Standards for Superconductor Characterization
Goals
Discussing the statistical analysis of residual resistivity ratio of highpurity Nb specimens.

This project develops standard measurement techniques for critical current, residual resistivity ratio, and hysteresis loss, and provides quality assurance and reference data for commercial hightemperature and lowtemperature superconductors. Applications supported include magneticresonance imaging, research magnets, faultcurrent limiters, magnetic energy storage, magnets for fusion confinement, motors, generators, transformers, transmission lines, magnets for crystal growth, highqualityfactor resonant cavities for particle accelerators, and superconducting bearings. Project members assist in the creation and management of international standards through the International Electrotechincal Commission for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on measurements of variabletemperature critical current, residual resistivity ratio, magnetic hysteresis loss, critical current of marginally stable superconductors, and the irreversible effects of changes in magnetic field and temperature on critical current.
Customer Needs
We serve the U.S. superconductor industry, which consists of many small companies, in the development of new metrology and standards. We participate in projects sponsored by other government agencies that involve industry, universities, and national laboratories.
The potential impact of superconductivity on electricpower systems makes this technology especially important. We focus on (1) developing new metrology needed for evolving, largescale 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 Strategy
International Standards
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).
Deliverables:
• Serve as Chairman of IEC TC 90 and as U.S. Technical Advisor to TC 90. (Ongoing)
• Develop Committee Drafts and maintain International Standards from Working Groups 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. (Ongoing)
Critical Current Measurements
Illustration of a superconductor's voltagecurrent characteristic with two common criteria applied.

The voltagecurrent characteristic of a Nb–Ti wire in a magnetic field of 2 teslas. The critical current I_{c} is determined using one of the criteria. The corresponding voltage V_{c} = E_{c} L = 5 microvolts when E_{c} =10 microvolts per meter and the voltagetap separation L = 0.5 meters. The inset is a plot of these same data on a logarithmic scale.

One of the most important performance parameters for largescale 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.
The next generation of Nb_{3}Sn and Nb_{3}Al wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The latest NbTi conductors are also pushing these limits. The resulting higher current required for criticalcurrent measurements turns many minor problems into significant engineering challenges. For example, heating of the specimen, from many sources during the measurement, can cause a wire to appear to be thermally unstable.
The figure below is an illustration of the voltagecurrent characteristic and two criteria for critical current. Typical criteria are electricfield strength of 10 microvolts per meter and resistivity of 10^{14} ohmmeters. An actual voltagecurrent characteristic for a Nb Ti wire is also shown.
This actual curve is much steeper than in the illustration. Typically, the curve can be approximated by the equation V = V_{0} (I/I_{0})^{n}, where V_{0}, I_{0} and the nvalue are constants. The nvalue is the slope of the voltagecurrent curve when plotted on a logarithmic scale (see inset in the plot).
Deliverables:
• Determine the current limits of a variabletemperature cryostat made for coil samples. Make variabletemperature criticalcurrent measurements on Nb_{3}Sn wire provided by Lawrence Livermore National Laboratory for the U.S. Department of Energy's (DOE's) Fusion program. (FY 2003)
• Design and construct a samplemounting fixture for marginally stable Nb_{3}Sn conductors with currents up to 1000 amperes. Participate in an interlaboratory comparison of critical current measurements on Nb_{3}Sn wires for the DOE Fusion and High Energy Physics programs. (FY 2003)
• Provide variabletemperature criticalcurrent measurements for the DOE Fusion program. (FY 2004)
• Measure marginally stable Nb_{3}Sn samples for U.S. companies and national laboratories. (FY 2005)
Metrology for Superconductors
We are comparing two methods of measuring the residual resistivity ratio (RRR) of highpurity Nb specimens. This comparison will set limits on the expected difference between the two methods and may lead to best procedures for acquiring and analyzing these data. The value of RRR is an indication of the purity and the lowtemperature thermal conductivity of the Nb, and is often used as a material specification in commerce. Pure Nb in its superconducting state is used for highqualityfactor resonant cavities for particle accelerators, synchrotron light sources, and neutron sources.
Another activity of the project is the measurement of the magnetic hysteresis loss in superconductors. A few years ago we demonstrated that flux jumps could be suppressed during the measurement of hysteresis loss by immersing marginally stable Nb3Sn conductors in liquid He. The increased thermal conduction affords dynamic stability against flux jumps, which allows AC losses to be estimated from the area of the magnetizationversusfield loop. Many measurements we do for superconductor wire manufacturers require special techniques to obtain accurate results.
Deliverables:
• Complete a statistical analysis on the comparison of two methods of measuring the RRR of highpurity Nb specimens. (FY 2003)
• Provide RRR measurements for U.S. companies and national laboratories. (Ongoing)
• Measure AC losses for U.S. companies and national laboratories. (Ongoing)
Removing an AC loss sample from the SQUID magnetometer.

