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Introduction of artificial pinning centers such as BaHfO 3 (BHO), BaZrO 3 (BZO) and BaSnO 3 (BSO) into REBa 2 Cu 3 O 7− δ (REBCO) coated conductor (CC) layers could improve the in-field critical currents ( I c ) in wide ranges of temperatures and magnetic fields. In particular, a combination of EuBCO + BHO has been found to be effective for attaining high in-field I c performance by means of IBAD/PLD process in short length samples. In this work, we have successfully fabricated a 93.7 m long EuBCO + BHO CC with 103 A/cm W at 77 K under a magnet field ( B ) of 3 T applied perpendicular to the CC ( B // c ). The 93.7 m long EuBCO + BHO CC had high uniformity of I c values and n -values without any trend of fluctuations, independent of the external field up to 0.3 T. I c – B –applied angle ( θ ) profiles of the 93.7 m long EuBCO + BHO CC sample showed the high in-field I c values in all directions of applied magnetic fields especially B // c (at θ ∼ 180°, I c = 157 A/cm W) at 77 K under 3 T. The profiles were about the same as those in a short length sample.

With technological advance of the REBaCuO (RE: Rare Earth elements, REBCO) coated conductors (CC), development of the superconducting applications for its practical use has been carried out recently in various fields. In addition, improvement of the magnetic properties of REBCO CCs makes superconducting devices feasible in the liquid nitrogen. The higher Ic values are required in high magnetic fields by the viewpoint of the requirement in conjunction with the superconducting CCs. Therefore the improvement of the Ic values in the magnetic fields is one of the most important problems to apply them to a device (e.g., NMR, MRI, others). In this work, we investigated the fabrication processing to improve the in-field properties of the YGdBaCuO (YGdBCO) CCs by introduction of artificial pinning centers (APCs). The TFA-MOD method using the batch type furnace for the conversion reaction as a cost effective process to fabricate REBCO CCs was improved by adopting the interim heat treatment during the ramp step of the conversion reaction. The improvement of the properties by the improved process was remarkable and the advanced CCs were named as nPAD-YBCO. A 124 m-long nPAD-YBCO consisted of YGdBCO CC with BZO nano particles was successfully fabricated, exhibiting the I cmin > 50 A/cm-w @77 K, 3 T (I c : Critical current) throughout the conductor. Analyzing the characteristics of the conductor, the BZO particles introduced into the YGdBCO layer worked as effective APCs in wide ranges of temperature and magnetic fields.

We have been developing long REBa 2 Cu 3 O 7-δ (RE: Y, Gd and Eu etc.) with BaMO 3 (M: Zr, Sn and Hf etc.) coated conductors by the combination of the ion-beam assisted deposition (IBAD) and the pulsed laser deposition (PLD) methods. REBa 2 Cu 3 O 7-δ with BaMO 3 coated conductors showed high in-field performance, therefore, these coated conductors could be expected for the industrial and commercial applications at high temperatures in magnetic fields. However, to realize the low production cost for long REBa 2 Cu 3 O 7-δ with BaMO 3 coated conductors, improvement of the production rate of the REBa 2 Cu 3 O 7-δ layers containing BaMO 3 rods with maintaining high superconducting properties is required. To solve these problems, we have tried deposition of the REBa 2 Cu 3 O 7-δ layers with high superconducting properties by the PLD method with high production rate. As a result, we have successfully fabricated EuBa 2 Cu 3 O 7-δ layers containing BaHfO 3 rods with high in-field J c and I c by the PLD method with high production rate. This EuBa 2 Cu 3 O 7-δ with BaHfO 3 coated conductor exhibit a high I c value of 412 and 48.7 A/cm-width at 77 K in self-field and 3 T, respectively at the deposition rate of about 40 μm/h and the production rate of 10 m/h for a 1.35 μm EuBCO layer thick.

Introduction of artificial pinning centers such as BaZrO3 (BZO), BaSnO3 (BSO), BaHfO3 (BHO) nano-rods is effective to improve in-field critical currents (Ic). In particular, the BHO doping into Gd1Ba2Cu3O7−δ had been found to exhibit high in-field Ic under wide ranges of temperatures and magnetic fields. Furthermore, a long coated conductor (CC) with high in-field properties and high uniformity was successfully fabricated by means of BHO doping. However, increase of the Ic values exhibited somewhat in a saturated manner with thickening the superconducting films above 3 μm. In this work, a new combination of Eu1Ba2Cu3O7−δ (EuBCO) + BHO system was investigated using the IBAD/PLD process to improve in-field performance especially in thick films. As a result, it was found that a EuBCO + BHO film with 3.6 μm in thickness showed extremely high property of 141 A/cm-w measured at 77 K, 3 T. This high in-field Ic value was tentatively explained due to suppression of a-axis oriented grains even in thick films. A 200 m long EuBCO + BHO CC with 55.5 A/cm-w and a 93.7 m long CC with 108 A/cm-w measured at 77 K, 3 T were successfully fabricated.

The five-year national project in Japan for RD a high in-field minimum Ic value over 54A/cm-width under 3T at 77K was realized in a 200m long EuBCO tape with artificial pinning centers of BaHfO3 by the pulsed laser deposition (PLD) technique on the IBAD template. On the other hand, the AC loss reduction was confirmed in the tapes fabricated by both PLD and the metal organic deposition (MOD) techniques by scribing 100m tapes into 10-filamments. Additionally, the mechanism of the delamination phenomenon was systematically investigated and the strength was improved by eliminating the origins of the weak points in the films. Through the development, all targeted goals were accomplished and the several results were appreciated as a world champion data.

TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors (CCs) with low cost. Applying YBCO CCs to the power electric devices such as transformers and power cables, the reduction of alternating current (AC) loss for long wire is necessary. Multi-filamentation process, which is one of the most effective approaches for AC loss reduction, has been developed by the scribing process. We have developed the filamentation process using chemical etching. MOD derived CCs are, however, easily damaged in the chemical etching process due to existence of pores in a YBCO layer, resulting in critical current ( I c )-degradation and weak delamination strength. Consequently, it is difficult to scribe MOD derived CCs into 1mm-wide filaments for long length using the chemical etching process. Accordingly, we have studied a scribing process using an excimer laser without chemical etching. We defined P’ value in this study as a function of irradiated laser power [J] divided by processing speed[m/s]. We studied relationship between the P’ value and the results of scribing. It was found that we could scribe the C.C. with a sufficient depth in the condition of large P’ value. Furthermore, we found that the I c was degraded with further increase of the P’ value. A 5 mm wide short sample was divided into 10 filaments by the excimer laser scribing process at the P’ of 9[J/(m/s)]. The sample revealed reduction of the hysteresis loss down to 1/10 which could be expected from a theoretical prediction using the numbers of the filaments. I c -degradation was suppressed as 28%, which was smaller than that of the scribed sample using chemical etching ( I c degradation was 38%). Subsequently, we applied the technique to a 100m long YBCO CC. The hysteresis loss of the 100m long MOD derived CC was reduced down to 1/10 (1/the number of filaments) after the multi-filamentation.

TFA-MOD derived YBCO tapes are expected for many applications due to cost-efficiency. In some applications, uniformity and mechanical strength are required for tapes. A 205 m-long YBCO tape was fabricated with high and uniform I c performance throughout the tape by adopting the interim annealing before the conversion process. The effect of the interim annealing on the crystal growth mechanism of YBCO has been studied focusing on the relationship between the interim annealing conditions and delamination, in this work. Delamination strength was evaluated in the samples prepared with and without interim annealing by the stud pull method. Measurements were carried out on 50 different points for each sample and the results were analyzed statistically. The difference between the two samples was remarkably seen in the delamination strength below 60 MPa. The conventionally annealed sample had more points with low delamination strength below 60 MPa than the interim annealed one. The cross sectional images of both samples observed by SEM showed that there were few pores within the interim annealed superconducting layer, although conventional superconducting layer had many pores. These results suggest that the pores within YBCO layer might be origins to be propagated for delamination at low strength.

TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process is of considerable practical concern for future applications since it can fabricate high performance coated conductors (CCs) with low cost. We have previously reported the success in fabricating a short sample of BaZrO 3 (BZO) nanoparticles dispersed Y 0.77 Gd 0.23 Ba 1.5 Cu 3 O y (YGdBCO) thick films with high I c values under magnetic field (67.8A/cm-w, @77K, 3T). To make thick CCs for long length, it is important to study the effect of process parameters varying with time. In this study, we investigated the effect of coating solution characteristics such as viscosity etc., which could vary with time due to evaporation of solvent, on the transverse film thickness distribution by using Reel-To-Reel (RTR)-dip coating method. We successfully fabricated BZO doped YGdBCO calcined films with 2.5 μm thickness without crack and delamination over 200 m in length. The precursor films were crystallized by the batch type furnace at SWCC Showa Cable Systems Co., Ltd. resulting in the 124 m long CC with high I c minimum value of 50 A/cm-w at 77 K and 3 T.

Y1−xGdxBa2Cu3O7−y with BaZrO3 film was fabricated on CeO2 buffered LaMnO3/ion beam assisted deposition-MgO/Gd2Zr2O7/Hastelloy C276TM substrates by the metal organic deposition process using trifluoroacetates. Both microstructural and analytical characterizations were performed by transmission electron microscopy with selected area electron diffraction method and energy dispersive X-ray spectroscopy. The thickness of Y1−xGdxBa2Cu3O7−y with BaZrO3 film was found approximately 700 nm and the film was composed of c-axis oriented Y1−xGdxBa2Cu3O7−y grains. Several types of particles, (Y,Gd)2Cu2O5, CuO and BaZrO3, were formed within the Y1−xGdxBa2Cu3O7−y film, whose sizes were about 100–200 nm for (Y,Gd)2Cu2O5 and CuO particles, and about 10–20 nm for BaZrO3 particles, respectively. In the Y1−xGdxBa2Cu3O7−y film, (Y,Gd)2Cu2O5 and CuO particles were dispersed heterogeneously, whereas BaZrO3 nanoparticles were found dispersed homogeneously with random orientation. In addition, electron tomographic observation was carried out to attain the three-dimensional information of polyhedral BaZrO3 nanoparticles.

We have been developing long REBa2Cu3O7-δ coated conductors with high performance by the combination of the IBAD and the PLD methods. To realize the low production cost for REBa2Cu3O7-δ coated conductors, growth conditions were optimized for long tape fabrication in the “in-plume PLD method”. As a result, the Ic performance was confirmed with a high production rate under the high oxygen gas pressure and high laser energy density of > 800 mTorr and > 3 J/cm2, respectively. We successfully fabricated a 35 m long GdBa2Cu3O7-δ coated conductor with high Ic value of 619 A/cm-w by the production rate of 30 m/h.

TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process is of considerable practical concern for future applications since it can fabricate high performance coated conductors (CCs) with low cost. In this study, we developed the process for fabricating thick films in BZO nanoparticle doped Y 0.77 Gd 0.23 Ba 1.5 Cu 3 O y (YGdBCO) CCs in order to realize high I c values under magnetic field. The effect of RTR (Reel To Reel)-dip-coating conditions on microstructure, J c(- B -θ) properties, etc. was investigated in order to fabricate crack-free thick and high performance films. A BZO doped YGdBCO film with uniform 3.0 μm thickness showed the high in-field I c and J c values of 55.6 A/cm-w and 0.18 MA/cm 2 @77.8K, 3T, respectively.

