Akira Ibi

Investigation of thick PLD-GdBCO and ZrO2 doped GdBCO coated conductors with high critical current on PLD-CeO2 capped IBAD-GZO substrate tapes

In order to increase the critical current, Ic, we have fabricated thick GdBa2Cu3O7?? (GdBCO) coated conductors (CCs) by the pulsed laser deposition (PLD) method on PLD-CeO2/ion-beam assisted deposition (IBAD)-Gd2Zr2O7 (GZO)/hastelloy metal substrate tapes. The highest critical current value was 522?A?cm?1 for a thickness of 3.6??m in self-field at 77?K. It was found that a low volume fraction of a-axis orientated grains was obtained in the thick GdBCO CCs, compared to YBa2Cu3O7?? (YBCO) CCs. Consequently, the GdBCO CCs showed higher critical current density (Jc) than YBCO CCs in all thicknesses from 0.2 to 3.6??m. Furthermore, we have succeeded in improving Ic in a magnetic field by the introduction of artificial pinning centres using a 5?mol% ZrO2 doped GdBCO target. In the measurement of the Ic dependence on the magnetic field angle, ?, Ic was much improved, especially at 0?, i.e., with the magnetic field parallel to the c-axis. The Ic value at 3?T was 59.5?A?cm?1 at 0? and it showed a minimum of 42.3?A?cm?1 at 82? for 2.28??m thick CC. The minimum value in the angular dependence of Ic at 3?T was about five times higher than that of YBCO CC and two times higher than that of pure GdBCO CC.

Magnetic field dependence of Jc for Gd-123 coated conductor on PLD-CeO2 capped IBAD-GZO substrate tapes

We prepared GdBa2Cu3O7?? (Gd-123) coated conductors by the pulsed laser deposition (PLD) method on PLD-CeO2/ion-beam assisted deposition (IBAD)-Gd2Zr2O7 (GZO)/hastelloy metal substrate tapes. The Gd-123 film showed a higher critical current density (Jc) in magnetic fields (B) and higher critical temperature (Tc) than those of YBa2Cu3O7?? (Y-123) film. The Gd-123 film exhibited a high Jc value of 0.19?MA?cm?2 at 3?T () in liquid nitrogen (77?K), and the Tc value was 93.8?K. The Jc value at 3?T was twice as high as that of Y-123 film. Moreover, the angular dependence in magnetic fields of Jc for Gd-123 was also superior to that of Y-123. The reduction of Jc by the magnetic field angle for Gd-123 was less sensitive than that in Y-123. Furthermore, Gd-123 clearly exhibited a peak at 0? () as well as at 90? () in the relationship of Jc on the magnetic field angle curve (Jc??), while Y-123 had no peak at 0?. These superior characteristics are considered to be due to the existing defects, such as stacking faults, as observed by transmission electron microscopy. PLD-Gd-123 coated conductors, with pinning-effective defects, high Jc and low anisotropy in Jc??, show promise for future applications.

Development of long GdBCO coated conductor using the IBAD/MPMT-PLD method

We have developed long GdBa2Cu3O7−X (GdBCO) coated conductors by a multi-plume and multi-turn pulsed laser deposition (MPMT-PLD) method and have successfully fabricated 32 and 60.7 m long GdBCO coated conductors with a high critical current, Ic, and high deposition rate. The Ic of the 32 and 60.7 m long GdBCO coated conductors were 205 A (Jc = 1.36 MA cm−2) and 183 A (Jc = 1.45 MA cm−2), respectively, at 77 K and 0 T. In addition, they exhibited higher Ic values in a magnetic field than a YBa2Cu3O7−X (YBCO) coated conductor: typically 20 A at 77 K and 3 T while the value for a YBCO coated conductor is 8 A. These high Ic values are due to the smaller number of a-axis oriented grains in GdBCO than in YBCO. Furthermore, the speed of production of the GdBCO layer was increased to 10 m h−1 while that of the former YBCO coated conductor was 3.75 m h−1. The material yield of long GdBCO layers using the MPMT-PLD method was about 26–28%. The high Ic of GdBCO in a magnetic field, the high production rate and the high material yield are promising for applications.

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.

