SPECIAL ISSUE PAPERS
The papers in this special issue of the Journal of Electronic Materials are a sampling of the exciting work that was presented at the symposium on Synthesis, Processing, and Large-Scale Applications of High-Temperature Superconductors during the 124th Minerals, Metals, and Materials Society (TMS) Annual Meeting in Las Vegas, NV, February 12-16, 1995. The symposium, sponsored jointly by the Electronic, Magnetic and Photonic Materials Division and the Structural Materials Division of TMS and the Electronic, Magnetic and Optical Phenomena Committee of American Society for Materials (ASM), was fortunate to comprise more than 75 presentations in eight technical sessions, with representation from North America, Europe, Asia, and Australia. All of the manuscripts published in this issue were reviewed by experts in the field of high-temperature superconductivity. The principal topics of the symposium included processing for high critical current densities and development of flux-pinning centers, flux-pinning mechanisms, processing for the film deposition, chemistry, phase relations, pathways to new superconductors, stoichiometry, effects of strain, processing/microstructure relationships, and prototype products for practical applications.
We hope that these papers, as well as the references cited therein, will provide the reader with a glimpse of the state of the art of the very exciting and growing field of high-temperature superconductivity. As guest editors, we express our appreciation to the speakers, attendees, and session chairs for making this endeavor possible. We thank the manuscript reviewers for their very timely reviews; Charles Malefyt of Argonne National Laboratory, for editing several manuscripts, and Sylvia Hagamann, also of Argonne, for preparing many of the manuscripts in proper format. Finally, we are grateful to Theodore C. Harman, Editor of the Journal of Electronic Materials, for his assistance in publishing this special issue.
Argonne National Laboratory
Argonne, IL 60439
University of Notre Dame
Notre Dame, IN 46556
Naval Research Laboratory
Washington, DC 20375
Application of Superconducting Materials to Power Equipment
GE Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301.
This paper deals with the requirements for successful application of superconducting materials to power equipment. Applications in the form of wires and leads are considered, with emphasis on high current and magnetic field situations. Both low-temperature and high-temperature superconductors are considered as a continuous family. While most important general requirements are touched upon, cost is discussed as an often under-emphasized criterion.
Power equipment, superconductors, wires and tapes
On The Manufacture of Silver-Sheathed Bi-2212 High Temperature Superconducting Wires
JAN KAJUCH, DAH-WEI YUAN, and MICHAEL J. POLLOCK
Concurrent Technologies Corporation, Johnstown, PA 15904
High temperature superconductors (HTS) are presently being considered for a variety of defense and commercial applications. However, problems associated with low critical current density (Jc) in long length conductors still need to be addressed. It is realized that success in the fabrication of long lengths of wire and tape (up to 1 km) with optimum superconducting properties relies on improved manufacturing technologies. Unfortunately, there is no systematic study concerning the effect of processing parameters on the green properties of wires and tapes during the deformation processes. The focus of this investigation centered on silver (Ag) sheathed Bi-2212 wire forming processes and use of the two most common techniques, wire drawing and hydrostatic extrusion. Billets of 6.35 mm outside diameter were reduced separately by five different die sequences to a final diameter of 1.63 mm. Wires made by these procedures were characterized for properties such as Bi-2212 relative packing density, sheath material thickness, deformation force, elongation, drawing friction, and microstructural and mechanical properties. These results indicated that the amount of reduction per pass has a significant impact on the characteristics of non-heat treated wires and on the mechanical aspects of the wire drawing operations.
Bi-2212, drawing, extrusion, superconducting wires
Finite Element Modeling of the Powder-in-Tube Process for Manufacture of BSCCO-2212 Superconducting Wires
R. SHAH, S. TANGRILA, S. RACHAKONDA, and M. THIRUKKONDA
Concurrent Technologies Corporation, 1450 Scalp Avenue, Johnstown, PA 15904.
High-temperature superconductors have recently attracted a great deal of attention owing to their potential use in a variety of applications including power generators, superconducting magnets for mine sweepers or ship propulsion motors, and magnetic levitation transportation systems. The powder-in-tube (PIT) process has emerged as one of the most promising and economically feasible techniques to produce long lengths high-Tc oxide based superconducting wires. The PIT method involves multi-pass wire drawing followed by rolling and heat treatment. This work focuses on the development of finite element models to simulate the PIT drawing process for fabrication of silver sheathed Bi-2212 superconducting wires. The numerical models were used to predict the density of the oxide powder, the wire drawing forces, and the silver-oxide ratio during drawing. A cap-type pressure dependent constitutive equation was implemented in the model to simulate the powder behavior. The model incorporated experimentally obtained material data for the silver and powder. Data from wire drawing experiments were used to verify model predictions.
BSCCO superconductor, deformation processing, finite element analysis, wires
Bi-2223/Ag-Sheathed Tape Processing Studies
J.O. WILLIS, R.D. RAY II, D.S. PHILLIPS, K.V. SALAZAR, J.F. BINGERT, T.G. HOLESINGER, J.K. BREMSER, and D.E. PETERSON
Superconductivity Technology Center, Mail Stop K763, Los Alamos National Laboratory, Los Alamos, NM 87545.
The production of high critical current density Jc Bi-2223/Ag sheathed conductors is a complex process involving interactions among many different parameters. The effects of three factors: 1) powder production path, 2) the first sinter temperature, and 3) the subsequent sinter temperatures were investigated. Statistical methods were used to design the experiment and interpret the results. Transport Jc was the main response for the analysis, but microstructural results were also used to assess the physical basis for the differences in performance. The powder variable had the largest main effect with only very weak main effects for the other factors.
Bi-2223/Ag sheathed tape, critical current density, thermomechanical processing, statistical analysis
Superconducting Joints for Silver-Clad BSCCO Tapes
J.Y. HUANG,1 R. JAMMY,1 A.N. IYER,1 U. BALACHANDRAN,1 and P.HALDAR2
1--Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439. 2--Intermagnetics General Corporation, Latham, NY 12110.
The advent of high-Tc superconducting tapes for a variety of applications has resulted in the need for superconducting joints. Although long-length tapes can be custom-made for some applications, interconnections between subcoils of a magnet or shorter-length conductors for coil winding and current leads, make the development of such joints imperative. Additionally, high-quality short- or medium-length conductors, which are easier to make, can be joined for improved performance. Employing a novel chemical etching technique, we have fabricated lap joints between short lengths of silver-clad BSCCO tapes. Each joint was formed by etching the silver away and bringing together the exposed superconductor cores of two tapes together. The joined tapes were then subjected to a series of thermomechanical treatments. Detailed microstructural and electrical characterization within and across the joint was performed. Critical currents of up to 37 A within the joint region and 10 A through the joint region(at 77K) have been achieved.
