All authors acknowledge the EU (EU-FP7 NMP-LA-2012-280432 EUROTAPES project), MINECO (COACHSUPENERGY, MAT2014-51778-C2-1-R and SUPERINKS, RTC-2015-3640-3) and Generalitat de Catalunya (2014SGR 753 and Xarmae). ICMAB authors acknowledge the Center of Excellence award Severo Ochoa SEV-2015-0496. TEM microscopy work was conducted in the Catalan Institute of Nanoscience and Nanotechnology (ICN2). Authors acknowledge the ICN2 Electron Microscopy Division for offering access to their instruments and expertise. Part of the STEM microscopy work was conducted in 'Laboratorio de Microscopias Avanzadas' at the Instituto de Nanociencia de Aragon-Universidad de Zaragoza. Authors acknowledge the LMA-INA for offering access to their instruments and expertise. JG and MC also acknowledge the Ramon y Cajal program (RYC-2012-11709 and RYC-2013-12448, respectively). ZL and BM acknowledge the China Scholarship Council (CSC) and F I MINECO for fellowships, respectively.

/pdf/epitaxial-yba2cu3o7-x-nanocomposite-films-and-coated-1sy65ysojk.pdf

Epitaxial YBa2Cu3O7−x nanocomposite films and coated conductors from BaMO3 (M = Zr, Hf) colloidal solutions

Obtaining a high critical current density (Jc) remains the main challenge in developing fluorine-free metal organic deposition (MOD) methods to fabricate YBCO superconducting thin films. Silver addition was used to raise the Jc values in this research work. By reacting with propionic acid and ammonia, AgNO3 was initially mixed with YBCO carboxylate precursors dissolved in methanol. High-temperature in situ XRD measurements on the YBCO-Ag powders revealed that silver addition lowers the incongruent melting temperature of YBCO to 760 °C and resulted in a smooth surface morphology of the YBCO films at a temperature as low as 760 °C. Grain growth and intergranular conductivity were also found to be improved by silver doping. After annealing under optimized conditions, a high Jc of 4.6 MA/cm2 was obtained in a YBCO-Ag thin film with 10 wt% Ag.

High-Jc YBa2Cu3O7−x-Ag superconducting thin films synthesized through a fluorine-free MOD method

High-field magnet application is one of most driving forces for the development of high temperature superconducting (HTS) materials. Recently, the magnet consisting of ReBa2Cu3O7- δ based coated conductors (RE123, also called as secondary-generation high-temperature superconducting tapes, 2G-HTS) has exhibited the magnetic field as high as 26.4 T, over than the limit of conventional NbTi-Nb3Sn magnets. As well, an overall superconductor magnet consisting of a 2G-HTS magnet inserted into the background with low temperature superconducting magnetic field of 17 T gave rise to the magnetic field up to 32 T, reaching a new world record with regard of all superconductor magnets. In reality, these breakouts take place thanks to the latest advances in practical HTS materials, showing the promising future for power applications. The great advantages and promising prospects of RE123 coated conductors over than other practical superconducting materials have spurred the related researches and rapid development of this field worldwide recently. The further development, however, will depend on the fundamental properties, producing yield, and optimum performance/price etc. In the present paper we give a brief introduction to RE123 coated conductor and its typical structure, as well as main technical routes for the scale-up preparation. We introduce the status and development for RE123 coated conductors around the world, including the existing research institutions and technologies they employed. In past decade, people have tested a variety of thin film techniques to realize the biaxial textured RE123 coated conductors epitaxially grown on flexible metallic tapes. For the most institutions working on the preparation of RE123 coated conductor, both IBAD and RABiTS have been used as popular technical routes due to their availability of long textured substrates, the feasibility of controlled buffer growth and the potential for cost-effective processing. At the same time, various thin film coating techniques including sputtering, pulse laser deposition (PLD), metallorganic chemical vapor or solution depositions (MOCVD, MOD) are employed to prepare the coated conductors for both buffer layers and superconducting layers. Particularly, some emerged MOD technologies for preparing superconducting layer with organic addition into the precursor solution or reduced fluorine content precursor solution have been addressed, due to their great potential for a cost-effective and rapid technical route to prepare RE123 coated conductor. It is also demonstrated that the considerable efforts for artificial flux pinning in RE123 coated conductor by introducing nanoparticles and substrate surface decoration have effectively improved the J c value of RE123 under applied magnet field. The thickness effect of 2G-HTS is also an important issue, because in most case the critical current density would decrease while the thickness of superconducting layer increases. Such an effect may be suppressed with controlled techniques and microstructures or compositions. The coated conductors may maintain good biaxial texture as the thickness of superconducting layer increase from 1 to 5 μm, leading to huge increase in current-carrying capability of resultant 2G-HTS tape. It is believed that the mechanical properties of 2G-HTS tape are better than 1G-HTS tape, which is very important during the magnet coil winding. Excessive stress relief or Lorentz force would make coils split or deformed. To improve the anisotropy and mechanical properties, people have designed various types of practical conductors with RE123 tape stacks or metal tubes covered. Moreover, the welding technique of 2G-HTS tape is investigated due to importance on improving the usage rate of short tapes. In summary, the present review introduces the state of the art of 2G-HTS tape and relevant key science and technical issues, including the technical routes, artificial flux pinning, field dependence and thickness induced degradation of critical current density, joint technologies, as well as power applications, especially in the field of HTS magnet.

