M. Yoshizumi

Overview of Materials and Power Applications of Coated Conductors Project

There are high expectations for coated conductors in electric power applications such as superconducting magnetic energy storage (SMES) systems, power cables, and transformers owing to their ability to contribute to stabilizing and increasing the capacity of the electric power supply grid as well as to reducing CO2 emission as a result of their high critical-current characteristics. Research and development has been performed on wires/tapes and electric power devices worldwide. The Materials and Power Applications of Coated Conductors (M-PACC) Project is a five-year national project in Japan started in 2008, supported by the Ministry of Economy, Trade and Industry (METI) and the New Energy and Industrial Technology Development Organization (NEDO), to develop both coated conductors that meet market requirements and basic technologies for the above-mentioned power applications using coated conductors. In this article, research and development results are reviewed and compared with the interim/final targets of the project, and future prospects are discussed.

Improvement of flux pinning performance at high magnetic fields in GdBa2Cu3Oy coated conductors with BHO nano-rods through enhancement of Bc2

Addition of BaHfO3 (BHO) nano-rods as pinning centers into a GdBa2Cu3Oy (GdBCO) coated conductor dramatically improves the critical current properties in high magnetic fields. This is partly ascribed to the strong flux pinning of these pinning centers. In this paper it is reported that this improvement is mainly caused by unexpected enhancement of the upper critical field, Bc2. The mechanism of the enhancement of Bc2 is not yet clear, although the strain around the interface between the pinning center and the superconducting matrix or the interface itself may scatter electrons. This result suggests that the critical current property of REBa2Cu3Oy (RE: rare earth elements, REBCO) coated conductors could be designed not only by tailoring the microstructure of pinning centers but also by controlling the intrinsic superconducting property. Especially, the selection of pinning material and its content that determines Bc2 is expected to be a key problem for further improvement of the pinning performance at high magnetic fields.

Enhancement of Flux Pinning in Y1-xSmxBa1.5Cu3Oy Coated Conductors with Nanoparticles

We introduced artificial pinning centers for trifluoroacetates (TFA)-metal organic deposition (MOD) Y1-xSmxBa1.5Cu3Oy coated conductors to enhance critical current density (Jc) under magnetic fields and to improve the magnetic field angular dependence of Jc (Jc–B–θ). The coated conductors showed high Jc values and isotropic Jc–B–θ property with the ratio Jc,min/Jc,max of 0.91 (77 K, B=1 T). From a microstructural observation, BaZrO3 (and/or ZrO2) nanoparticles were uniformity dispersed in the films. The uniform dispersion of nanoparticles was maintained not only along the lateral direction but also in the thickness one. It could be considered that the nanoparticles in the TFA-MOD REBCO coated conductors may act as pinning centers to enhance the Jc in all field orientations.

Development of a laser scribing process of coated conductors for the reduction of AC losses

Expectations are high for using coated conductors for electric power applications not only because of better cost performance but also higher Jc–B properties compared with Bi2Sr2Ca2Cu3O10 (BSCCO) tapes. Furthermore, the coated conductors could also reduce AC losses by the use of various methods in post-treatment. When a YAG laser was used for scribing the superconductive layer, an AC loss reduction due to a decrease in the width of the superconductivity layer could be confirmed in short samples. In the case of AC applications using long tapes, high resistance between scribed filaments is necessary. However, the resistance between the filaments scribed by a laser was as low as 10−3 Ω cm−1. The reason for the low resistance was the existence of dross in the filament spacings, which was confirmed by a cross-sectional compositional mapping observation. Although the superconductivity layer was divided by the laser, the dross of the metal substrate made a bridge over the superconductivity filaments. Post-annealing in an O2 atmosphere to oxidize the dross was carried out in order to increase the resistance. Consequently, the resistance between the filaments was improved to be as high as 50 Ω cm−1. We succeeded in controlling the resistance value between filaments in a range five orders of magnitude higher than that of the as-scribed YBCO film, through oxygen atmosphere post-annealing.

Rare Earth Substitution Effects and Magnetic Field Dependence of Critical Current in Y1-xRExBa2Cu3Oy Coated Conductors with Nanoparticles (RE=Sm, Gd)

In order to enhance the Jc characteristics under the magnetic fields, we prepared Y1-xRExBa2Cu3Oy coated conductors (CCs) with artificial pinning centers by using the starting solution containing Zr-salt, where RE is Gd or Sm, in the trifluoroacetates-metal organic deposition. From microstructure observation, BaZrO3 (BZO) nanoparticles were uniformly dispersed in these films. High Jc in the magnetic fields and significant enhancement of Jc for all applied magnetic field angles were achieved. Especially, YGdBCO+BZO CCs showed the highest self-fields Jc values (Jcsf) of 4 MA/cm2 and its minimum value of 0.64 MA/cm2 at 77 K and 1 T.

