Superconductor Electromagnetic Measurements
Project Goals
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Preparing to measure the electromechanical properties of a superconductor tape.
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This project specializes in measurements of the effect of mechanical strain on superconductor properties for applications in magnetics, power transmission, and electronics. Recent research has produced the first electromechanical data for the new class of flexible high-temperature superconductors, one of the few new technologies expected to have an impact on the large electric power industry and the next generation of accelerators for high-energy physics. The Strain Scaling Law, previously developed by the project for predicting the axial-strain response of superconductors in high magnetic fields, is now being generalized to three-dimensional stresses for use infinite-element design of magnet structures. Recent research also includes extending the high-magnetic-field limits of electromechanical measurements for development of 23.5 tesla nuclear magnetic resonance spectrometers operating at 1 gigahertz. The project's research, which previously led to the first four-contact patents for high temperature superconductors, is being broadened to develop electrical contacts with ultra-low interface resistivity for coated high-temperature superconductors.
Customer Needs
The project serves industry primarily in two areas. First is the need to develop a reliable measurement capability in the severe environment of superconductor applications: low tem-perature, high magnetic field, and high stress. The data are being used, for example, in the design of superconducting magnets for the magnetic-resonance imaging (MRI) industry, which contributes 2 billion dollars per year to the U.S. economy.
The second area is to provide data and feedback to industry for the development of high performance superconductors. This is especially exciting because of the recent deregulation of the electric power utilities and the attendant large effort being devoted to developing reliable superconductors for power conditioning and enhanced power transmission capability. We have received numerous requests, both from industry and from government agencies representing industrial suppliers, for reliable electromechanical data to help guide their efforts in research and development in this critical growth period.
The recent success of the second generation of high-temperature superconductors has brought with it a new set of measurement problems in handling these brittle conductors. We have the expertise and equipment to address these prob-lems.
Technical Strategy
Our project has a long history of unique measurement service in the specialized area of electromechanical metrology. Significant emphasis is placed on an integrated approach. We provide industry with first measurements of new materials, specializing in cost-effective testing at currents less than 1000 amperes. Consultation is also contributed to industry on developing their own measurements for routine testing. We also pro-vide consultations on metrology to the magnet industry to predict and test the performance of very large cables with capacities on the order of 10 000 amperes, based on our tests at smaller scale. In short, our strategy has consistently been to sustain a small, well connected team approach with industry.
We have developed an array of specialized measurement systems to test the effects of mechanical stress on the electrical performance of superconducting materials. The objective is to simulate the operating conditions to which a superconductor will be subjected in magnet applications. Among these measurement systems are apparatus for measuring the effects of axial tensile stress and transverse compressive stress, and a unique system for determining the electromechanical properties of reinforced superconducting composite coils.
These measurements are an important element of our ongoing work with the U.S. Department of Energy (DOE). The DOE Office of High Energy Physics sponsors our research on electrome-chanical properties of candidate superconductors for particle-accelerator magnets. These materials include low-temperature superconductors (Nb3Sn and Nb3Al), as well as high-temperature super-conductors (Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O). The purpose of the database produced from these measurements is to allow the magnet industry to design reliable superconducting magnet systems.
Some of our research is sponsored in part by the DOE Office of Energy Efficiency and Renewable Energy. Here, we focus on high-temperature superconductors for power applications, including transformers, power-conditioning systems, motors and generators, superconducting magnetic energy storage, and transmission lines. In all these applications, the electromechanical properties of these inherently brittle materials play an important role in determining their successful utilization.
In the area of low-temperature superconductors, we have embarked on a fundamental program to generalize the Strain Scaling Law (SSL), a magnet design relationship we discovered more than a decade ago. Since then, the SSL has been used in the structural design of most large magnets based on superconductors with the A-15 crystal structure. However, this relationship is a one-dimensional law, whereas magnet design is three-dimensional. Current practice is to generalize the SSL by assuming that distortional strain, rather than hydrostatic strain, dominates the effect. Recent measurements in our laboratory suggest that this assumption is invalid. We are now developing a measurement system for carefully determining the three-dimensional strain effects in A-15 superconductors. The potential financial consequences of these measurements for very large accelerator magnets are considerable.
Milestones
• By 2003, perform parametric studies of axial and transverse stress effect on the electrical performance of second-generation high-temperature superconductors.
• By 2002, create the first data base of mechanical properties of coated superconductor substrates at cryogenic temperatures.
• By 2003, develop the measurement techniques and obtain the data needed to generalize the Strain Scaling Law from one to three dimensions.
