Magneto-Mechanical Measurements for High Current Applications
Project staff members in the laboratory for
superconductor strain measurements.
This project specializes in measurements of the effect of mechanical strain on materials for highcurrent applications. Recent research has produced the first electromechanical data for the new class of high-temperature coated superconductors, one of the few new technologies expected to have an impact on the U.S. electrical power grid and the large electric power industry. The project’s research has also lead to the first four patents on contacts for high-temperature superconductors. Recent research also includes extending the highmagnetic- field limits of electromechanical measurements for development of nuclear-magneticresonance (NMR) spectrometers operating at 23.5 teslas and 1 gigahertz, 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 low-temperature superconductors in high magnetic fields, is now being extended to high compressive strains for use in finite-element design of magnet structures.
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 electromechanical problems. Stress and strain management is one of the key technology areas 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 scale-up phase of coatedconductor development. 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 magnets for the magnetic-resonance-imaging (MRI) industry, which provides invaluable medical data for health care, and contributes $2 billion 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 large effort being devoted to develop superconductors for grid reliability and enhanced power-transmission capability. We receive numerous requests, from both industry and government agencies, for accurate electromechanical data to help guide their efforts in research and development in this decisive growth period.
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 in areas where there is significant research potential.
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. 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 variabletemperature magnetoresistance measurements. Our electromechanical test capability for superconductors is one of only a few in the world.
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 rollingassisted, biaxially textured substrates (RABiTS) and conductors made by ion-beam-assisted deposition (IBAD). Our research is also sponsored by the DOE Office of Electric Transmission and Distribution. Here, we focus on high-temperature 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.
Characterization of Superconductors For Electric Power Grid Reliability — Improved superconductors are being developed by U.S. companies and demonstrated for power transmission. Superconductors’ greater current carrying capability is advantageous for upgrading real-estate-limited transmission lines in cities. Superconductors are also being developed for use in superconductor magnetic energy storage (SMES). Our work on characterizing superconducting properties at high stresses and high strains, and over variable temperatures is critical for the development of these superconductors. This work is also supported by DOE. Significant progress in second-generation superconductors was reported in 2005-2006. These thin, highly textured YBCO films are deposited with mainly non-vacuum techniques on fl exible metal substrates. They are now available in lengths of over 300 meters, carrying very high currents of over 2.5 to 3.0 mega-amperes per square centimeter at 77 kelvins. These superconductors have the potential to replace and improve parts of the ageing power grid in the United States. However, with the first coils fabricated from second-generation conductors, manufacturers learned that the layered architecture of the conductor may pose a problem: delamination of the ceramic layers under transverse tensile stress. This is important for rotating machinery, because of the centrifugal forces on the conductors, and more generally, because differential contraction in coil structures can place the conductors under severe transverse tensile stresses.
Scaling Laws for Magnet Design — In the area of low-temperature superconductors, we are generalizing 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. We are 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 SSL is also being developed for high-temperature superconductors, since we recently discovered that practical high-temperature superconductors also exhibit an intrinsic axial-strain effect.
- New Apparatus Developed to Measure Delamination in YBCO Coated Conductors — Over the past two years we have measured the delamination strength of second-generation YBCO superconductors. This required the design and construction of a new test apparatus. The test fixture head of the apparatus consists of two anvils made of Ni-5at.%W, which is the same material as the substrate of most coated conductors. The bottom anvil is soldered to the substrate side of the sample, while the top anvil is soldered to the silver cap layer on top of the ceramic YBCO layer. The choice of fabricating both anvils from the same material as the substrate of the superconductor ensures the absence of thermal shear stress between the sample and the anvils when they are cooled to 77 kelvins. The transverse tensile strength of the conductor is measured in two steps. First, the internal strength of the ceramic layers is measured, without edge effects. Second, the overall strength is measured, including edge effects. Edge damage may arise from, for instance, slitting of the conductor to smaller widths (a common procedure in the manufacturing process), and we anticipated that it may play a role in initiating delamination. Indeed, successful testing with the new apparatus showed that slit conductors have a relatively low transverse tensile strength. The overall strength of the conductor is reduced significantly by the slitting process. One company developed a structure to reinforce the slit conductor by soldering copper strips around the conductor (three-ply structure). The added solder joints at the edges of the conductor help restore the overall strength. We are currently collaborating with another company to find other novel solutions to avoid edge damage caused by slitting. In particular, we are pursuing solutions that produce a strong conductor without the need for extra reinforcement that lowers the conductor’s overall critical current density.
Test fixture head of a new apparatus to measure the
delamination strength of second-generation high-
temperature superconductors. The photo shows
a YBCO coated conductor tape mounted between
the two anvils of the apparatus.
Conductor slitting reduces the overall transverse tensile strength to
an average of 17.3 megapascals (numbers in the bars indicate the
average value), which is far below the average internal strength of
26.5 megapascals. Reinforcing the slit sample by soldering copper
strips (three-ply structure) raised the average transverse tensile
strength to 24.8 megapascals. The solid parts of the bars give an
indication of the spread in strength among samples.
