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We have shown that a persistent current mode operation was possible in various types of HTS coils wound by the joint-less winding technique with 2G REBCO tape. However, in a previous test, a solenoid type of the joint-less HTS coil showed very poor magnetic field quality which was caused by the inevitable gap located at the very center part of the solenoid coil. In this paper, to improve the homogeneity of the magnetic field of the solenoid joint-less HTS coil, we proposed a joint-less HTS coil which had two separable parts of the solenoid coil arranged concentrically, just like a conventional high field magnet. By this arrangement, we could improve the homogeneity of the center magnetic field remarkably. A prototype coil was made and operated at the temperature of 77.3 K by liquid nitrogen cooling and charged up with a persistent current switch that was also a part of the perfect superconducting closed loop in the joint-less HTS coil.

In order to achieve a persistent current mode operation with high-temperature superconducting (HTS) coils, we have been trying to improve the concept of a jointless HTS coil. The coil has a perfect closed loop without any joints in a coil, and we have proven that a persistent current can be induced in the coil by several charge up methods such as PCS, flux pump, or field cooling with a background magnet. In this paper, based on the previous experiment, which showed partially successful results [10], we designed and fabricated a small solenoid type jointless HTS magnet with a 100-m-long ReBCO wide conductor and tested it in a liquid nitrogen bath. After the persistent current mode operation of 15 h, we tried to measure temporal and spatial homogeneity by a field mapper we developed under the DSV of 10 mm .

The concept of joint-less winding with 2G high-temperature superconductors (HTS) wide conductors proposed by the authors made HTS magnets possible to realize a persistent-current-mode operation. It could successfully maintain reasonable amount of the persistent current in multiple stacked HTS pancake coils with perfectly closed superconducting loops. In this paper, we applied the same technique to HTS layer winding, so called solenoid winding, to make an improvement of the magnetic field quality. To avoid unfavorable gap near at the center plane of the joint-less winding, we made two identically designed joint-less coils and attached them symmetrically. By this particular design of the coil, we could locate the intrinsic gap of the coil far away from the important magnetic center point. We fabricated the joint-less coil according to the design. During the experiment of the coil to measure the central magnetic field, one of the two coils was found to be damaged after the successful cooling down below 20 K and the charging up the healthy one with a persistent magnetic field of 0.5 T, which was just a half of the expected value. The damaged coil will be replaced with a new one, and tested again in the near future.

A flux pump is a kind of power source that energizes superconducting magnets usually embedded in the cryogenic system. Therefore, heat loss through current leads is eliminated, allowing for more efficient cooling design. A winding technique, the so-called “wind-and-flip,” had been proposed by the authors to realize a jointless HTS coil for a perfect resistanceless closed loop, which may enable to induce a persistent current in an HTS coil wound with second-generation HTS tape. In this paper, we investigated charging/discharging characteristics and a possibility of persistent current operation of a jointless HTS coil with a flux pump. Results are discussed with regard to current capacity and temporal stability with a prototype jointless HTS coil.

We had proposed a winding method, the so-called “wind-and-flip,” which enables a persistent current-mode operation of a high-temperature superconducting (HTS) pancake coil. Two pancake coils had been fabricated using this method and tested in a liquid nitrogen bath with an external magnetic field. The result showed that the time stability was not good enough to use for magnetic resonance imaging or nuclear magnetic resonance when they were magnetized by field cooling at 77 K. In this paper, the test results of a stacked HTS pancake coil for a center magnetic field of 1 T are presented. The coils were cooled to 20 K by conduction cooling using a Gifford-McMahon cryocooler. We measured the time stability of the persistent current-mode operation at 20 K, which shows a strong possibility of a high-field HTS magnet using this method.

We fabricated superconducting magnet using second-generation (2G) high-temperature superconducting wire by SuNAM. Magnetic field strength at the center is 4 T, and room temperature bore diameter is 203 mm. The magnet consists of 30 double pancake coils (DPCs) with the inner diameter of 245 mm and outer diameter of 297 mm. All double pancakes were wound by no-insulation method and performance were tested separately before assemble. Tested DPCs were resistively connected by HTS tape(splice joint), and assembled coil was conduction cooled by a two-stage Gifford-McMahon cryo-cooler to the operating temperature of 8 K. The size of magnet is 452 mm in height. Current, voltage, and field strength were measured as a function of time with various ramping up and down conditions and results were compared with the simulated behavior. The coil generates 4 T when operating current ramped to 205 A by 0.03 A/s without quench. Initial cool down time was 72 h and the measure field homogeneity in 10 mm DSV was 0.015% and 0.012% in radial axis and vertical axis, respectively. The results showed that no-insulation winding method is a possible option for making compact magnet coil with sufficient structural integrity, thermal and electrical stability at the same time. The magnet showed quench at field strength of 4.49 T when ramped with 0.2 A/s to 235 A. The magnet showed same performance after recovery from quench.

