M Coll

Progress towards all-chemical superconducting YBa2Cu3O7-coated conductors

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 (~1.5xa0h), 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≈4–5xa0MAxa0cm−2 and Ic≈300xa0Axa0cm−1 width at 77xa0K). 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 (≈10xa0m) 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≈140xa0Axa0cm−1xa0width) 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.

Growth, nanostructure and vortex pinning in superconducting YBa2Cu3O7 thin films based on trifluoroacetate solutions

Chemical solution deposition (CSD) is a very competitive technique to obtain epitaxial films and multilayers of high quality with controlled nanostructures. Based on the strong attractiveness from the cost point of view, the production of long length coated conductors based on the CSD approach is being extensively developed. The trifluoroacetate route (TFA) is the most widely used route to achieve epitaxial YBa2Cu3O7 (YBCO) layers with high critical currents, however a deep understanding of all the individual consecutive processing steps, as well as their mutual influence and relationship, is required to achieve superconducting materials with high performance. In this work, we review advances in the knowledge of all the steps relevant to the preparation of YBCO thin films based on TFA precursors as a CSD methodology: solution preparation and deposition, pyrolysis processes, intermediate phase evolution, nucleation and growth phenomena, microstructural evolution and its influence on percolating supercurrents, as well as vortex pinning by natural existing defects. Finally, we discuss the open issues still existing in the TFA approach, particularly that of film nanostructuration, and we provide a future outlook for this outstanding methodology.

Chemical solution deposition: a path towards low cost coated conductors

The achievement of low cost deposition techniques for high critical current YBa2Cu3O7 coated conductors is one of the major objectives to achieve a widespread use of superconductivity in power applications. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films at a low cost, so an intense effort is being carried out to develop routes for all chemical coated conductor tapes. In this work recent achievements will be presented towards the goal of combining the deposition of different type of buffer layers on metallic substrates based on metal–organic decomposition with the growth of YBa2Cu3O7 layers using the trifluoroacetate route. The influence of processing parameters on the microstructure and superconducting properties will be stressed. High critical currents are demonstrated in all chemical multilayers.

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

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 790–830u2009°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.

Nanostructural control in solution-derived epitaxial Ce1?xGdxO2?y films

A novel mechanism based on aliovalent doping, allowing fine tuning of the nanostructure and surface topography of solution-derived ceria films, is reported. While under reducing atmospheric conditions, non-doped ceria films are inherently polycrystalline due to an interstitial amorphous Ce(2)C(3) phase that inhibits grain growth, a high quality epitaxial film can be achieved simply by doping with Gd(3+) cations. Gd(3+) [Formula: see text] Ce(4+) substitutions within the lattice are accompanied by charge-compensating oxygen vacancies throughout the volume of the crystallites acting as an efficient vehicle to reduce the barrier for grain boundary motion caused by interstitial Ce(2)C(3). In this way, the original nanostructure is self-purified by pushing the amorphous Ce(2)C(3) phase towards the free surface of the film. Once a full epitaxial cube-on-cube oriented ceria film is obtained, its surface morphology is dictated by the interplay between faceting on low energy {110} and/or {111} pyramidal planes and truncation of those pyramids by (001) ones. The development of the latter requires the suppression of their polar character which is thought to be achieved by charge compensation between the dopand and oxygen along [Formula: see text] directions.

Atomically flat MOD La0.7Sr0.3MnO3 buffer layers for high critical current YBa2Cu3O7 TFA films

Superconducting YBa2Cu3O7 (YBCO) thin films were grown by the trifluoroacetates route onto metallorganic deposited La0.7Sr0.3MnO3 (LSMO) buffer layers. The growth conditions of the LSMO films have been studied in detail and fully strained epitaxial layers with atomically flat surfaces have been obtained with local roughness ~1xa0nm, as determined by atomic force microscopy. We prove that this surface morphology is a key parameter for achieving YBCO/LSMO multilayers with high critical current densities in self-field, Jcsf (77xa0K) = 1.8xa0MAxa0cm−2. Angular pinning studies demonstrate that the defect structure is modified with respect to standard TFA–YBCO films as a pronounced peak in Jc is observed for at high temperatures. This indicates the presence of anisotropic defects playing an important role in vortex pinning even at the irreversibility line stressing the suitability of these all-chemical TFA–YBCO/LSMO multilayers as conductive coated conductor architectures.

All-chemical YBa2Cu3O7 coated conductors on IBAD-YSZ stainless steel substrates

We report on the fabrication of all-chemical YBa2Cu3O7 coated conductors on IBAD-YSZ (IBAD stands for ion beam assisted deposition; YSZ is yttrium stabilized zirconia) stainless steel substrates. YBCO films were grown by the trifluoroacetates route on top of CeO2 buffer layers made by metal–organic decomposition. The achievement of atomically flat CeO2 surfaces is found to be a key factor for obtaining clean interfaces with YBCO and high performance. Coated conductors with percolative critical currents of JcGB(65xa0K) = 1.8xa0MAxa0cm−2 were achieved. The determination of the intra-grain critical current JcG from inductive measurements suggests that the limiting factor for JcGB is the YBCO in-plane texture, which is already of higher quality than that of the IBAD-YSZ cap layer.

Role of twin boundaries on vortex pinning of CSD YBCO nanocomposites

We study the effect of twin boundaries (TBs) on the critical current density of YBa2Cu3O (YBCO) films and nanocomposites grown on different substrates. Varying both the direction of the current and magnetic field, we show that the TB orientation is a crucial parameter to consider in the optimization of Jc for particular applications. A quantitative and detailed analysis of the role of TBs on vortex dynamics has allowed us to infer that extended TB planes in pristine YBCO films can reduce Jc by 60% at low temperatures due to vortex channeling effects or increase it by a 98% at high temperatures due to directional vortex pinning. Moreover, we demonstrate that TB length and distribution can be strongly modified in YBCO nanocomposites. We observe that TB planes with no vertical coherence are still effective for vortex pinning while are not to create channels for easy vortex flow.

Nanostrain induced pinning in YBa2Cu3O7−x nanocomposites even close to the irreversibility line

It has been recently shown that solution-derived YBa2Cu3O7−x (YBCO) nanocomposites can reach extraordinary pinning forces at 77xa0K, thanks to the isotropic nanostrain introduced by nanoparticles. In this paper we demonstrate that nanostrain pinning is even effective very close to the irreversibility line (IL). Ac susceptibility and transport measurements are used to determine the isotropic and anisotropic contributions to the activation energy for thermal depinning in different (BaZrO3, Y2O3, Ba2YTaO6) YBCO nanocomposites. Results demonstrate that the activation energy associated with isotropic defects correlates linearly with the nanostrain.

High J/sub c/ YBCO thin films and multilayers grown by chemical solution deposition

Chemical solution deposition (CSD) has emerged as a very competitive technique to obtain superconducting films of high quality. However, there is still few knowledge about how underlying microstructure can affect the performances of the YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films. Here we will summarize our recent progress in this subject. We have grown YBCO thin films by the so called trifluoroacetate route on top of buffer layers also grown by CSD. By modifying the growth conditions of SrTiO/sub 3/ and BaZrO/sub 3/ buffer layers we have investigated the influence of surface roughness and grain size. The role of lattice parameter mismatch has been studied by producing multibuffered architectures such as SrTiO/sub 3//BaZrO/sub 3//LAO. It has been proved that YBCO thin films with critical currents in excess of 1 MA/cm/sup 2/ at 77 K in self field can be achieved, thus demonstrating the abilities of the CSD technique.