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Project Leader: 325 Broadway Boulder, Colorado 80305 Phone 303-497-5477 Fax 303-497-5316 magtech@boulder.nist.gov July 11, 2002 |
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Standards for Superconductor CharacterizationGoals
This project develops standard measurement techniques for critical current and provides quality-assurance and reference data for commercial high-temperature and low-temperature superconductors. Applications supported include magnetic-resonance imaging, research magnets, fault-current limiters, magnetic energy storage, magnets for fusion confinement, motors, generators, transformers, transmission lines, magnets for crystal growth, and superconducting bearings. Project members assist in the creation and management of international standards for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on critical-current measurements of marginally stable superconductors, on temperature-variable critical-current measurements, and on measuring the irreversible effects of changes in magnetic field and temperature on critical current. Customer NeedsWe serve the U.S. superconductor industry, which consists of many small companies with limited resources for committing to the development of new metrology and standards. We participate in projects sponsored by other government agencies that involve U.S. industry, universities, and national laboratories. The potential impact of superconductivity on electric-power systems makes this technology very important. We focus on (1) developing new metrology needed for evolving, large-scale superconductors, (2) participating in interlaboratory comparisons needed to verify techniques and systems used by U.S. industry, and (3) developing international standards for superconductivity needed for fair and open competition and improved communication. Technical StrategyOne of the most important performance parameters for large-scale superconductor applications is the critical current. Critical current is difficult to measure correctly and accurately; thus, these measurements are often subject to scrutiny and debate. Another activity is the measurement of the magnetic hysteresis loss in superconductors. With each significant advance in superconductor technology, new procedures, interlaboratory comparisons, and standards are needed. International standards for superconductivity are created through the International Electrotechnical Commission (IEC), Technical Committee 90 (TC 90). The next generation of Nb3Sn and Nb3Al wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The latest Nb-Ti conductors are also pushing these limits. The resulting higher current required for critical-current measurements turns many minor problems into significant engineering challenges. For example, specimen heating, from many sources during the measurement, can cause a wire to appear to be thermally unstable. DeliverablesStandardsDuring FY 2002, we will continue to solicit and incorporate comments from U.S. participants in IEC/TC 90 standards development. We will review, edit, and comment on draft standards in the 11 Working Groups in IEC/TC 90. During FY 2002, Loren Goodrich will continue to serve as Chairman of IEC/TC 90 and manage and coordinate the development of standards for superconductivity. Characterization of Superconductors
During FY 2002-2004, we will further develop our variable-temperature critical-current measurement capability and provide a critical-current database to the U.S. Department of Energy, Office of Fusion Energy Sciences program. The primary focus will be on Nb3Sn wires. During FY 2002-2004, we will develop routine high-current testing of marginally stable Nb3Sn conductors for the U.S. Department of Energy, Office of High Energy Physics program. The commonly used techniques have been shown to be inadequate for many of the latest conductors. More precise measurements are needed to evaluate conductor performance and provide reliable feedback to the development process. The development of testing will involve input from the other U.S. testing laboratories and will likely include interlaboratory comparisons. During 2002, we will continue to provide measurements of critical current, residual resistivity ratio, and hysteresis loss for U.S. companies and national laboratories. AccomplishmentsstandardsIEC Technical Committee, Led by NIST, Publishes Five New Superconductivity Standards - Five new international standards on superconductivity were recently published by the IEC/TC 90. The documents are:
We have worked extensively on these documents and helped resolve many difficulties encountered during the development process. Loren Goodrich serves as Chairman of TC 90 and manages the international work. Thirteen countries participate in TC 90. The vocabulary was created under TC 90, but all vocabulary publications are listed under TC 1. This vocabulary contains 301 terms and their definitions. The standard on the critical current of oxide superconductors is the first IEC standard on the newer high-temperature superconductors. This brings the number of IEC TC 90 published standards to seven. Currently, seven more documents are at various stages of development within TC 90.