The mechanical, electrical and thermal characteristics of insulating materials may significantly affect the performance and reliability of electrical devices using superconductors. General method to provide insulating layer between coated conductors is wrapping coated conductor with Kapton tape. But uniform and compact wrapping without failure or delamination in whole coverage for long length conductor is not a simple task and need careful control. Coating of insulating layer directly on coated conductor is desirable for providing compact insulating layer rather than wrapping insulating layers around conductor. Ceramic added polymer has been widely used as an insulating material for electric machine because of its good electrical insulating properties as well as excellent heat resistance and fairy good mechanical properties. The insulating layer of coated conductor should have high breakdown voltage and possesses suitable mechanical strength and maintain adhesiveness at the cryogenic temperature where it is used and withstand stress from thermal cycling. The insulating and mechanical properties of polymer can be improved by adding functional filler. In this study, insulating layer has been made by adding ceramic particles such as $SiO_2$ to a polymer resin. The size, amount and morphology of added ceramic powder was controlled and their effect on dielectric property of the final composite was measured and discussed for optimum composite fabrication.

A large bore magnet coil wound without turn to turn insulation was fabricated and cooled down to 10 K by conduction cooling. The coil has room temperature bore of 102 mm in diameter and consists of 22 double pancakes wound with 2G HTS conductor on aluminium alloy bobbin. Tests for 22 double pancake coils were performed at 77 K for individual single DPC, coupled 2 DPCs, 4 DPCs, 8 DPCs and 22 DPCs. Tested DPCs were connected by splice joint and assembled coil structure was installed to the vacuum cryostat with radiation shield and cooled down to 10 K for 28 h. The coil generate 4.1 T when operating current ramped to 200 A by 0.1 A/s without quench. Test results agreed well with the designed performance showing field homogeneity of 0.021% for radial axis and 0.056% for vertical axis in 10 mm DSV at 4.1073 T. The magnet showed quench at 210 A when ramping rate was 0.15 A/s at 10 K. The results showed that without turn to turn insulation winding method is a possible option for making compact magnet coil with sufficient structural integrity, thermal and electrical stability at the same time.

CeO2 has been used as a buffer layer of a coated conductor because of good chemical and structural compatibility with YBCO. But cracks were often observed at the surface for films thicker than 100 nm deposited at a high temperature because of a large difference in a thermal expansion coefficient between metal and CeO2. The deposition rate was limited to be slow for getting good epitaxy. In order to increase the film deposition rate, while maintaining the epitaxy till a final thickness, two-step deposition process was tested. The thin seed layer with a thickness less than 10 nm was deposited with a deposition rate of 3 A/s, and the homo-epitaxial layer at a thickness more than 240 nm was deposited at a deposition rate of 30 A/s. The resulting CeO2 films deposited at 600 °C showed a good texture with a Δφ of 5.3°, Δω of 4.2° and Ra of 2.2 nm. The two-step process may be option for a low cost buffer layer for Ni–3%W metal substrates for the coated conductor.

Coated conductor by MOCVD shows the best Ic × L performance currently, but cost reduction is still ongoing issue. R&D effort for a process capable of utilizing cheap alternative precursors were tried by many research groups but few of them showed results having potential for replacing current MOCVD. Spray pyrolysis method adopting ultrasonic atomization was tried as one of the possible options. Y123 films have been deposited on LaAlO3 (100) single-crystal and IBAD substrates by aerosol assisted spray pyrolysis method. Ultrasonic atomization was used in order to generate fine droplets of precursor solution made of Y, Ba, Cu nitrate. A pre-heater was located between spraying nozzle and substrate for fast drying and enhancing decomposition of precursors. SEM and XRD observation revealed that deposited films have smooth and dense microstructure. The influence of operating parameters such as cation stoichiometry, oxygen partial pressure, substrate temperature on the microstructure, formation of superconducting 123 phases and superconducting properties of deposited films were tested. Ex-situ conversion was tried to decrease the possible reactions between precursor compounds and buffer layer materials of metal substrate showed the possibility of adopting this technique for epitaxial growth of 123 phase on metal substrate.

[ ]has been widely used for single buffer layer of coated conductor because of superior chemical and structural compatibility with (Re

Silver stabilizing layer of coated conductor has been prepared by dip coating method using organic silver complexes containing 10 wt.% silver as a starting material. Coated silver complex layer was dried in situ with hot air and converted to crystalline silver by post heat treatment in flowing oxygen atmosphere. A dense continuous silver layer with good surface coverage and proper thickness of 230 nm is obtained by multiple dip coatings and heat treatments. The film heat treated at 500 °C showed good mechanical adhesion and crystallographic property. The interface resistivity between superconducting YBCO layer and silver layer prepared by dip coating was measured as 0.67 × 10 −13 Ω m 2 . Additional protecting copper layer with the thickness of 20 μm was successfully deposited by electroplating. The critical current measured with the specimen prepared by dip coating and sputtering on same quality YBCO layer showed similar value of ∼140 A and proved its ability to replace sputtering method for industrial production of coated conductor.

It is well known that high temperature superconductors (HTS) are promising for a magnet application, but it has been almost impossible so far to apply them to the magnet in persistent current mode because of poor qualities of their superconducting joint at higher current as well as lower index values. In this paper, in order to get rid of the joints from an HTS superconducting magnet, a prototype HTS pancake coil using YBCO coated conductors by the technique of wind-and-flip pancake coil was made and will be tested later. This paper presents a design, fabrication, and thermal analysis of the prototype pancake coil. The coil will be cooled down around 20 K by conduction cooling using a GM cryocooler. The expected current drift rate for an MRI application has not been achieved, probably caused by large flux flow resistance of coated conductor at higher temperature, 77.3 K. A temperature dependency of temporal stability of the persistent current will be measured by monitoring the long-term drift of the magnetic field at different operating temperatures by Hall sensors.