Accomplishments
International Standards
• New IEC Superconductivity Standards — New international standards on superconductivity were recently published by IEC TC 90. The documents are:
 IEC 617884 Superconductivity  Part 4: Residual resistance ratio measurement  Residual resistance ratio of NbTi composite superconductors
 IEC 617887 Superconductivity  Part 7: Electronic characteristic measurements  Surface resistance of superconductors at microwave frequencies
 IEC 6178810 Superconductivity  Part 10: Critical temperature measurement  Critical temperature of NbTi, Nb_{3}Sn, and Bisystem oxide composite superconductors by a resistance method
 IEC 6178812 Superconductivity  Part 12: Matrix to superconductor volume ratio measurement  Copper to noncopper volume ratio of Nb_{3}Sn composite superconducting wires
We worked extensively on these documents and helped resolve many difficulties encountered during the development process. The standard on surface resistance of superconductors at microwave frequencies is the first IEC standard for electronic applications of superconductivity. This brings to 10 the number of IEC TC 90 published standards. Currently, 4 more documents are at various stages of development within TC 90.
TC 90 Working Groups and Status 
1 
Terms and definitions (301 terms) 
IS 
2 
Critical current measurement of Cu/NbTi 
IS 
3 
Critical current measurement of Bibased superconductors 
IS 
4 
Residual resistivity ratio measurement 
IS & CDV 
5 
Room temperature tensile test 
IS 
6 
Matrix composite ratio measurement 
two ISs 
7 
Critical current measurement of Nb_{3}Sn 
IS 
8 
Electronic characteristic measurements 
IS 
9 
AC loss measurement 
two CDVs 
10 
Trapped flux density measurements of oxides 
CD 
11 
Critical temperature measurement 
IS 
Document stages: Working Draft (WD), Committee Draft (CD), Committee Draft for Voting (CDV), Final Draft International Standard (FDIS), International Standard (IS). 
IEC Technical Committee 90 
Secretariat 
Japan 
Chairman 
L. F. Goodrich 
Secretary 
K. Sato 
Participating Countries 
13 
Observing Countries 
15 
Critical Current Measurements
• CriticalCurrent of NbTi — We continue to provide measurements of critical current of Cu/NbTi samples for U.S. wire manufacturers. The current or magnetic field requirements are occasionally beyond their measurement capabilities. One recent sample was an Alclad Cu/NbTi wire where we had to make a difficult lowresistance solder connection to the Al. Another difficult sample had a wire diameter of only 1.9 millimeters and carried more than 900 amperes in a magnetic field of 9 teslas.
• Magnetoresistance Correction for Resistance Thermometers — We constructed and used a new cryostat to determine the magnetoresistance correction for eight resistance thermometers as a function of magnetic field (0 to 12 teslas) and at several temperatures (4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20, 25, 30, and 35 kelvins). These corrections will be used in our future variabletemperature criticalcurrent measurements where we use three or four thermometers simultaneously. The literature on magnetoresistance corrections indicated a wide range of possible corrections (as large as 0.15 kelvins at 12 teslas) for individual thermometers, especially in the temperature range from 4 to 10 kelvins. Thus, we needed to determine the correction for our individual thermometers and reduced the temperature uncertainty due to magnetoresistance to about 0.01 kelvins. As expected, we found differences among the thermometers, although thermometers from the same batch seemed to have very similar magnetoresistance. The main differences were the maximum value of magnetoresistance and the temperature at which the maximum occurred. The magnitude varied by a factor of 2 for different thermometers. In some cases the maximum effect was at 5 kelvins and in other cases the maximum effect was at 8 kelvins.
• CriticalCurrent of BiSrCaCuO films — We made critical current measurements on two Bi_{2}Sr_{2}CaCu_{2}O (Bi2212) thinfilm samples for researchers at a national laboratory. Our transport results were lower by a factor of 10 than they expected based on their magnetization measurements. This prompted them to make a microstructural analysis, which showed the presence of voids that explained the difference between the magnetization and currenttransport measurements. Since most applications require a transport current, this result confirmed that periodic verification of current transport is worth the extra difficulty.
Metrology for Superconductors
• CriticalCurrent of Marginally Stable Nb_{3}Sn — We recommended that simple measurements be made that would show that criticalcurrent density measured at another laboratory was too high by a factor of about 6. We based this recommendation on our 1995 paper ("Anomalous Switching Phenomenon in CriticalCurrent Measurements when Using Conductive Mandrels," IEEE Trans. Appl. Supercond. 5, 34423444) in which we showed that a subtle effect creates misleading results. The researchers made the additional measurements, verified our explanation, and presented the results at the 2002 Applied Superconductivity Conference.
Two common myths about criticalcurrent measurements are that the highest measured value is correct and that the repeatable value is correct. The study supports our suggestion that the endtoend sample voltage is an important diagnostic, especially when measuring marginallystable conductors.
Threedimensional resistance surface of a pureNb specimen.