Document Stages: Working Draft (WD), Committee Draft (CD), Committee Draft for Voting (CDV), Final Draft International Standard (FDIS), International Standard (IS). Old Standards Withdrawn - The two American Society for Testing and Materials (ASTM) standards for superconductors were withdrawn, and Subcommittee B01.08 on superconductors was dissolved by Loren Goodrich, the former Chairman of B01.08. These two ASTM standards (B 713-82 and B 714-82) were used to draft parts of three IEC standards under TC 90. These two ASTM standards are now superseded by three IEC standards (IEC 61788-1, IEC 61788-2, and IEC 60050-815). The members of ASTM B01.08 had agreed that when these new IEC standards were published, then the above actions should be taken. These ASTM standards have served the superconductor industry well during the last 18 years, and they live on in the new IEC standards. Characterization of Superconductors
Verified Performance of Conductor for Large Hadron Collider - A national laboratory involved in the U.S. program for the international Large Hadron Collider program asked NIST to conduct critical-current verification on several Cu/Nb-Ti strands. Loren Goodrich was a co-author on a paper that detailed the characterization of 2000 kilometers (about 18 tonnes) of superconducting strand for this program. The paper included an interlaboratory comparison among three laboratories on measurements of critical current, n-value, and residual resistivity ratio. This was the highest current comparison ever on a single strand (as opposed to a cable), with an average critical current of about 2000 amperes at 5 teslas and 4.2 kelvins. In the conductor design, the amount of Cu stabilizer in the strand was kept to a minimum and additional high-purity Al and alloy Al were added to the cable to improve the stability and mechanical strength of the final product. However, the individual strands need to be tested before they are cabled in order to avoid introducing inferior strand into the cable, which would waste even more strand. This resulted in the need to develop new measurement procedures on strands that are designed to be marginally stable. This type of work gives us the experience needed to develop future measurement standards and keeps us up to date on the latest conductors and measurements challenges. There is no good substitute for the experience and insight gained by performing routine measurements on the latest conductors in the advancing technology of superconductors. The unexpected scientific and practical discoveries continue to be reviewed by this work. The next accomplishment below, on a new source of misinterpretation in superconductor measurements, is one example.
New Source of Misinterpretation in Superconductor Measurements - As part of our program to develop standard measurement techniques for superconductors, we have identified and studied a new source of misinterpretation in critical-current measurements of superconductors. The critical current is the maximum current a conductor can carry before a quench, when it reverts to the normal, resistive state. Researchers can tell when the critical current is reached by measuring the resistive voltage on pairs of voltage taps soldered to the superconductor wires. However, we discovered that anomalous inductive voltages can be induced in the loop formed by the voltage taps. The inductive voltages vary systematically with current, current sweep direction (increasing or decreasing), applied magnetic field, and whether the specimen was driven into the normal state in an immediately previous measurement. Furthermore, the decay time of the inductive voltage signal, after ending the current ramp, is longer near the onset of the resistive transition. These decay times are even longer during a superconductor's first current sweep after a quench. Many superconductor applications now require higher current densities, larger wire diameters, and less copper stabilizer, all of which results in marginally stable conductors with high critical currents above 1000 amperes. Variable induced voltages and long decay times become a concern when currents or current-ramp rates are high, or when voltage curves need to be extrapolated for measurements on marginally stable conductors. The resulting data can be mistakenly attributed to (1) a bad conductor, (2) a damaged specimen, (3) an electrical ground loop, (4) a low critical current, or (5) specimen motion in the background magnetic field. To avoid anomalous induced voltages, we recommend cycling the current before acquiring data after a quench, avoiding data acquisition while the current is being ramped, and allowing 3 seconds of settling time after current levels are changed before measurements are made near the critical current.
