Superconductor Electromechanics

The Magnetics Group's program in superconductor electromechanics measures the critical current of superconductor wires as a function of magnetic field, temperature, and mechanical strain, and investigates methods to improve the performance of high-temperature superconductors. Energy losses in the transmission of electricity from generation plants to cities amount to over 5 percent. Underground electrical distribution lines in cities are at the limits of their current-carrying capacities. These are examples of problems that could be mitigated with superconductors. The Superconductivity Program conducts enabling research in superconductivity for energy, power grid reliability, medical instruments, and fundamental physics. The program has a world-wide reputation for its measurements of the effects of mechanical strain and temperature on the ability of superconductor wires to carry extremely high current. Additionally, the program develops standard measurement techniques and international documentary standards for measurements to characterize superconductors. It provides quality assurance and reference data for commercial and prototype high-temperature and low-temperature superconductors.

Figure 2. Two copper-alloy spring sample holders.Superconductor wires are soldered to the outer circumference of the springs. Torque applied to the spring puts the wires into tension or compression.

Figure 3. Upper part of new high-current apparatus. The worm gear that is applied to torque the spring can be seen through the window.

The global market for superconductors is about $5 billion, largely dominated by superconducting magnets for magnetic resonance imaging (MRI) machines. However, recent successful demonstration projects point to increasing applications in power transmission lines, fault current limiters, transformers, synchronous condensers, magnetic energy storage devices, and other components for the electrical power grid. Measurements to characterize superconductors require specialized expertise, and the measurements must be reliable in order to solve engineering problems that prevent wider use of both conventional and high-temperature superconductors.

To further its unique role as the de facto reference laboratory for superconductor critical current measurements, the program recently developed the world’s first high-current unified apparatus to measure critical current as a function of strain, temperature, and magnetic field. It is being used for fundamental research at NIST, the U.S. contribution to the International Thermonuclear Experimental Reactor (ITER), and in support of the Department of Energy’s High Energy Physics particle accelerator program. NIST’s data on the irreversible strain limit and alternating-field losses of Nb3Sn superconductor wires have already impelled ITER to change its wire specifications and heat-treatment schedule. The reason strain is such an important parameter is that, in most applications, superconductors are subject to powerful Lorentz forces due to the current they carry in the magnetic field of neighboring wires. These forces strain the wires, usually to their detriment.

Recent research in the Program has shown that the reduction of critical current with compressive strain in the high-temperature Y Ba Cu O superconductor, which is being developed for electric transmission lines, is intrinsic to the superconductor grains and not the grain boundaries. In fact, the reversible strain effect in single-crystal thin films is comparable to that of multi-granular Y Ba Cu O coated conductors. This could have profound implications for the development of practical high-temperature conductors. Other research in support of the commercial development of high-temperature superconductors has shown how to prevent Y-Ba-Cu-O from delaminating from their metal substrates when under strain.

Figure 4. Critical-current measurement of a single of a single-crystal thin film of Y-Ba-Cu-O deposited on a SrTiO3 crystal. Axial compressive strain is applied by bending the beam
  

• Designed and constructed a unified temperature-strain apparatus.
• Demonstrated that the reversible strain effect in Y-Ba-Cu-O is intragranular in origin
• Constructed new electromechanical test structures for second-generation, high-temperature superconductors, including a new apparatus to measure the strain effect in variable magnetic-field angles.
• Showed how low irreversible strain limits could explain serious damage in ITER prototype magnets. 
• Formulated a scaling law for the flux pinning force in MgB2 superconductors.
• Updated 12 International Electrotechnical Commission (IEC) standards, including measures of uncertainty. 

Associated Publications/Reports:

• D. C. van der Laan, J. W. Ekin, “Large Intrinsic Effect of Axial Strain on the Critical Current of High-Temperature Superconductors for Electric Power Applications,” Applied Physics Letters, vol. 90, 052506, January 2007.

• N. Cheggour, J. W. Ekin, L. F. Goodrich, “Critical-Current Measurements on an ITER Nb3Sn Strand: Effect of Axial Tensile Strain,” IEEE Transactions on Applied Superconductivity, vol. 17, pp. 1366-1369, June 2007.

• L. F. Goodrich, N. Cheggour, J. W. Ekin, T. C. Stauffer, “Critical-Current Measurements on ITER Nb3Sn Strands: Effect of Temperature,” IEEE Transactions on Applied Superconductivity, vol. 17, pp. 1398-1401, June 2007.