Materials for information science and electronics – Page 5

Vortex pinning in chemical solution nanostructured YBCO films

Written by Albert Moreno on February 20, 2008. Posted in Materials for information science and electronics, Scientific Highlights.

T. Puig, J. Gutierrez, A. Pomar, A. Llordes, J. Gazquez, S. Ricart, F. Sandiumenge and X. Obradors Supercond. Sci. Technol. 21, 034008 (2008) DOI:10.1088/0953-2048/21/3/034008 A methodology of general validity to study vortex pinning in YBa₂Cu₃O_7−δ (YBCO) is described. It is based on measuring the angular-dependent critical current density in a large range of temperatures and magnetic fields and it permits one to identify, separate and quantify the three basic vortex pinning contributions associated to anisotropic–strong, isotropic–strong and isotropic–weak pinning centers. The corresponding vortex pinning phase diagrams for these three contributions are built up. This methodology is applied to the recently discovered solution-derived YBCO–BaZrO₃ nanocomposites which display a maximum vortex pinning force of 78 GN m⁻³ at 65 K, being five times larger than that of NbTi at 4.2 K. These nanocomposites show an effective anisotropy tending to unity and the best superconducting properties observed so far in YBCO. We present the richness and complexity of their microstructure and the correlation with the vortex pinning properties. The application of the vortex pinning methodology and comparison with a standard solution-derived YBCO film enable us to identify the isotropic–strong defect contribution being at the origin of their unique properties.

Self-organization of heteroepitaxial CeO2 nanodots grown from chemical solutions

Written by Albert Moreno on November 17, 2007. Posted in Materials for information science and electronics, Scientific Highlights.

M. Gibert, T. Puig, X. Obradors, A. Benedetti, F. Sandiumenge and R. Hühne Adv. Mater. 19, 3937-3942 (2007) DOI: 10.1002/adma.200700361 Self-organized rows of CeO₂ nanodots (see image) have been achieved through centering of islands within the terraces of vicinal perovskite substrates. The extraordinary effectiveness for lateral confinement, with islands’ heights ∼ 20 times larger than the lattice steps, results from the non-coherent vertical interfaces at the substrate steps between dissimilar crystallographic structures.

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

Written by Albert Moreno on April 22, 2007. Posted in Materials for information science and electronics, Scientific Highlights.

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 YBa₂Cu₃O_7-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

Written by Albert Moreno on December 6, 2006. Posted in Materials for information science and electronics, Scientific Highlights.

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 YBa₂Cu₃O_x 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 Ba_1-_xY_xF₂₊_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 YBa₂Cu₃O_x phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba_1-_xY_xF₂₊_x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa₂Cu₃O_x at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa₂Cu₃O_x 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 YBa₂Cu₃O_x 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 YBa₂Cu₃O_x is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa₂Cu₃O_x grains and films with a low mosaic spread.

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

Written by Albert Moreno on May 21, 2006. Posted in Materials for information science and electronics, Scientific Highlights.

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 H_m(T ) below the upper critical field H_c2(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 H_l(T ) betweenH_m(T ) and H_c2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field H_l(T ) sets an upper limit to the irreversibility line H_irr(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

Written by Albert Moreno on January 20, 2006. Posted in Materials for information science and electronics, Scientific Highlights.

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 (J_c≈4–5 MA cm⁻² and I_c≈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 (I_c≈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

Written by Albert Moreno on January 15, 2005. Posted in Materials for information science and electronics, Scientific Highlights.

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.