Off-stoichiometric (Y 1 RE x )Ba 2 Cu 3 O 7− δ ( x = 0–0.4, RE = Y or Sm) films have been prepared on LaAlO 3 (1 0 0) single-crystal substrates by a metal-organic deposition using dichloroacetic acid as chelating solution. Coating solutions with excess Sm or Y were prepared by dissolving Y, Sm, Ba and Cu acetates in dichloroacetic acid and distilled water followed by refluxing, drying to obtain blue gel. The final precursor solution for dip coating was prepared by diluting blue gel in 2-methoxyethanol. Coated films were calcined at low temperature up to 500 °C in flowing humid oxygen atmosphere. Conversion heat treatment was performed at 800 °C for 2 h in flowing Ar gas containing 1000 ppm oxygen with a humidity of 9.45%. The viscosities of the precursor solution were increased as the content of added Sm or excess Y increased. The transport critical current density ( J c ) of (Y 1 RE x )Ba 2 Cu 3 O 7− δ film was enhanced with the addition of excess Sm and Y up to 20% and then decreased with further addition. Scanning electron microscope (SEM) observation showed that surface roughness was increased with the addition of excess Y.

It is well known that HTS (high temperature superconductor) is promising for magnet applications, but it has been almost impossible so far to apply them to magnets in persistent current mode because of poor qualities of their superconducting joints at higher current as well as their lower index values. To get rid of the non-superconducting joints from an HTS superconducting magnet, a wind-and-flip technique for demonstrating a persistent current mode operation of HTS magnet using YBCO coated conductors had been suggested. This paper presents experimental results of measured drift of a persistent current in the YBCO pancake coil which was made by using the wind-and-flip technique. The pancake coil was wound by YBCO coated conductor with a width of 12 mm, which was slit longitudinally at the center of the conductor along the conductor by a specially designed slitter. A stainless steel heater was attached on the one end of the YBCO conductor as a persistent current switch to energize the pancake coil. A temporal stability of the magnetic field was analyzed by monitoring the long-term drift of the magnetic field at 77.3 K by Hall sensors.

We fabricated BSCCO-2212 (2212) rods using a casting process and evaluated the effects of annealing time and size to improve critical current ( I c ). Diameters of rods were 1.0 cm and 1.6 cm and annealing was performed from 60 h to 200 h at 800 °C. The I c of rods increased with increasing annealing time, probably due to increased grain size of 2212. Annealing time of the highest I c for the smaller rod was shorter (150 h) than that of the larger rod (200 h). This size effect seems to be related to different grain sizes of the intermediate phases such as 2201 and secondary phases in the ingot. In addition, we successively fabricated 2212 tubes from the rod by making a hole in the center region which contained inhomogeneous microstructures. This method was simple and I c did not significantly decrease. However, critical current density ( J c ) improved when as much as 60% of the diameter in the center region was machined out of the rod.

High J c over 1 MA/cm 2 YBCO film has been successfully prepared using nitrate precursors by a spray pyrolysis method. Precursor mists generated using a concentric spray nozzle were directly sprayed on a LaAlO 3 (1 0 0) single crystal substrate. The cation ratio of precursor solution was Y:Ba:Cu = 1:2.65:1.35. The distance between nozzle and substrate was 15 cm. Deposition temperature was ranged from 760 to 800 °C and deposition pressure was 100 Torr. Oxygen partial pressure was varied from 10 Torr to 50 Torr. The microstructure, phase formation, texture development and superconducting properties of deposited films were largely changed with oxygen partial pressure and deposition area. Deposited films showed a texture with (0 0 1) planes parallel to substrate plane. High quality film was obtained when the film was deposited at 760 °C with an oxygen partial pressure of 30 Torr. The critical current density ( J c ) of the YBCO film was 1.75 MA/cm 2 at 77 K and self-field.

We present results of Raman scattering studies of superconducting YBa 2 Cu 3 O 7− x (YBCO) films grown by chemical vapor deposition and metal-organic deposition methods. It is shown by X-ray diffraction that all the as-grown YBCO films have a highly c -axis oriented and in-plane aligned texture. Raman scattering measurements were used to investigate optical phonon modes, oxygen contents, structural properties, and second-phases of the YBCO coated conductors. Raman spectra of YBCO films with lower-transport qualities exhibit additional phonon modes at ∼300 cm −1 , ∼600 cm −1 , and ∼630 cm −1 , which are related to second-phases such as Ba 2 Cu 3 O 5.9 and BaCuO 2 . Our results strongly suggest that Raman scattering be useful for optimizing YBCO film growth conditions.

