Verlag des Forschungszentrums Jülich
JUEL-3779
Maus, Martin
HTS rf-SQUID-Planargradiometer mit langer Basislänge für die Inspektion von Flugzeugfelgen
136 S., 2000
An important task in the field of nondestructive evaluation (NDE) of aircraft components is the reliable detection of small cracks hidden very deep in the material. This will become even more important in the future, because the thickness of aircraft parts is going to increase further. Conventional eddy current testing systems reach their limit here, whereas SQUIDs (Superconducting QUantum Interference Devices) show an excellent sensitivity for the low excitation frequencies needed. As it is required to suppress large magnetic disturbances without magnetic shielding, a gradiometer SQUID is utilized for these measurements.
By calculations, it is proved in this work that an optimum signal-to-noise ratio is reached when the baseline of the SQUID gradiometer is enlarged from the so far used 3.6 mm to about 10 mm. However, an enhancement of the baseline leads to a significant increase of the SQUID inductance and hence of the white noise. Consequently, the SQUID sensor cannot be operated reliably in the flux locked loop any more. It is shown that the inductance may be reduced significantly by placing a superconducting cover strip on top of the gradiometer slit. Thus, the gradiometer slit cannot be penetrated by magnetic flux any more. Calculations of the geometric and kinetic inductance confirm that a distinct reduction of the inductance can be reached this way. Furthermore, the calculations show that it is possible to raise the baseline to 9 mm without an increase of the inductance, compared to an uncovered gradiometer with 3.6 mm baseline.
The experimental realization of the gradiometer is very complicated because it is necessary to use three epitaxial layers for the fabrication and multilayer technology for high temperature superconductors is still under development. It is especially important to avoid electrical shorts between the superconducting layers. In order to achieve that, gradiometer structures with varying steepness at the edges as well as different widths of the cover strip are examined. In order to exclude electrical shorts, caused by particles on top of the surface of the lower superconducting layer, sputtered YBaCuO thin films are used. They typically form very smooth surfaces almost free of particles.
The experiment shows that by applying the superconducting cover strip, the white noise of the gradiometer is reduced from 144 [my][Phi]0/Hz to 73 [my][Phi]0/Hz, nearly by a factor of 2. Simultaneously, the fieldgradient-to-flux-coefficient rises from 15.1 nT/([Phi]0 cm) to 33.4 nT/([Phi]0 cm). This proves that the lower white noise is really due to a smaller pick-up area for the magnetic flux, and thus the shielding strip works. The common-mode-rejection-ratio (CMRR) of the gradiometer shows a decrease to 10, compared to an uncovered sample having a typical value of about 1000. On one hand, this can be partially attributed to an observed lateral shift of the cover strip caused by inevitable alignment errors of the photolithographic process. Consequently, both pick-up areas for the magnetic flux slightly vary in size. On the other hand, scanning electron microscope (SEM) photographs show that the crystal quality of the upper YBaCuO thin film is not distributed homogeneously. This leads to a different shielding effect on both sides of the cover strip.
The work shows that planar rf SQUID gradiometers with long baseline can be realized by covering the gradiometer slit with a superconducting film. However, some technological difficulties have to be solved before the gradiometers can go into production. Future development should aim especially at the improvement of the crystal quality of the upper YBaCuO thin film. Optimization of the multilayer technology for high temperature superconductors is a key issue in future SQUID development.
Neuerscheinungen
Schriften des Forschungszentrums Jülich
Ihre Ansprechperson
Heike Lexis
+49 2461 61-5367
zb-publikation@fz-juelich.de