Chemical etching, interconnections, magnetic coils, silver clad BSCCO tapes, superconducting joints
Fabrication of Long Length Bi-2223 Superconductor Tape Using Continuous Electrophoretic Deposition on Round and Flat Substrates
L.D. WOOLF, T.L. FIGUEROA, R.A. OLSTAD, F.E. ELSNER, and T. OHKAWA
Pacific Superconductors Division, General Atomics, San Diego, CA 92186.
We have developed a continuous fabrication process for producing long lengths of Bi-2223 superconductor tapes. The process involves sequentially electrophoretically depositing and sintering superconductor and then silver layers on a substrate, followed by rolling and thermal processing. Both round and flat silver substrates have been used. Bi-2223 tapes made using flat silver substrates require only a few processing steps. Transport critical current densities at 77K in zero applied magnetic field exceeding 20000 A/cm2 have been obtained.
Electrophoretic deposition, rolling, sintering, thermal processing
Sandwich Rolling, Jc, and Pinning Energy in Ag-Sheathed BPSCCO
S.X. DOU,1 M. IONESCU,1 W.G. WANG,1 H.K. LIU,1 E. BABIC,2 and I. KUSEVIC2
1--Centre for Superconducting and Electronic Materials, University of Wollongong, Northfields Av. 2522, Wollongong, Australia. 2--Department of Physics, University of Zagreb, 41001, Zagreb, Croatia.
A "sandwich rolling" process was developed to prevent the formation of sausaging and cracks in the longitudinal direction. The stress-strain state of the tape in "sandwich" rolling is the same as that of uniaxial pressed tape because the deformation of steel sheets is negligible in comparison to that of Ag-clad tape. Critical current densities of 3.2 x 104 A/cm2 at 77K and 2.7 x 105 A/cm2 at 4.2K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U0(B) = U(T-0, B) for Bi-2212 tape and Bi-2223 tapes consisting of different fraction of 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that for the same fields, the Uo for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films after taking into account the difference of the Tc between 2223 tape and 2212 tape, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centers such as dislocations, introduced during processing, affect the flux motion at lower B.
Critical current density, pinning energy, sandwich rolling
Critical Current Response of BSCCO Tapes in Tension
W.R. BLUMENTHAL, R.A. MOORE, J.Y. COULTER, J.F. BINGERT, and K.V. SALAZAR
Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545.
Uniaxial tension experiments were performed on monofilament Bi1.9Pb0.4Sr2Ca2Cu3Ox (BSCCO)-silver and pure silver tapes. Nearly silver-free BSCCO tapes were also tested to study the influence of the silver phase. In situ measurements of the critical current density (Jc) vs applied strain provided evidence for the strength and failure behavior of the superconducting oxide core. Final rolling deformation was used as the primary processing variable. BSCCO-Ag tapes exhibited strain tolerance of up to 0.4% before the superconducting performance began to degrade. The stress-strain response of a near silver-free core was typical of ceramics with brittle failure occurring after an elastic strain of 0.19%. Although not optimized, the yield strength and Jc improved with greater final rolling deformation. Even greater strain tolerance is postulated by improving the structure and integrity of the oxide core and by increasing the residual compressive stress imposed on the core by the thermal expansion mismatch with the silver sheath.
Ag-clad tapes, compressive stress, strain effect
Recent Status on High Temperature Superconducting
Bi2Sr2CaCu2O8+x Wire Development at NYSIS: 1-90 Meter Length Jcs and 3 Meter Diameter Ring Furnace Design
T. HAUGAN, S. PATEL, M. PITSAKIS, F. WONG, S.J. CHEN, and D.T. SHAW
New York State Institute on Superconductivity and Department of Electrical and Computer Engineering, State University of New York at Buffalo, Buffalo, NY 14260-1900.
The status of long length, Bi2Sr2CaCu2O8+x (Bi-2212) wire development at the New York State Institute on Superconductivity (NYSIS) is reviewed and updated. Transport Jcs (4.2K, 0 T) of Bi-2212/Ag oxide powder-in-tube single-filamentary tapes have reached 70,000-80,000, 50,000-60,000, and 30,000-40,000 A/cm2 for 1, 4-15, and 40-90 meter length tapes, respectively. The decrease in Jc as the tape length was increased from 15 to 90 meters was attributed to the (measured) sensitivity of Jc to temperature nonuniformities (+/-3°C) in the box-type furnace used for annealing. To reduce this problem, a ring-type high-temperature furnace (~3 meter diameter) was designed and constructed which provides a large-volume (~13w x 10h x 1000l cm) processing zone with expected excellent temperature uniformity (+/-0.5°C). The advantages of the ring-type furnace for processing of kilometer-length conductors are described.
Bi-2212, long length conductors, ring furnace
Fabrication of Ag-Sheathed Bi-2223 Tapes Using Powders
Produced by Aerosol Spray Pyrolysis
YUAN-LIANG WANG,1 W. BIAN,1 Y. ZHU,1 Y. FUKUMOTO,1 H.J. WIESMANN,1 M. SUENAGA,1 T.R. THURSTON,2 K. MERKEN,3 and S. HONG3
1--Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973. 2--Department of Physics, Brookhaven National Laboratory, Upton, NY 11973. 3--Oxford Superconducting Technology Inc., Carteret, NJ 07808.
Bi-2223 tapes were manufactured from a fine "two-powder" product produced by using an aerosol spray pyrolysis technique. Critical current density of 22000 A/cm2 at 77K and 0 T was achieved. Nondestructive transmission x-ray diffraction study indicated good alignment of the superconducting grains. The texturing process of the superconducting phase was found to be nearly complete after the first 24 h of heat treatment for the samples studied. Pressing was found to play a little role in the texturing process. The texturing can be enhanced by Ag-doping. Jc, however, was not found to be improved significantly, presumably due to the reduced effective cross-sectional area. A new phase, Bi-4435, was identified which may play a significant role in the formation of 2223.
Aerosol spray, texturing, two-powder process
Powder Processing of High Temperature Ceramic Superconductor Tl2Ba2Ca2Cu3O10
Y. XIN,1 B.R. XU,2 W.S. HE,1 G.F. SUN,1 R. FARR,3 D.F. LU,1 K.W. WONG,2 and D. KNAPP1
1--Midwest Superconductivity, Inc., 1315 Wakarusa Dr., Lawrence, KS 66049. 2--Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045. 3--Department of Geology, University of Kansas, Lawrence, KS 66045.
