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Project Leader:
Loren Goodrich
                        
Staff-Years (FY 2002):
1 professional
1 technician

Previous Reports:
2002
2001

Standards for Superconductor Characterization

Goals

Discussing the statistical analysis of residual resistivity ratio of high-purity Nb specimens

Discussing the statistical analysis of residual resistivity ratio of high-purity 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 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, high-quality-factor 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 variable-temperature 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 electric-power systems makes this technology especially 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 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 voltage-current characteristic with two common criteria applied

Illustration of a superconductor's voltage-current characteristic with two common criteria applied.

The voltage-current characteristic of a Nb–Ti wire in a magnetic field of 2 teslas. The critical current Ic is determined using one of the criteria. The corresponding voltage Vc = Ec L = 5 microvolts when Ec =10 microvolts per meter and the voltage-tap separation L = 0.5 meters. The inset is a plot of these same data on a logarithmic scale

The voltage-current characteristic of a Nb–Ti wire in a magnetic field of 2 teslas. The critical current Ic is determined using one of the criteria. The corresponding voltage Vc = Ec L = 5 microvolts when Ec =10 microvolts per meter and the voltage-tap 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 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.

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, 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 voltage-current characteristic and two criteria for critical current. Typical criteria are electric-field strength of 10 microvolts per meter and resistivity of 10-14 ohm-meters. An actual voltage-current 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 = V0 (I/I0)n, where V0, I0 and the n-value are constants. The n-value is the slope of the voltage-current curve when plotted on a logarithmic scale (see inset in the plot).

Deliverables:

• Determine the current limits of a variable-temperature cryostat made for coil samples. Make variable-temperature critical-current measurements on Nb3Sn 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 sample-mounting fixture for marginally stable Nb3Sn conductors with currents up to 1000 amperes. Participate in an interlaboratory comparison of critical current measurements on Nb3Sn wires for the DOE Fusion and High Energy Physics programs. (FY 2003)

• Provide variable-temperature critical-current measurements for the DOE Fusion program. (FY 2004)

• Measure marginally stable Nb3Sn 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 high-purity 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 low-temperature thermal conductivity of the Nb, and is often used as a material specification in commerce. Pure Nb in its superconducting state is used for high-quality-factor 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 magnetization-versus-field 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 high-purity 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. 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:

  1. IEC 61788-4 Superconductivity - Part 4: Residual resistance ratio measurement - Residual resistance ratio of Nb-Ti composite superconductors
  2. IEC 61788-7 Superconductivity - Part 7: Electronic characteristic measurements - Surface resistance of superconductors at microwave frequencies
  3. IEC 61788-10 Superconductivity - Part 10: Critical temperature measurement - Critical temperature of Nb-Ti, Nb3Sn, and Bi-system oxide composite superconductors by a resistance method
  4. IEC 61788-12 Superconductivity - Part 12: Matrix to superconductor volume ratio measurement - Copper to non-copper volume ratio of Nb3Sn 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/Nb-Ti IS
3 Critical current measurement of Bi-based 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 Nb3Sn 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

• Critical-Current of Nb-Ti — We continue to provide measurements of critical current of Cu/Nb-Ti samples for U.S. wire manufacturers. The current or magnetic field requirements are occasionally beyond their measurement capabilities. One recent sample was an Al-clad Cu/Nb-Ti wire where we had to make a difficult low-resistance 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 variable-temperature critical-current 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.

• Critical-Current of Bi-Sr-Ca-Cu-O films — We made critical current measurements on two Bi2Sr2CaCu2O (Bi-2212) thin-film 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 current-transport 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

• Critical-Current of Marginally Stable Nb3Sn — We recommended that simple measurements be made that would show that critical-current 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 Critical-Current Measurements when Using Conductive Mandrels," IEEE Trans. Appl. Supercond. 5, 3442-3444) 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 critical-current measurements are that the highest measured value is correct and that the repeatable value is correct. The study supports our suggestion that the end-to-end sample voltage is an important diagnostic, especially when measuring marginally-stable conductors.

Three-dimensional resistance surface of a pure-Nb specimen Three-dimensional resistance surface of a pure-Nb specimen.

• Residual Resistivity Ratio Measurements of High-Purity Nb — We compared two methods of measuring the RRR of high-purity 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 low-temperature 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

Components and assembled unit for the magnetic levitation demonstration.

The two methods to obtain this extrapolated normal-state resistance are (1) measure the normal-state resistance as a function of field at 4.2 kelvins and extrapolate to zero field (field extrapolation), or (2) measure the normal-state 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 micro-ohms 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 Hands-On 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.

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.