K.J. Song

Use of SrTiO/sub 3/ as a single buffer layer for RABiTS YBCO coated conductor

SrTiO/sub 3/ (STO) films were epitaxially deposited by PLD on textured Ni-3wt%W (Ni-3W) substrates. The STO film was deposited in two steps-reducing atmosphere first, and then oxidizing atmosphere-to prevent the oxidation of the metal surface from hindering the epitaxial relationship between the film and the substrate. The deposited STO films exhibited sharp biaxial texture and dense surface. YBCO films were epitaxially deposited on STO/Ni-3W and showed J/sub c/(77K, self field)=1.2MA/cm/sup 2/ and T/sub c/(onset)=88 K. Auger electron spectrometry depth profile analyses show that the STO film can block the diffusion of the metal from the substrate. This work has shown that STO films epitaxially deposited on textured Ni-3W substrate can be used as a single buffer layer of the high current carrying YBCO coated conductor.

Effects of deposition rate and thickness on the properties of YBCO films deposited by pulsed laser deposition

YBCO films with various thicknesses from 100 nm to 1.6 µm were deposited on single crystal SrTiO3 substrates by pulsed laser deposition (PLD). The effects of thickness and deposition rate—by means of controlling the pulsed laser frequency—on the critical current density (Jc) were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the orientation, crystallization and surface quality. The amount of a-axis YBCO component evaluated from the ratio of XRD chi-scan intensities of the a-axis and c-axis for the YBCO (102) plane increased as the YBCO film became thicker. SEM was used to analyse the surface of YBCO film, and it was shown that the surface of YBCO film became rougher with increasing thickness. There were many large singular outgrowths and networks of outgrowths on the surface of the YBCO films with thickness greater than 0.4 µm. The increased amount of a-axis YBCO component and the coarse microstructure of the thick YBCO film caused degradation of Jc with increasing thickness.

YBCO coated conductor using biaxially textured clad composite Ni–Mn/Ni–Cr substrate

A new biaxially textured composite tape of Ni?4.5%?Mn/Ni?1.5%?Cr was used as a substrate for a YBCO coated conductor through the RABiTS approach. Multi-layer CeO2/YSZ/Y2O3 buffer layers and YBCO film were deposited on the substrate by pulsed laser deposition. The deposition conditions of the buffer layers and the YBCO were studied and compared. Good biaxial textures have been obtained for buffer layers on composite Ni?4.5%?Mn/Ni?1.5%?Cr substrates. Scanning electron microscopy on sample cross-sections was used to examine the interface and diffusion of oxygen. The uniform formation of an Ni?Mn?O layer between NiO and the Ni?4.5%?Mn layer was observed, and the Ni?Mn?O layer restricted the further growth of NiO layer, which was thin and not continuous, within the coated conductor during YBCO deposition at higher temperature and higher oxygen pressure. The Jc of YBCO films on these metal substrates was 1.5 ? 106?A?cm?2 at 77?K, 0?T.

Single-filament composite MgB2/stainless-steel ribbons by powder-in-tube process

Abstract We report the successful fabrication of single-filament composite MgB 2 /stainless-steel ribbons, as an ultrarobust conductor type. The transport critical current of the non-sintered MgB 2 /stainless-steel ribbons is remarkably high, as an unexpected result. Transport critical currents I c ∼316 A at T =4.2 K and I c ∼82 A at T =20 K were observed at self-field for the non-sintered composite MgB 2 /stainless-steel ribbons. The I c value of 316 A is the higher than any other value reported so far. In addition, the persistent current density, that was estimated by Bean formula, of the sintered composite MgB 2 /stainless-steel ribbons was more than J p ∼6×10 5 A/cm 2 at T =5 K with H =0 G, and J p ∼1.1×10 5 A/cm 2 at T =30 K with H =0 G.

Development of Bi-2223 HTS tape and its application to coil and current leads

Abstract We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.

Influence of Filament Number on Workability and Critical Current Density of Bi-2212/Ag Superconducting Wires

The important merit of Bi-2212/Ag high Tc superconducting wire is to apply cable as round wire geometry. Bi-2212/Ag wires were fabricated in order to develop Rutherford cable for SMES near the future. Various Ag ratio and various numbers of filaments for PIT (powder-in-tube) process were applied to investigate the workability during drawing and to improve critical current property of the wires. In order to find proper working process, we investigated the variation of drawing stress during working. Post-annealing of Bi-2212/Ag wires at various atmospheres was carried out after main heat-treatment in order to increase critical current density. The results of post-annealing in nitrogen were better than that of post-annealing in oxygen. Jc of Bi-2212/Ag wire was increased as increased maximum temperature,Tm , for partial melting until 890degC, and highest Jc was over than 2000 A/mm2. The shape of grain was observed by SEM to investigate Bi-2212 phase formation in filaments. The critical current density increased in proportion to the increase of Bi-2212/Ag wires filaments number.