YBCO films were fabricated by the TFA-MOD method using the “211-process”, and the effects of the film thickness on phase formation, microstructure, texture evolution, and critical properties were evaluated. Various film thicknesses ranging from 0.41 μm to 2.14 μm were obtained by repeating the dip coating and calcining processes one to five times. The critical properties varied significantly with the film thickness. The I c increased from 35 to 105 A/cm-width with increasing the film thickness from 0.41 μm to 1.17 μm. On the other hand, the corresponding J c remained almost constant in the range of 0.76–0.90 MA/cm 2 . With further increases in thickness, these values decreased drastically, which was attributed to the degraded microstructure, i.e., the formation of BaF 2 and a -axis grains and degraded texture and surface morphology arising from the insufficient heat treatment time. It is believed that the optimum thickness for improving both the I c and J c values is approximately 1.17 μm.

Yb 2 O 3 films were successfully deposited on (1 0 0) SrTiO 3 (STO) single crystal substrates and on a cube-textured Ni–5at%W by MOCVD (metal organic chemical vapor deposition) method. H 2 O vapor was used as an oxidant in order to avoid the oxidation of Ni substrate. The working pressure, Ar flow rate and H 2 O vapor flow rate were 10 Torr, 600 sccm and 700 sccm, respectively. Epitaxial Yb 2 O 3 (4 0 0) films were deposited on STO substrates at high temperatures above 950 °C, while small Yb 2 O 3 (2 2 2) peak from XRD was detected below 920 °C. The AFM surface roughness of Yb 2 O 3 film grown on STO was in the range of 10 nm for the film deposited at 950 °C with a H 2 O vapor partial pressure of 5.5 Torr and deposition time of 5 min. For cube-textured Ni–5at%W substrate, both Yb 2 O 3 (2 2 2) and Yb 2 O 3 (4 0 0) textures were grown at deposition temperatures above 850 °C. The Δ ϕ values of Yb 2 O 3 /STO were in the range of 2–3°, while Yb 2 O 3 /Ni–5at%W were in the range of 8–10.5.

Many superconductor applications such as magnetic levitation train (MAGLEV), magnetic resonance imaging (MRI) and superconducting magnetic energy storage (SMES) must be operated on persistent current mode to eliminate loss. Research on PCS using coated conductor (CC) tape has not been actively in progress, compare with that using BSCCO tape. In this paper, current charging rate was adjusted to 0.5 A/s to investigate operating characteristics of PCS. To analyse current decay during persistent current mode, new electrical equivalent model considering the n-value of CC was applied. Experimental results were compared with the theoretical results which were derived by applying finite deference method (FDM). We concluded that the characteristics of initial current decay were determined by the voltage generated in the load due to n-value, joint resistance and current charging.

Bi-2212/SrSO4 bulk superconductors were fabricated using a casting process, and the effects of the powder mixing method, annealing temperature, and SrSO4 content on the texture, microstructure, and critical properties were evaluated. The Bi-2212 powders were mixed with SrSO4 either by hand-mixing (HM) or planetary ball milling (PBM), followed by melting at 1100degC-1200degC, solidification, and annealing. The rod produced by PBM had a more homogeneous microstructure and smaller grain size of SrSO4 and minor phases, resulting in increased Ic, than the rod made by HM. The Ic of the rod was also dependent on the annealing temperature with the highest Ic of 200 A being obtained at an annealing temperature of 810degC. This was attributed to the moderate density and 2212 texture as well as the smaller and less minor phase than that observed at higher temperature. In addition, the optimum SrSO4 content was found to be 6 wt.% for an increased Ic. SEM, EPMA, and DTA analyses indicated that the differences in microstructural evolution caused the variations of the Ic with the powder mixing method, annealing temperature, and SrSO4 addition.

We present results of Raman scattering studies of superconducting YBa2Cu3O7-x (YBCO) films grown by a high-rate electron-beam co-evaporation method. It is shown by X-ray diffraction that the as-grown YBCO films have a highly c-axis oriented and in-plane aligned texture. Raman scattering measurements were used to study optical phonon modes, oxygen content and second phases of the YBCO coated conductors. Raman spectra of YBCO films with lower-transport properties exhibit additional phonon modes at ~ 300 cm-1, ~ 600 cm-1, and ~ 630 cm-1, which are related to second-phases such as Ba2Cu3O5.9 and BaCuO2. We also performed micro-Raman scattering measurements, which enable the local characterization of the material over a few micrometer scales. We could clearly determine the chemical homogeneities of the films on which we could not have information using XRD. Our results strongly suggest that micro-Raman scattering can be a very effective tool for characterizing structural and chemical properties, which is essential for optimizing growth conditions.

High quality REBa 2 Cu 3 O x (RE = Y, Eu, Gd) films have been successfully fabricated by metalorganic deposition method using dichloroacetic acid. Metal acetates or oxide powders were dissolved in dichloroacetic acid and blue-colored get was obtained by drying. Coating solution was prepared by diluting blue gel in methanol. Coating solution was coated on single crystal (001) LaAlO 3 (LAO) by dip coating method. Processing parameters such as oxygen partial pressure, water vapor, ramping rate and pyrolysis temperature etc were controlled in order to obtain a good epitaxial film. 0.5 μm-thick film was obtained by single coating and with no crack after calcination heat treatment. Oxygen partial pressure was varied in the range of 100-1000 ppm and conversion heat treatment was carried out at the temperature range of 725-765 °C. A critical transition temperature (T c0 ) of 90 K and a critical transport current density (J c ) of >0.5 MA/ cm 2 (77 K and self-field) were demonstrated for the GdBCO film grown on LAO single crystal substrate with a thickness of 0.5 μm. I c was determined by utilizing a transport measurement. SEM observations showed that the growth behavior of Y123 grains is different from that of TFA-MOD process.

