Superconductor Electromagnetic Measurements
Goals
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Preparing to measure electromechanical properties of superconductor tape.
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This project specializes in measurements of the effect of mechanical strain on superconductor properties, such as critical-current density, 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, which previously led to the first four patents on contacts 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 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 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, 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.
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 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.
Electromechanical Measurements of High-Temperature Superconductors
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Measurement probe for Jc as a function of axial tensile strain.
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We have developed an array of specialized measurement systems to test the effects of mechanical stresses 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. In particular, since most technologically important superconductors are brittle, it is crucial to know the value of strain at which fractures occur in the superconductor. This value is referred to as the irreversible strain, since the damage caused by the formation of cracks is permanent. The effect of cracks is extrinsic. In contrast, below the irreversible strain, there exist an elastic strain regime where the effect of strain is intrinsic to the superconductor. In this elastic regime, the variation in the critical-current density (Jc) with strain, if any, is reversible and is primarily associated with changes in the superconductor's fundamental properties, such as the critical temperature (Tc) and the upper critical field (Hc2), as well as changes in the superconductor's microstructure due to the application of strain.
Among the measurement systems we have are apparatus for measuring the effects of axial tensile stress, the effects of transverse compressive stress, and the stress-strain characteristics. We have a unique system for determining the electromechanical properties of reinforced superconducting composite coils. Our electromechanical test capability for superconductors is one of the few of its kind in the world, and the only one providing specialized measurements for U.S. superconductor manufacturers.
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 (BSCCO) and Y Ba-Cu O (YBCO) — 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.
Deliverables:
• Measure the stress-strain characteristics of nominal-size high-quality substrates for RABiTS and IBAD conductors at room temperature, 76 kelvins, and 4 kelvins. (FY 2003)
• Measure the dependence of Jc on transverse stress in Ni-alloy YBCO coated RABiTS and IBAD conductors having different substrate compositions. Study the microstructure after transverse stress testing to evaluate failure modes. (FY 2003)
• Measure the dependence of Jc on axial tensile strain in YBCO coated pure-Ni and Ni-alloy RABiTS samples and IBAD conductors, having various YBCO film thickness and different substrate materials. Determine the microstructural crack patterns of the samples after axial tensile strain testing to evaluate failure modes. (FY 2003)
• Measure the dependence of Jc on axial tensile strain in new high-current Bi-2212 wires, multifilamentary Bi-2212 wires, and three-ply Bi-2223 conductors. (FY 2003)
• Measure the dependence of Jc on transverse stress and fatigue cycles in new three-ply Bi-2223 conductors. (FY 2003)
• Measure the dependence of Jc on axial tensile strain and on transverse stress in recently developed MgB2 tapes and wires. (FY 2004)
Electromechanical Measurements of Low-Temperature Superconductors
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.
Deliverables:
• Complete the data set of transverse stress effects in two series of Nb3Sn tape conductors. Measure the effect of axial strain on Jc. Test for a correlation between the magnitude of the intrinsic (reversible) strain effect and phonon anharmonicity in superconductor crystal structures. (FY 2003)
• Measure the effect of axial strain on Jc of Nb3Sn wires developed for a new generation of accelerator magnets. (FY 2003)
• Measure the Young's modulus in Nb3Sn tape conductors and relate stress and strain for developing the multidimensional SSL. Complete the data set of axial strain measurements in two series of Nb3Sn tape conductors. Combine the two orthogonal sets of data into a unified model. (FY 2004)
• Conclude the study of the correlation of uniaxial strain effects with phonon anharmonicity in the A-15 superconductors. Determine the hydrostatic (volume-change) and deviatoric (shape-change) coefficients to generalize the SSL from one to three dimensions. Apply the model to finite-element strain designs of large superconducting magnet systems. (FY 2004)
Textbook on Cryogenic Measurement Apparatus and Methods
We are in the process of finishing a textbook on experimental techniques for cryogenic measurements. This book covers the design of cryogenic measurement probes, and provides cryogenic materials data in the appendices for their construction. Topics include thermal techniques for designing a cryogenic apparatus, selecting materials appropriate for such apparatus, how to make high-quality electrical contacts to a superconductor, and how to make reliable critical-current measurements. The textbook is written for beginning graduate students, industry measurement engineers, and materials scientists interested in learning how to design successful low-temperature measurement systems. The appendices are written for experts in the field of cryogenic measurements and include electrical, thermal, magnetic, and mechanical properties of technical materials for cryostat construction; properties of cryogenic liquids; and temperature measurement tables and thermometer properties. These appendices aim to collect in one place many of the data essential for designing a new measurement apparatus.
Deliverables:
• Edit chapters on superconductor critical-current measurement techniques and analysis. Complete appendix material. Send drafts to international reviewers. (FY 2003)
• Check and return publisher's proofs of the book. (FY 2004)
Accomplishments
Electromechanical Measurements of High-Temperature Superconductors
• YBCO on Alloy Substrate Exhibits Tolerance to Compressive Stress — One of the major challenges facing the development of an economical, practical high-temperature superconductor has been the extremely weak mechanical behavior of YBCO coated onto RABiTS. Of all the manufacturing processes, the RABiTS process is the one that is most easily scalable to fabricate industrial quantities of this promising superconductor.
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Effect of transverse stress on Jc in YBCO films on pure Ni and Ni-W alloy RABiTS. The results illustrate the benefits of work-hardening pure Ni and provide a comparison between YBCO coatings on pure Ni and Ni-W alloy substrate materials.
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Recently, a U.S. company has produced a new RABiTS coated conductor with substrates made of Ni plus 5 atomic percent W. We completed a series of experiments using our specialized equipment for both transverse stress and transport current to measure the electromechanical performance of this new coated superconductor.
