Recent activities in the R&D of coated conductors are reviewed. Coated conductors were classified into two groups based on the architecture. One group is textured metal substrates such as RABiTS. In this group, a high performance of Ic=186 A/cmw in a 34 m-long tape was realized using a TFA-MOD process with several PVD buffer layers. Remarkable results have also been reported by the other R&D group, which uses IBAD buffered substrates. Combined with the PLD method for the YBCO layer, the highest values of Ic·L have been obtained. The highest Ic·L value is 13230 Am by Fujikura. Aiming at lower processing costs, TFA-MOD and MOCVD processes have also been investigated using IBAD substrates. SRL achieved an extremely high Ic value of 413 A/cm-w in a tape produced using the TFA-MOD process. R&D efforts in long tape processing have led to a 8.6 m-long tape with 119 A/cm-w as the Ic value.
In order to shorten the calcination time in the metal-organic deposition (MOD) process using metal trifluoroacetate (TFA), a new combination of starting materials was developed using the F-free salt of Cu and TFA salts of Y and Ba as a so-called "new solution". Moreover, a new process temperature profile for this solution was developed as an advanced TFA-MOD process in which a high heating rate was achieved. Consequently, an overall transport Ic value of 210 A and a Jc value of 1.5 MA/cm2 (77 K, Self-field), similar to Ic and Jc values using TFA for all salts, were obtained. In order to obtain higher Jc performance, the effects of process parameters such as the heating rate in the calcination process for the Jc characteristics were investigated. As a result, a YBCO film with a thickness of 1.2μm was fabricated on a CeO2/IBAD-Gd2Zr2O7 layer-buffered Hastelloy substrate with Δφ∼7°. The overall Ic value of 251 A (77 K Self-field) corresponding to the Jc value of 2.1 MA/cm2 was achieved by calcination at a heating rate of 2°C/min.
We developed a low-temperature growth (LTG) technique as a novel deposition process to obtain RE1+xBa2-xCu3Oy (REBCO) thin films with high superconducting properties. SmBCO films fabricated using the LTG-technique (LTG-SmBCO) showed complete c-axis orientations at a low substrate temperature (Ts). This Ts is about 100°C lower than the Ts when fabricating c-axis oriented SmBCO films using the conventional vapor growth method. Moreover, the critical current density (Jc) of the LTG-SmBCO in magnetic fields at 77 K has been improved reaching a high value of 0.17 MA/cm2 at 5 T (B//c). The magnetic field angular dependence of the Jc at 77 K in LTG-SmBCO had a smaller anisotropy than that in PLD-SmBCO. From the microstructure observation, we found that the LTG-SmBCO showed a compositional fluctuation in Sm/Ba. We assume that the high Jc of LTG-SmBCO was affected by the solid solution acting as three-dimensional pinning center. From the viewpoint of diffusion in the solid phase, we speculated that this solid solution was generated by the low Ts.
We found that a PLD-CeO2 cap layer on an IBAD-Gd2Zr2O7 (GZO) tape can make a high grain alignment without ion-beam assistance such as the IBAD process. We call this phenomenon “self-epitaxy in a PLD-CeO2 cap layer”. At present, we are developing long tapes by the Reel-to-Reel process for the self-epitaxial PLD-CeO2 cap layer on an IBAD-GZO tape. A 108 m-long tape with a PLD-CeO2 cap layer was obtained. The delta phi values of the buffer layer were improved to 4.3-4.8 degrees of PLD-CeO2 from 13.3-14.0 degrees of IBAD-GZO. Although the fabrication rate of the PLD-CeO2 cap layer was as fast as 5-6 m/h, that of the IBAD-GZO was 1 m/h. Then, we tried to improve the overall fabrication rate of the total buffer layers using a thin IBAD layer and the self-epitaxial PLD-CeO2 cap layer. A PLD-CeO2 cap layer was deposited at a tape transfer speed of 2.5 m/h on a 55 m-long IBAD-GZO tape with the delta phi values of 23.1-24.0 degrees fabricated at 2 m/h, which was two times faster than the conventional IBAD process. As a result, the delta phi values of the PLD-CeO2 cap layer were in the range of 8.6-10.4 degrees. The fabrication rate throughout all of the processes for buffer layers was achieved to be 2 m/h. It was found that PLD-CeO2 was effective for enhancing both the fabrication rate and grain alignment.
