Magneto-Mechanical Measurements for High Current Applications

Goals

Mike Abrecht transferring liquid He to an experiment dewar for the measurement of magnetoresistance.

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 electric-power industry. The Strain Scaling Law, previously developed by the project for predicting the axial-strain response of low-temperature superconductors in high magnetic fields, is now being generalized to three-dimensional stresses, for use in finite-element design of magnet structures, and to high-temperature superconductors. Recent research includes extending the high-magnetic-field limits of electromechanical measurements for development of nuclear-magnetic-resonance (NMR) spectrometers operating at 23.5 teslas and 1 gigahertz, and the next generation of accelerators for high-energy physics. The project has diversified its research to include magnetoresistance studies on a new class of carbon nanostructures using our highfield superconducting magnet facility and a newly developed, variable-angle, variable-temperature measurement capability.

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 (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 develop superconductors for power conditioning and enhanced power-transmission capability. We receive 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 hightemperature superconductors has brought with it new measurement problems in handling these brittle conductors. We have the expertise and equipment to address these problems. Stress and strain management is one of the key parameters needed to move the second-generation high-temperature coated conductors to the market place. The project utilizes the expertise and unique electromechanical measurement facilities at NIST to provide performance feedback and engineering data to companies and national laboratories fabricating these conductors in order to guide their decisions at this critical phase of coated-conductor development.

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 its own measurements for routine testing. We also provide consultation 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 Superconductors — 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, we need to know the value of strain at which fractures occur in the superconductor. This value is referred to as the irreversible strain limit, since the damage caused by the formation of cracks is permanent. The effect of cracks is extrinsic. In contrast, below the irreversible strain, there exists 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 (J_c) with strain, if any, is reversible and is primarily associated with changes in the superconductor's fundamental properties, such as the critical temperature (T_c) and the upper critical field (H_c2), as well as changes in the superconductor's microstructure due to the application of strain.

Measurement Facilities — Extensive, advanced measurement facilities are available, including high-field (18.5 teslas) and split-pair magnets, servohydraulic mechanical testing systems, and state-of-the-art measurement probes. These probes are used for research on the effects of axial tensile strain and transverse compressive strain on critical current; measurement of cryogenic stress-strain characteristics; composite magnetic coil testing; and variable-temperature magnetoresistance measurements. 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.

Collaboration with Other Government Agencies — 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, Nb₃Al, and MgB₂), 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. Our research is also sponsored by the DOE Office of Electric Transmission and Distribution. Here, we focus on hightemperature superconductors for power applications, including power-conditioning systems, motors and generators, transformers, 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.

Scaling Laws for Magnet Design — 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. The Strain Scaling Law is now also being developed for high-temperature superconductors since we recently discovered that practical high-temperature superconductors exhibit an intrinsic axial-strain effect.

Pre-compressive strain ε_max versus Nb fraction for several niobium-tin wires with high niobium density. Data were obtained using a new measurement method developed by EEEL researchers for marginally stable superconductor wires.

Accomplishments

New Measurement Method for Marginally Stable Superconductor Wires — The next generation of particle accelerators for high-energy physics, and magnet systems for nuclear magnetic resonance (NMR) spectroscopy, will require the development of a new type of superconducting niobium-tin wire able to carry extremely high currents at high magnetic fields. One way to achieve high currents is to push the density of superconductor filaments in composite wires to new limits. Oxford Superconductor Technology (Carteret, NJ) has successfully demonstrated the feasibility of this concept. However, this could significantly reduce the beneficial "pre-compressive strain" in these conductors upon cooling, an important parameter for magnet design. Our superconductor electromechanical testing system is the only one in the U.S. that utilizes stress-free cooling, which is essential for a direct measurement of pre-compressive strain. Unfortunately, the new niobium-tin wires, owing to their relatively small amount of copper stabilizer, are only marginally stable, which makes electrical characterization extremely challenging. Hence, a new measurement technique was required that did not compromise the stress-free cooling advantage.

The technique consists of measuring critical-current density (the maximum lossless current density that a superconductor can carry) versus axial strain for a number of copper-plated specimens of the same wire with different amounts of copper. We then deduced the strain properties of the virgin (noncopper-plated) wire by an extrapolation technique. Copper plating made the niobium-tin wires electrically stable enough to characterize, but the extra copper also influenced the value of the pre-compressive strain (ε_max); hence the need for extrapolation. We confirmed that ε_max indeed decreased linearly with increasing niobium fraction. However, we found that other parameters such as the matrix material and wire diameter also influence ε_max.

