A nondestructive inspection (NDI) system using an HTS SQUID for copper heat-exchanger tubes has been constructed. An eddy-current-based NDI method using an HTS SQUID gradiometer cooled using a cryocooler was employed for the detection of micro flaws in thin copper tubes 6.35 mm in diameter and 0.8 mm in thickness. With an excitation field of 1.6 μT at 5 kHz, a flaw in the tube, measuring 30 μm deep and 15 mm long, was successfully detected by the system with a high signal-noise ratio of at least 20. Numerical simulations were also conducted to determine how many sensors would be required for inspection around the circumference of an entire tube.
A practical robotic three-dimensional (3D) SQUID-NDE system was developed for the first time. This system is based on a previously developed traveling SQUID-NDE (i.e., one that is not stationary, but the SQUID sensor itself moves or scans along the surface of the object), which incorporates SQUID sensor travel technologies such as the `pseudo' third-order SQUID gradiometer. The system developed is composed of a traveling SQUID gradiometer, an articulated-type robot used as a SQUID sensor manipulator, a laser CCD displacement sensor, and a numerical interpolation procedure to reconstruct the surface of the evaluated object without the need for CAD data. To demonstrate the capability of this imaging system, it was used to detect artificial damage in a stainless steel (type-304) tube. The system successfully detected the artificial damage. It is anticipated that the robotic 3D SQUID-NDE imaging system will be in practical use in the near future.
Distribution measurement of thermal effusivity was carried out for c-axis aligned YBCO thin-film superconducting material using a thermal microscope based upon a thermoreflectance technique. The 800 nm-thick YBCO film synthesized on MgO substrate showed line-shaped damage by scanning a processing laser beam on the surface. The damaged YBCO film was prepared to be coated with a Mo thin film 100 nm in thickness before measurement. The thermal effusivity YBCO film with no damaged part was measured and determined to be 1890 J s-0.5 m-2 K-1, on the basis of the calibration result of reference materials, Corning7740 and glassy carbon. The scatter of the measurement is estimated to be lower than 1.4%. A line-shape contrast image of thermal effusivity due to damaging the YBCO film was clearly observed. The thermal effusivity of the damaged part in the YBCO film was estimated to be 1.7% lower than that of non-damaged film. It was confirmed that a thermal microscope is applicable for homogeneity evaluation or defect inspection with about 1% resolution in the distribution measurement of thermal effusivity.
We developed a novel non-destructive and non-contact method for the measurement of critical current density (Jc) using a permanent magnet. A high-temperature superconductor (HTS) thin-film was fixed on the stage of the system and a small cylindrical permanent magnet (Sm2Co17) was set above the HTS film. The repulsive force (Fr) and attractive force between the small magnet and HTS film were measured by a high-resolution load sensor changing the distance (L) between the HTS film and the magnet. The maximum repulsive force (Fm) could be estimated from an experimental result of the Fr vs. L curve. We investigated the relationship of Jc to Fm in several HTS samples. The standard Jc of the HTS films was determined using the inductive measurement system made from THEVA GmbH. We found that the Jc is approximately proportional to the Fm normalized by the film thickness. This result indicates that the permanent magnet technique can easily estimate the Jc of HTS films. We also found that the system can measure Jc with sufficient reproducibility and in a short time. Moreover, it was checked to confirm that Jc mapping of a YBCO thin film could be also measured.
We have investigated an inductive method measuring third-harmonic voltages to evaluate the local critical current density Jc in superconducting films and bulk materials. When a sinusoidal drive current I0cosωt flows in a small coil close to the flat surface of superconductors, harmonic voltages Vncos(nωt+θn) are induced in the coil, because of the nonlinear response of superconductors. We have considered the AC response of superconductors and the induced third-harmonic voltage V3 as the function of I0, and we present the precise technique to measure the critical current density Jc in superconducting films as well as Jc in bulk superconductors.