In the context of high temperature superconductors, pulsed laser deposition derived GdBa

Three types of superconducting GdBa2Cu3O7−x (GdBCO) layers containing rods of either BaHfO3 (BHO), BaZrO3 (BZO), or BaSnO3 (BSO) were fabricated by pulsed laser deposition on Hastelloy substrates with a CeO2 based textured buffer layer. The critical currents (J c) values of the GdBCO layers containing those nano-rods are enhanced compared with those of pristine GdBCO layer in high magnetic fields. In order to investigate the relationships between their superconductive properties and their nanostructures, they were characterized in detail by transmission electron microscopy (TEM). TEM is the only method for direct observation of these nano-rods in the GdBCO grains. The GdBCO layers were mainly composed of c-axis oriented GdBCO grains containing numerous nano-sized rods. The crystal orientation relationships between the GdBCO and the nano-rods were as follows; (001)GdBCO//(001)nano-rods and (100)GdBCO//(100) nano-rods. The average diameters of the BHO and the BZO nano-rods were 4.5 and 5.6 nm, respectively. The BSO nano-rods were thicker than other rods. These nano-rods in the central region of the c-axis oriented GdBCO grains were aligned parallel to the c-axis of the GdBCO, while nano-rods in the outer region of the c-axis oriented grains were tilted away from the c-axis. With increase in the thickness of the GdBCO layers, the ratio of the BZO or the BSO nano-rods aligned parallel to the c-axis to those tilted away from the c-axis decreased, so that the J c-B-θ profiles of the thicker GdBCO layers containing the BZO or the BSO nano-rods became flatter. The BHO nano-rods were homogeneously distributed throughout the GdBCO, and their average length of was less than that of the other nano-rods. The homogeneous distribution and short length of the BHO nano-rods enhanced the J c values of the GdBCO layers containing them in high magnetic fields. The J c-B-θ profiles of the GdBCO layers containing the BHO were independent of the layer thickness. From these results, we will discuss about the morphologies and distributions of suitable vortex pinning for applications of GdBCO coated conductor in high magnetic fields.

The authors have explored mechanical and transport characteristics of coated conductors by hoop stress tests at 4.2 K, 11 T. Two monolayer coils, which were YBa 2 Cu 3 O 7 (YBCO) and GdBa 2 Cu 3 O 7 (GdBCO) coils, and a double-stacked single-pancake coil were tested. The mechanical and electrical behavior of monolayer coils were simple, thus they were well explained by simple forms. The stress and strain were easily evaluated by using BJR relation and strain gauge measurements. On the other hand, the behavior of the pancake coil was not simple. The analytically evaluated azimuthal strain was qualitatively consistent with measured strain. However, the measured radial strain was not explained by analytically because of non-monolithic deformation of the coil. The transport characteristic was influenced by the non-monolithic deformation.

A GdBa 2 Cu 3 O y (GdBCO) layer was fabricated on Hastelloy with a textured CeO 2 /LaMnO 3 /MgO/Gd–Zr–O buffer layer by in-plume pulsed laser deposition using a reel-to-reel tape transfer system. The microstructures of the coated conductor were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a focused ion beam (FIB)–SEM dual beam system. Numerous outer-growth grains form on the GdBCO surface. Their shapes of these grains can be broadly categorized into three types: rectangular, pyramidal and circular. Cross-sectional TEM images of the GdBCO layer indicate that it is predominantly composed of grains with their c -axis aligned normal to the substrate. The circular grains were identified as being oriented along the c -axis, but rotated 45° in-plane relative to the matrix grains, while the rectangular grains were identified as those with their a -axis parallel to the substrate normal, and the pyramidal grains with their c -axis tilted away from the substrate normal. In addition, TEM revealed that CuO grains formed under the tilted grains and the rotated grains. A 3D reconstructed image of the GdBCO layer using an FIB–SEM dual-beam system clearly shows the distribution of those outer-growth grains and the CuO.

Mechanical property of GdBa2Cu3Oy (GdBCO) coated conductor was investigated at 4.2 K in a magnetic field by two ways, which are the tensile test and the hoop stress test. The tensile stress/strain dependence of critical current (Ic) of a 2 mm width conductor was explored at 4.2 K, 18 T. The result provided that Ic -reversible strain limit existed in between 0.43% and 0.46%, corresponding to 907 MPa and 960 MPa in stress, and the elastic constant was 203 GPa. The hoop stress test has been performed at 4.2 K, 11 T. A test coil was fabricated by winding a 5 mm width conductor on a 270 mm diameter GFRP bobbin by 1.5 turns. The maximum value of applied hoop stress, which was deduced from a product of magnetic field, current density and coil radius, was 1322 MPa. Five strain gauges glued on the conductor surface showed almost the same values, which were in a range of 0.64% to 0.67%, indicating the uniform longitudinal deformation. Furthermore, the hoop stress-strain characteristics were linear, suggesting an elastic deformation. The deduced elastic constants were in a range of 196-204 GPa. It was confirmed that the GdBCO coated conductor performance was deteriorated irreversibly by 1322 MPa hoop stress, whereas not by 1302 MPa hoop stress.

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.

A Nd:YAG (neodymium-doped yttrium aluminum garnet) laser is environmentally safe, due to the absence of any poisonous or hazardous gases, and also economical, without the need for expensive gases. We can thus expect a safe and low cost REBa2Cu3Oy (REBCO)-coated conductor fabrication process by using an Nd:YAG laser. We investigated the fabrication of YBa2Cu3Oy (YBCO) films using a PLD method with a fourth harmonic Nd:YAG laser. The YBCO films showed good superconducting properties; for example, on SrTiO3 single crystalline substrates, Tc and Jc were about 90 K and 2.5 MA/cm2, respectively, at 77 K. In order to enhance Jc in magnetic fields, BaSnO3 was doped into YBCO films by means of a combinatorial Nd:YAG-PLD method. The combinatorial method enabled the growth and study of a library of samples ranging from pure-YBCO to BSO-doped-YBCO. We demonstrated rapid optimization of BaSnO3 content by the combinatorial method and concluded that the optimal BSO content was around 3.6 wt%. In addition, in order to enhance material yield of the Nd:YAG-PLD method, we optimized the oxygen pressure and distance between the target and substrate. On a substrate with dimensions of 10 × 10 mm2, the material yield of a YBCO film reached about 19.0%. These results suggest that the Nd:YAG-PLD method is promising for forming YBCO-coated conductors.

The microstructure of the recently developed coated conductors was investigated by using electron back scatter diffraction pattern (EBSP). We prepared TFA (trifluoroacetates)-MOD (metal organic deposition) derived YBa 2 Cu 3 O 7− x (YBCO) films on CeO 2 /LaMnO 3 /IBAD-MgO/Gd 2 Zr 2 O 7 /Hastelloy C276 substrates of 1 cm-width. The EBSP observation showed that there was a difference of surface microstructure between the midsection and the end of TFA-MOD YBCO film layer in the direction of width. This is attributed not to the local difference of the biaxial texture of CeO 2 top layer but to the local difference of growth condition during TFA-MOD process.