The pre-compressive strain for high-niobium-fraction wires can be reduced to about 0.1 percent, a very small strain window for magnet design. Fortunately, we also found that the use of copper alloys, instead of pure copper — along with small wire diameters — substantially mitigates the problem and provides reasonable strain operating margins in these high performance conductors. The data were used by Oxford Superconductor Technology to make immediate decisions regarding the conductor design for a new NMR system.

Copper Stabilizer Improves Coated Superconductors' Strain Tolerance — High-temperature superconductor (HTS) wires are now being fabricated in kilometer lengths, providing the basis for a new generation of electric power devices, including high power-density motors and generators, transmission lines, and power conditioners. The development of HTS technology is expected to play a crucial role in maintaining the reliability of the power grid and upgrading power delivery to core urban areas. The most promising superconductor candidate for replacing ageing utility equipment is the highly textured Y-Ba-Cu-O (YBCO) compound deposited on buffered flexible metallic substrates. These "coated conductors" have a much higher current-carrying capacity compared to the Bi-Sr-Ca-Cu-O (BSCCO) tapes now commercially available. Whereas BSCCO tapes experience permanent damage when subjected to axial strains less than 0.2 percent, we demonstrated last year that the formation of cracks in the new YBCO system does not commence until subjected to strains higher than 0.38 percent, almost a two-fold increase in strain tolerance. This resilience of YBCO to strain is providing a strong motivation to produce commercial lengths of this "second generation" conductor, especially for the design of electric generators for which strain tolerance requirements have been raised to 0.4 percent.

This year, we found that adding a Cu layer to the YBCO coated-conductor architecture extends the irreversible strain limit (ε_irr) of this composite even further, from 0.38 percent to more than 0.5 percent. This markedly widens the strain window for coated-conductor applications and takes it beyond even the most demanding benchmark for large-scale superconducting generators. These measurements were undertaken in close collaboration with conductor manufacturers American Superconductor (Westborough, MA) and SuperPower (Schenectady, NY), who are incorporating the stabilizer layers either by Cu-lamination or Cu-plating. The original motivation for adding the Cu layers was to improve the electric and thermal stability of the conductor; the strain-tolerance dividend was unexpected. We can relate this remarkable result to the mismatch of thermal contraction between Cu and the other components of the composite. During sample cooling from processing temperatures to the cryogenic operating temperatures, the Cu layer exerts an additional pre-compressive strain on the YBCO film, and hence extends the irreversible strain ε_irr where permanent damage occurs. The Cu may also be acting as a crack arrester, which further improves the strain tolerance.

Normalized critical current density as a function of mechanical tensile strain for unlaminated and Culaminated YBCO coated conductor. The Cu stabilization layer extends the irreversible strain limit ε_irr of the composite from 0.38 percent to more than 0.5 percent.

Photograph of a new magnetoresistance probe designed to investigate carbon nanostructures. At the right end of the probe, the photo shows the steppermotor-controlled worm-gear system and sample stage, which allow precise angle-dependent, high-field measurements.

New Magnetoresistance Apparatus to Probe Carbon Nanostructures — Electronic properties of materials change markedly as their dimensions approach those of a few atomic layers. Carbon nanostructures (including graphite sheets, singlewalled carbon nanotubes, and multi-walled carbon nanotubes) are prime examples of such potentially useful materials, although some of their very fundamental properties remain controversial. Characterization of these structures at high magnetic fields is one of the principal methods for determining the existence of ballistic conduction, for example, which could be the foundation for a new generation of nanoelectronic devices.

We have designed and recently commissioned an apparatus to measure magnetoresistance of these highly directional structures in fields up to 18.5 teslas. (For comparison, the Earth's magnetic field is only about 0.05 millitesla.) The apparatus automatically acquires data as a function of magneticfield magnitude, angle, and temperature. It was designed to also be compatible with the very-highfield magnet facilities at the National High Magnetic Field Laboratory at Florida State University, permitting the extension of EEEL's measurements to fields up to 30 teslas. Magnetic field mapping has commenced for nanotubes fabricated at NIST and Rice University as well as for graphitic sheet structures manufactured by a nanotechnology research team at Georgia Institute of Technology. Magnetic-field angle can be varied with a resolution of better than 0.1 degree over a range of 130 degrees, and sample temperature can be varied over an extended range of 4.1 to 120 kelvins, with a stability of better than 3 millikelvins at 4.2 kelvins.
Textbook on Cryogenic Measurement Apparatus and Methods — A new textbook has been written on experimental techniques for cryogenic measurements to be published by Oxford University Press. It covers the design of cryogenic measurement probes and provides cryogenic materials data 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 criticalcurrent 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 new cryogenic measurement apparatus.