The results are striking. The data show that, in repeated testing, Jc is degraded by only 1 to 5 percent at the benchmark stress level of 100 megapascals. This result opens the path for commercialization of RABiTS coated conductors. Projections are that this conductor could be manufactured at about $10 per kiloampere-meter, a cost that would be competitive with copper in transformers and in other electric-utility applications, and far less expensive than Cu for increasing the capacity of underground transmission lines in urban areas.
Until now, the RABiTS process had worked only with soft, pure Ni substrates. Our earlier measurements showed that its Jc degraded by as much as 28 percent at the benchmark 100 megapascal stress level, which made it unacceptable for use as a practical conductor. Our results provide evidence that the mechanical properties of the substrate material play a dominant role in determining the response of these samples to transverse compressive stress. Another possible source for the degradation of Jc could be delamination of the ceramic layers due to application of stress. More comprehensive data are still required to draw definitive conclusions.
• Mechanical Properties of Candidate Substrate Materials — Our electromechanical testing showed a correlation between the mechanical properties of the RABiTS substrate material and the tolerance of a YBCO-coated conductor to transverse stress. In order to guide manufacturers in their selection of a suitable substrate material and in designing processing equipment for the manufacturing of the coated conductors, we characterized the mechanical properties of several substrate materials that are potential candidates both for the RABiTS and IBAD processing technologies. The tensile yield strength, Young's modulus, and proportional limit of elasticity of these materials at 295 kelvins, 76 kelvins, and 4 kelvins were tabulated and compared. This database has been generated to guide the development of the YBCO coated conductors. We will expend it as new substrate materials being developed become available.
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Comparison of stress-strain curves at 76 kelvins of pure Ni, Ni-Cr, Ni-W-Fe, and Ni-W alloy substrate materials for RABiTS technology.
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Comparison of stress-strain curves at room temperature, liquid-nitrogen temperature, and liquid-helium temperature of annealed Inconel-625 substrate material for IBAD technology.
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Inserting a thick, nonmagnetic spacing layer between the YBCO film and the magnetic cap layer can significantly mitigate the magnetic substrate effect.
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MgB2 with a pure Ni sheath exhibits a small reversible increase in Jc as a function of strain before reaching the irreversible strain limit.
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• Limiting the Magnetic Substrate Effect in YBCO Coated Conductors — We have shown that the degradation in Jc of YBCO due to the magnetic substrate effect can be significantly mitigated if a thick spacing layer is inserted between the YBCO film and the magnetic cap layer. High-temperature superconducting tapes based upon coatings of YBCO on biaxially textured, buffered, magnetic Ni-W-Fe substrates showed a degradation of 12 percent in Jc when the YBCO layer is sandwiched between two Ni-W-Fe substrates. We found that this degradation of Jc can be reduced dramatically to less than 1 percent if a 300 micrometer-thick Kapton tape is placed between the YBCO film and magnetic cap layer. Such a spacing layer could naturally be incorporated into the manufacture of YBCO coated conductors as an insulating coating on the conductors. A systematic study as a function of the thickness of the Kapton layer showed that the degradation of Jc is reduced substantially to 3 percent by a separation of just 50 micrometers.
The magnetic substrate effect resulting from sandwiching YBCO between two magnetic layers may occur in some applications where the coated conductor needs to be wound or cabled. The spacing tape, which limits the magnetic interaction of the top and bottom Ni-W-Fe layers, represents an engineering solution for limiting the magnetic substrate effect in low magnetic field applications such as underground power-transmission cables. The separation layer could be made of a high conductivity material, such as Cu, to enhance the electrical and thermal stability of the cable.
• Mechanical Tests on Magnesium Diboride — We made a preliminary investigation of the electromechanical properties of the newly discovered MgB2 superconductor as a function of magnetic field. A Ni-sheathed MgB2 tape was tested as a function of axial strain in high magnetic fields. MgB2 was found to exhibit a small reversible increase of Jc as a function of strain, driven by an intrinsic strain effect on the effective upper critical field of this material. The data indicate that the strain sensitivity of MgB2 is significantly smaller than that of Nb3Sn superconductor. As the applied strain is increased beyond the irreversible strain limit of the conductor, Jc shows a dramatic drop as cracks form in the superconductor due to the application of strain.
• Loss in Magnesium Diboride Multifilamentary Wires Suppressed at Low Fields by Magnetic Shielding — We measured the magnetization-field curves of multifilamentary MgB2 superconductors in an Fe matrix. The magnetic measurements demonstrated that the filaments were shielded from external fields by the Fe material. The hysteresis loops, measured at different temperatures, suppressed magnetization at low fields up to about 0.3 tesla. These results suggest that the use of a magnetic material in the architecture of MgB2 wires could be beneficial for low-field applications such as transformers.
Electromechanical Measurements of Low-Temperature Superconductors
• Electromechanical properties of Nb3Sn Tapes — We carried out measurements of the effect of strain in Nb3Sn tapes made by chemical vapor deposition and found results that seem to contradict the deviatoric-strain model. At a field of 11 teslas and a strain of 0.26 percent, the effect of axial strain on Jc amounts to about +15 percent, whereas that of transverse compressive stress is only about -0.5 percent at 170 megapascals. This result does not support the deviatoric-strain model, which would predict an increase of Jc with transverse stress, just the opposite of the sign of the observed effect and far different from the measured magnitude. One possibility to explain the results obtained is to consider a large hydrostatic strain effect, which was previously neglected by the deviatoric-strain model.
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Magnetization curves of a Fe-sheated multifilamentary MgB2 square wire. The filaments are shielded from external fields up to 0.3 tesla by the Fe matrix.
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