In February 2011, R&D magazine reported that Danko van der Laan, a scientist working at NIST, had invented a method of making HTS cables that are thinner and more flexible than ever before. Van der Laan provided Cold Facts with more detail on his work and that of his colleagues at NIST as follows:
The superconducting material that we used to make the cables is a high-temperature superconducting “coated conductor” that consists of a 50-micron-thick Hastelloy substrate, coated with ceramic buffer layers and a 1-micron-thick gadolinium-barium-copper-oxide (GBCO) superconducting film. The superconducting film is similar to yttrium-barium-copper-oxide (YBCO), but with the Y fully substituted by Gd. The superconductor was purchased from SuperPower Inc. in Schenectady, NY.
[These materials’] tolerance to compressive strain is two-fold. First, the ceramic films can withstand relatively large compressive as well as tensile strain before mechanical damage occurs. This is mainly due to the very high level of grain alignment in almost all REBCO (rare-earth-barium-copper-oxide, with RE=Y, Gd, Dy, etc.) coated conductors. Applications are always designed in such a way that the conductor doesn’t exceed the strain levels at which mechanical damage occurs. Second, the superconducting properties (critical current, magnetic flux pinning strength, etc.) change reversibly with strain, even before any mechanical damage occurs in the ceramic films. We are very close to proving that this reversible change is caused by the pressure dependence of the critical temperature of the superconductor. Earlier this year, we published a paper that proves this for bismuth-strontium-calcium-copper-oxide-2223 (Bi-2223) superconductors, and are close to providing the same evidence for REBCO coated conductors. The pressure dependence of the critical temperature (Tc) depends on the type of high-temperature superconductor. Some show a higher pressure dependence of Tc, which most likely causes a larger change in critical current with strain, while others show a relatively small change in Tc with pressure. We think, but haven’t had a chance to measure this, that Tc of GBCO is less pressure dependent than that of YBCO, causing the “higher tolerance” to strain of GBCO, as demonstrated in the paper.
I was aware that REBCO coated conductors are highly tolerable to strain, since I have studied many REBCO coated conductors under various strain conditions over the years. I came up with the idea that a coated conductor should tolerate a very tight bend around a former, thus allowing for a new compact cable design. The first challenge was to verify this. I therefore wrapped coated conductors around round formers with 1/8″ (3.2 mm) diameter, and measured their performance. The concept was verified when the performance of the coated conductor wrapped around the former was similar to a straight sample that was put in the same strain state.
The second challenge was to actually make high-current cables using this approach. After learning from our mistakes (by burning out several smaller cables), we came up with a method that worked. The next problem was to measure the performance of the cable at currents up to 3000A. This required a lot of low-noise current supplies that all had to work together to reach that level of current. To put it in perspective, as far as I know, the highest current in any conventional transmission line is about 3000A (this is one in Brazil), while most of them operate at currents below 1000A. Fortunately, Loren Goodrich from NIST had a lot of experience with low-noise, high current power supplies, having designed and constructed several power supplies from submarine batteries. His help really made the high-current cable tests successful.
The cabling concept will likely benefit some of the high-temperature superconducting applications that have been developed largely through the superconductivity program of the Department of Energy, Office of Electricity Delivery & Energy Reliability. The NIST research on high-temperature superconductors in which the cabling concept was developed was also largely funded through this program. The cabling concept could directly benefit high-temperature superconducting transmission lines, motors and generators, fault current limiters and superconducting magnetic energy storage systems by making them more compact and in some cases mechanically more robust.
One of the main additional advantages of the cabling concept is that the concept allows REBCO coated conductors to penetrate and even create new markets. For instance, there are several military applications that require light-weight, high-current power transmission cables (for instance on aircraft, or on ships), where even “conventional” high-temperature superconducting transmission cables will not work due to their size and weight. Another very important new market is ultra high-field magnets for high-energy physics and other scientific applications. Conventional low-temperature superconductors are fundamentally limited to operation below a critical field of about 20-25 Tesla. No other superconducting material besides high-temperature superconductors can operate above this limit in magnetic field. The problem with high-temperature superconductors was always that they couldn’t be cabled into very compact high-current windings that are needed for these ultra-high field magnets. Our cabling concept will most likely be the first way to provide such cables, therefore allowing the use of high-temperature superconductors in this new class of magnets. Our cabling concept could also be used for other superconducting magnet applications that are currently design using low-temperature superconductors, such as in magnets for fusion plasma confinement (ITER) and medical applications, such as proton cancer treatment facilities. High-temperature superconductors have the benefit that the application can operate efficiently at relatively high temperatures without the use of liquid helium.
I strongly believe that the price of high-temperature superconductor will steadily continue to come down. Especially when new markets for the technology emerge, hopefully with the help of our cable.