M. Miura

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.

Formation mechanism of BaZrO3 nanoparticles in Y1?xSmxBa2Cu3Oy-coated conductors derived from trifluoroacetate metal?organic deposition

Grains of BaZrO3 (BZO) in REBa2Cu3Oy (REBCO) films exhibit microstructural differences, depending on whether they were deposited by pulsed-laser deposition (PLD) or metal–organic deposition (MOD). In order to understand the origins of these differences, we examined the formation mechanism of BZO nanoparticles in the MOD process with detailed observations of the quenched films by transmission electron microscopy. The BZO nanoparticle was found to form in the precursor during the temperature ramp prior to the formation of the Y1−xSmxBa2Cu3Oy (YSmBCO) crystals. The YSmBCO grew layer by layer while entrapping the BZO particles, which resulted in random dispersion of the BZO particles in the YSmBCO layer. Consequently, uniformly dispersed BZO nanoparticles were formed in the YSmBCO matrix derived from the TFA-MOD process. These findings indicate that a key factor in achieving fine dispersion of BZO nanoparticles in the superconducting matrix is strongly related to nucleation of the BZO phase crystals in the precursor before growth of the YSmBCO layer, which is unique to the MOD process. In subsequent testing, YSmBCO-coated conductors with uniformly dispersed and densely concentrated BZO nanoparticles showed striking isotropic magnetic-field angular dependence of critical currents.

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.

Development of TFA-MOD Process for Coated Conductors in Japan

Progresses in the development of the TFA-MOD process for the coated conductors in Japan are reviewed. In the fundamental approaches, some important advantages of Ba-poor starting solution were found such as for obtaining higher Jc, shorter reaction time and wider process temperature window. The high Ic value of 735 A/cm-w@77 K was attained. Additionally, introduction of artificial pinning centers by Sm-substitution and Zr-addition to the Ba-poor films was successfully investigated. The films showed a high minimum Ic value of 42 A/cm-w at 77 K, 1 T. Furthermore, isotropic Ic properties in the angular dependence of applied magnetic fields was realized by the uniform distribution of nanoprticles of Zr-compound. On the other hand, two different process of MOD including batch and reel-to-reel (RTR) types have been developed as an engineering process for low cost and long tape production with high superconducting performances at a high rate. In the batch type, long tape synthesis process could be developed by the optimization of the growth conditions in a large furnace. A 500 m long tape was heat-treated and a high Ic value of 310 A/cm-w in a 250 m tape was confirmed. Concerning the RTR type, a high production rate system was developed after confirmation of the ability of the continuous process in a 90 m long tape. A multi-turn with a vertical gas flow system was developed for this purpose. This system was applied to the fabrication of superconducting films on the lower cost buffer layers including a high deposition rate IBAD-MgO layer. Currently, a 5 m long tape with 250 A/cm-w was obtained at the production rate of 3 m/h using a part of the multi-turn system, which is equivalent to 15 m/h in the entire use of the furnace.

Increase of the production rate and crystal growth mode of GdBa2Cu3Oy-coated conductors using an in-plume growth technique for a reel-to-reel pulsed-laser deposition system

We developed a new fabrication technique which we call the in-plume growth (IPG) technique. A REBa2Cu3Oy layered film for a coated conductor (REBCO CC) is grown in the plume using a pulsed-laser deposition (PLD) method with a short distance between the target and the substrate (dT−S) to increase the production rate. In general, the critical current density (Jc) of PLD-REBCO CCs using an RTR system decreases as dT−S decreases since the amount of the dead layer increases and the composition of the REBCO layer becomes off-stoichiometric. In this work, we fabricated high- Jcxa0GdBa2Cu3Oy (GdBCO) CCs using the IPG technique by varying the target composition and the tape moving speed to control the composition of the REBCO layer and to suppress the formation of a-axis-oriented grains. As a result, the IPG-GdBCO CCs, which were fabricated at 2xa0 mxa0h−1 (deposition area = 1-turn × 6.5xa0cm = 6.5xa0cm2, laser power = 300xa0mJ, f = four-plumes × 30xa0Hz), showed the following characteristics: Ic = 312xa0Axa0cm−1 width (Jc = 2.6xa0MAxa0cm−2) with 1.2xa0µm in thickness and 1xa0m length. The production speed and the Jc value are 3.0 and 1.8 times higher, respectively, compared to those deposited under standard conditions without using the IPG technique. This result indicates that the PLD-REBCO CCs could be more competitive for production of CCs for practical electric power applications in the near future.