We found that YBCO films deposited by the controlled process showed improved superconducting properties in magnetic fields, even without any artificial pinning centers such as BaZr0 3 and BaSn0 3 . The YBCO films with a thickness of 450 nm were deposited on hastelloy tapes with a textured MgO buffer deposited by IBAD (Ion Beam Assist Deposition) method (IBAD-MgO) by PLD method. In the YBCO films on the IBAD-MgO (IBAD-YBCO), the critical current density of 2.9 MA/cm 2 at 77 K in self-field and the ratio of J c (B//c)/J c (B//a, b) at 1 T was almost unity. Furthermore, we found that the J c anisotropy changed with surface morphology on underlying buffer layer and YBCO thickness. This means that some c-axis correlated pinning centers are included in the films. From the cross-sectional TEM images of the IBAD-YBCO, there were not so many defects such as dislocations, Y 2 O 3 inclusions and so on. Thus it is considered that some defects such as oxygen heterogeneity, displacement of a local composition and so on are pinning centers, although such defects can not be detected by a present analytical technique.

We have investigated the influence of partial pressure of water vapor [P(H 2 O)] in the crystallization process on the superconducting properties of YBa 2 Cu 3 O 7-y (YBCO) films fabricated by a trifluoroacetate-metal organic deposition (TFA-MOD) method. The starting solution with different compositions of Y: Ba:Cu = 1.0:2.0:3.0 and 1.0:1.5:2.0 were studied. The critical current density (J c ) values of YBCO films fabricated from Y:Ba:Cu = 1.0:2.0:3.0 starting solution significantly increase (1.71 → 2.55 MA/cm 2 ) with increasing P(H 2 O) from 12.3 to 47.4 kPa. In the YBCO films fabricated from Y:Ba:Cu = 1.0:1.5:2.0 starting solution, high J c values of over 2.5 MA/cm 2 were recognized in a wide range of P(H 2 O) (12.3-47.4 kPa). One of the reason for J c improvement is the suppression of coarsening of the secondary phases grains such as Y 2 Cu 2 O 5 and CuO due to increase in growth rate of YBCO layer in the crystallization process. The J c values of all YBCO films decreased as P(H 2 O) increased up to 70.1 kPa. This degradation of J c values may be caused by difference of crystal growth mechanism in high growth rate.

A new Japanese national project, called M-PACC, to develop high temperature superconducting electric power devices started in June last year (FY2008–FY2012). This project aims to develop three different types of electric power devices that are expected to provide stable power supplies with large capacity and small size by using YBCO coated conductors. The first program is the development of a 2 GJ class superconducting magnetic energy storage system to control stable electric power systems. It is planned to develop several sets of element coils for a 20 MJ class system as a technological feasibility study for a 2 GJ class coil. The second program is the development of two different types of power cables with higher performance than existing power cables; one is a three-core 66 kV–5 kA class large current cable and the other is a single-phase 275 kV–3 kA class high voltage cable. These cable were required several technological developments, namely, large current and low AC loss, high voltage insulation and low dielectric loss, and power and heat balance for both cables. The third program is the development of a 20 MVA class power transformer with 66 kV/6.9 kV as a distribution transformer. In this project, power transformer systematization technology including 2 kA class large current coil technology, anti short-circuit wire winding technology, AC loss reduction technology, and winding technology will be developed.

The flux pinning properties in a magnetic field of various directions are investigated for trifluoroacetates-metal organic deposition (TFA-MOD) processed Y 1− x Gd x Ba 2 Cu 3 O y (YGdBCO)-coated conductors with the superconducting layer of 0.5 μm thick with artificial BaZrO 3 (BZO)-nanoparticles. It is found that the critical current density is enhanced in a wide range of field angle except around the direction parallel to the a − b plane, resulting in low field-angle anistropy. The enhancement of J c in the wide range of field angle is attributed to the isotropic pinning of nanoparticles. The decrease in the critical current density in the field direction parallel to the a − b plane is considered to be caused by limitation of growth of stacking faults by distributed BZO nanoparticles. The observed results were theoretically explained by the flux creep-flow model and the above speculation of the pinning mechanism was confirmed.

On the research and development of coated conductors in Japan, the I c characteristics and the length have been remarkably improved in the national project. Five hundred meter-long tapes with higher I c values than 300 A/cm-width were realized by the pulsed laser deposition (PLD) and the metal organic deposition using trifluoroacetates (TFA-MOD) processings for the superconducting layer on the IBAD-GZO buffered substrates. In order to realize the low cost by the increasing the production rate of the ion beam assisted deposition (IBAD) layer, the process of IBAD-MgO was developed and a 1000 m-long IBAD buffer tape was fabricated at an extremely high production rate of 1 km/h. On the other hand, the artificial pinning center has been introduced in both PLD and MOD processing. The BaZrO 3 nano-rods were aligned along the c -axis of GdBCO superconducting films by the PLD process. The J c value in the magnetic field parallel to the c -axis was remarkably improved. Additionally, the BaZrO 3 nano-particles were uniformly dispersed in YGdBCO films by the TFA-MOD process and the J c – B–θ property was confirmed to be isotropic. Based on the above-mentioned achievements, the new national project, whose main objective is the development of some electric power applications using a coated conductor, started in 2008. In this project, the improvement of the coated conductor includes the five sub-themes: (1) degradation of tapes, (2) high I c under magnetic field, (3) low AC loss, (4) high mechanical strength and high J e and (5) low cost and high yield.

Dependences of crystallization time on microstructures and superconducting properties such as critical current density (Jc) were investigated for YBa2Cu3Oy (Y123) films by metal organic deposition (MOD) using starting solutions with trifluoroacetate (TFA) salts. We have prepared the Y123 films on Hastelloy C-276® metal tapes with CeO2/Gd2Zr2O7 buffer layers at various crystallization times and investigated the microstructures and Jc properties. As a result, Jc depended strongly on crystallization time and showed a peak at optimum time. Microstructure observations suggested that the cause of the Jc degradation with longtime crystallization was the decomposition of Y123 crystal grains.

