Strong isotropic flux pinning in solution-derived YBa2Cu3O7-x nanocomposite superconductor films

nmat1893-f1 J. Gutiérrez, A. Llordés, J. Gazquez, M. Gibert, N. Romá, S. Ricart, A. Pomar, F. Sandiumenge, N. Mestres, T. Puig and X. Obradors Nat. Mater. 6, 367-373 (2007) DOI:10.1038/nmat1893 Power applications of superconductors will be tremendously boosted if an effective method for magnetic flux immobilization is discovered. Here, we report the most efficient vortex-pinning mechanism reported so far which, in addition, is based on a low-cost chemical solution deposition technique. A dense array of defects in the superconducting matrix is induced in YBa2Cu3O7-x–BaZrO3 nanocomposites where BaZrO3 nanodots are randomly oriented. Non-coherent interfaces are the driving force for generating a new type of nanostructured superconductor. Angle-dependent critical-current measurements demonstrate that a strong and isotropic flux-pinning mechanism is extremely effective at high temperatures and high magnetic fields leading to high-temperature superconductors with record values of pinning force. The maximum vortex-pinning force achieved at 65 K, 78 GN m-3, is 500% higher than that of the best low-temperature NbTi superconductors at 4.2 K and so a great wealth of high-field applications will be possible at high temperatures.

Precursor evolution and nucleation mechanism of YBa2Cu3Ox films by TFA metal-organic decomposition

cm0617891n00001 J. Gazquez, F. Sandiumenge, M. Coll, A. Pomar, N. Mestres, T. Puig, X. Obradors, Y. Kihn, M. J. Casanove and C. Ballesteros Chem. Mat. 18, 6211-6219 (2006) DOI: 10.1021/cm0617891 We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa2Cu3Ox superconducting ceramics on (001)-oriented LaAlO3substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa2Cu3O7. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba1-xYxF2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa2Cu3O7 nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y2O3 which reacts with the CuO particles forming Y2Cu2O5 at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa2Cu3Ox phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba1-xYxF2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa2Cu3Ox at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa2Cu3Ox and bulk Y2Cu2O5. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO3 substrate:  (001)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 and (111)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c-axis-oriented YBa2Cu3Ox on the buried (001)-LaAlO3 substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa2Cu3Ox is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa2Cu3Ox grains and films with a low mosaic spread.

The loss of vortex line tension sets an upper limit to the irreversibility line in YBa2Cu3O7

nphys311-f1 J. Figueras, T. Puig, X. Obradors, W. K. Kwok, L. Paulius, G. W. Crabtree and G. Deutscher Nat. Phys. 2, 402-407 (2006) DOI:10.1038/nphys311 In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field Hm(T ) below the upper critical field Hc2(T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be ‘pinned’ individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line Hl(T ) betweenHm(T ) and Hc2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field Hl(T ) sets an upper limit to the irreversibility line Hirr(T ) marking the onset of dissipation. For that reason it determines a new magnetic-field–temperature region in which a superconductor can remain useful.

Progress towards all-chemical superconducting YBa2Cu3O7-coated conductors

X. Obradors, T. Puig, A. Pomar, F. Sandiumenge, N. Mestres, M. Coll, A. Cavallaro, N. Roma, J. Gazquez, J. C. Gonzalez, O. Castano, J. Gutierrez, A. Palau, K. Zalamova, S. Morlens, A. Hassini, M. Gibert, S. Ricart, J. M. Moreto, S. Pinol, D. Isfort and J. Bock Supercond. Sci. Technol. 19, S13-S26 (2006) DOI:10.1088/0953-2048/19/3/003 Chemical solution deposition (CSD) has recently emerged as a very competitive technique for obtaining epitaxial films of high quality with controlled nanostructure. In particular, the all-CSD approach is considered to be one of the most promising approaches for cost-effective production of second-generation superconducting wires. The trifluoroacetate (TFA) route is a very versatile route for achieving epitaxial YBa2Cu3O7 (YBCO) layers with high critical currents. In this work, recent advances towards improvement of the performance of several conductor architectures based on the YBCO TFA process will be presented. We show that new improved anhydrous TFA precursors allow a significant shortening of the pyrolysis time (~1.5 h), and we have increased the total film thickness in a single deposition using polymeric additives. On the other hand, further understanding of the YBCO nucleation and growth process has allowed us to obtain a controlled microstructure and high critical currents (Jc≈4–5 MA cm−2 and Ic≈300 A cm−1 width at 77 K). The growth conditions (CSD) and post-processing conditions (sputtering and CSD) for the underlying oxide cap and buffer layers (CeO2, BaZrO3, SrTiO3, La2Zr2O7, (La,Sr)MnO3) and of self-organized nanostructures (CeO2, BaZrO3) deposited by CSD have been investigated to obtain high-quality interfaces in multilayered systems. Different single-crystal or metallic substrates (YSZ-IBAD (yttrium stabilized zirconia-ion beam assisted deposition) and Ni-RABiT (rolling assisted biaxial texturing)) have been investigated and long (≈10 m) CSD biaxially textured buffers (CeO2, La2Zr2O7) have been grown on Ni-RABiT substrates using a reel-to-reel system. High-performance TFA-YBCO-coated conductors have been obtained on vacuum-based buffer layers (Ic≈140 A cm−1 width) and on CSD buffer layers grown on IBAD YSZ-SS (stainless steel) substrates. Finally, we report on recent analysis of the magnetic granularity and vortex pinning properties of TFA-YBCO conductors.

The influence of growth conditions on the microstructure and critical currents of TFA-MOD YBa2Cu3O7 films

T. Puig, J. C. Gonzalez, A. Pomar, N. Mestres, O. Castano, M. Coll, J. Gazquez, F. Sandiumenge, S. Pinol and X. Obradors Supercond. Sci. Technol. 18, 1141-1150 (2005) DOI: 10.1088/0953-2048/18/8/020 The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa2Cu3O7 film growth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.

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