Yoshitaka Tokunaga

Fabrication and growth mechanism of YBCO coated conductors by TFA-MOD process

Abstract In the development for the coated conductors, the trifluoroacetates-metal organic deposition (TFA-MOD) process using the multi-coating method was applied to form thicker YBa 2 Cu 3 O y (YBCO) films on the CeO 2 buffered IBAD(Zr 2 Gd 2 O 7 )/Hastelloy tape. As a result, in the 5 times coated film with 1.38 μm in thickness, the J c and I c values achieved to be 1.5 MA/cm 2 and 210 A, respectively. Then, we fabricated a 1 m long YBCO tape by the continuous reel-to-reel system. As a result, we confirmed the tape to be homogeneous by XRD analysis. Additionally, we report the theoretical analysis of YBCO growth during post-annealing in the TFA-MOD process considering both the diffusion in the gas boundary layer and the growth kinetics at the precursor/YBCO interface. This model reveals a basic idea of the growth mechanism to define the steady state growth rate, and could explain the experimental results. In addition, it predicts that the growth rate could be estimated from the partial pressure of the water vapor of the inlet gas.

Research & development of superconducting fault current limiter in Japan

R&D of fundamental technologies for superconducting AC Power equipment (Super-ACE project) started as a national project, for a five-year plan since fiscal 2000 by MITI and NEDO. This project is to research and develop the basic technology of super-conductive cable, fault current limiter and transformer. This paper summarizes the research and development of following 2 types FCL. The R&D of the SN transition resistive type superconducting thin film fault current limiter aims to establish large-current/high-voltage technology by arranging films in multi-parallel/series, and increasing the current-carrying capacity through widening the area of the thin film of the current limiting element. The R&D of the reactor for a rectifier-type fault current limiter using coils of superconducting wire of the Ag sheath Bi2223 tape, aiming to apply it to high-voltage systems.

High-Ic YBCO coated conductors by metal organic deposition method using trifluoroacetates

For the development of the YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) coated conductors, processing of the thicker film and long tape have been investigated using the metal organic deposition (MOD) method with the trifluoroacetates solution. About 1 micro-meter thick MOD film was realized by the control of the P/sub H2O/ condition during annealing for the formation of the YBCO layer. The improvement was explained by the competition between the nucleation of the undesirable grains and the epitaxial growth up to the surface. By applying the suitable condition to the metallic substrate system, high critical current density (Jc) and critical current corresponding to 1 cm wide tape (Ic/sup */) were realized as to be 1.6 MA/cm/sup 2/ and 153 A/cm-w in the film with 1 micrometer thickness, respectively. For the fundamental investigation of long tape processing, influence of the gas flow direction during annealing for the YBCO formation was investigated. Although nonuniformities in the X-ray diffraction (XRD) intensities and Jc values were recognized in the film grown under the gas flow parallel to the long direction of the tape, both of them was improved by changing the gas flow direction to the transverse one.

High critical current YBCO thick films by TFA-MOD process

Abstract As a method of the fabrication processes of YBa 2 Cu 3 O 7− x (YBCO), the metalorganic deposition (MOD) process using metal trifluoroacetete (TFA) is considered to be a strong candidate due to its low cost fabrication process for coated conductors with high J c . In our previous work, a triple coated film with 1 μm in thickness was fabricated on a CeO 2 /IBAD–YSZ layer buffered Hastelloy substrate by optimizing the condition of heat treatments such as P H 2 O in the multi-coating method [Physica C 378–381 (2002) 1013]. The J c value of 1.6 MA/cm 2 (77 K in self-field) in this film patterned 100 μm width and the I c * value of 153 A/cm-width at 77 K in self-field were achieved. In order to obtain a thicker film with high overall I c * for 1 cm width, the influence of the heat treatment conditions of P H 2 O , P O 2 , and the temperature in the MOD process was investigated. Subsequently, a 5 times coated film was obtained on a CeO 2 /IBAD–Zr 2 Gd 2 O 7 layer buffered Hastelloy substrate by optimizing the conditions of heating and dip coating. As a result, the overall transport I c value was improved to 210 A and J c value of 1.53 MA/cm 2 was obtained (77 K in self-field).

Low temperature scanning laser microscopy of YBCO coated IBAD tapes

Spatially resolved measurements of dissipative state in YBCO coated conductors have been carried out by use of the low temperature scanning laser microscopy (LTSLM). It has been shown that the current dependent dissipation peak is well scaled by the Gaussian distribution, and its normalized distribution becomes narrower in inversely proportion to the square root of power index in the local current-voltage characteristic. From the LTSLM analysis, we can collect positional information of current limiting coordinate as well as spatial variation of local critical current. Optimizing the measurement condition, we also succeeded to visualize the dissipation even in the tape which is covered by a 10 /spl mu/m thick Ag stabilization layer.