It was previously shown that the apparent pinning potential U 0 ∗ associated with the relaxation of superconducting current at low magnetic fields depended on the thickness of superconducting layer for IBAD/PLD-processed YBCO-coated conductors. It is necessary to clarify the thickness dependence of the relaxation property at high magnetic fields for application of YBCO-coated conductors in a persistent current mode at such magnetic fields. In this study, the thickness dependence was investigated in the field range of 1–6 T at 20 K for IBAD/PLD-processed YBCO-coated conductors with the superconducting layer thicknesses of 0.25 – 1.0 μ m . It was found that U 0 ∗ decreases monotonically with decreasing thickness at 6 T. This seems to be attributed to the two-dimensional pinning mechanism because of the longitudinal flux bundle size limited by the film thickness.

To achieve large current capacity and high mechanical flexibility, YBa 2 Cu 3 O 7 (YBCO) superconducting cables consist of a number of YBCO coated conductors that are assembled and wound spirally on a Cu former. In practical applications, superconducting cables are vulnerable to short-circuit fault currents that are 10–30 times greater than the operating current. Therefore, in order to ensure the stability of YBCO superconducting cables in such a situation and to protect them from the fault currents, it is important to investigate the redistribution of the transport current and electromagnetic coupling between the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 10-m-long YBCO model cable, which was manufactured by Furukawa Electric. An over-current with a peak of 31.8 kA rms and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a novel computer program developed using the 3D finite element method to elucidate the electromagnetic and thermal behavior of the YBCO model cable in the presence of an over-current.

In Japan we started a national project to develop a 66/6.9 kV–20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV–20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design.

To improve J C properties in a magnetic field ( J C – B ) of YBa 2 Cu 3 O 7− δ (YBCO) films by a TFA-MOD (trifluoroacetates metal organic deposition) method, we fabricated YBCO films with SnO 2 -doping as artificial pinning centers and investigated the superconducting properties and the crystalline structures. TFA-MOD is expected as a cost-effective method with a non-vacuum system to fabricate YBCO films with high superconducting properties. However, YBCO films have problems that J C decreases in a magnetic field ( B ). In this study, the TFA solutions with SnO 2 for pinning centers were used as starting materials. In the transmission electron microscope image, existence of second phase particles with size of 20–30 nm has been observed in the YBCO film. These nano-particles contained Sn-element and distributed randomly in the film. The J C / J C (self field) values of the SnO 2 -doped YBCO films were enhanced in all magnetic field angles. Therefore it is considered that 3D pinning centers of Sn-compounds were introduced into YBCO film.

Simple 4-layered buffer architecture of Ion Beam Assisted Deposition (IBAD)-MgO is proposed. Using a new barrier and nucleation layer of amorphous Gd2Zr2O7 (GZO) and self-epitaxy of PLD-CeO2 layer, the new 4-layered structure, GZO/IBAD-MgO/LaMnO3 (LMO)/CeO2, was realized, which decreased the number of buffer layers by one layer compared to the conventional 5-layered IBAD-MgO structure. Self-epitaxial CeO2 layer developed the in-plane grain texturing in the order of 3–4°. Several tens meter long IBAD-MgO substrates were fabricated by a reel-to-reel method at the production speed of 24 m/h for IBAD-MgO. Using this highly textured substrate, a 41 m long GdB2Cu3Ox (GdBCO) coated conductor was fabricated. The conductor exhibited high in-plane texturing degree around 4° and high Ic values of 500–600 A at 77 K and self-field. Furthermore, improvement in production speed of each buffer layers was carried out: a new IBAD-MgO technique by DC-reactive sputtering brought about a high production rate of 150 m/h in spite of the small deposition area (6 × 20 cm2) and exhibited a high Ic value of 286 A; PLD-CeO2 and sputter LMO were also improved in the production speed by the multi-turn method and the deposition temperature control, respectively.

We have visualized non-uniform current flow in RE123 coated conductors by using a scanning Hall-probe magnetic microscopy (SHPM). Newly developed SHPM system allows us to measure two-dimensional magnetic field distribution with high spatial resolution in micro-meter scale. Corresponding current density distribution can be obtained from the magnetic field image by solving inverted Biot–Savart’s law. One of the most important advantages of the present system is to visualize the current density distribution in practical high transport current and also in wide scanning area. For example, the system has current leads with large capacity up to 500 A, and the operating distance can be 15 cm by 15 cm with a micro-meter step distance. Using the SHPM system, we have successfully visualized current density distributions in the coated conductor, and clarified different kinds of non-uniform current flow. Those insights are very useful to identify local defects as well as non-uniform tape quality. These results indicate that the SHPM system is a powerful diagnostic tool not only to observe spatial inhomogeneities of transport property but also to understand their reason in practical coated conductors.

For high temperature superconductor (HTS) applications such as a power cable, a motor, and a transformer, a refrigerator enough to cool them below liquid nitrogen temperature is required to attain the HTS system stable operation. Since no refrigerator matched for the HTS application both in temperature and in cooling power is available up to now, a neon turbo-Brayton cycle refrigerator of 2 kW at 70 K was developed. Neon gas was used as the working fluid in the refrigerator. Although hydrogen and helium could be selected as a working fluid for the refrigerator, neon is better energy carrier since it has higher heat capacity and larger molecular weight than hydrogen and helium at a temperature around 70 K in order to make a refrigerator smaller in size. Neon has an advantage in using a small turbo-expander with an impeller of 25 mm in diameter. The turbine rotated at a maximum speed of 100,000 rpm. The information for developing a suitable refrigerator for HTS applications was collected and a new type of neon refrigerator was proposed.

YBa 2 Cu 3 O 7− δ (YBCO) films were grown by using precursor films with barium concentration gradient in film thickness by an advanced metal organic deposition process using trifluoroacetates (TFA-MOD). We have reported previously that a lot of non-reacted particles such as Y- and Cu-oxides were remained for the YBCO film surface grown by the precursors using a starting solution with barium-poor (cation ratio as Y:Ba:Cu = 1:1.5:3). Then, the barium concentration was increased in the film surface to complete the reaction among these Y and Cu residues and Ba and to increase the Y123 growth thickness for realizing higher J c performance. Transmission electron microscopy (TEM) observation showed the increase of film thickness of YBCO grown by the precursors with barium concentration gradient in film thickness, indicating that the reaction between these Y and Cu-oxides and Ba proceeded to form Y123 phase. Consequently, higher J c was obtained for the YBCO film by the precursors mentioned above than that of the YBCO film by the precursors with a constant concentration. Effects of barium concentration gradient in the precursors on the superconducting properties and microstructures in the YBCO film were discussed.