• Residual Resistivity Ratio Measurements of HighPurity Nb — We compared two methods of measuring the RRR of highpurity Nb and achieved agreement within 6 percent. The RRR is typically defined as the ratio of the electrical resistivities or resistances measured at 273 kelvins and 4.2 kelvins (the boiling point of helium at standard atmospheric pressure). However, pure Nb is superconducting at 4.2 kelvins, so the lowtemperature resistance is defined as the resistance in the normal (nonsuperconducting) state extrapolated to 4.2 kelvins and zero magnetic field.
Components and assembled unit for the magnetic levitation demonstration.

The two methods to obtain this extrapolated normalstate resistance are (1) measure the normalstate resistance as a function of field at 4.2 kelvins and extrapolate to zero field (field extrapolation), or (2) measure the normalstate resistance as a function of temperature in zero field and extrapolate to 4.2 kelvins (temperature extrapolation). Both methods require the precise measurement of resistance as small as 0.5 microohms on a specimen that resists wetting by solder. Both methods have their difficulties and typically would be performed with different experimental apparatus. In our experiment we can make both types of measurements during a single sequence with one apparatus to directly compare methods on a given specimen.
The resistance surface as a function of temperature and magnetic field is shown above. When the combination of field and temperature are low enough, the sample is in the superconducting state and the resistance is zero. The transition from superconducting to normal state occurs at lower magnetic fields as the temperature is increased. For temperatures above 9.4 or 9.5 kelvins, the sample is normal at zero magnetic field. The surface was generated with measurements of resistance R versus temperature T at zero magnetic field H and measurements of R versus H at various T.
Outreach
• Demonstration Experiments — We hosted a high school physics teacher under the Practical HandsOn Application to Science Education (PHASE) program during the summer of 2002 to develop and construct demonstrations in superconductivity and magnetism for outreach programs. He constructed multiple kits of five different demonstrations. A set of instructions was written for the two superconductivity demonstrations (magnetic bearing and levitated train) that will be used by NIST staff and local science teachers through the Career Awareness and Resource Education (CARE) program. Two of the magnetism demonstrations were detailed in a paper to be published in The Physics Teacher. One illustrated the diamagnetic properties of water and the other demonstrated diamagnetically stabilized magnetic levitation.

Demonstrating diamagnetically stabalized levitation for his high school class.

Standards Committees
• Loren 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.