YBa 2 Cu 3 O y superconducting films were deposited on LaAlO 3 (1 0 0) single crystal substrates by spray pyrolysis method. Two types of ultrasonic and concentric nebulizer were used in order to generate fine droplets of metal-inorganic precursor solution. c -Axis oriented films were obtained at deposition temperature of 750–850 °C and at working pressures of 100 Torr and 500 Torr. In case of ultrasonic nebulizer, the films showed rough surface morphology due to the presence of enormous droplets, whereas smooth and dense films were obtained for concentric nebulizer. The good c -axis oriented YBCO films were formed at the wide range of the oxygen partial pressure. Oxygen which is generated via the decomposition of nitrate precursors is considered to participate in the formation reaction of YBCO film. Microstructures of YBCO films varied depending on oxygen partial pressure and rod-like grains were appeared when the oxygen partial pressure was lower than 30 Torr. YBCO films were deposited epitaxially on LAO(1 0 0) substrate. Δ ϕ of in-plane and Δ ω of out-of-plane texture were measured as 3.3° and 1.0°, respectively. A transport J c value of 0.50 MA/cm 2 at 77 K and self-field was achieved for the YBCO film deposited on LaAlO 3 (1 0 0) single crystal substrate.

We fabricated 2212-SrSO 4 composites by the melt casting process and evaluated the effect of melt flowing on the critical current ( I c ) by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. It was observed that the 2212-SrSO 4 rod processed by the tilt casting method had a higher I c value than that processed by the vertical casting method. This improved I c is considered to be related to the more homogenous microstructure due to the laminar flow of the melt. In addition, we evaluated the variation of the I c with the SrSO 4 content. The I c value was found to be significantly dependent on the SrSO 4 content: the I c of the 2212 increased as the SrSO 4 content increased and reached a peak value (260 A at 77 K) at an SrSO 4 content of 6 wt.%, and then decreased when the SrSO 4 content was further increased. This increase is probably due to the lower porosity, larger grain size and stronger texture, resulting from the enhanced diffusion kinetics. The possible causes of the variation in the I c with the SrSO 4 content based on the microstructural observations were discussed in detail.

Surface oxidation behavior of cube-textured polycrystalline nickel was investigated at various oxidation conditions. Cube-textured nickel substrate, which was prepared by rolling and texture anneal, was used. Cube-textured NiO film was formed on a cube-textured polycrystalline nickel regardless of oxidation conditions but different growth behavior of NiO crystals was observed depending on the oxidation conditions. The introduction of water vapor in O2 did not affect on the texture evolution but rough and porous microstructure was developed. Microstructure of NiO film tends to be denser as the oxygen partial pressure is increased. It is interesting that (1 1 1) oriented crystal grew favorably in the substrate oxidized in air atmosphere while (2 0 0) plane was major in the substrate oxidized in high purity argon gas. Other crystallographic planes than (2 0 0) plane were developed when N2O was used as an oxidant while only (2 0 0) plane crystal was formed in dry O2 atmosphere. The morphology and texture of NiO crystal were changed from smooth one into faceted one when oxidation was performed at 900 °C in dry O2 gas followed by the reduction under a reducing atmosphere. It is thought that absorption behavior of oxygen or OH varies depending on the oxidizing species.

We fabricated YBCO films on LAO substrates using the TFA-MOD method and evaluated the effects of the heat treatment temperature on the microstructure, degree of texture, and critical properties. The phase formation and microstructure were characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM) and the degree of texture was evaluated by pole-figure analysis. The firing was performed in the temperature range of 750–800 °C and we found that the phase purity, grain size and orientation, degree of texture, and oxygen content varied with the firing temperature. The films fired at 775 °C showed the highest critical temperature (TC-onset) of 89.5 K and critical current (IC) of 40 A/cm-width, which corresponds to a critical current density (JC) of 1.8 MA/cm2. According to the results of the XRD, pole-figure, SEM and Raman analyses, these optimum critical properties can probably be attributed to the formation of a pure YBCO phase, stronger c-axis orientation and higher oxygen content.

Superconductive YBCO thin films have been prepared by trifluoroacetate (TFA) metalorganic deposition (MOD) method. Non-stoichiometric precursor solutions with cation ratios of Y:Ba:Cu = 1 + x:2:3 (x = 0, 0.05, 0.1, 0.15 and 0.2) have been prepared by dissolving high purity Y 2 O 3 , BaCO 3 and CuO powders into trifluoroacetate. Coated film was calcined at lower temperature in a moisture-containing oxygen atmosphere. Superconducting YBCO films have been obtained by conducting a conversion heat treatment at higher temperature in a moisture-containing Ar (1000 ppm oxygen) atmosphere. Critical current density (J c ) of YBCO film was increased with the amount of excess yttrium in YBCO film and revealed maximum J c for x = 0.15 and decreased again for x = 0.2. Four times higher J c was obtained for the film with x = 0.15 compared to that of the film prepared from stoichiometric solution of Y:Ba:Cu = 1:2:3. It is considered that the densification of microstructure and absence of a-axis grains attributed to the enhancement of J c in the film made of non-stoichiometric solution.