High-temperature superconducting (HTS) power cables transmit bulk power with lower loss than conventional cables. Moreover, HTS cables are expected to be constructed as a new underground cable in urban areas at lower cost compared to a high voltage XLPE cable. To put promising HTS cables to practical use, we need (RE)BCO tapes with long length, high critical current, and low cost. Recently many organizations have improved the performance of the (RE)BCO tapes, such as YBCO tapes, or other coated conductor tapes that are made with a variety of different processes. We have fabricated the conductors for the HTS power cable that was constructed of different kinds of (RE)BCO tapes and measured the I c and AC losses. We achieved significantly low AC loss of 0.1 W/m at 1 kA in the HTS conductor using narrow slit tapes that were cut by laser. Moreover, a 20 m long HTS power cable model and a cable intermediate joint were developed. Short circuit current tests were conducted on the cable system that consisted of two 10 m cables, a cable joint, and two terminations. The cables and the joint withstood the short circuit current of 31.5 kA for 2 s without damage.

GdBa 2 Cu 3 O y (GdBCO) films were fabricated on pulsed laser deposition (PLD)-CeO 2 /ion-beam-assisted deposition (IBAD)-Gd 2 Zr 2 O 7 /Hastelloy C276 metal substrate tapes using the continuous in-plume PLD method. The control of the target composition is important to improve the critical current density ( J c ) property, especially in the case of a short target-substrate distance. As a result, the critical current ( I c ) value of 320 A/cm-W and the J c value of 3.2 MA/cm 2 were obtained by using the target composition of Gd:Ba:Cu = 1:2:3.4 (GdBa 2 Cu 3.4 O y ) in a short sample. It was also found that a high growth rate for the GdBCO film could be realized by crystal growth in the plume (in-plume) for the target. A twice higher growth rate was achieved by the in-plume PLD technique comparing with that in the conventional condition in which crystal growth takes place near the plume edge. In addition, in order to improve the I c values in the external magnetic fields, we introduced the artificial pinning centers using a 5 mol%BaZrO 3 doped GdBa 2 Cu 3.4 O y target by the in-plume method. The BaZrO 3 nanorods were effectively introduced by in-plume PLD and the obtained sample showed improved I c –B– θ properties due to enhanced B || c ; the minimum I c value of 25.6 A/cm-W at 3 T in the 1.6 μm thick film.

Five years of Japanese national project (FY2003–FY2007) was ended last spring with remarkable success. The national project was originally aimed for development of coated conductors which have high superconductivity performance and long length enough to fabricate high temperature superconducting (HTS) electric power devices. Preliminary research and development of HTS electric power devices were carried out as well. A series of R&D results will be summarized and reviewed in this paper. The new 5 years Japanese national project has started last June (FY2008–FY2012) to develop HTS electric power applications including SMES, cables and transformers and to develop/produce coated conductors satisfying the requirements from the respective power devices. Collaborative R&D efforts by national laboratories, universities and private companies have been started with the supports of NEDO and METI. The accomplishment of the previous project will be summarized and the plans and goals of the new project will be presented in this paper.

We have characterized current transport properties in a GdBa2Cu3O7−δ coated conductor fabricated by a combination of ion-beam assisted deposition (IBAD) method and pulsed laser deposition (PLD) method with reel-to-reel systems, and have applied them to a coil design for a high-field magnet. The J-E characteristics in the conductor are given as a function of temperature, magnetic field and the field angle based on percolation transition model, and its potential as a high-field magnet coil is discussed based on a finite element method analysis. As a result, we have found that the conductor with Ic = 350 A/cm-w at 77 K and self-field has a possibility to realize a 40 T magnet coil with very compact size (300 mm in outside diameter) and small usage of the coated conductor (8 km). Furthermore, we have also investigated the relationship between the scale of the magnet coil and the corresponding requirements for the conductor. Consequently, it has been suggested that the improvement of critical current characteristics of the conductor greatly advances the downsizing of the coil without large increment in hoop tensile force. In addition, it has also been found that such downsizing of the coil contributes to the reduction in stored energy and total magnetization loss of the coil. This means that the recent improvement of critical current characteristics in coated conductors is especially worthwhile for high-field magnet coils in terms not only of downsizing but also of quench protection and magnetizing/demagnetizing operation.

On the research and development of a reel-to-reel TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process, a present status is reviewed and its future strategy is addressed. As a base of the study, the 90 m long tape with uniform I c distribution of the 300 A/cm-width level was obtained on the CeO 2 buffered IBAD-Gd 2 Zr 2 O 7 /Hastelloy C276 substrate. The tape has the 56 m region with the end-to-end I c value of 250 A, which corresponds to maximum product of I c × L of 14,000 Am. Based on the results, several directions on R&D have been studied such as “higher I c ”, “higher I c – B ”, “higher production rate both in coating/calcinations and crystallization steps” and lower cost buffer/substrate”. Then, an extremely high I c value of 735 A/cm-width was achieved in a short tape by the compositional control (e.g. Ba-deficient), in the starting solutions. On the efforts for achieving higher I c – B properties, high I c values of 115 and 35 A/cm-width under the magnetic fields of 1 and 3 T were obtained by the RE mixture of Y and Gd in REBCO, addition of Zr and a growth rate control process. On the other hand, the production rate for the coating/calcinations process was improved by development of new starting solutions, which uses F-free Y salt instead of TFA salt of Y. The high J c value of 1.9 MA/cm 2 was confirmed using the precursor films fabricated at a high traveling rate of 10 m/h. Concerning a higher rate in the crystallization step, the multi-turning system with a vertical gas flow system was developed. The validity of the concept was confirmed using 2-turn parts of the furnace. The high I c value of 250 A/cm-width was realized in the 5 m tape crystallized with a traveling rate of 3 m/h, which is equivalent to 15 m/h for usage of entire area of the furnace of 10-turns. Furthermore, in order to achieve the lower cost, the architecture of the coated conductor with a low cost buffer/substrate system has been developed. An IBAD buffered substrate using IBAD-MgO layer (CeO 2 /LMO/IBAD-MgO/Hastelloy C276) was developed and a high production rate of 24 m/h was realized for IBAD-MgO layer using a small ion gun system with the area of 6 × 22 cm 2 . The grain texturing of the substrate was reached the Δ ϕ value of 4° in the CeO 2 layer. This substrate was applied to the above mentioned multi-turning crystallization furnace for TFA-MOD process. Then, a 5 m long tape with 260 A/cm-width (@77 K. s.f.) was achieved. According to the TFA-MOD process in the above achievements, the prospects of each issue for the future stage were independently confirmed. Consequently, R&D combining the above-mentioned achievements for longer tapes are expected in the next stage.