We have developed 100-m class YBCO conductors using reel-to-reel vacuum apparatuses for ion-beam-assisted deposition (IBAD) and pulsed-laser deposition (PLD). IBAD-Gd2Zr2O7 films 100m long were obtained with Δφ of 10°on non-textured Hastelloy tapes. Second buffer layers of Y2O3 or CeO2 that were 100m long were grown by PLD on the IBAD templates with Δφ of 7°or 5°, respectively. YBCO films were formed on a Y2O3/Gd2Zr2O7/Hastelloy substrate using PLD. An end-to-end Ic of 38 A and Jc of 0.76 MA/cm2 (77 K, self-field) were obtained in a 100m-long sample.
We have developed a defect analysis procedure that combines a magneto-optical (MO) imaging system with scanning ion microscopy (SIM) and transmission electron microscopy (TEM). Low Ic areas of coated conductors were inspected using the MO imaging system. The defective area revealed in the MO image was prepared for cross-sectional SIM specimen using a focused Ga ion beam. The porous YBCO layers were found by SIM observation. Furthermore, defective areas were picked up using micro-sampling and then thinned using a focused Ga ion beam. TEM observation and energy dispersive X-ray spectroscopy indicated that the YBCO layers were composed of Y-rich phases and Ba-Cu-O.
The stress and strain dependence of the critical current (Ic) was examined for YBCO-coated conductors with IBAD-CeO2/YSZ or ISD-MgO buffer layers. For all samples, Ic increased as the stress was increased and reached a maximum. For larger tensile stress, Ic started to decrease. The reversibility of Ic showed a different behavior depending on the buffer layer. For the tape buffered with IBAD-CeO2/YSZ, Ic recovered to the initial value after the applied strain reached 0.30%. On the other hand, a smaller reversible strain was observed for the ISD-buffered tape. The difference of the reversible strain was discussed based on a microstructure observation. For all samples, quenching occurred at a strain of more than 0.30%. Compared to the stress-strain curve of composite tapes, it was confirmed that quenching strains were determined by the yield strain of the Hastelloy substrate. The strain scaling law for the present YBCO-coated conductors was found to be similar to that for A15-type superconductors.
We have investigated the high-speed deposition of YBa2Cu3O7-x (YBCO) using PLD on a CeO2-capped IBAD buffer layer. In order to speed up the coated conductor process, we introduced a continuous PLD system mainly consisting of high-power laser equipment and a vacuum chamber with a reel-to-reel substrate transfer system. The laser system was designed so that the laser beam scans YBCO targets during deposition and raises plural and discrete plumes (multi-plume). The aim of this is to perform high-speed production of YBCO layers. Although high laser pulse frequency deposition makes high-speed deposition possible, it causes roughening of the surface morphology and lowers the Jc. The multi-plume gave us a high Jc during low laser pulse frequency deposition and high-speed deposition at a higher frequency. The substrates were turned three times to enlarge the deposition area (multi-turn). Within 15 degrees aslant to a plume center, we obtained a high Jc for all turns. Using this multi-plume and multi-turn PLD equipment, we have succeeded in deposition of a 21m-long YBCO layer with a high end-to-end critical current (Ic) of 85 A and high Jc exceeding 1 MA/cm2. Ic data measured in 1.4m intervals showed good uniformity, with a standard deviation of 3.43%. This layer also showed high in-plane alignment, attaining 5.2 degrees of full-width-at-half-maximum as observed by X-ray phi scans. These results indicate the availability of utilizing multi-plume and multi-turn PLD for YBCO-coated conductor preparation.
