A technique for determining inhomogeneities in a specimen of a high-critical-temperature superconductor involves scanning a small eddy-current probe over a flat surface of the specimen at room temperature while monitoring the inductance of the probe. In contrast, older techniques for obtaining the same or similar information involve cooling the specimen below its superconducting-transition critical temperature (TC), with attendant difficulty and cost of operating a cryogenic apparatus.

Figure 1. A Flat Specimen of a High-TC Superconductor is translated under an eddy-current probe to obtain local probe-inductance measurements at the intersections of a rectangular grid.

The present technique is based on the following two concepts:

  1. The inductance of a test coil placed near a highly electrically conductive object increases with increasing electrical resistivity of the object.
  2. Previous measurements on conventional low-TC superconductors have revealed correlations between their TC and normal resistivity ratios. High-TC superconductors are assumed to behave similarly.

Applying these concepts to a specimen of a high-TC superconductor, one can infer inhomogeneities (specifically, spatial variations of TC, including intergrain contacts and structural defects) from spatial variations of room-temperature resistivity and thus from spatial variations of the inductance of an eddy-current probe scanned over the specimen.

Figure 2. Images of Inhomogeneities in two high-TC-superconductor specimens were synthesized from Hall-probe scans at low temperature and from room-temperature scans by the present technique. A qualitative correlation between the two images for each specimen is apparent.

Figure 1 schematically depicts an apparatus for implementing the technique. An eddy-current probe is held stationary just above the flat surface of a high-TC specimen, which is mounted on a horizontal two-dimensional (x,y)-translation table. Stepping motors controlled by a computer actuate the translation stages to move the specimen to commanded x,y positions. By use of a variable-frequency inductance/capacitance/resistance meter, the coil is excited at a frequency (typically about 10 MHz) chosen to obtain the best signal-to-noise ratio and the inductance of the coil is measured. This measurement is repeated at regular increments (typically 0.5 mm) of x and y to obtain a map of eddy-current-probe inductance on a rectangular grid that spans the specimen surface. The inductance data at the grid points can be used to synthesize a gray-scale or false-color image of inhomogeneity in the specimen (see Figure 2).

This work was done by Robert C. Sisk and Palmer N. Peters of Marshall Space Flight Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel
Marshall Space Flight Center; (256) 544-0021

Refer to MFS-31249

NASA Tech Briefs Magazine

This article first appeared in the July, 1999 issue of NASA Tech Briefs Magazine.

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