We have developed a technique to produce high quality Tl2Ba2Ca2Cu3O10 powders used for making superconducting wire, tape, lead, shield, and other large scale bulk applications. Starting with Tl2O3, BaO2, CaO, and CuO, we mix and grind these chemicals with a machine ball mill and then press the ground mixture into pellets. The pellets are sintered at about 895°C for at least 30 h in an oxygen atmosphere. The sintered material is mainly the Tl2Ba2Ca2Cu3O10 compound. To get more homogeneous superconductor powders, we pulverize the sintered material and use a magnetic superconducting material selector to separate and grade the material. Finally, the top grade material has a phase purity of >98% and a Tc (r=0) of 123-126K.
Magnetic separation, phase purity, synthesis, superconductor, Tl2Ba2Ca2Cu3O10
The Phase Transformation of Bi-Pb-Sr-Ca-Cu-O Bulk Materials are Rapidly Melted and Solidified by a CO2 Laser
C.H. KAO,1 H.L. CHERN,2 and C.J. WANG2
1--Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan, ROC. 2--Department of Mechanical Engineer, National Taiwan Institute of Technology Taipei, Taiwan, ROC.
The phase transformation of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy bulk materials rapidly melted and solidified by a CO2 laser with the scanning speed of 40 mm/s were investigated. Results of x-ray diffraction pattern, scanning electron microscopy and energy-dispersive x-ray analysis showed the decomposition of the high-Tc phase in the laser irradiated region. Nonsuperconducting phases such as CaO and (Sr1-xCax)CuOy were found to be in the melting zone. On the other hand, (Sr1-xCax)CuOy and 2212 phase were also found in the heat-affected zone. When the irradiated samples were treated with 835°C for 72 h in air, the laser treated region changed into the high-Tc as a major phase, in addition to the low-Tc phase and nonsuperconducting phase. However, the high-Tc phases are piled up randomly. The transport critical-current density of the laser treated samples after annealing is lower than that of the original sintered one, i.e. at 77K and zero magnetic field.
Bi-Pb-Sr-Ca-Cu-O, CO2 laser, high-Tc superconductor, phase transformation
Phase Relations and Homogeneity Region of the High Temperature Superconducting Phase (Bi,Pb)2Sr2Ca2Cu3O10+d
STEFANIE KAESCHE, PETER MAJEWSKI, and FRITZ ALDINGER
Max-Planck-Institut für Metallforschung, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, D-70569 Stuttgart, Germany.
For the nominal composition of Bi2.27-xPbxSr2Ca2Cu3O10+d, the lead content was varied from x = 0.05 to 0.45. The compositions were examined between 800 and 890°C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi2Sr2Ca2Cu3O10+d) is stable. Only compositions between x = 0.18 to 0.36 could be synthezised in a single phase state. For x >0.36, a lead-containing phase with a stoichiometry of Pb4(Sr,Ca)5CuOd with a small solubiliy of Bi is formed, for x< 0.18 mainly Bi2Sr2CaCu2O8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890°C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850°C was found for the 2223 phase.
Bi-Pb-Sr-Ca-Cu-O, high-Tc superconductor, phase relations
Effect of Lead Content on Bi-2223 Formation by a Two-Powder Process
S.E. DORRIS, M.A. PITZ, J.T. DAWLEY, and D.J. TRAPP
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439-4838.
Decreasing the total lead content from Pb0.4 to Pb0.3 significantly stunts Bi-2223 phase development and greatly reduces the critical current density (Jc) of powder-in-tube tapes made by a two-powder process. This effect can be explained on the basis of the solubility limit for lead in 2212. Pb0.4 samples exceed the solubility limit for lead in 2212, so lead is rejected to create a lead-rich liquid that enhances the kinetics of 2223 formation during heat treatment. By contrast, a liquid does not form in the Pb0.3 sample because its lead content is below the solubility limit. As a result, 2223 formation is much slower and Jc is much lower in the Pb0.3 sample.
Heat treatment, kinetics of 2223, lead-rich liquid
Mechanical and AC Loss Properties in Mono- and
Multi-Filamentary Bi-2223 Ag-Sheathed Tapes
S. YUHYA,1 M. HIRAOKA,1 T. FUKUNAGA,2 and A.
1--Central Research Laboratory, Mitsubishi Cable Industries, Ltd., 8 Nishino-cho, Higashimukaijima, Amagasaki, Hyogo 660, Japan. 2--Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441, Japan.
The evaluation of the bending strain tolerance and AC loss properties for mono- and multi-filamentary Bi-2223 Ag-sheathed tape were carried out at liquid nitrogen temperature. For tapes with a filament number of over 19, the critical current (Ic) was maintained at the same values up to the bending strain of 0.3%, although the Ic of the mono-filamentary tape at the condition of 0.2% strain degraded to 90% of the value for the no-strain condition. The AC loss of the mono- filamentary tape was the hysteresis type. On the contrary, the AC loss of the multi-filamentary tape was substantially dominated by the eddy current loss in the Ag matrix.
AC loss, bending strain tolerance, Bi-2223 Ag-sheathed tape, multi-filamentary tape, bending strain tolerance
Superconducting Coil Development and Motor Demonstration: Overview
Naval Research Laboratory, Washington, DC 20375-5343.
Superconducting bismuth-cuprate wires, coils, and magnets are being produced by industry as part of a program to test the viability of using such magnets in Naval systems. Tests of prototype magnets, coils, and wires reveal progress in commercially produced products. The larger magnets will be installed in an existing superconducting homopolar motor and operated initially at 4.2K to test the performance. It is anticipated that approximately 400 Hp will be achieved by the motor. This article reports on the initial tests of the magnets, coils, and wires as well as the development program to improve their performance.
Navy motor, prototype magnets, wires
Shape-formed Ceramic Superconductors by Slip-Casting
ATIT BHARGAVA, IAN D.R. MACKINNON, TORU YAMASHITA, and JOSÉ
Centre for Microscopy and Microanalysis, The University ofQueensland, Q4072 Australia.
In order to rigorously test emerging applications using prototypes and pilot designs, high temperature superconductor (HTS) materials must be fabricated into a variety of shapes in an economical manner. We have developed a simple, economical, ceramic slip-casting approach to form complex shaped monolithic HTS articles for which high bulk density has been achieved. The sintered articles exhibit good Meissner signal and consist of phase-pure HTSC phase. A low transport critical current density is observed and is explained on the basis of densification and grain growth.
