National Institute of Materials Physics, Laboratory of Magnetism and Superconductivity, Romania

TYBCO PROJECT

Thick YBa2Cu3O7 films with improved parameters for superconducting coatings

Thick YBa2Cu3O7 films with improved parameters for superconducting coatings

SUMMARY

relaxationThe project is devoted to the fabrication of thick YBa2Cu3O7 (YBCO) films with improved critical current density Jc for superconducting coatings. It is well known that Jc decreases with increasing film thickness. Our aim is to introduce artificial pinning centres in a few micron thick films so that Jc to remain above 1 MA/cm2 at 77 K in self magnetic field, and the resulting coatings to be competitive for large scale applications. The fabrication route will involve Pulsed Laser Deposition (PLD) and chemical deposition (CSD), and the optimal conditions will be decided after the structural characterization (XRD, SEM, TEM, AFM, STM) of nanostructured thick YBCO superconducting films, correlated with a detailed investigation of the supercurrent transport properties (resistive measurements, transport measurements, and SQUID magnetometry). The main idea of this proposal is to use a combined vortex pinning effect resulting from the presence of columnar defects, randomly distributed nanoparticles, the substrate decoration with metallic nanodots, as well as the quasi-multilayer approach, in order to diminish the detrimental effects of different vortex excitations. The expected results are nanostructured thick YBCO films on various substrates, including La-zirconate and Gd added ceria-buffered Ni-W (RABiTS approach), suitable for superconducting cable production. The proposed project is multi-disciplinary one: materials science, nanotechnology, surface science, condensed matter physics, applied and fundamental superconductivity and magnetism. A special care will be devoted to the changes in the vortex dynamics with the modification of the complex film microstructure, which necessitates a better understanding of the effects of many vortex excitations and vortex creep regimes appearing in this situation: half vortex loops, double vortex kinks, super-kinks, variable range vortex hopping, plastic and elastic vortex creep.

Specific Objectives

O1. Verification of the Jc(d) dependence, where d is the film thickness, using high-quality YBCO films and (YBCO)n/(PrBCO)m super-lattices to avoid the influence of the substrate (feasibility objective). High quality thin and thick films and super-lattices will be prepared mainly by dc sputtering, in the framework of our long-standing collaboration with the University of Mainz (Institute of Physics);

O2. Finding the best materials and architectures of metallic nanodots for substrate decoration and quasi-multilayers which lead to the highest increase in super-current capability (technical feasibility);

O3. Growing nanostructured thick films by PLD using the best nanodot materials (including noble metals) and architectures (super-lattices in a thick YBCO film) in combination with YBCO targets for doping with various nanoinclusions (BZO and/or BaSnO3) on STO, MgO, and CeO2-buffered, textured Ni-W alloys. Our present intergovernmental research project with Beijing Technical University and the collaboration with Univ. of Birmingham will be helpful in this respect;

O4. Development of chemical methods (CSD) for thick YBCO and YBCO/BZO films on various substrates (including CeO2-buffered, textured, non-magnetic Ni-W alloys).

O5. Performing a detailed structural characterization of nanostructured films and artificially-induced pinning centres (XRD, SEM, TEM, AFM, STM);

O6. Finding the best technique to determine the effective pinning energy and the types of vortex excitations (experimental development);

O7. Performing continuous studies of superconducting properties (critical temperature Tc, Jc, vortex melting line - irreversibility line, pinning force and pinning potential), in various magnetic fields;

O8. Optimization of Jc and other properties of nanostructured films for various application conditions;

O9. Developing robust and low cost buffer layers (La-zirconates, doped ceria,) on Ni-5 at.% W bi-axially textured substrates using CSD methods of deposition;

O10. Epitaxial growth of YBCO nanocomposite thick films by CSD using low fluorine and/or fluorine-free coating solutions. The YBCO nanocomposite film will contain BZO or Y2O3 randomly oriented nanoparticles which generate nanostrain in the YBCO epitaxial film acting as pinning centres;

O11. Elaboration and characterization of a YBCO based HTS superconducting cable prototype with a transport capacity using chemical methods developed within the present project.

Activities and results

I. Preparation and characterization of thin YBCO films and superlattices with nanoinclusions on different substrates

Target preparation

Targets for dc sputtering and PLD were prepared by conventional sintering and by SPS (Spark Plasma Sintering), see Fig. 1.

relaxation

The structural evolution of the calcined YBCO powder, sintered discs, compacted discs, and after annealing in O2 (100 h at 460 – 465 ºC), including the orthorhombicity, indicated by XRD, as well as Tc are shown in Table 1.

Table 1. Target characteristics

relaxation

Preparation of thin YBCO films and superlattices:

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