We fabricated 1 m high temperature superconducting conductor (HTS conductor) using YBa 2 Cu 3 O 7−x coated conductors (YBCO tapes) on textured metal substrates, which are expected to be lower in cost than YBCO tapes using ion-beam assisted deposition. Those substrate and intermediate layers were manufactured by Furukawa Electric, and YBCO and a protective layer were applied to the intermediate layer by Chubu Electric Power. Before fabricating the conductor, a 0.1 mm thick copper tape was soldered to the YBCO tape, and 10 mm wide YBCO tape was divided into three strips by a YAG laser. To have sufficient current capacity for 1 kA, a two-layer conductor was fabricated, and its critical current ( I c ) was 1976 A, but the magnetic properties of the textured metal substrates affected the increase in AC loss. In a low current region, the AC loss in this conductor was much higher than the Norris strip model, but approached the Norris strip model in the high current region because the magnetization was almost saturated. Low AC loss of 0.144 W/m at 1 kA rms was achieved even though the conductor had a small outer diameter of 20 mm and was composed of YBCO tapes with magnetic substrates.

Transport loss characteristics of polygonal assemblies of coated conductors with textured-metal substrate were studied. Coated conductors using two types of textured-metal tape as their substrate were prepared for the experiments: a Ni alloy substrate and a clad-type substrate with reduced magnetism. First, the transport losses of hexagonal assemblies of coated conductors with non-magnetic substrate were measured where the textured-metal tapes were under-lied or over-lied on the coated conductors to simulate the coated conductors fabricated with these textured-metal substrates. The over-lied Ni alloy tapes affect transport loss characteristics substantially; the over-lied or under-lied clad tapes do not substantially affect them. Next, two types of hexagonal assemblies were made using coated conductors fabricated with the textured-metal substrates: the substrates were placed inside in one of them, and they were placed outside in the other. In case of the coated conductors fabricated with the Ni alloy substrate, the measured transport loss of the latter was much larger than that of the former, because the substrates placed outside were exposed to the magnetic field generated around the assembled coated conductors. The measured transport losses reasonably agree with the numerically-calculated ones. In case of the coated conductors fabricated with the clad substrate, the difference between the transport losses of the two types of the assemblies is much smaller; the influence of the magnetism of the clad substrate on AC losses is reasonably small.

YBa 2 Cu 3 O 7− δ (YBCO) films were grown on LaAlO 3 substrates by a metal organic deposition using trifluoroacetates (TFA-MOD). In this study, effects of calcination condition such as temperature on microstructures and J c properties for the YBCO films were studied. The TFA starting solution was coated on a substrate by a spin-coating method. Then, a two-step heat treatment was applied to the coated films. In the calcination step, the coated film was calcined to form a homogeneous amorphous precursor by increasing the temperature which was varied from 300 °C to 500 °C at a constant heating rate of 2 °C/min. The prepared precursor films were crystallized at a constant condition. As a result, J c value depended on the calcination temperature, YBCO film calcined at 400 °C showed the highest J c of 2.5 MA/cm 2 . J c ’s for the YBCO films calcined at 300 °C and 350 °C were 1.9 MA/cm 2 and 2.0 MA/cm 2 , respectively. On the other hand, YBCO film calcined at 500 °C showed the lowest J c of 1.4 MA/cm 2 . Cross-sectional TEM images showed that the YBCO film calcined at 400 °C had less pores and small size of pores in the crystallized film randomly throughout the film. Large pores were observed in the YBCO film calcined at 500 °C. Also, these large pores were existed near substrate in the YBCO film. It is considered that the differences of these pore formation mechanisms are the main cause of the difference of J c characteristic.

As one of the programs in the Ministry of Economy, Trade and Industry (METI) project regarding R&D on superconducting coated conductor, three-phase superconducting fault current limiter (SFCL) for 6.6 kV application was developed and successfully tested. The developed SFCL was mainly comprised three-phase set of current limiting coils installed in a sub-cooled nitrogen cryostat with a GM cryocooler, circuit breakers and a sequence circuit. The whole system was installed in a cubicle. Two tapes of coated conductor were wound in parallel in each coil to obtain the rated current of 72 A rms. After developing the whole SFCL system, short circuit experiments were implemented with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted the prospected short circuit current over 1.6 kA to about 800 A by the applied voltage of 6.6 kV. The SFCL was installed in a user field and connected with a gas engine generator, followed by a consecutive operation. In this program, 600 A class FCL coil, with which four coated conductor tapes were wound, was also developed. The coil showed sufficiently low AC loss at the rated current. With these results, the program attained the planned target of the fundamentals for the 6.6 kV/600 A SFCL.

The thickness dependence of the critical current characteristics was investigated for YBa 2 Cu 3 O 7− δ (YBCO) coated conductors fabricated by Ion Beam Assist Deposition (IBAD)/Chemical Vapor Deposition (CVD) method in the range of 0.18–0.90 μm to find out the optimum thickness for various applications. The transport and magnetization critical current densities were estimated using the electric field criterion of E c = 1.0 × 10 −4 V/m and E c = 1.0 × 10 −9 V/m, respectively. The critical current density decreased with increase thickness in the low magnetic field region in the both electric field region because of the structural degradation of superconducting layer. This decreasing rate was lower than in Pulsed Laser Deposition (PLD)-processed YBCO coated conductors. It is found that the thickness dependence of irreversibility field B i differed between the low and normal electric field region. In the low electric field region, B i increased with increasing thickness. On the other hand, B i is almost independent of the thickness in the normal electric field region. The thickness dependence of B i was also superior to that of PLD-processed YBCO coated conductors.