Densification, grain growth, slip casting, superconductors
2 Tesla Class Magnet Fabrication Using Bi-2212 Ag-Sheathed Tape
S. YUHYA and M. HIRAOKA
Central Research Laboratory, Mitsubishi Cable Industries, Ltd., 8 Nishino-cho, Higashimukaijima, Amagasaki, Hyogo 660, Japan.
Three test magnets of pancake-shaped coils using Bi-2212 tape were prepared by the wind-and-react technique. At liquid helium temperature, 16 pancake coils which were stacked in a volume of 14x48x75H mm generated a magnetic field of 2.25 Tesla (T), which was within 1% of the calculated B0. The load lines of the magnet at every temperature from 4.2 to 30K coincided with the Ic of the short tape, up to the magnetic field of 6 T. For the quadruple pancake coil, the steady-state operational current, which produced increasing voltage with the lapse of time in a cryogenic atmosphere, was a value between the critical current (Ic) determined by the criteria of 1 uV/cm and 10-13 ·m.
Bi-2212 Ag-sheathed tape, load line, oxide superconducting magnet
Hydrostatic Extrusion of BSCCO/Ag Composite Wire
J.F. BINGERT, B.L. BINGHAM, R.J. SEBRING, and A.M. KELLY
Los Alamos National Laboratory, MS G770, Los Alamos, NM 87545.
The application of hydrostatic extrusion processing to composite wire fabrication offers several advantages over conventional reduction techniques. These include enhanced uniformity, increased reduction capability, and successful extrusion of normally brittle materials. Specifically, this work involved an experiment to screen the effects of seven extrusion process parameters on the uniformity and density of as-extruded BSCCO-core wire, and the required extrusion pressure. Results suggest the following factors to be potentially significant: 1) back pressure, extrusion ratio, and die angle on core uniformity, 2) packing method, extrusion ratio, silver powder additions, and temperature on density, and 3) extrusion ratio and temperature on extrusion pressure.
High temperature superconductor (HTSC), hydrostatic extrusion, wire fabrication
Microtexture and Mesotexture in High-Jc Bi-2223
A. GOYAL,1 E.D. SPECHT,1 D.M. KROEGER,1 T.A. MASON,2 D.J. DINGLEY,2 and G.N. RILEY3
1--Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831. 2--TEXSEM Corporation, Provo, UT. 3--American Superconductor Corporation, Westborough, MA 01581.
The microstructure of high-Jc Bi-2223 powder-in-tube tapes was studied using x-ray and electron diffraction. Although the c-axis is nominally aligned perpendicular to the tape surface (FWHM~20°), x-ray phi scans and pole figures show no evidence of any in-plane texture, either macroscopically or locally. Electron backscatter diffraction patterns acquired in a scanning electron miscroscope (SEM) were used to measure individual grain orientations. Grain boundary misorientation between adjacent grains was described by rotation angles and axes (i.e. the disorientation) and compared with theoretical values of ideal coincidence site lattices (CSLs). Data collected from over 113 spatially correlated grains resulting in 227 grain boundaries, show that over 40% of the boundaries are small angle. In addition, 8% of the boundaries were found to be within the Brandon criterion for CSLs (larger than 1 and less than 50). Grain boundary "texture maps" derived from the SEM image and orientation data reveal the presence of percolative paths between low energy boundaries.
Misorientation, pole figures, texture maps, x-ray diffraction
Silver Alloys for High-Temperature Superconducting Wire
B.N. HUBERT,1 R. ZHOU,2 T.G. HOLESINGER,2 W.L. HULTS,2 A. LACERDA,2 A.S. MURRAY,2 R.D. RAY II,2 C.M. BUFORD,3 L.G. PHILLIPS,3 A. KEBEDE,3 and J.L. SMITH2
1--Permanent address: Department of Mechanical Engineering, MIT, Cambridge, MA 02139. 2--Los Alamos National Laboratory, Los Alamos, NM 87545. 3--Permanent address: Departmentof Physics, North Carolina A & T State University, Greensboro, NC 27411.
The silver cladding for high-temperature superconducting wire can be modified for various applications by alloying. For example, for powder-in-tube wire, stiffer cladding improves the smoothness of the interface. For large coils, higher strength is needed at low temperatures to hold them together. Power applications require more resistance in the cladding. We have made a survey of the properties of alloys to check their feasibility for various applications. Alloys with several elements added to silver have been prepared and evaluated for hardness, electrical properties, and compatibility with high-temperature superconductors during processing.
Electrical resistivity, hardness, high-temperature superconductivity, silver alloys
Recent Issues in Fabrication of Ag-Clad BSCCO Superconductors
A.N. IYER,1 R. JAMMY,1 U. BALACHANDRAN,1 M. SUENAGA,2 and P. HALDAR3
1--Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439. 2--Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973. 3--Intermagnetics General Corporation, Latham, NY 12110.
Long lengths of mono- and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (Jc) up to 12,000 A/cm2 has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with 770 m of BSCCO tape, generated fields of 1 T at 4.2K and 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono- and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain 1%.
High temperature superconductors (HTSC), long length conductors, powder-in-tube process (PIT), strain tolerance
Aspects of the Current Understanding of the Supercurrent Transport in (Bi,Pb)2Sr2Ca2Cu3O10 Silver-Sheathed Tapes the "Railway-Switch" Model
B. HENSEL, G. GRASSO, and R. FLÜKIGER
Université de Genève, Département de Physique de la Matière Condensée, 24, quai Ernest-Ansermet, 1211 Genève 4, Switzerland.
The "railway-switch" model describes the superconducting current transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries ("railway-switch") constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger, Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary "powder-in-tube" samples [jc(T=77K, B=0 T) = 20..30 kA/cm2] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density jcc along the c axis (or the even lower critical current density jct across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes for applications
Brick-wall model, "railway-switch" model, small angle boundary, texture
Superconducting Magnet System Containing Bi-2212/Ag Coil
Generates 21.8 T at 1.8K
HITOSHI KITAGUCHI,1 HIROAKI KUMAKURA,1 KAZUMASA TOGANO,1 TSUKASA KIYOSHI,1 KIYOSHI INOUE,1 HIROSHI MAEDA,1 NARUAKI TOMITA,2 JUNICHIRO KASE,2 EIJI YANAGISAWA,2 and KOKI KATO3
1--National Research Institute for Metals, 1-2-1, Sengen Tsukuba 305, Japan. 2--Research Center, Asahi Glass Co., Ltd., 1150, Hazawa-Cho, Kanagawa-ku, Yokohama 221, Japan. 3--Hitachi Cable, Ltd., 3550, Kidamari-Cho, Tsuchiura 300, Japan.
