films was evaluated using X-ray diffraction and transmission electron microscopy. Critical temperature(T_c), J_c at 77-20 K, and irreversibility temperature were measured. In addition, density functional theory calculation was performed to understand nanorod structure in the films based on thermodynamic and elastic parameters. T_c was 91-85 K in the films, and T_c-nanorod volume fraction relation depended on nanorod material. In YBCO+BZO and YBCO+BHO films, J_c at low field was not large, but high J_c was obtained at high magnetic field at 77 K compared with BSO nanorods, showing high density of BZO and BHO nanorods in the films. Based on the systematic results, influence of nanorod density, diameter, interface sharpness, and elastic strain on T_c and J_c is analyzed, and structure for high J_c is discussed.

This work was supported by Fundamental Research Developing Association for Shipbuilding and Offshore (REDAS) and KAKENHI, Grant-in-Aid for Science Research (S), Grant Number 23226014.

This work was supported by the U.S. D.O.E through ARPA-E under contract No. DE-AR0000190, the Center for Emergent Superconductivity and the Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357.
 

The possibility of strongly improve YBa₂Cu₃O_7-x (YBCO) films flux-pinning properties by introducing nano-scale artificial pinning centres (APCs) has attracted in recent years a remarkable interest. In fact, by enhancing the transport film performances as a function of both applied magnetic field and temperature it is possible to reach the current requirements needed in a wide range of present and future applications. In particular, the controlled growth of self-assembled nano-columns oriented along the YBCO c-axis by means of pulsed laser deposition (PLD), using composite targets, has been shown to be a promising and efficient technique for both improving the critical current density magnitude and modifying its intrinsic angular anisotropy.
In this work we consider PLD grown YBCO films with the inclusion of 5at% Ba₂YTaO₆ (BYTO) and 2.5at% Ba₂YTaO₆ + 2.5at% Ba₂YNbO₆ (BYTO + BYNO) nano-rods. The films are deposited on (00l)-oriented SrTiO₃ single crystals by using a XeCl excimers laser radiation at a wavelength of 308 nm and with a repetition rate up to 10 Hz. The transition temperature drop with respect to pristine YBCO is almost completely recovered by tuning the sample deposition temperature. Transmission electron microscopy analysis reveals a well-defined APCs growth in the YBCO matrix, with almost straight and continuous along the whole film thickness nano-rods. These columns often grow associated with nanoparticles of Y₂O₃. Correspondingly, transport properties exhibit an excellent flux-pinning behaviour: D.C. electrical measurements are carried out in a wide range of temperatures (from 77 K to 4.2 K) and applied magnetic field intensities (from 0 to 12 T) and directions, allowing evaluating the effect of this rich pinning landscape. In particular, the BYTO + BYNO APCs combination displays a very effective pinning also in the low temperature and high field region of the YBCO H-T phase diagram which is extremely interesting looking at fusion applications.

This work has partially received funding from the European Union’s Seventh Programme under grant agreement N. 280432 EUROTAPES

Since the production of the first electronic thin films in 1980 Chemical Solution Deposition (CSD) has emerged as a highly flexible and cost-effective technique for the fabrication of a wide variety of functional oxide films. Among the different application areas we have explored CSD-MOD technique for the production of High Temperature Superconductors (HTSC). Metal-organic decomposition has been established as the methodology to grow low cost, scalable, high performance epitaxial YBa2Cu3O7 films for coated conductors.
New designed solutions are proposed in accordance with the new requirements concerning environmental safety. Looking at these objectives we will present here our work in the preparation of precursor solutions with reduction of fluorine content, which should be adapted to the requirements of Superconducting ReBCO layers production, leading to high production rates with optimal performance. Solutions with low and non-fluorine precursors (acetates, ethylhexanoates) in different amounts of additives (thriethanolamine, propionic acid) have been stabilized and their rheology modified for substrate wetability. Thermal decomposition analysis performed directly in films, have revealed differences in decomposition and growth steps. Upon optimization of growth process parameters, Tc and Jc(77K)of around 90 K and 3 MA/cm2 are obtained.
Changes in solvent and use of inorganic salts and polymers produce changes in the rheological properties of the solution improving the thickness and homogeneity of the final layers. Besides, understanding of the thermal decomposition processes, represents a powerful tool to produce ceramic layers with improved structural properties.
This knowledge has been also used in the new approach to nanostructured YBCO layers by exploiting the capabilities of combining two materials with distinct functionalities. Preparation of solutions with mixed salt precursors or colloidal solutions of oxide nanoparticles adapted to form stable solutions with Y, Ba and Cu salts will be presented. The steps to be surpassed to produce stable and homogeneous colloidal solutions with oxide nanoparticles (CeO2, ZrO2 and BZO) and to use them in combination with TFA-YBCO solutions and Low Fluorine YBCO solutions to obtain final nanostructured superconducting layers will be discussed.

The research leading to these results has received funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES project and MAT2011-28874-C02 national project.

The recent progress of enhancing the current carrying capability for YBa₂Cu₃O_7-d (YBCO) coated conductors by nano-engineering makes this high-temperature superconductor a suitable candidate for many applications such as low-loss conduction cables and high-energy physics fusion magnets. The necessary immobilization of fluxoids can be achieved by artificial pinning centers such as BaHfO₃ (BHO) and the mixed double-perovskite Ba₂Y(Nb/Ta)O₆ (BYNTO). Density, size and distribution of such pinning centers can be adjusted to yield best results for the intended application conditions in temperature and magnetic field range. The incorporation of up to 6 mol% BHO on technical substrates has already improved the in-field performance at liquid nitrogen temperature [1], whereas BYNTO showed an astonishing J_c(B,Q) behavior on SrTiO₃ single crystals in magnetic fields up to 9 T [2,3].
In this work we present results on ~1 µm thick YBCO films with up to 15 mol% BHO and 5 mol% BYNTO, respectively, grown by pulsed laser deposition on biaxially textured metal tapes. In particular, we studied the influence of the deposition conditions on the growth of incorporated nanorods. Structural characterization carried out by X-ray diffraction and transmission electron microscopy confirms epitaxial growth of both, YBCO on the tape and homogenously distributed nanorods within the superconducting matrix. Both additions expand the YBCO lattice and thereby generate strain leading to further structural defects, e.g. stacking faults. Transport current measurements in a standard four-point assembly show significant improvement of the in-field behavior and J_c (B,Q) anisotropy for both types of additions due to the presence of nanorods.

[1] M. Sieger et al., IEEE Trans. Appl. Supercond. 25 (3), 6602604, 2015
[2] G. Ercolano et al., SUST 24 (9),095012, 2011
[3] L. Opherden et al., submitted to SUST

The authors acknowledge financial support from EUROTAPES, a collaborative project funded by the European Community's Seven Framework Programme (FP7 / 2007 - 2013) under Grant Agreement n.280432.

This work was supported in part by Grant-in-Aid for Scientific Research (Nos. 23226014 and 25289358) from the JSPS. This work was supported by the NEDO. We would like to thank Prof. T. Matsushita belonging to Kyushu Institute of Technology for fruitful discussions.