Electrically superconductive outer layers are supported by highly thermally conductive skeletons.
A heterogeneous material construction has been devised for sensing coils of super-conducting quantum interference device (SQUID) magnetometers that are subject to a combination of requirements peculiar to some advanced applications, notably including low-field magnetic resonance imaging for medical diagnosis. The requirements in question are the following:
- The sensing coils must be large enough (in some cases having dimensions of as much as tens of centimeters) to afford adequate sensitivity;
- The sensing coils must be made electrically superconductive to eliminate Johnson noise (thermally induced noise proportional to electrical resistance); and
- Although the sensing coils must be cooled to below their superconducting- transition temperatures with sufficient cooling power to overcome moderate ambient radiative heat leakage, they must not be immersed in cryogenic liquid baths.
For a given superconducting sensing coil, this combination of requirements can be satisfied by providing a sufficiently thermally conductive link between the coil and a cold source. However, the superconducting coil material is not suitable as such a link because electrically superconductive materials are typically poor thermal conductors.
The heterogeneous material construction makes it possible to solve both the electrical- and thermal-conductivity problems. The basic idea is to construct the coil as a skeleton made of a highly thermally conductive material (typically, annealed copper), then coat the skeleton with an electrically superconductive alloy (typically, a lead-tin solder) [see figure]. In operation, the copper skeleton provides the required thermally conductive connection to the cold source, while the electrically superconductive coating material shields against Johnson noise that originates in the copper skeleton.
This work was done by Inseob Hahn, Konstantin I. Penanen, and Byeong Ho Eom of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-45929, volume and number of this NASA Tech Briefs issue, and the page number.
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