We fabricated a Bi-2212/Ag double stacked pancake coil of 13 mm in inner bore and of 46.5 mm in outer diameter, by using Bi-2212/Ag tapes prepared by the combination of continuous dip-coating process and melt-solidification technique. This small superconducting magnet was used as an insert magnet of a conventional superconducting magnet system and tested at saturated superfluid helium temperature (~1.8K) in various bias fields. The generated field of Bi-2212/Ag coil was 0.9 T, with Ic of 310 A(criterion 10-13*m), in the bias field of 20.9 T. Thus, this superconducting magnet system achieved generation of magnetic field of 21.8 T in the full superconducting state.
Bi-2212 conductor, dip coating, high temperature superconductor (HTSC) magnet, pancake coils
Explosive Consolidation of (Bi, Pb)-Sr-Ca-Cu-O Superconductor
H. THOMAS,1 O.T. INAL,1 and U. BALACHANDRAN2
1--Materials and Metallurgical Engineering Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801. 2--Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439.
Superconducting (2212 phase) Bi-Sr-Ca-Cu-O (BSCCO) and BSCCO-Ag composites were explosively consolidated in silver tubing and then drawn and rolled into tapes. These silver-sheathed tapes were then subjected to repeated cycles of pressing and heat treatment, which resulted in enhanced texturing and grain growth and a subsequent increase in critical current density (Jc). The effect of Ag powder additions to the superconducting powder further increased texturing and Jc, with optimal properties occurring with 10 vol.% silver powder additions. Scanning electron microscopy indicated that grain alignment increased and grain structure became refined after the thermomechanical treatment. X-ray diffraction studies indicated that grain orientation and conversion of 2212 phase to 2223 phase are improved when explosive consolidation is introduced before the drawing step in the powder-in-tube process.
Defects, shock consolidation, strain rate
Fabrication and Characterization of Bi(Pb)-Sr-Ca-Cu-O/Ag2O
C.H. KAO,1 L.T. LIN,1 Y.D. CHIU,2 M.J. WANG,1 C.L. LIN,1 and M.K. WU1
1--Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan, ROC. 2--Department of Mechanical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan, ROC.
Superconducting magnetic shields of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6Oy with or without Ag2O superconductor were fabricated in tubular form with one end closed by using a cold isostatic pressing method. It was found that the addition of Ag2O in the BPSCCO samples resulted in no significant effect on Tc when sample were treated at 835°C in air. However, the critical current density and the shielding magnetic flux density (at 77K) were found to be decreased, compared to the pure BPSCCO sample. The correlations of superconducting properties with microstructure of these materials are discussed in this paper.
Ag2O, Bi(Pb)-Sr-Ca-Cu-O, magnetic shields, microstructure
Effect of Ag on Processing and Properties of Bi- and Tl-based
C. PARK, S.T. MISTURE, D. SRIRAM, and R.L. SNYDER
Institute for Ceramic Superconductivity, New York State College of Ceramics at Alfred University, Alfred, NY 14802.
High temperature x-ray diffraction was used to determine the effect of Ag on the peritectic melting sequence of low carbon content Bi2Sr2CaCu2O8. Only slight changes in the solid solution compositions were found with 20wt.% Ag added to the system, with the phases and stability regions remaining the same. Microstructure, sintering behavior, lattice parameters, and superconducting property changes in Tl0.5Pb0.5Sr2CaCu2O7 were investigated with different amounts of Ag added to investigate the effect of Ag. From the results in both systems, the presence of Ag does not seem to change the intrinsic properties of these superconductors, but affects the processing variables, like reaction temperatures, which change the optimum temperatures for obtaining best properties. The temperature at which melting occurs and the amount of liquid formed depends both on the amount of Ag and carbon content of the materials.
Bi, carbon, silver, superconductors, Tl
Processing with Colloidal Ag Particles in Hot Forged
Ag-Coated BSCCO-2223 Composites
NICHOLAS W. MEDENDORP, JR., KEITH J. BOWMAN, and KEVIN P. TRUMBLE
Purdue University, School of Materials Engineering, West Lafayette, IN 47907-1289.
Multilayer Ag-BSCCO-2223 composites have been fabricated by open-die hot forging to study the role of Ag in densification and microstructural development. The composite microstructures were produced using Ag-coated BSCCO particles produced from a chemical precipitation technique. Grain alignment and Ag microstructure effects were characterized via optical and electron microscopy.
High temperature superconductors, hot forging, silver precipitation
Process and Mechanism of Growth of YBa2Cu3O6+x Single Crystal
by the Pulling Method
M. EGAMI, Y. YAMADA, Y. NAMIKAWA, and Y. SHIOHARA
Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan.
Continuous growth of YBa2Cu3O6+x(Y123) single crystals was achieved by the modified pulling method1 using BaO+CuO(3:5) solvent and Y2BaCuO5(Y211) solute. Y211 solid was placed at the bottom of the crucible. Temperature was set ten degrees higher at the bottom than on the surface of the solution. The crystal rotation speed was 120 rpm at first; but as the crystal radius increased, it was reduced to prevent increasing the interface temperature. The maximum crystal growth rate was about 0.2 mm/h. The crystal growth direction was controlled by the seed crystal direction.2 Two-dimensional numerical simulation was performed using a finite-difference method by a supercomputer to investigate the solution flow and temperature distribution.
Growth rate, numerical simulation, pulling method, single crystal, YBa2Cu3O7-x
Process Parameters, Orientation, and Functional Properties of
Melt-Processed Bulk Y-Ba-Cu-O Supeconductors
I.V. ZAKHARCHENKO,1 K.M. TERRYLL,1 K.V. RAO,1 U. BALACHANDRAN2
1--Department of Condensed Matter Physics, Royal Institute of Technology, Stockholm, Sweden. 2--Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439.
This study compared the microstructure, texturing, and functional properties (critical currents) of YBa2Cu3O7-x-based bulk pellets that were prepared by the quench-melt-growth-process (QMGP), melt-textured growth, and conventional solid-state reaction approaches. Using two x-ray diffraction methods, -2, and rocking curves, we found that the individual grains of two melt-processed pellets exhibited remarkable preferred orientational alignment (best rocking curve width=3.2°). However, the direction of the preferred orientation among the grains was random. Among the three types of bulk materials studied, the QMGP sample was found to have the best Jc values, 4500 A/cm2 at 77K in a field of 2 kG, as determined from superconducting quantum interference device magnetic data.