- Discovery of Large Universal Effect of Axial Strain on the Critical Current of High- Temperature Superconductors — Although remarkable technical advances have been achieved during the past several years in the development of high-temperature superconductors (HTS) for use in large-scale applications, these have occurred without a clear understanding of the underlying mechanism of superconductivity in these materials. One of the main areas not fully understood is the change of the superconducting current density with applied strain. We have discovered a very large universal, reversible change in the Jc of YBCO coated conductors, which is symmetric under both high compressive and high tensile strain. We anticipate these findings will initiate detailed research on the effect of strain on the underlying mechanisms of superconductivity in practical HTS. For instance, strain fields at grain boundaries in HTS may be the main limiting mechanism for supercurrents.
The superconducting current density in its self-field decreases reversibly by more than 40 percent under compressive strain in a wide range of conductors fabricated by vastly different processes. The effect is nearly the same for all high-current conductors measured, including YBCO deposited by metalorganic chemical vapor deposition (MOCVD) (which results in a columnar YBCO grain structure), metal-organic deposition (MOD) (which results in a laminar YBCO grain structure), and a hybrid conductor fabricated with a double YBCO layer (consisting of YBCO that is doped with a large amount of Dy particles that provide extra flux pinning centers). The effect is symmetric under both compressive and tensile strains.
A major reversible strain effect is a consistent and intrinsic phenomenon of high-current YBCO biaxially aligned coated conductors, encompassing a number of different types fabricated with widely varying YBCO deposition techniques. The universality and symmetry provide evidence that the mechanism behind the reversible strain effect is an intrinsic feature of the YBCO grain structure (columnar for MOCVD-IBAD and laminar for MOD-RABiTS). The results open the door for detailed studies into the mechanism of superconductivity in HTS.
Normalized superconducting current density plotted as
function of intrinsic strain ε0 for three different types of
samples: MOCVD-IBAD, MOD-RABiTS, and (hybrid)
MOD-RABiTS, for both bare samples and those with
copper added for stability. The solid lines describe a
power-law function. The values of the strain-sensitivity
parameter a are included in the figure. Compressive
strain is indicated by negative values of ε0; tensile strain
Illustration of method for applying large axial strains to
superconducting sample that is soldered on top of a
Cu-2%Be bending beam. Axial tension is applied by
bending the beam in the direction shown in (a), whereas
axial compression is applied by bending the beam in the
opposite direction (b). Strain is uniform over the
thickness of the superconducting films to within about
1 part in 2500.
- Slit YBCO Coated Conductors Prove Mechanically Robust Under Fatigue Cycling — In order to evaluate the effect of slitting, we used fatigue cycling under transverse compressive stress, since earlier experiments had shown this to induce crack propagation. Stress was applied to the tape sample by means of two stainless-steel anvils. Uniformity of stress over the pressed area of the conductor was achieved by beveling the edges of the top anvil, and attaching it to a biaxially gimbaled pressure foot so that this anvil conforms precisely to the bottom anvil and sample surfaces. Stress was cycled between positive and negative 150 megapascals (about twice the stress level of most applications) at a frequency of 1 hertz for up to 20,000 fatigue cycles. Jc was measured at 76 kelvins in self-field. These tests simulate conditions in applications such as electric generators and industrial magnets, and evaluate whether fatigue exacerbates cracks by propagating them into the middle of the conductor. The measurements also help discriminate between different slitting techniques. The samples investigated exhibited no significant degradation under fatigue testing, thus demonstrating that, unlike transverse-tension testing, slitting does not affect the sample performance under transverse-compressive-stress cycling.
Fatigue test fixture showing the top anvil, biaxially gimbaled
to uniformly apply pressure to the conductor.
Effect of fatigue cycling under transverse compressive stress
in a YBCO RABiTS sample laminated with Cu foils on both
sides (three-ply geometry). Jc showed no significant
degradation under fatigue testing up to 20,000 cycles at
Jack Ekin’s new textbook, Experimental Techniques
Cover of new book on cryogenic
for Low-Temperature Measurements, was published by Oxford University Press in October 2006. The text covers the design of cryogenic measurement probes, and the appendix provides cryogenic materials data for carrying out that design. 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. Topics include heat-transfer techniques for designing a cryogenic apparatus, selecting materials appropriate for such apparatus, how to make high-quality electrical contacts to a superconductor, and techniques for reliable critical-current measurements.
The appendix is a data handbook of materials properties and cryostat design consisting of 70 tables compiled from over 50 years of literature. The tables were compiled for experts in the field of cryogenic measurements and include electrical, thermal, magnetic, and mechanical properties of materials for cryostat construction; properties of cryogenic liquids; and temperature measurement tables and thermometer properties.
Superconductivity for Electric Systems Program, Office of Electricity Delivery and Energy Reliability, U.S. Department of Energy, recognition as top ranked project, 2005 and 2006 (Jack Ekin, Najib Cheggour, and Danko van der Laan).