Alan Lauder

Magnetic susceptibility imaging for nondestructive evaluation (using SQUID magnetometer)

High-resolution superconducting magnetometers such as MicroSQUID (superconducting quantum interference device) have been shown to be effective for nondestructive evaluation. MicroSQUID can also be used with a room-temperature magnet to image the magnetic susceptibility of materials. A diamagnetic or paramagnetic sample is scanned in the applied field, and the local perturbations are measured. For thin samples, such as plates, sheets, or thin sections of rock, the data are deconvolved to generate two-dimensional susceptibility images. Three-dimensional structures can be imaged with magnetic susceptibility tomography: deconvolution of a large data set obtained by applying the field and scanning in multiple orientations. Extremely small surface defects on nonmagnetic or weakly magnetic samples are imaged by decorating the sample with paramagnetic microspheres prior to scanning. Magnetic susceptibility imaging demonstrates the feasibility of SQUID nondestructive evaluation on materials that could previously be examined only with X-rays or ultrasound.<<ETX>>

High performance superconducting thin films on large area substrates

The first fabrication of 3-in-diameter, thin films of thallium-based superconductors is reported. (Tl/sub 0.5/Pb/sub 0.5/)Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9/ thin films on LaAlO/sub 3/ are found to display the best surface resistance (R/sub s/) properties of any superconductor at higher temperatures and show approximately 50* better performance than copper at 100 K and 10 GHz. A Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ thin film, processed as a 1.8-m-long, 10- mu m-wide meander line carries about 1*10/sup 7/ A/cm/sup 2/ at 4.2 K and 2*10/sup 6/ A/cm/sup 2/ at 80 K. The thallium-based superconductors are compared to YBa/sub 2/Cu/sub 3/O/sub 7/ with respect to T/sub c/, R/sub s/, and Q.<<ETX>>

A high resolution imaging susceptometer

A high-resolution, MicroSQUID (superconducting quantum interference device)-based susceptometer has been constructed for imaging the susceptibility distribution in diamagnetic and paramagnetic objects. The maximum available applied field, produced by the Helmholtz pair, is 300 mu T, which provides an adequate signal-to-noise ratio for common susceptible materials. The calculated variation in the applied field within 20 mm of the center is less than 0.1% of the field at the center. In a 100-Hz bandwidth, the minimum detectable susceptibility-induced field change, (limited by SQUID noise) is 3*10/sup -9/ of the applied field. The sensitivity of the system may be improved by increasing the applied field, for instance by incorporating a superconducting magnet into the magnetometer dewar, and by using lower-noise SQUIDs.<<ETX>>

A Materials System Approach to Optimum Performance in High Temperature Superconducting Thin Films

Leading microwave performance data are reported for epitaxial YBa2Cu3O7, Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3O10, Tl0.5.Pb0.5Sr2CaCu2O7, and Tl0.5Pb0.5Sr2Ca2Cu3O9 high temperature superconducting compositions on a variety of substrates. An in situ process for thallium superconductors is reported. No deterioration in superconductor performance was encountered during subsequent photolithography and metallization processing. A 1.8 meter long patterned meander line with 10 μm lines and spaces demonstrated a critical current of 2 x 106 amp/cm2 at 80 K. Contact resistivity between superconductor and silver pads was below the detection limit of 0.1 x 10-8 Ω-cm2 at 77 K.