Superconductor Electromagnetic Measurements
Preparing to measure electromechanical properties of a superconductor tape at 18.5 tesla field.
This project specializes in measurements of the effect of mechanical strain on properties of superconductors for applications in magnetics, power transmission, and electronics. Recent research has produced the first electromechanical data for the new class of high-temperature coated conductors, 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 in finite-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 on electrical contacts, which previously led to the first four-contact patents for high temperature superconductors, is being broadened to develop contacts with ultra-low interfacial resistivity for coated high-temperature superconductors.
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 temperature, 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 provides invaluable medical data for health care, and 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, from both industry and 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 new measurement problems in handling these brittle conductors. We have the expertise and equipment to address these problems.
The unique measurement services that Ekin, Cheggour, and Clickner have provided us and other U.S. wire manufacturers have allowed us to develop superconducting magnets of much higher field, helping our company compete in the international marketplace. Their critical current vs. strain instrument is one of very few in world, and their new, sensitive, stress vs. strain measurement apparatus at liquid-helium temperature is the only one in the U.S.
Dr. Seung Hong
Vice President, Engineering and Development
Oxford Superconducting Technology
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 provided to industry on developing their own measurements for routine testing. We also provide 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, the effects of transverse compressive stress, and the stress-strain characteristics, 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 electromechanical properties of candidate superconductors for particle-accelerator magnets. These materials include low-temperature superconductors (Nb3Sn and Nb3Al), and high-temperature superconductors (Bi-Sr-Ca-Cu-O and Y-Ba-Cu-O), including conductors made on rolling assisted, biaxially textured substrates (RABiTS) and conductors made by ion-beam-assisted deposition (IBAD). 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, 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 two decades 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, however, that this assumption is invalid. We are now developing a measurement system to carefully determine the three-dimensional strain effects in A-15 superconductors. The importance of these measurements for very large accelerator magnets is considerable.
Electromechanical Performance of High-Temperature Superconductors
In FY 2002, we will perform cryogenic mechanical testing of metal substrates for RABiTS and IBAD development.
During FY 2002-2003, we will perform parametric transverse stress studies of YBCO coated IBAD and RABiTS conductors at 76 kelvins.
During FY 2002-2003, we will perform parametric axial tensile strain studies of YBCO coated IBAD and RABiTS conductors at 76 kelvins.
During FY 2002-2003, we will perform axial tensile strain and transverse stress measurements of new BSCCO conductors.
In FY 2002, we will complete a preliminary survey of axial strain and transverse stress effects in recently discovered MgB2 tape conductors.
Absolutely essential data are being generated here.
Excellent results. Such creation of basic engineering data is necessary for effective systems.
Important basic work, essential to successful system designs.
U.S. Department of Energy
Annual Peer Review
Electromechanical Performance of Low-Temperature Superconductors
In FY 2002, we will complete the data set of axial strain and transverse stress effects in two series of Nb3Sn tape conductors, and measure the Young's modulus at 4 kelvins in these two samples in order to relate stress and strain for developing a multidimensional model.
In FY 2003, we will complete the data correlation and determine the hydrostatic and deviatoric coefficients to generalize the Strain Scaling Law from one to three dimensions. Publish a generalized 3-dimensional model of strain effects in A-15 superconductors for use within finite-element strain designs of large superconducting magnet systems.
In FY 2003, we will publish a paper on the results of the study testing the correlation of uniaxial strain effect with phonon anharmonicity in the A-15 superconductors.
Textbook on Cryogenic Measurement Apparatus and Methods
During FY 2002, we will edit the introduction and chapters on heat transfer and on superconductor critical-current measurement techniques and analysis.
In FY 2003, we will complete the appendix and send the book to the publisher.
First experimental evidence of a significant drop in Jc of coated conductors when a Y-Ba-Cu-O film is sandwiched between two magnetic substrates.
Magnetic Substrates Shown to Reduce the Performance of Y-Ba-Cu-O Coated Conductors - Measurements on Y-Ba-Cu-O coating on buffered pure-nickel RABiTS revealed the first experimental evidence that the use of magnetic substrates can result in a significant and reversible reduction of the current-carrying capacity of the tape when the tape is arranged in a stack of two or more layers. This configuration is readily used in many potential applications where one tape is wound on top of another, or crosses over another as in a braided cable. When the Y-Ba-Cu-O layer is sandwiched between two magnetic nickel substrates, the interaction of the top and bottom nickel layers increases the perpendicular component of magnetic flux at the superconductor tape edges, and hence reduces the critical current density of the tape.
A model was successfully developed to quantify this phenomenon and showed that the reduction of the current-carrying capacity (Jc) depends on the geometry of the sample. The estimated drop in Jc can reach about 26 percent if the thickness of Y-Ba-Cu-O film is 1 micrometer and width is 3 millimeters, instead of 15 percent measured for tapes having a thickness of Y-Ba-Cu-O layer of 0.3 micrometer and width of 3 millimeters. The estimated drop in Jc for a thicker Y-Ba-Cu-O layer of 4 micrometers exceeds 40 percent. This information is highly relevant since the coated-conductors' manufacturers are developing tapes with thick Y-Ba-Cu-O layers (1 to 4 micrometers). This finding put more emphasis on the magnetic-substrate effect, which limits the potential use of Y-Ba-Cu-O coated conductors on magnetic buffered substrates, particularly in low-field applications such as underground power-transmission lines.