Effect of oxygen partial pressure on quasi-ternary phase diagram of NdO1.5–BaO–CuO system

Abstract Quasi-ternary phase diagrams of the NdO 1.5 –BaO–CuO system under low and high oxygen partial pressure atmospheres ( P O 2 =0.01, 1.00 atm) were constructed. Liquidus curves, solidus lines, and the equilibrium tie-lines in the two-phase field of liquid and Nd 1+ x Ba 2− x Cu 3 O 6+ δ were determined experimentally. As the oxygen partial pressure of atmosphere increased, liquidus curves were shifted to the Nd-rich region and the Nd solubility in the solvent with lower BaO/CuO ratio was further increased. The stable region of Nd 1+ x Ba 2− x Cu 3 O 6+ δ , which was under the influence of temperature and P O 2 , was widened at lower temperature and/or higher P O 2 . Especially, its sensitivity to P O 2 was high. By investigating equilibrium tie-lines, the x value of Nd 1+ x Ba 2− x Cu 3 O 6+ δ was reduced and became insensitive to the BaO/CuO ratio of liquid phase equilibrated with Nd 1+ x Ba 2− x Cu 3 O 6+ δ as the BaO/CuO ratio increased and slightly reduced as temperature decreased. Those tendencies were maintained at different P O 2 . It was discussed thermodynamically why NdBa 2 Cu 3 O 6+ δ (Nd123) superconductors, fabricated in a reduced oxygen atmosphere, show high superconductive critical temperature. One of the reasons for this phenomenon was the decrease of the solubility of Nd 1+ x Ba 2− x Cu 3 O 6+ δ with the oxygen partial pressure of atmosphere.

Crystal growth of giant magnetostrictive Tb-Dy-Fe alloy

Crystal growth of the giant magnetostrictive Tb-Dy-Fe alloy has been investigated. 〈112〉 and 〈110〉 twinned ‘single’ crystals were obtained utilizing the preferred growth of the Tb-Dy-Fe alloy. The twinning could be effectively suppressed by low growth rates such as 3 mm h−1. With seed crystals cut from the 〈112〉 twinned ‘single’ crystal, 〈111〉 oriented single crystals were successfully grown. High growth rates such as 60 mm h−1 induced the twinning and a growth tilted from axial heat flow direction, and thus destroyed the twinless growth along 〈111〉 direction. In the initial growth regions of the 〈111〉 crystals, a REFe3 banded structure was usually observed.

Flux pinning properties of TFA-MOD (Y,Gd)Ba2Cu3Ox tapes with BaZrO3 nanoparticles

A high critical current density Jc with a nearly isotropic angular dependence at 77.3 K was recently obtained by the introduction of nanoparticles to (Y,RE)Ba2Cu3Oy (RE123, RE = Sm, Gd)-coated conductors prepared by trifluoroacetate metal–organic deposition (TFA-MOD). The flux pinning properties were evaluated on the basis of detailed measurements of the upper critical field, the irreversibility field and the critical current density of the TFA-MOD RE123-coated conductor. We found that strong random pinning by the BaZrO3 nanoparticles and weak c-axis correlated pinning by twin boundaries functioned cooperatively in nanoparticles introduced to (Y, Gd)123-coated conductors prepared by TFA-MOD. The coexistence of two different pinning centers played an important role in the nearly isotropic angular dependence of Jc.

Present Status & Future Prospects of Coated Conductor Development in Japan

Since 2003, three years of the current 5-year national project for development of coated conductors using Y-system superconductors have passed and remarkable results have been achieved. In this paper, the current status and future prospects are reviewed. The group of Fujikura and SRL has worked on a long tape with high performance in the PLD-YBCO superconducting tapes on the IBAD-GZO buffered substrates. The highest value of the tape is over 50 k Am which is 212 m in length and 245 A of critical current. Additionally, a 500 m long IBAD-buffered tape with a reasonable in-plane texture was produced using a large scale IBAD apparatus by Fujikura. In the other group, long tape processing has been developed focusing on lowering production cost. An extremely high value of 692 A was obtained in a short sample by the advanced TFA-MOD method on a substrate. In the efforts for the long tape processing, a 25 m long tape with the value of 100 A by a continuous reel-to-reel system and a 40 m long tape with 155 A by a batch system were realized. 100 m class long tapes were also obtained with the MOCVD and PLD-HoBCO processes. Both groups are aiming for 500 m long tapes with the high value of 300 A/cm-w with a production rate of 5 m/h. Furthermore, feasibility studies for applications using coated conductors have already began due to the above mentioned success of long tape production. Several kinds of coils using the long coated conductors such as solenoid and pancake types and spiral shaped conductors for the cable were made. Reasonable high performance results were confirmed in the trials. New approaches on the preliminary studies for the applications of cables, transformers, motors and current limiters etc. are planned in this project.

Development of Long Length IBAD-MgO and PLD Coated Conductors

For high speed and low cost production of coated conductors, a simplified 4-layered buffer architecture consisting of Gd₂Zr₂O₇ (GZO)/Ion Beam Assisted Deposition (IBAD)-MgO/LaMnO₃/CeO₂ was realized, which is compared with the conventional 5-layered IBAD-MgO buffer layers. A new cation diffusion barrier and nucleation GZO layer was first reported. The fourth layer of Pulsed Laser Deposition (PLD)-CeO₂ showing the self-epitaxial growth effect resulted in the in-plane texturing (Deltaphi) of about 3 to 4 degrees. Consequently, high critical current, I_c, values above 600 A/cm-width were attained for PLD-GdBa₂Cu₃O_7-X (GdBCO) short samples. Using this result, a 41 m long GdBCO coated conductors was successfully fabricated at the production rate of 24 m/h for IBAD-MgO. This conductor exhibited superior Deltaphi values of 3.7 to 3.8 degrees and high I_c values of 500 to 600 A/cm-width at 77 K and self -field (s.f.) along almost the entire length. The highest I_c recorded 608 A/cm-width with the J_c value of 2.43 Ma/hboxcm². Furthermore, a new IBAD-MgO deposition method was developed, using DC-reactive sputtering which is expected to be inexpensive and easy to scale up. This system brought about a high production rate of 150 m/h in spite of the small deposition area (6 times 20 cm²). The GdBCO on this buffered substrate also showed a high I_c value of 286 A/cm-width at 77 K and s.f., corresponding to the J_c value of 1.43 Ma/hboxcm².