Melt processing, orientation, rocking curve
Magnetic Flux Pinning in Epitaxial YBa2Cu3O7- Thin Films
ALEXANA ROSHKO, LOREN F. GOODRICH, DAVID A. RUDMAN, RICHARD MOERMAN, and LEILA R. VALE
National Institute of Standards and Technology, Boulder, CO 80303.
The influence of microstructure on the critical current density of laser ablated YBa2Cu3O7- thin films has been examined. Scanning tunneling microscopy was used to examine the morphologies of YBa2Cu3O7- films and the morphology data were then correlated with measurements of the critical current density. The films were found to grow by an island nucleation and growth mechanism. The critical current densities of the films are similar to those of films with screw dislocation growth, indicating that screw dislocation growth is not necessary for good pinning. The data suggest that the critical current density in applied magnetic field may be higher in films with higher densities of growth features.
Dislocations, flux pinning, microstructure, nucleation mechanism
Melt Processing for Strong Flux Pinning in RE-Ba-Cu-O (RE: Nd, Sm, Eu, Gd) Superconductors
S.I. YOO, M. MURAKAMI, N. SAKAI, T. OHYAMA, T. HIGUCHI, M. WATAHIKI, and M.
Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-16-25, Shibaura, Minato-Ku, Tokyo 105, Japan.
Factors affecting a characteristic flux pinning in light rare earth (RE)-Ba-Cu-O (RE: Nd, Sm, Eu, Gd) superconductors fabricated by the oxygen-controlled-melt-growth (OCMG) process have been investigated through a comparative study. At 77K and for the applied field parallel to the c-axis of the sample (H//c), the flux pinning of all OCMG-processed REBa2Cu3Oy (RE123) samples studied was very sensitive to the oxygen partial pressure (PO2) controlled during the melt growth and thus, with lowering PO2, the peak field (Bpk) in the M-H loops shifted to a high field and the irreversibility line (IL) shifted to a high H-T region. For a Nd123 sample, as the oxygen annealing temperature increased above 300°C, both Bpk and IL were systematically depressed. However, Bpk for all systems was insensitive to the amount of the second phase (Nd4Ba2Cu2O10 (Nd422) and RE2BaCuO5 (RE211) for the other) inclusion in the superconducting RE123 matrix, supporting that the characteristic flux pinning is due to the superconducting matrix.
Irreversibility line, oxygen-controlled-melt-growth (OCMG) process, oxygen partial pressure, peak field, strong flux pinning
Understanding the Phase Relations and Cation Disorder in
R.W. MC CALLUM, M.J. KRAMER, K.W. DENNIS, M. PARK, H. WU, and R. HOFER
Ames Laboratory, Iowa State University, Ames, IA 50011.
Unlike Y123 which forms only a stochiometric compound, the light rare earth elements (LRE) form a solid solution LRE1+xBa2-xCu3O7+ (LRE123ss), with increasing substitution of the LRE3+ for the Ba2+ with increasing ionic radii of the LRE. Charge balance is maintained by increasing oxygen occupation on the anti-chain sites. The range of solubility is partially controlled by the oxygen partial pressure (PO2). The peritetic decomposition temperature also increases with increasing ionic radii. At doping levels of 0 < x < 0.1, there is an increase in Tc when the high temperature annealing (T ~ 940°C) is performed in low PO2 (< 0.1 bar). The maximum Tc occurs at a doping level of ~x = 0.05 for Nd and Gd. When annealing is performed in 1 bar PO2, there is a gradual decrease in Tc with increasing x. These phenomenon can be understood in terms of the number and distribution of the LRE which substitute on the Ba site.
Light rare earth elements, oxygen partial pressure, solid-solution
Precipitation and Pinning in Ca and Sr-Rich High-Tc
Superconducting "Bi2Sr2CaCu2O8" Ceramics
PETER MAJEWSKI, STEFFEN ELSCHNER, and FRITZ ALDINGER
Max-Planck-Institut für Metallforschung, Heisenbergstr. 5, 70569 Stuttgart, Germany.
From a consideration of the phase equilibrium diagram of the system Bi2O3-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi2+xSr2-xCuO6+d, Sr14Cu24O41-x, and Bi2Sr3O6 in high-temperature superconducting Sr-rich "Bi2Sr2CaCu2O8" ceramics and Ca2CuO3 and a liquid in Ca-rich "Bi2Sr2CaCu2O8" ceramics. The transformation results in an increase of the critical current density of the which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.
Depinning, flux pinning, pinning energy, thermal activation
Effects of Processing Variables on the Y2BaCuO5 Size and Magnetic Properties of Melt-Processed YBa2Cu3Ox
H.F. YANG, C. VARANASI, and P.J. MC GINN
Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556.
Samples of YBa2Cu3Ox (123) with excess Y2BaCuO5 (211) in the molar ratio of 5:1 (123/211) were processed using the "solid liquid melt growth" (SLMG) technique. The effect of hold time above the peritectic on the magnetic properties was examined. Extended hold times above the peritectic during processing degrade the magnetic properties of SLMG processed 123. In SLMG 123, the very fine (<100 nm) 211 particles produced by this processing route are the primary magnetic flux pinners. Extended hold periods reduce the number and/or coarsen the average size of these fine precipitates, resulting in a reduced magnetization. These results were compared to undoped Y123 processed by the more traditional melt texture growth (MTG). In MTG processing, extended hold times above the peritectic are found to result in improved magnetic behavior because of increased defect densities.
Grain alignment (texturing), magnetization, Y2BaCuO5
Nd Substitution in Y/Ba Sites in Melt Processed YBa2Cu3O7-
Through Nd2O3 Additions
CHAKRAPANI VARANASI,1 PAUL J. MCGINN,1 HOWARD A. BLACKSTEAD,2 and DAVID B. PULLING2
1--Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556. 2--Department of Physics, University of Notre Dame, Notre Dame, IN 46556.
YBa2Cu3O7- (Y123) samples with excess Nd2O3 and Y2O3 additions in the same molar ratios were melt textured in air. In the Nd-doped samples, in addition to Y ion site substitution, partial substitution into the Ba2+ sites is anticipated because of the similar ionic sizes of Nd3+ and Ba2+. The microstructure, Tc, and magnetic properties of Nd-doped samples were analyzed and compared with undoped Y123 and samples with excess Y2O3. The Nd2O3 additions lead to significant magnetization improvements, likely due to both rare earth- and Ba-site substitution by the doped Nd3+ ions, and to increases in Tc. Y2O3 additions resulted in no marked property enhancement.