We fabricated YBCO coated conductors using a new TFA-MOD method and evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperature and oxygen heat treatment time, in order to explore its possible application in CCs fabrication. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors, instead of Y-, Ba- and Cu-based acetate or organics, and dissolved in trifluoroacetic acid followed by firing and oxygen heat treatment. We observed that for the films fired at 725 °C, BaF2 and other second phases formed and Tc was measured to be 66.5 K. In contrast, when the film was fired at 775 °C, strong and sharp (0 0 l) diffraction peaks of YBCO were observed, without the formation of the BaF2 phase. In addition, X-ray pole-figure analysis indicated that a strong biaxial texture was developed and the full-width at half-maximums of in-plane and out-of-plane textures were 4.38° and 4.26°, respectively. The critical temperature and current were also increased to 85.3 K and 35.8 A/cm-width, respectively, at 77 K when the oxygen heat treatment time was increased to 20 h. Microstructure observation indicated that a crack-free surface was obtained and that there was no a-axis grain growth.

We fabricated cube textured Ni substrates for YBCO coated conductors from the initial specimens prepared by powder metallurgy (P/M) and casting and evaluated the effects of annealing temperature on microstructural evolution and texture formation. To characterize the roles of the two preparation methods, the initial specimens were prepared to be of the same size and experienced the same rolling conditions, and were then annealed in the temperature range of 600/spl deg/C to 1200/spl deg/C. The texture was evaluated by X-ray pole-figure analysis and transmission electron microscopy (TEM). The texture analysis indicated that a strong cube texture formed for the substrate made by P/M and this texture did not vary significantly with annealing temperature in the range of 600-1200/spl deg/C. On the other hand, the texture of the substrate made by casting was more dependent on the annealing temperature, and a twin texture ({221} ) and several minor texture components began to form at 1000/spl deg/C. Microstructural observation showed that the morphology of deformation band together with micro-band in the as-rolled tape prepared by P/M was more homogeneous and of higher density than that produced by casting, resulting in a strong cube texture in wider range of annealing temperatures and a finer microstructure after recrystallization.

AC loss is one of the important parameters in HTS (High Temperature Superconducting) AC devices. Among the HTS AC power devices, the transformer is an essential part in electrical power system. But, AC loss is one of the most serious problems of the HTS transformer, especially with pancake windings, because high alternating magnetic field is applied perpendicularly to the surface of BSCCO wire in HTS windings of that, comparing with the other HTS AC power devices. For the reason above the calculation of AC loss generated in the HTS windings should be carried out in advance when designing the HTS transformer. In the paper we performed study for optimization of winding design to minimize the magnetization loss of HTS winding such as the spaces between pancake windings and operating temperature of HTS wire. The calculation of the AC loss was accomplished by 2-dimensional Finite Element Method.

A 1 MVA single phase high temperature superconducting (HTS) transformer was manufactured and tested. The rated voltage of primary and secondary of HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 HTS tape was used for HTS windings of 1 MVA HTS transformer. In order to reduce AC loss generated in the HTS winding, the concentrically arranged winding was adopted to 1 MVA HTS transformer. Single HTS tape for primary windings and 4 parallel HTS tapes for secondary windings were used considering the each rated current of the HTS transformer. A core of HTS transformer was fabricated as a shell type core made of laminated silicon steel plate. And a GFRP cryostat with a room temperature bore was also manufactured. The characteristic tests such as no load test, short circuit test and several insulation tests were performed at 65 K using sub-cooled liquid nitrogen. Through these test results, the validity of design of HTS transformer was ascertained.

AC loss is one of the most important parameters in high-temperature superconducting (HTS) ac devices. Among the HTS ac power devices, the transformer is the essential part in the electrical power system. But unfortunately, the transformer is the worst device concerning ac loss because of very large magnetization loss due to high magnetic field applied to the HTS wire. We calculated the magnetization losses in HTS pancake windings for transformer according to the operating temperature. Two kinds of arrangements of HTS pancake windings were adopted for calculation of ac loss of shell-type transformer, and the analysis results were presented and discussed.

We fabricated pure Ni and Ni–W alloy substrates for YBCO-coated conductor applications and evaluated the effect of W in Ni on texture, grain size, grain boundary and surface morphology, and hardness of the substrates. Pure Ni, Ni–2at.%W, and Ni–5at.%W alloy substrates were prepared by plasma arc melting, cold rolling, and recrystallization heat treatment at various temperatures (700–1300 °C). Substrate texture was evaluated by pole-figure and microstructure and surface morphology were investigated by optical and atomic force microscopy (AFM). We observed that the Ni–W alloy substrates had stronger cube texture that was maintained at higher annealing temperatures than seen for the pure Ni substrate: Full-width at half-maximum of in-plane texture was 13.40° for the Ni substrate and 4.42–5.57° for the Ni–W substrate annealed at 1000 °C. In addition, the Ni–W substrate had smaller grain size, less thermal grooving, and higher hardness, compared to those of the pure Ni substrate, indicating that the presence of W in Ni effectively restricts grain growth and enhances thermal stability by strengthening the grain boundary in the Ni substrate.