There are great expectations for YBCO tape as a “next-generation superconductor tape” because the Jc characteristic has better magnetic field dependence at high temperatures and/or in a high magnetic field. It is important to consider stability for superconducting applications; however, the stability criterion for YBCO has not yet been distinctly established. Thus, for the sake of establishing stability criterion for YBCO, we examined the normal-zone initiation and propagation characteristics of a YBCO tape sample experimentally and numerically, and the experimental results were compared with a simulation conducted utilizing a newly developed computer program based on the finite element method (FEM). We also investigated the influence of Ag stabilizer thickness and Hastelloy substrate on the transient thermal behavior of the YBCO tapes.
Using a new substrate produced by depositing a self-epitaxial CeO2 cap layer with a pulsed laser deposition (PLD) method on a Gd2Zr2O7 (GZO) buffered Hastelloy tape, which was applied using the ion-beam assisted deposition (IBAD) method, YBa2Cu3O7-δ (YBCO) layers by the PLD method were studied in terms of the delta phi (Δφ), Ra values, amount of existing a-axis oriented crystals, and the critical current, Ic, values of the YBCO coated conductors. To obtain a YBCO coated conductor with high Ic, good in-plane texturing of the CeO2 layer is required. The values of Ic were saturated at a certain critical YBCO thickness, which is thicker with a better Δφ of the CeO2 layer. As the YBCO layer thickness increased, the surface morphology became much rougher. Consequently the surface temperature of the YBCO layer fell and the amount of a-axis oriented crystals in the YBCO increased. To solve this problem, we fabricated YBCO films by multi-layer deposition using the Reel-to-Reel system, which was carried out at different temperatures settings and thus the values of Ic increased.
Recently, much more attention is being paid to YBa2Cu3O7-δ (YBCO) coated conductors (CCs) as a next generation superconducting wire because of high transport current performance, even in magnetic fields. Especially, YBCO CCs based on templates obtained by ion beam-assisted deposition have exhibited excellent results. For electric power applications, however, high performance must be realized at a large scale (i.e., of more than hundreds of meters). In this article, the fabrication of IBAD templates of more than one-hundred meters for YBCO CCs is reported.
YBa2Cu3O7-δ (Y123) films were deposited under various water vapor partial pressures (P(H2O)) in the crystallization process by an advanced metalorganic deposition (MOD) method using trifluoroacetates. Microstructures of the films were evaluated by means of transmission electron microscopy. As a result, in the Y123 film crystallized under lower P(H2O) such as 2.1%, large pores were remarkably observed. Furthermore, there were many amorphous layers of non-superconducting phases in the film. In contrast, small pores were distributed in the films deposited under higher P(H2O) of 4.2, 6.3 and 13.5%. Considering that the critical current (Ic) values become higher as P(H2O) increases, the existence of large pores and non-superconducting phase compounds causes a reduction in Ic. By investigating the microstructures of the films quenched during the crystallization process, it was found that the pores in the films are generated by the reaction of the non-superconducting phases trapped in the growing Y123 film, which then transform to Y123 afterwards accompanied by volume reduction of about 30%. The sizes of the entrapped phase crystals in the Y123 film depend on the P(H2O), and this may determine the pore sizes in the final film.
The YBCO growth rate dependence on flow-direction during the TFA-MOD process was investigated in order to apply the model to long tape processing. As a result, the YBCO growth rate strongly depended on the substrate position from the windward and decreased as distance increased from the windward position along the film-width direction. This suggests the effect of two-dimensional diffusion is not negligible. The use of two-dimensional analysis of the YBCO growth rate for the gas flow-direction during the post-annealing process has been discussed to understand substrate position influence on the growth rate by considering the gas flow rate and mass diffusion in the gas boundary layer and precursor. From the analysis, it was suggested that the YBCO growth rate decreases toward the leeward region with the distribution of both H2O and HF gases, which rise out of the YBCO growth. Consequently, the growth rate estimation from the model was in good agreement with the experimental results.
We measured total AC losses in a YBCO tape conductor using a calorimetric method for which the validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. Our data is accurate to 5x10-5∼5x10-2 J/m/cycle at 64.1 Hz. The data can be reduced to one curve plotted against a corrected abscissa of the external magnetic field plus the internal magnetic field. The AC loss in different phases of transport current and external magnetic field varies with the phase. Our finding demonstrates the possibility of reducing loss in the case of tri-axial superconducting cables.