Magnetization, melt texturing, rare earth substitution, YBa2Cu3O6+x
Relationships Between Dislocations, Second Phases, and Pinning in Y-Ba-Cu-O
E.P. KVAM,1 V. PAVATE,1 C. VARANASI,2 and P.J. MC GINN2
1--School of Materials Engineering, Purdue University, W. Lafayette, IN 47907-1289. 2--Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556.
The pinning effects and the critical current density predicted to occur in bulk YBa2Cu3O7 from second phase particles and from dislocations are examined. It is shown that core or strain pinning by dislocations provides the best explanation of pinning forces, and that strain pinning by dislocations or particle pinning of individual (but not multiple) flux lines best fit the pinning energy. An experiment in which critical current is increased when particles are coarsened by long molten holding times during melt processing is correlated with increased dislocation density, despite a drop in volume fraction and increase in size of the added normal state particles. The combined aspects of these suggest that strain induced pinning from dislocations may be the dominant mechanism in high critical current bulk materials.
Dislocations, flux pinning, melt processing, particle pinning
Engineered Flux-Pinning Centers in Bi2Sr2CaCu2Ox and TlBa2Ca2Cu3Ox Superconductors
K.C. GORETTA,1 V.R. TODT,1 D.J. MILLER,1 M.T. LANAGAN,1 Y.L. CHEN,1 U. BALACHANDRAN,1 J. GUO,2 and J.A. LEWIS2
1--Argonne National Laboratory, Argonne, IL 60439. 2--Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801.
Three methods were used to introduce flux-pinning centers into Bi2Sr2CaCu2Ox (Bi-2212) and TlBa2Ca2Cu3Ox (Tl-1223) samples. It was found that carbon induced local decomposition, that nanosized Al2O3 additions created stable reaction products, and that second phases could be isolated in Tl-1223 during synthesis. Each of these defects enhanced flux pinning and was of most benefit at temperatures <=35K.
Carbon induced decomposition, flux pinning, nanosize particles
Achieving Superconductivity Through Reductive Treatments of YSr2(Cu1-xFex)3Oy
J.R. O'BRIEN,1 H. OESTERREICHER,1 and R.D. TAYLOR2
1--Department of Chemistry, University of California, San Diego, La Jolla, CA 92093-0317. 2--Los Alamos National Laboratory, Los Alamos, NM 87545.
In order to obtain massive Fe nano clustering coexisting with superconductivity, the dependence of the structural, magnetic, and superconductive properties of YSr2(Cu1-xFex)3Oy (x = 0.13 or x = 0.167) on annealing conditions is explored. Materials are initially prepared by conventional solid state technique with an annealing for two days at T = 1000K and is termed oxygenating preparation (OP). The materials treated by OP method show no signs of superconductivity above 5K. Subsequently, Gd getter annealing is used to control reductions at T = 1000K for four days followed by reoxidation at 800K for 1 h and is termed reducing preparation (RP). For x = 0.13, the RP treatment results in the development of superconductivity with Tc-onset = 40K. For x = 0.167, a Tc-onset = 30K is achieved. A local thermodynamic model based on the stabilities of various Fe cluster arrangements is used to explain the effect of the thermal treatments.
Cluster arrangements, magnetic property, thermodynamic model
High Pressure-High Temperature Effect on the HTSC Ceramics Structure and Properties
V.N. Bakul Institute for Superhard Materials of the Ukrainian National Academy of Science, 2, Avtozavodskaya Str., 254074 Kiev, Ukraine.
The present paper looks at the high pressure and temperature effect on the structure, phase composition, superconductive, and mechanical behavior of high temperature superconducting ceramics MeBa2Cu3O7- (Me = Y, La, Nd, Gd, Sm, Eu), ceramics based on superconductive compounds of the Bi-Pb-Sr-Ca-Cu-O system, the so-called 2212 and 2223 phases, and also on the infinite-layer compounds of the nominal composition Sr0.7Ca0.3CuO2. It has been shown that under high pressure-high temperature conditions, materials with high superconductive and mechanical characteristics can be obtained and the increase in critical current density can be the result of such a treatment.
High pressures-high temperatures (HP-HT), high temperature superconductors (HTSC), mechanical properties, structure, superconductive (SC)
Superconductivity Above 100K in Compounds Containing
M.S. OSOFSKY, W.L. LECHTER, L.E. TOTH, E.F. SKELTON, A.R. DREWS, C.C. KIM, B. DAS, S.B. QADRI, A.W. WEBB,and R.J. SOULEN, JR.
Naval Research Laboratory, Washington D. C. 20375-5000.
Several members of a new family of (Hg,Tl)(Ba,Sr)CaCuO high temperature superconductors have been synthesized. These compounds, which are analogs to the Hg-Ba-Ca- and Tl-Ba-Ca-layered cuprates, are multi-phased and have superconducting transition temperatures above 100K. Several samples with a nominal composition of HgTl2Ba4Ca4Cu6Oy had Tcs near 130K. Details of the characterization of these samples are presented.
Hg-based superconductor high pressure pressing, magnetic moment
Phase Relations in Cu-RO1.5 -O (R = Ho,Er,Yb) and Gibbs Energy of Formation of Cu2R2O5 (R = Ho,Er,Yb) Between 1000 and 1325K
G.M. KALE and D.J. FRAY
Department of Mining and Mineral Engineering University of Leeds, Leeds, LS2 9JT, UK.
Phase relations in Cu-RO1.5-O(R = Ho,Er,Yb) ternary systems at 1273K have been established by isothermal equilibration of samples containing different ratios of Cu:R(R = Ho,Er,Yb) in flowing air or high purity argon atmosphere for four days. The samples were then rapidly cooled to ambient temperature and the coexisting phases were identified by powder x-ray diffraction analysis. Only one ternary oxide, Cu2R2O5(R = Ho,Er,Yb) was found to be stable. The chemical potential of oxygen for the coexistence of the three phase assemblage, Cu2O + R2O3 + Cu2R2O5(R = Ho,Er,Yb) has been measured by employing the solid-state galvanic cells,
Chemical potential, free energy of formation, phase diagram
in the temperature range of 1000 to 1325K. Combining the measured emf of the above cells with the chemical potential of oxygen at the reference electrode, using the Nernst relationship, gives for the reactions,
respectively. Combing the chemical potential of oxygen for the coexistence of Cu2O + R2O3 + Cu2R2O5(R = Ho,Er,Yb) obtained in this study with the oxygen potential for Cu2O + CuO equilibrium gives for the reactions,
It can be clearly seen that the formation of Cu2R2O5(R = Ho,Er,Yb) from the component oxides is endothermic. Further, Cu2R2O5(R = Ho,Er,Yb) are an entropy stabilized phases. Based on the results obtained in this study, the oxygen potential diagram for Cu-R-O(R = Ho,Er,Yb) ternary system at 1273K has been composed.