YBa 2 Cu 3 O y (YBCO) coated conductors were fabricated by trifluoroacetates (TFA)–metal organic deposition (MOD) previously using a single reel-to-reel (RTR) crystallization furnace with a gas flow system parallel to the tape surface which could realize only a low production rate. In this study, we developed a new multi-turn (MT) RTR furnace with a vertical gas flow system to increase the production rate of the crystallization step and attained the uniform reaction in the tapes of the multi-turn system. We fabricated YBCO films with different conditions partial pressure of water vapor, the total pressure and the gas flow volume using the MT-RTR furnace with the vertical gas flow system, in order to investigate the influence of the processing parameters on the growth rate and the superconducting properties of the YBCO films. It was found that the growth rate of the YBCO phase film increased under the conditions of the high gas flow rate, the low total pressure and the high water vapor partial pressure. As a result, 21 times higher J c value was attained at 17 times faster growth rate than that without controlling the processing parameters in the crystallization step. This is thought to be due to the suppression of coarsening of Y 2 Cu 2 O 5 and CuO particles in the precursor film. Consequently, the high I c,end-to-end value of 250 A was achieved in a 5 m long tape fabricated at the production rate of 3 m/h using the only two lanes of the entire MT-RTR crystallization furnace with 10 lanes.

YBa 2 Cu 3 O y (YBCO) films grown by starting solutions with Ba-poor (Y:Ba:Cu = 1:1.5:3, Ba/Y = 1.5) have been reported to perform higher J c than that for YBCO films with stoichiometric one due to smaller and less pores in YBCO films by metal organic deposition (MOD) using tri-fluoroacetates (TFA). However, we have reported that a lot of non-reacted particles such as Y and Cu oxides were remained for the YBCO film surface grown by the precursors using Ba-poor solution. In this study, influences of YBCO growth rate on microstructures of the YBCO film were discussed to control microstructures of these second phase particles. As a result, microstructures of the YBCO film depended strongly on the YBCO growth rate, residues of Y and Cu oxides were dispersed randomly throughout the YBCO film grown at higher growth rate. Consequently, it was succeeded to control the segregated phases as seen in the previous YBCO film with Ba/Y = 1.5 at the film surface into the YBCO film. J c of the YBCO film grown at higher rate was a value almost equal to that grown at previous growth rate.

We have experimentally investigated thermal diffusion in a SrTiO3 based YBCO bicrystalline thin-film by means of one-dimensional low temperature scanning microscope (LTLSM). Grain boundary (GB) being a weak link is temperature sensitive, thus it can be used as a local thermal sensor. In experiments DC bias current was higher than the GB critical current, but much lower than the intra-grain one. At 10 Hz modulation frequency the detected GB trace was found to be about 300 micrometers. Upon the frequency increase, the trace diminishes and for 200 kHz was about 10 micrometers. Theoretical analysis shows that thermal length varies inversely as the square root of the frequency, and phase delay is in direct proportion. We have shown phase delay between modulating and response signals to be in a good agreement with theoretical frequency dependence. Effective thermal diffusivity coefficient calculated from the above dependence is close to that of the SrTiO3 and indicates that heat diffusion is much influenced by the substrate. Hence this result demonstrates the potential of the LTLSM for obtaining effective in-plane thermal diffusivity of multilayered superconducting tapes and films.

We designed and fabricated a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor was 6-pole type and has no iron core. The armature winding was wound with copper wires. 6 pieces of racetrack-shaped field coils were wound with 5 mm wide RE1Ba2Cu3O7-delta (REBCO, RE:Rare Earth) superconducting tapes produced by an ion beam assisted deposition method and a pulsed laser deposition method. The total length of REBCO tape was 400 m per a field coil. We confirmed the good transport properties of the respective field coils up to 80 A at 40 to 50 K before and also after the assembly to a motor. The maximum magnetic field in the field coils was 0.67 T. The superconducting rotor was cooled down to around 40 K by forced-flowed helium gas and the completed motor was tested in a similar way to conventional motors. As a result of a load test, the designed operation of 7.5 kW at 360 rpm was verified for the rated field current of 60 A. Even when the field current was increased up to 70 A at 40 K, the temperature at each part of the superconducting rotor was held constant and the quite stable operation up to 11 kW at 360 rpm was also achieved.

YBCO tapes are expected to be used in future high temperature superconducting (HTS) applications because of their good J c characteristics at high temperatures and in high applied magnetic fields. In applications to electric power devices such as transmission cables, transformers, and fault current limiters, the HTS conductors will be subjected to short-circuit fault currents that are 10 to 30 times the normal operating current. These overcurrents are greater than the critical current, and degrade or burn-out the HTS conductors. Therefore, it is important to clarify the mechanism of the degradation caused by such overcurrent pulses. We carried out preliminary experiments on damage caused by overcurrent pulse drive, focusing on the temperature limitation without suffering degradation for overcurrent pulse operation. A 10-mm-wide YBCO tape was cut into 2-mm-wide sample tapes by a laser beam, and the sample tapes were soldered on silver-deposited 100-mum-thick copper plates. Overcurrent tests were carried out on these sample tapes and Ic degradation was investigated. In addition the contact interface between YBCO and the Ag layer or buffer layer before and after the overcurrent drives has been investigated in order to clarify the correlation between the degradation and delamination of sample tapes.

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.

We have carried out observations of current flow in processed RE123 coated conductors using a scanning Hall-probe microscope. Using this system, the position of defects and the resulting disturbed current flow could be observed. The two-dimensional current distribution in a 10-filament formed coated conductor (400 mum width and 100 mum space) was also investigated. Current sharing and the current distribution in each filament were quantitatively estimated. Furthermore, current penetration in quasi-static AC current mode was also measured.