/pdf/advance-and-challenge-of-secondary-generation-high-2j3ava374n.pdf

Advance and challenge of secondary-generation high- temperature superconducting tapes for high field applications

Several low fluorine solutions containing different contents of fluorine were prepared by a chemical process. The fluorine contents in these solutions with respect to the conventional all-trifluoroacetate solution were calculated as 0%, 7.7%, 15.4%, 23.1%, 30.8%, 38.5% and 53.8%.
YBa2 Cu3 O7−x (YBCO) films were deposited on LaAlO3 and CeO2/ MgO/ Al2 O3/ Hastelloy substrates using these low fluorine solutions. The phase formation, texture and microstructure of the YBCO films were characterized by x-ray diffraction and scanning electron microscopy. The decomposition mechanism of the low fluorine solution was also discussed. The results indicate that the ratio of F/Ba and the carbon content in decomposed powders could be controlled by adjusting the fluorine content in the precursor solutions. Fluorine had a great influence on the phase transformation, nucleation and growth of YBCO film during the crystallization process. The optimization of fluorine content was in the range of 15.4%– 23.1%, which contributed to the good texture, homogeneous microstructure and high Jc value of the YBCO crystallized films.

Optimization of fluorine content in TFA-MOD precursor solutions for YBCO film growth

Achieving high-critical current densities (J(c)) with small artificial pinning centers is a crucial challenge for YBa2Cu3O7- (YBCO) nanocomposite thin films fabricated using chemical solution deposition methods. In this work, the YBCO texture, structure purity, and its J(c) properties were improved by understanding the influence of preformed ZrO2 nanocrystals (Ba2+ consumption) during the nucleation and growth mechanism. This comprehensive study leads to an additional intermediate dwelling step during thermal process to increase the YBCO nuclei density before the YBCO growth, resulting to a self-field J(c) of 5-6MA/cm(2) at 77K for undoped and ZrO2-doped YBCO films. Counter-intuitively, the space and size distribution of the ZrO2 nanocrystals in the YBCO matrix are independent of this intermediate dwelling step.