Processing for long YBCO coated conductors by TFA-MOD process

Abstract The metal organic deposition (MOD) process of YBa 2 Cu 3 O 7− X (YBCO) using metal trifluoroacetate (TFA) precursors is considered to be a strong candidate as a low cost fabrication process in coated conductors since the TFA-MOD process is a non vacuum process and can provide high J c films. In order to develop long tape conductors using this process, the influence of the gas flow direction in the heat treatment was investigated. The J c value decreases toward to leeward side in the case of the parallel gas flow condition. On the other hand, in the case of the perpendicular gas flow condition. The J c value reveals uniformity in all over the tape and the high J c level is maintained. In the case of long tape production, it was found that the perpendicular gas flow to the tape length is effective to fabricate the uniform films. It could minimize the influence of consumption of H 2 O and generation of HF gas in the up-stream area to the leeward region. The J c and I c properties and these distribution of the YBCO tape fabricated by this method on CeO 2 /IBAD-Gd 2 Zr 2 O 7 /Hastelloy is presented. Furthermore, a large scale equipment for the continues long tape process was developed based on the above findings. Consequently we have fabricated a 1 m long Y123 tape conductor by a reel to reel system. Even in the first trial, the uniform reaction for the entire length was recognized.

Design and Test Results of 66 kV High-Tc Superconducting Fault Current Limiter Magnet

This paper describes the design of a 66 kV high-Tc superconducting fault current limiter (FCL) magnet and the test results obtained. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of coils wound with Bi2223 tape. The rated current is 750 A. The insulation voltage of the magnet is of the 66 kV class. In the final year of the project, all six sets of the coils are set connected in the cryostat and evaluation tests have been implemented. In the cooling test, sub-cooled nitrogen of 65 K was obtained, with homogenous temperature distribution in the cryogen. The rated current of 750 A was successfully obtained for both direct and alternating current tests. In addition, the magnet passed the simultaneous current and voltage application test. The dielectric test results showed that the magnet satisfied the insulation for 66 kV apparatus in the Japanese Electrotechnical Committee Standard. Finally, the magnet was combined with a rectifier bridge circuit to form an FCL. In a fault current limiting test with a short-circuit generator, the FCL successfully limited short circuit current up to 20 kV applied voltage. The obtained result was in good agreement with the design and the Electro-Magnetic Transients Program (EMTP) analysis

Influence of the second buffer layer for alignment of seed film on Ni tape with NiO buffer layer

Abstract In order to investigate the influence of the buffer layer for the seed film quality which is important to obtain high quality liquid phase epitaxy film, we prepared several different kinds of SmBa 2 Cu 3 O y (Sm123) seed films on SOE-NiO buffered biaxially textured Ni tapes with and without the second buffer layer of BaZrO 3 by the pulsed laser deposition method. As a consequence of the investigation, it was found that BaZrO 3 is a suitable material as a buffer layer to obtain highly in-plane aligned Sm123 seed films on SOE-NiO buffered biaxially textured Ni tapes.

66 kV/1 kA high-T/sub c/ superconducting fault current limiter magnet

One of the items included in the Super-conductive AC Equipment (Super-ACE) project, being performed from 2000 to 2004, is development of a 66 kV/1 kA class high-Tc superconducting (HTS) fault current limiter (FCL) magnet. This research focuses on fundamental technical items essential for a 66 kV class fault current limiter, that is, high current-capacity, high voltage-insulation, and sub-cooled nitrogen cooling. This paper describes the design of the magnet and the test results obtained. The magnet mainly consists of a vacuum vessel, a nitrogen bath, a pair of current leads, cryocoolers, and six sets of coils wound with Bi2223 tape. The rated current of the magnet is 750 Ap. The insulation voltage of the magnet is of the 66 kV class. In the final year of the project, all six sets of the coils are set connected in the cryostat and evaluation tests have been implemented. In the cooling test, sub-cooled nitrogen of 65 K was obtained, with homogenous temperature distribution in the cryogen. The rated current of 750 Ap was successfully obtained for both direct and alternating current tests. In addition, the magnet passed the simultaneous current and voltage application test. Finally, the dielectric test results showed that the magnet satisfied the insulation for 66 kV apparatus in the Japanese Electrotechnical Committee Standard (JEC) standard.

All Japan efforts on fundamental materials technology developments for HTS applications: Focusing on R&D of coated conductors

Abstract The coated conductors using REBa2Cu3O7−δ (RE=rare earth element, Y, Nd, Sm etc.) materials are expected for a lot of electric power applications, since it has a high critical current density at the liquid nitrogen temperature even under high magnetic fields. However, it has been difficult to obtain a bi-axially textured coated conductor, which is necessary to realize its high performance, and there has not been a tape fabricated longer than 1 m. Several different processes have been investigated for realizing the bi-axially textured buffer layers/metal substrates as well as thicker film deposition processes and non-vacuum processes for a superconducting layer. Recently, a 10 m long YBa2Cu3O7−δ tape with high Jc was successfully fabricated. This paper reviews the recent progress of the research and development for the coated conductor with its prospects in Japan.