Current Transport Properties of 200 A-200 m-Class IBAD YBCO Coated Conductor Over Wide Range of Magnetic Field and Temperature

Current transport properties in 200 A-200 m-class IBAD-YBCO coated conductor (CC) have been investigated over a wide range of temperature and magnetic flux density up to 26 T. YBCO CC fabricated by reel-to-reel pulsed laser deposition equipment with a multi-plume and multi-turn deposition system showed high critical current of 245 A in length of 212.6 m. Critical current density, Jc, in 5 T parallel to c-axis at 65 K and 4.2 K were 0.44 MA/cm2 and 5.5 MA/cm2, respectively. Moreover, even in very high magnetic flux density, the superior Jc property remains at lower temperature, e.g. Jc at 4.2 K in 26 T was 2 MA/cm2. We also showed analytical expressions of electric field vs. current density (E-J) characteristics based on a framework of percolation model and scaling law of the flux pinning properties. Using the expressions, E-J characteristics and Jc value at any conditions of temperature and magnetic flux density can be predicted quantitatively.

Reversible strain limit of critical currents and universality of intrinsic strain effect for REBCO-coated conductors

Intensive research work has been carried out in order to develop industrially available HTS REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in the project succeeded in the development of high performance coated conductors. Their characteristic features have been evaluated in terms of mechanical properties and their influence on critical currents. The mechanical properties at RT and 77 K were analyzed on the basis of the rule of mixtures. The force-free strain (Aff) was analytically deduced, which indicates the strain at which the residual stress exerted on the superconducting layer becomes zero. Tensile strain dependence on critical currents could be divided into elastic and brittle regions. The reversible strain limit (Arev) was defined as a strain at which the critical current recovers elastically to the level of 99% Ico. Within the elastic region, the critical current showed a convex strain dependence, which is explained as Ekins intrinsic strain effect. The degradation beyond the reversible strain limit was attributed to a fracture of the superconducting layer. As a whole, the present study made clear quantitatively the tensile strain behavior of critical currents and proposed a reasonable definition for the reversible strain limit.

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.

Dependence of critical current properties on the thickness of the superconducting layer in YBCO coated tapes

The dependence of the superconducting layer thickness on the critical current properties was investigated in the range of 0.27–2.0 µm for YBCO coated tapes made by PLD processing on IBAD substrates. The critical current density at low fields in the direction of the c-axis was found to decrease with the increasing thickness d in proportion to d−1/2 up to 1 µm. This seems to agree with the prediction of the two-dimensional collective pinning of random point pins. However, this dependence did not change over a wide temperature range of 5–60 K and the critical thickness for the two-dimensional pinning was found to be much smaller than 1 µm. This result suggests that the observed thickness dependence does not come from the pinning mechanism but simply from the change of the superconducting layer structure with increasing thickness. The effect of flux creep on the critical current properties at high temperatures is also investigated, and the observed thickness dependences of irreversibility field and n-value are compared with the theoretical predictions of the flux creep-flow model.

Rapid Formation of 200 m-long YBCO Coated Conductor by Multi-Stage CVD

Toward the industrialization of YBa2Cu3O7-x (YBCO) coated conductors for practical applications, metal-organic chemical vapor deposition (MOCVD) has been investigated. We applied multi-stage MOCVD to achieve high-speed production of coated conductor because the increase of the number of depositing stages placed in a line could enlarge deposition area. Using 12-stage CVD, we succeeded in deposition of highly biaxially oriented YBCO layer at tape transfer speed of 50 m/h on the CeO2 capped ion-beam-assisted-deposition (IBAD) - Gd2Zr2O7 (GZO) buffered Hastelloy tape, and achieved high critical current density ( Jc) exceeding 2 MA/cm2 in a short sample. And it was found that lowering substrate temperature according to YBCO layer growth was effective to increase Ic. Using this method, high critical current (Ic) of 174 A was achieved by 1 mum thick YBCO layer which passed through 12-stage CVD, and 227 A by 1 mum thick YBCO layer deposited by 6-stage one. Based on these results, three 200 m-class YBCO layers were deposited by multiple depositions at tape speed of 50 m/h each, using the 12-stage CVD system. A 203 m-long YBCO layer was successfully deposited with end-to-end Ic of 92.8 A, so that the multiplication of Ic and length reached 18.8 kA ldr m. These results indicate that a multi-stage CVD technique is useful for a rapid fabrication of long YBCO coated conductor.

Visualizing Transport Properties in IBAD Based YBCO Coated Conductors by Multiple Analysis Techniques

Current transport properties in Y1Ba2Cu3O7-delta (YBCO) coated conductors obtained by pulsed laser deposition process on a CeO2 capped Gd2Zr2O7-IBAD template have been studied by spatially resolved measurements. We utilized low temperature scanning laser microscopy along with laser induced Seebeck effect imaging and scanning SQUID microscopy that allow us to visualize 1) distributed flux flow dissipation, 2) current blocking obstacles and 3) local current flow, respectively. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the dissipation, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens mum to hundreds mum. Detailed correlation between local current flow and dissipation has been observed. Present methods have great potential as tools for basic understanding of current limiting mechanisms in the coated conductors.