Effect of transverse stress on Jc in a Y-Ba-Cu-O film on pure nickel RABiTS. The results illustrate the role played by substrate work-hardening.
Higher Yield-Strength Substrates Required for the Coated-Conductors to Achieve a Better Tolerance to Stress - Critical current measurements as a function of transverse compressive stress were made on Y-Ba-Cu-O coated conductors with textured pure nickel substrates. The results show that the tapes have much better tolerance to transverse stress when the soft nickel substrate is work-hardened. The samples tested under transverse compressive stress exhibit a degradation of the critical current density of about 28 percent at 100 megapascals. After work-hardening, however, the critical current density at 100 megapascals degraded by only 6 percent. This result lends support to the conclusion that substrate yield-strength is playing a major role. This suggests that in magnet applications using the coated conductors on soft substrates, a good practice would be to energize the magnet to its maximum magnetic field during the first run after manufacture in order to improve the robustness of the windings against transverse compressive stress. These results emphasize the need for development of non-magnetic substrates with higher yield-strength for RABiTS technology.
Microstructural characterization of the samples was carried out after static and cyclic transverse stress testing. Scanning electron microscopy (SEM) was used to examine the top Y-Ba-Cu-O layer of the samples. We found isolated regions of cracks both longitudinal and transverse to the direction of electrical current flow. The cracked regions are randomly distributed throughout the entire sample. These cracked regions cover areas in the sample that are a few micrometers to more than 600 micrometers wide. The total degradation of Jc correlates with the crack density in these defective regions. The cracks in the Y-Ba-Cu-O layer are found to extend through the buffer layers. The crack pattern, fundamentally different between RABiTS and IBAD samples we have studied previously, may reflect the influence of certain parameters on the robustness of the coated conductors, such as the mechanical properties of substrate material, the buffer layers, or the size of the Y-Ba-Cu-O grains.
Scanning electron micrographs of the Y-Ba-Cu-O layer in RABiTS tape, after static and cyclic transverse-stress testing. Multi-patterned cracks in the Y-Ba-Cu-O and buffer layers are apparent. The vertical axis of the images coincides with the direction of the electric current applied to the sample.
Stress-strain measurement probe.
Stress-strain curves measured on pure nickel and nickel-alloy RABiTS materials, which are candidates for the fabrication of Y-Ba-Cu-O coated-conductors. The results show the benefit of developing nickel-alloy substrates for RABiTS technology.
A probe for measuring stress-strain characteristics developed - The mechanical test apparatus for axial stress-strain measurements required modifications to accommodate very thin, long and soft samples with very low yield strength.
We characterized mechanical properties of pure-nickel and nickel-alloy materials at room temperature, 76 kelvins and 4 kelvins. The two materials, which are candidates for use as substrates for Y-Ba-Cu-O coated superconductors, were compared in terms of yield strength, modulus of elasticity and proportional limit of elasticity. This information is important to the manufacturers in their selection of a suitable substrate material and in designing processing equipment for the manufacturing of the coated conductors.
Superconductor Wire with High Niobium Content Has Unexpectedly Good Electromechanical Properties - The fabrication of the next generation of particle accelerators for high energy physics will require the development of new niobium-tin/copper superconductors able to carry extremely high current densities at high magnetic fields. One technique for accomplishing this is to push the density of superconductor in the composite wire to new limits. Such an experimental, high-niobium composite was recently fabricated by Oxford Superconducting Technology. A concern in the high-energy-physics community was that the conductor would have very low tolerance to mechanical strain. To test the conductor, we modified our axial electromechanical test apparatus and used a new 16.5 tesla, high-field magnet. Surprisingly, the conductor had electromechanical tolerance similar to standard Nb3Sn composites. The irreversible strain, beyond which the conductor shows permanent degradation, had a relatively high value of 0.73 percent. The peak critical current was measured at a strain of 0.29 percent. This result clears the way for wire manufacturers to push the niobium density to even higher values, which could provide a significant extension of the magnetic field limit of present accelerator magnets.
Electromechanical properties of a new generation of Bi-2212 wires improved - The axial strain measurements carried out on a new generation of Bi-2212 multifilamentary wires at 16.5 teslas and 4 kelvins, revealed that the tolerance to strain of this conductor has been greatly improved. The irreversible strain at which the critical current density starts to degrade is found to be as high as 0.6 percent, representing an improvement by a factor of three with respect to early Bi-2212 wires made a decade ago. This new finding opens very promising perspectives for the use of Bi-2212 multifilamentary wires in fabricating large electromagnets for high-energy-physics accelerators. These new multifilamentary wires, developed by IGC, were designed so that the porosity of Bi-2212 powder is reduced. This resulted in a significant enhancement of both the critical current density and its tolerance to strain.
The tolerance to tensile strain of Bi-2212 multifilamentary wires has tripled compared to earlier Bi-2212 conductors made in the early 1990s. this improvement is a result of a substantial reduction in powder porosity.
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