Influence of Ba2+ consumption and intermediate dwelling during processing of YBa2Cu3O7 nanocomposite films

Improvement of Production Rate of YBCO Coated Conductors Fabricated by TFA-MOD Method☆

A modified heat-treatment procedure, which consisted of three steps, i.e., 1) calcination, 2) intermediate heat-treatment (IHT), and 3) crystallization, was developed for the metal organic deposition method using trifluoroacetates (TFA). BaZrO3-doped Y0.77Gd0.23Ba2Cu3O6+δ (YGdBCO) coated conductors fabricated by this modified procedure provided the minimum critical current density $(J_{\mathrm{c}})$ value of 0.21 MA/cm2 at 77 K in 3 T, which was as high as 2.1 times the value obtained by the conventional one, and a homogeneous dispersion of BaZrO3 nanoparticles with finer size in the matrix. Furthermore, the 1-D theoretical analysis of the reaction kinetics in the IHT process, considering diffusion and growth kinetics at the interface between the precursor layer and the calcined layer, was proposed, and the estimated reaction rate was compared with experimental ones.

Enhancement of $J_{\mathrm{c}}$ Properties in Magnetic Fields of BaZrO 3 Doped REBa 2 Cu 3 O 6+δ Coated Conductors by a Newly Modified TFA-MOD Process

The crystallization process of trifluoroacetate-metal-organic deposition derived YBa2Cu3O7-y (YBCO) crystals has been investigated to understand the influence of the starting solution composition on the crystal growth process. The starting solutions with molar ratios of Y: Ba: Cu = 1.0: 2.0: 3.0 (stoichiometry) and 1.0:1.5:3.0 (Ba-deficient) were examined. The highest Ic values were achieved at 100 min holding time of the conversion step for 0.8- μm-thick YBCO films, which were independent of the starting solutions. The degradation of Ic performance was observed only in the films prepared from the stoichiometric solution with longer holding time than 100 min. The longer holding times for disappearing of the X-ray diffraction peak of BaF2 derived from the stoichiometric solution is required compared to that from the Ba-deficient solution. On the other hand, the epitaxial growth rate measured from cross-sectional scanning electron microscopic observation for the YBCO films showed no difference between each other. X-ray diffraction analysis on the YBCO films fabricated from stoichiometric starting solution suggested that BaCuO2 and/or Y2O3 phases formed during the solid-state reaction process. The degradation of Ic performance might be caused by the formation of BaCuO2 phases.

Effect of Solution Composition on Superconducting Properties of YBCO Derived From TFA-MOD Process

High-rate fabrication of YBCO coated conductors using TFA-MOD method

REBa2Cu3O y (RE: rare-earth element, REBCO) coated conductors (CCs) have a high critical current density (J c) under a magnetic field at the liquid nitrogen temperature and a potential performance for high-magnetic field applications. In order to prepare a biaxially highly oriented REBCO, a highly-textured buffer layer is required. The ion-beam assisted deposition (IBAD) technique is one of the promising techniques for preparation of the highly-textured layer even on the non-textured metallic substrate such as Hastelloy™. However, excellent surface smoothness and uniformity are indispensable to the non-textured metallic substrate using the IBAD technique. Therefore, an inexpensive surface planarization process has been required for fabrication of REBCO CCs with low-cost. In this study, we have investigated the surface planarization using the metal-organic deposition (MOD) method for the IBAD-MgO texturing. The Gd-Zr-O (GZO) planarization layer was prepared by MOD method on roughly-polished Hastelloy™ C-276 substrate. The surface roughness values (R a) of MOD-GZO planarization layer became smaller than that of the Hastelloy™ C-276 with increase in coating times. Minimum R a of the MOD-GZO planarization layer achieved less than 2 nm, which is smooth enough for obtaining the highly-textured IBAD-MgO film. The in-plane grain alignment of CeO2 film deposited on sputter-LaMnO3 / IBAD-MgO / MOD-GZO / Hastelloy™ C-276 substrate by the pulsed laser deposition was 4°, and high J c value of 2.5 MA/cm2 (1.8 μm-t) was obtained for MOD-YBCO film fabricated on it. The CCs using MODGZO planarization layer provided the delamination strength over 70 MPa, which was compatible with conventional structure of CCs.

http://iopscience.iop.org/article/10.1088/1742-6596/1054/1/012034/pdf

Development of surface planarization process using MOD-Gd-Zr-O layer

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