A 1 MVA single phase high temperature superconducting (HTS) transformer was fabricated and a part of experimental test for characteristics of the transformer was performed. The rated voltages of primary and secondary of the HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 wire was used for HTS windings whose shapes are double pancake arranged reciprocally. Single HTS wire for primary windings and 4 HTS wires for secondary windings were used considering the rated current of the transformer. Four HTS wires in secondary windings were transposed at the connection between double pancakes 3 times. The electrical insulation and transport current test were performed at 77 K using liquid nitrogen. The rest of the test for the characteristics of the transformer is in progress.

In high temperature superconducting (HTS) transformer with double pancake windings, the perpendicular component of leakage magnetic flux density (B/sub r/) applied to HTS tapes of double pancake windings is larger than that of solenoid windings, which acts as a cause to decrease the critical current in HTS tape and increase AC loss. In this paper, in order to reduce the B/sub r/ applied to HTS windings, the use of ferrite flux diverters is presented. Flux diverters are located in between the primary windings and the secondary windings, and must be made of low-loss materials to minimize the heat. And electromagnetic analysis of a single-phase 1 MVA 22.9 kV/6.6 kV HTS transformer with flux diverters in HTS windings was accomplished. The variation of B/sub r/ applied to the secondary windings and magnetic flux density in flux diverters were analyzed by changing their relative permeability of flux diverters respectively. Suitable relative permeability of flux diverters to reduce B/sub r/ applied to HTS windings was selected, considering flux diverter losses by increment of B in flux diverters.

NiO buffer layers for YBCO coated conductors were deposited on textured Ni substrates by a metal-organic chemical vapor deposition(MOCVD) method. Processing variables were the oxygen partial pressure and substrate temperature. The degree of texture and the surface roughness of the deposited NiO surface were analyzed by X-ray pole figure, atomic force microscopy (AFM), and scanning electron microscope (SEM). The (200) textured NiO layer was formed at 450 /spl sim/ 470 /spl deg/C and oxygen partial pressure of 1.67 Torr. Out-of-plane(/spl omega/-scan) and in-plane(/spl Phi/-scan) texture were 10.34/spl deg/ and 10.00/spl deg/, respectively. The surface roughness estimated by atomic force microscopy was in the range of 3.1 /spl sim/ 4.6 nm which was much smoother than that prepared by an oxidation method. We discuss the development of the (200) texture in the MOCVD-NiO films in terms of processing variables.

It is common practice to wind coils in pancake shape for conventional high voltage transformers because of electrical insulation problems. In HTS superconducting transformers, the perpendicular component of magnetic flux density (B/sub r/) applied to HTS tapes of pancake windings becomes larger than that of solenoid winding, thereby decreasing the critical current in the HTS tapes. This paper introduces several methods to reduce B/sub r/ applied to the HTS tapes in the transformer with double pancake windings by changing winding arrangements and the relative permeability of flux diverters. We have conducted a winding design for a single-phase 1 MVA 22.9 kV/6.6 kV HTS transformer. The magnetic field in the spaces occupied by the windings and its vicinity was numerically simulated. We observed a change of B/sub r/ due to various gap arrangements of high voltage and low voltage windings and various reciprocal arrangements. We also observed a change of B/sub r/ on the HTS tapes due to variation of relative permeability of flux diverters placed between the high voltage winding and the low voltage winding.

We fabricated textured Ag substrates for YBCO coated conductor and evaluated the effects of annealing temperature on microstructural evolution, texture formation, and surface morphology. An Ag ingot, prepared by melting in plasma arc and quenching, had a fine equiaxed microstructure. A strong {110} textured Ag substrate was obtained by cold rolling and annealing at 800 /spl deg/C; the full-width at half-maximum (FWHM) value of {110} poles was as sharp as 10/spl deg/. Atomic force microscopy (AFM) profiles show that the surface morphology was very smooth and root-mean-square (RMS) roughness of the substrate annealed at 800 /spl deg/C was 39.2 nm. However, it was found that the thermal grooving and faceting became remarkable as annealing temperature increased. The substrate of strong texture and smooth surface, fabricated in our study, is considered to be suitable for use as a substrate for the epitaxial deposition of superconductor films.

Powder metallurgy has been widely used to make high purity metals and alloys. We applied the powder metallurgy route to the fabrication of the bi-axially textured Ni and Ni-W alloy tapes for YBCO coated conductors. The Ni and Ni-W powder compacts were cold-rolled into tapes. The Ni powder used in this study was 5 /spl mu/m in size and 99.99% in purity. To densify them, the Ni and Ni-W rods were sintered at 1100 /spl deg/C for 6 h in 96% Ar-4% H/sub 2/ atmosphere, and cold-rolled into thin tapes of 80-100 /spl mu/m thickness. Heat-treatment condition for the development of cube texture were established for the tapes. The out-of plane and in-plane texture of the Ni and Ni-W tapes were about 8/spl deg/. In this paper, we discuss the effect of the processing variables on the texture development of the Ni and Ni-W tapes.