The Use of Nanophase Powders for High Jc Supercondcutor Tape
M. MUHAMMED,1 L.N. WANG,1 and K.V. RAO2
1--Department of Inorganic Chemistry, Royal Institute of Technology, Stockholm, Sweden. 2--Department of Condensed Matter Physics, Royal Institute of Technology, Stockholm, Sweden.
Advances in processing of superconductors has demonstrated that ceramic powders with exact composition, super-homogeneity, and extremely fine particle size can be prepared by chemical processing to achieve high density and uniform microstructure. The chemical processing routes have great advantage in preparing powder with high reactivity thus promoting solid state reactions required to produce `purer' superconducting phase. We have studied extensively the coprecipitation technique for the preparation of superconducting ceramic powders. Chemical solution methods have been used in order to model several superconducting systems and to predict optimum conditions for obtaining exact composition. Several improvements on the control of precipitation reaction have been implemented in order to obtain powders of few nanometers in size. This has been done by carrying out the reaction in the bulk of a solution or in microemlusion systems. The coprecipitated precursor powder has been used for production of 20-30 meter long Ag-clad tapes. These tapes require considerably reduced processing times. By controlled heat treatment conditions, these tapes have been processed to carry transport critical current density, Jc >4 x 104 A/cm2 at 77K in zero applied field.
Ceramic superconductors, chemical synthesis, coprecipitation, powder-in-tube, YBCO, BSCCO
REGULAR ISSUE PAPERS
Fabrication of Amorphous Ni-Si Films with Small Temperature Coefficient of Resistance by New Flash Evaporating Method
Department of Electronic Engineering, Faculty of Science & Technology, Kinki University, 3-4-1 Kowakae Higashiosaka City Osaka, Japan 577.
Utilizing a heat treatment processes, Ni-Si thin films which have various resistivity and various and/or small temperature coefficients of resistance (TCR) can be fabricated by new flash evaporating equipment assembled in our laboratory. In relation to the increase of Si content in the film, the TCR of the as-deposited film increases negatively. By using heat treatment to stabilize it, the film' s resistance decreases and its TCR shifts in a positive direction. These trends become more marked if the heat treatment is performed at high temperature for longer durations. Under optimal heat treatment conditions, samples with a range of resistivity from 250 to 3500 µ·cm and a small TCR can be obtained. By using Auger electron spectroscopy, we also clarify the composition change of the film's depth direction fabricated by this method is more uniform than that of direct evaporating method.
Flash evaporating method, Ni-Si, thin films
A Composition-Dependent Model for the Complex Dielectric Function of In1-xGaxAsyP1-y Lattice-Matched to InP
LEONARD I. KAMLET and FRED L. TERRY, JR.
Department of Electrical Engineering & Computer Science, 111 DTM Bldg., The University of Michigan, 2360 Bonisteel Blvd., Ann Arbor, Ml 48109-2108.
Accurate and numerically efficient models for the complex dielectric function vs wavelength and material characteristics are essential for the use of nondestructive optical techniques such as spectroscopic ellipsometry or reflectometry. These optical techniques are commonly used for real-time and run-to-run monitoring and control of growth and etch processes to determine a material's composition and thickness. In this work, we discuss an improved composition-dependent model for the complex dielectric function for lattice-matched In1-xGaxAsyP1-y/InP systems valid over the entire composition range 0¾y¾1. We describe our model, which is an extension of the critical point parabolic band method and is based on the model proposed by Charles Kim et al. for the AlxGa1-xAs/GaAs system. We demonstrate the quality of the model in fitting optical data for individual compositions and compare our results to other established models including the harmonic oscillator approximation and the model of Adachi. Using results obtained from the individual fits, we generate a composition-dependent model that is valid for the entire range of lattice-matched compositions. Also, we show how this model can be used to accurately determine the composition (+/-0.01) of an unknown material whose dielectric response has been obtained using spectroscopic ellipsometry or a similar technique.
Dielectric function, InGaAsP, optical properties
SiO2 Gate Insulator Defects Spatial Distributions, Densities, Types, and Sizes
A. REISMAN and C.K. WILLIAMS
Department of Electrical and Computer Engineering, Box 7911, North Carolina State University, Raleigh, NC 27695.
Standard IC processes, as well as those involving the use of ionizing radiation, such as x-ray lithography etc., result in the generation of bulk defects, and interface states in the gate insulator, or underlying substrate, respectively, of insulated gate field effect transistors. Bulk defects are believed to be present as positively and negatively charged electron and hole traps, respectively, as well as neutral hole and "large" and "small" neutral electron traps. This paper provides a perspective of the current state of knowledge about the spatial distributions of large bulk defects, their areal densities, sizes, possible interrelationships among them, and the special cases of defects created by ion implanted silicon and oxygen, where knock-on effects have been simulated. It appears that bulk defects may all have their origin in neutral hole traps, (so-called E´ centers) and that when the insulator thickness is decreased to about 6-7 nm, defects are either no longer present, or, more likely, are incapable of trapping charge at room temperature because trapped carriers can either tunnel to one of the interfaces, or be annihilated by a reverse process. It appears possible also that the precursor of the several types of defects only forms at a "grown" silicon-silicon oxide interface. In theory, this would make it possible to grow defect free insulators by a combination of deposition and oxidation processes.
Bulk SiO2 defects, charged gate insulator defects, gate insulator defects, neutral electron traps, neutral hole traps, SiO2 insulator defects
Electron Microscope Studies of InxGa1-xAs/GaAs/Si Grown by Metalorganic Chemical Vapor Deposition
K. KAMEI,1 W.M. STOBBS,2 and K. FUJITA3
1--Research and Development Center, Sumitomo Metal Industries Ltd. 1-8 Fuso-cho, Amagasaki 660 Japan. 2--Department of Materials Science & Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK. 3--ATR Optical and Radio Communications Research Laboratories Seika-cho Soraku-gun, Kyoto 619-02 Japan.
The microstructure of InxGa1-xAs/GaAs (5 nm/5 nm, x=0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450°C, and subsequently annealed at 750°C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.
GaAs/S heteroepitaxy, InGaAs, metalorganic chemical vapor deposition (MOCVD), transmission electron microscopy (TEM)
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