Critical-Current of Nb-Ti - We continue to provide critical current measurements of Cu/Nb-Ti samples for U.S. wire manufacturers. The current or magnetic field requirements are occasionally beyond their measurement capabilities. These measurements for a modest charge allow a small company to avoid the expense of maintaining specialized equipment and personnel. Significant Paper on High Temperature Superconductors Published - We published a comprehensive paper on variable-temperature critical-current (Ic) measurement of oxide superconductors in the July-August 2001 issue of the NIST Journal of Research. This paper presents results on magnetic hysteresis in transport Ic measurements of Ag-matrix (Bi,Pb)2Sr2Ca2 Cu3O10-x (Bi-2223) and AgMg-matrix Bi2Sr2Ca Cu2O8+x (Bi-2212) tapes. Magnetic hysteresis causes Ic to have a different value depending on the history of the magnetic field, the magnetic-field angle, and the temperature. This effect is completely reversible. The value of Ic at the same magnetic field, magnetic-field angle, and temperature can be different by as much as 74 percent, depending on what sequence of parameters the specimen experienced. Which value is correct is addressed in the context that the proper sequence of measurement conditions reflects the application conditions. The hysteresis in angle-sweep and temperature-sweep data is related to the hysteresis observed when the field is swept up and down at constant angle and temperature. The necessity of heating a specimen to near its transition temperature to reset it to an initial state between measurements at different angles and temperatures is discussed. A copy of this paper can be obtained at www.nist.gov/jres/ (NIST J. Res., vol. 106, no. 4, pp. 657-690). Resistivity Measurement Problems Identified - We have been collaborating with a U.S. company and two U.S. universities on measurements of residual resistivity ratio (RRR) on high-purity Nb specimens. The RRR is typically defined as the ratio of the electrical resistivities measured at 273 kelvins (ice point) and 4.2 kelvins (in liquid helium). However, Nb is superconducting at 4.2 kelvins, so the low-temperature resistivity is defined as the normal-state resistivity extrapolated to 4.2 kelvins and zero magnetic field. The value of RRR is an indication of the purity and the low-temperature thermal conductivity of the Nb, and is often used as a material specification in commerce. One future purchase is planned for 600 tonnes of high-purity Nb. There are two ways to obtain this extrapolated normal-state resistivity: measure the normal-state resistivity as a function of field at 4.2 kelvins and extrapolate to zero field, or measure the normal-state resistivity as a function of temperature in zero field and extrapolate to 4.2 kelvins. Both approaches have their difficulties, but it is generally thought that the extrapolation of the normal-state resistivity as a function of field at 4.2 kelvins has less uncertainty. Both approaches require the precise measurement of resistance as small as 0.5 micro-ohms on a specimen that resists wetting by solder. We conducted an interlaboratory comparison of RRR measurements. The agreement and repeatability between results obtained at one university and NIST was within a few percent, which is quite acceptable. The difference between the average results obtained at another university and NIST was about 12 percent. That university's results also varied by more than 22 percent for five repeat measurements. We suggested seven changes to their procedure. Centennial Open House Demonstrations - We prepared new exhibits that demonstrate some of the properties of superconductors for the NIST-Boulder Centennial Open House on May 11-12, 2001. The demonstrations showed zero resistance, magnetic-flux expulsion (Meissner effect), a magnetic bearing, and levitated and suspended motion using high-temperature superconductors and Nd-Fe-B permanent magnets. An estimated 3000 visitors toured the NIST-Boulder site and we performed the demonstrations almost continuously for a total of about 12 hours. The suspended model train was received by the crowds with the most enthusiasm.
Standards CommitteesLoren Goodrich is the Chairman of IEC/TC 90, the U.S. Technical Advisor to TC 90, the Convener of Working Group 2 (WG2) in TC 90, the primary U.S. Expert to WG4, WG5, WG6 and WG11, and the secondary U.S. Expert to WG1, WG3, and WG7. Ted Stauffer is Administrator of the U.S. Technical Advisory Group to TC 90. |
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