CeO/sub 2/ buffer layers for YBa/sub 2/Cu/sub 3/O/sub 7/(YBCO) coated conductors were deposited on biaxially textured Ni substrates by metalorganic chemical vapor deposition (MOCVD) method. The variables were the oxygen partial pressure (P/sub O2/), deposition temperature and time. The [200] texture of CeO/sub 2/ was formed at T = 500 /spl deg/C - 520 /spl deg/C, t = 3 - 15 min and P/sub O2/ = 2.30 torr, while the [111] and [200] texture were competitively formed at other condition. The surface roughness of CeO/sub 2/ films was as good as 5 - 15 nm up to 500 /spl deg/C, while it rapidly increased as a result of grain growth of the CeO/sub 2/ at T /spl ges/ 520 /spl deg/C. The surface roughness of the CeO/sub 2/ films also increased as the deposition time increased. The growth rate of the CeO/sub 2/ films at T = 520 /spl deg/C and P/sub O2/ = 2.30 torr was 200 nm/min, which is much higher than those prepared by other physical deposition methods.

Bi-axially textured Ni tapes for YBCO coated conductors were fabricated by forming, sintering, cold rolling and heat treatment of Ni powder compacts. The powder metallurgy process consists of filling of fine Ni powders in a rubber mold, cold isostatical pressing in a water chamber and sintering of the powder compacts. The sintered compacts were cold-rolled and made into tapes with a thickness of 100 micron and then heat-treated at 1000 °C for various time periods for the development of the (2 0 0) texture. The (2 0 0) texture of Ni tape was successfully formed through the optimization of the recrystallization heat treatment condition for the cold rolled Ni tapes. The full width half maximum of the Ni tapes was 8–10° and the atomic force microscopy surface roughness was 3–5 nm.

We studied the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors. The use of the multi-seeded melt growth process can reduce the processing time remarkably compared with the single-seeded melt growth process, but the levitation forces and the trapped magnetic field value decreased as the number of seeds increased due to the formation of weakly linked grain boundaries at the grain junctions. The degradation of the trapped magnetic field was attributed to the presence of the residual liquid at the grain boundary. To minimize the degradation of the magnetic properties, various processing parameters such as seed distance and grain orientation were tested.

Various YBCO grain boundary junctions were prepared by a top-seeded melt growth process using two seeds and the grain boundary properties were investigated. The results of the levitation forces and trapped magnetic fields for the top surfaces of the samples showed that the (1 1 0)/(1 1 0) grain junction was more strongly connected than other grain boundaries. We discussed the connectivity of the grain boundaries in terms of the wetting angle of a melt at the boundaries.

Effects of processing variables on the top surface properties (the levitation force and trapped magnetic field) of the multi-seeded melt growth process were studied. The variables were the number of seeds, seed distance, initial composition of a YBCO compact, variety of the grain junctions. The processing time was reduced by providing many growth sites for 123 grains. However, the properties were decreased as the number of seeds increased due to the poor connection of the grain junctions. We summarize the ways to minimize the degradation of the magnetic and levitation properties at the grain junctions.

Two-grain YBCO samples with various grain junctions were prepared by a multi-seeded melt growth process and the levitation force and the trapped magnetic field of the samples were examined. The trapped magnetic field was dropped at all grain junctions and the degree of the deterioration depended on their crystallographic planes of the grains. The levitation forces of the samples with top surfaces normal to the c -axis (e.g., (1 0 0)/(1 0 0) and (1 1 0) (1 1 0) junctions) were higher than that of the samples with top surface partly and completely parallel to the c -axis (e.g., (1 0 0)/(0 0 1) and (0 0 1)/(0 0 1) junctions).

We evaluated the degree of texture and the interface irregularity between the Ag sheath and the superconductor core of Bi-Sr-Ca-Cu-O (BSCCO) superconductor tape during drawing and rolling processes. The degree of texture and the interface irregularity were characterized by pole figure analysis and the coefficient of variation in thickness (COV) factor, respectively. It was observed that the interface became gradually irregular during the drawing and rolling processes. On the other hand, the degree of texture improved significantly during rolling process, but little during the drawing process. The critical current of the tape depended remarkably on the combined effects of the interface irregularity and the degree of texture. As the dimension of the wire/tape was changed from a diameter of 3.25 mm to a thickness of 0.20 mm, the critical current increased by 10 times. Microstructural investigation showed that grain alignment was locally degraded by the existence of both second phases and interface irregularity. It was observed that larger grain size and better texturing developed near the relatively straight interface compared to those inside the superconducting core.

CeO/sub 2/ and NiO films have been grown by metal-organic chemical vapor deposition (MOCVD) on cube-textured Ni substrate. At a lower deposition temperature of 400/spl deg/C, an amorphous film was formed. Deposited CeO/sub 2/ film showed a mixed texture of [100] and [100] orientations. Depending on the deposition condition, the transition from [100] texture to [100] orientation was observed for the CeO/sub 2/ film. The NiO film was deposited at 470/spl deg/C for 10 min. Deposition pressure was 10 Torr and the oxygen partial pressure was 0.91 Torr. The X-ray rocking curve and /spl phi/-scan showed that the NiO film has a bi-axial texture with a [100] orientation. The out-of-plane and the in-plane deviation were measured as 4.2/spl deg/ and 6/spl sim/7/spl deg/ from the FWHM of [200] and [111] planes, respectively.