In the News: Cold Facts

As published by the Cryogenic Society of America in Cold Facts, Vol 27, No 2:

In February 2011, R&D magazine report- ed that Danko van der Laan, a scientist work- ing 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 con- sists 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 compres- sive strain is two-fold. First, the ceramic films can withstand relatively large compressive as well as tensile strain before mechanical dam- age 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 pub- lished a paper that proves this for bismuth- strontium-calcium-copper-oxide-2223 (Bi- 2223) superconductors, and are close to pro- viding 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 depend- ent than that of YBCO, causing the “higher tol- erance” to strain of GBCO, as demonstrated in the paper.

“I was aware that REBCO coated conduc- tors 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 com- pact cable design. The first challenge was to verify this. I therefore wrapped coated con- ductors around round formers with 1/8″ (3.2 mm) diameter, and measured their perform- ance.The concept was verified when the per- formance 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 cur- rents 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 con- structed several power supplies from subma- rine batteries. His help really made the high- current cable tests successful.”