Accomplishments
Measurements of Transverse Stress on Coated High Temperature Superconductors
We completed the first measurements of trans-verse stress effects in second-generation high temperature superconductors, Y-Ba-Cu-O-coated, rolling-assisted, biaxially-textured substrates (RABiTS) and ion-beam-assisted deposition (IBAD) tapes. The critical current of these materials is over 1 000 000 amperes per square centimeter at 77 kelvins, making possible magnet and power transmission-line applications at liquid-nitrogen temperature. Our measurements program is the only one looking at the electrome-chanical performance of these new materials. Before these measurements, the available electromechanical data on these conductors were limited to a few measurements of bending strain versus critical current. The electromechanical performance of the RABiTS conductor, which consists of brittle superconductor and buffer layers deposited on a substrate of soft, pure Ni, was particularly suspect. The results showed that the ion-beam-textured material behaves well under stress, but the deformation-textured conductor, which is currently made using pure Ni, may have to be constructed from stronger Ni alloys to withstand the magnetic stress encountered in commercial magnet applications. Our data are being sought both to guide government funding decisions for new conductor develop-ment, as well as to provide feedback to industry on the technical development of these new conductor materials.
Comparison of the effect of transverse stress (in megapascals) on the critical-current density Jc of various high-temperature tape superconductors: Y-Ba-Cu-O on an Inconel IBAD substrate, Y-Ba-Cu-O on a pure Ni RABiTS, Bi2Sr2Ca2Cu3Ox with Ag and oxide dispersion strengthened Ag matrices, and Bi2Sr2Ca1Cu2Ox with oxide-dispersion-strengthened Ag matrix and double sheathed with Ag and Ag-Al matrices.
The timing is particularly important for the upgrading of electric power service to urban areas, as well as for new magnet technology being developed for stabilizing electric power grids operating at high capacity levels. Using our recently developed fatigue testing facility, we also made the first high-cycle fatigue measurements on Bi2Sr2Ca2Cu3Ox superconductors. The initial results indicate there may be significant accumulated damage on mechanically cycling these materials, such as they would experience in repeatedly charged electromagnets.
New Class of Superconductors Shown to Tolerate Stress
Our measurements on a new class of flexible high-temperature coated super-conductors have shown that the use of frictional support from a high-yield structural material co-wound with the superconductor can greatly improve their transverse stress tolerance. This should allow these new superconductors to be used in high-field magnet applications. The effects of static and cyclic transverse stress on the critical current of Y-Ba-Cu-O coated tapes of textured Ni were carried out at 76 kelvins. When subjected to monotonic loading, critical current degraded by about 1 percent. However, when samples were tested with frictional support removed between each measurement, critical current degraded by 7 percent to 26 percent. Microstructural data also show that improvement in stress tolerance may be achieved by providing lateral transverse support to the superconductors in magnet applications.
Scanning electron micrograph of the Y-Ba-Cu-O on an Inconel IBAD substrate after static and cyclic transverse stress testing to 122 megapascals, showing the pattern of longitudinal cracks along the length of the tape near the tape edge.
Scanning electron microscopy of ion-beam-textured tapes subjected to transverse stress show a series of thin longitudinal cracks (along the direction of the transport current) near the tape edges; the center of the tape, however, was free of any observable cracks. This indicates that the failure is due to in-plane transverse tensile strain.
Stress Effects on Nb3Sn Produced by Chemical-Vapor Deposition
As part of an investigation of three-dimensional stress effects in Nb3Sn superconductors, the effect of trans-verse stress was measured in several specimens of a Nb3Sn tape produced using a chemical-vapor deposition process. This conductor was selected for its relatively simple structural geometry, which lends itself to mechanical modeling. Previous measurements of numerous other Nb3Sn conductors have shown large degradation of critical-current density (about 40 percent) with transverse stress; however, the degradation of this conductor was less than 1 percent at 170 mega-pascals. Understanding this unexpected and seemingly remarkable behavior will be an important step in extending the existing Strain Scaling Law from two to three dimensions.
New Monograph on Contact Techniques
We completed a monograph on contact techniques for high-temperature superconductors. The information is particularly relevant for critical-current measurements of the new coated superconductors, where films carrying 500 amperes of current are now being fabricated. The higher currents have resulted in severe contact heating problems in critical-current tests, usually resulting in vaporization of the samples. This chapter details techniques for reducing the contact resistivity of such contacts by up to two orders of magnitude. Many requests were received for this chapter at a recent Department of Energy peer review on superconductor development for electric-power utilities.
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