Fabrication of high-jc YBCO films by the TFA-MOD Process using YBCO powder as precursor

YBCO thin films were prepared on LaAlO/sub 3/ (100) single-crystal substrates using the metal-organic deposition of trifluoroacetates (TFA-MOD). Unlike the normal TFA-MOD process, TFA-based solutions were prepared dissolving YBCO powder into the trifluoroacetic acid solvent. The solutions were coated on the substrates by spin coating, fired at the temperature up to 400/spl deg/C in 4.2% humid and oxygen atmosphere, fired again at the temperature region of 725-850/spl deg/C for 2 h in 4.2% humidity and 1000 ppm oxygen partial pressure, and finally oxygenated at 450/spl deg/C for 1 h. Similar to the normal TFA-MOD process, both microstructure and superconducting properties of YBCO films were highly sensitive to the firing temperature. Optimally processed YBCO films of /spl sim/250 nm thickness exhibited T/sub C,zero/ of 90 K and J/sub C/ of 2.9 MA/cm/sup 2/ at 77 K in a self-field. The present results clearly evidence that our cost-effective modified TFA-MOD process is applicable to the fabrication of YBCO coated conductors.

Fabrication of meter-long coated conductor using RABiTS-PVD methods

The practical application of HTS materials in power devices requires the HTS conductor which is long, powerful (high current carrying), and affordable. Many different methods for making practical coated conductor include multiple ways of obtaining texture templates and a variety of film deposition methods which include both physical and chemical routes. In this work, three film deposition systems (pulsed laser deposition, sputtering, and evaporation) equipped with reel-to-reel metal tape moving apparatus were installed and used to make meter-long coated conductor. Buffer architecture of CeO/sub 2//YSZ/Y/sub 2/O/sub 3/ was continuously deposited on Ni alloy using sputtering, evaporation, and PLD. YBCO superconducting layer was continuously deposited on buffered metal tape by pulsed laser deposition. End-to-end critical current I/sub c/ of 80 A and critical current density J/sub c/ of 1.3 MA cm/sup -2/ for 600 nm-thick YBCO at 77 K, self-field have been achieved in 1 cm-wide tape over 1 meter length. Longitudinal distribution of the I/sub c/ values was between 72 A/cm and 90 A/cm.

Complementary study of heat capacity and magnetization for intermetallic YNi2B2C single crystal

The superconducting and magnetic properties of YNi2B2C single crystal have been investigated by heat capacity and dc-magnetic methods, with magnetic field applied parallel to the c-axis, i.e., H kð 001 Þ-direction. In the framework of heat capacity and magnetization analyses, we obtain the thermodynamic critical field Hc from both heat capacity and magnetization data. The heat capacity data deviate from predictions for both weak- and strong-coupling superconductivity, but are described relatively well in a medium-coupling analysis. The precise t 3 -dependence of the electronic heat capacity Ces indicates the gap anisotropy with the presence of point nodes for YNi2B2C single crystal.

Fabrications and properties of MgB/sub 2//stainless-steel tapes by PIT process

We have successfully fabricated single-filament composite MgB/sub 2//Stainless-Steel (SS) tapes using powder-in-tube processes such as swaging and cold rolling. The transport critical currents, that were measured by a standard dc four-probe method, I/sub c//spl sim/316 A and /spl sim/105 A were observed at T=4.2 K with H=0 G for as-rolled and sintered MgB/sub 2//SS tapes, respectively. In addition, the isothermal magnetizations M(H) of sintered MgB/sub 2//SS tapes and commercial MgB/sub 2/ powder were measured at temperatures T between 5 and 50 K in fields up to 6 T, employing a PPMS. The critical current density (J/sub c/) values obtained from the M(H) data, using Bean model, were >5 /spl times/ 10/sup 5/ A/cm/sup 2/ and >5 /spl times/ 10/sup 7/ A/cm/sup 2/ at T=10 K with H=0 G for the sintered MgB/sub 2//SS tapes and the commercial MgB/sub 2/ powder, respectively. On the other hand, we investigated the cross sections of both the sintered and the as-rolled tapes, using SEM and EPMA. No evidence of significant diffusion between MgB/sub 2/ and SS tube was found in the SEM/EPMA. However, traces of reaction can be seen in the interface region between MgB/sub 2/ core and SS tube for the sintered MgB/sub 2//SS tapes, from the EPMA line profiles. In addition, impurity phases such as MgO or MgB/sub 4/, which can be generated inside the superconductor MgB/sub 2/ core during fabrication, were observed by XRD analyses.