Studies On High Temperature Superconducting Dc-SQUIDs And Their Method Of Eddy Current Nondestructive Evaluation

In this thesis, we describe the basic physical principle of direct current superconducting quantum device (dc-SQUID). The equations describing the characteristics of dc-SQUID are derived according to the equivalent circuit theory of dc-SQUID. By normalizing the related parameters in these equations, a simple simulating way based on Matlab tool software for symmetrical high temperature superconducting (high-T_c) dc-SQUID with bicrystal junctions is proposed. The simulating results are very consistent with the theoretical analysis and provide reliable basis for ascertaining dc-SQUID parameters when we practically design and fabricate high-Tc dc-SQUID devices. We design a washer type of flux-focused dc-SQUID based on bicrystal grain boundary Josephson Junction. The central square hole and the outer square washer of the SQUID are 30μm×30μm and 8mm×8mm, respectively. The SQUID inductance is estimated about 47pH. High quality high-Tc YBCO thin films with pure c-axis orientation growth and few outgrowths are fabricated on STO(100) substrates by using pulsed laser deposition (PLD). The dc-SQUID devices are patterned with the fabricated YBCO thin films by standard mask photolithography and etching methods. The measurements of the characteristics of the dc-SQUIDs are carried out with a set of computer-controlled virtual instrument data acquisition system based on Labview which is developed by ourselves. The testing results show that the critical current of the fabricated washer-type high-Tc dc-SQUID with two bicrystal junctions is about 55μA with its IcR of about 110μV. For the magnetometer operated on flux locked mode with 50kHz modulation frequency, its magnetic field sensitivity is about 333 fT/Hz^1/2 at white noise zone and the corresponding flux sensitivity is about 14.5μΦ₀/Hz^1/2 in non-superconducting shielded environment. The optimization for the configuration of first-order planar high-Tc dc-SQUID gradiometer which is widely used in unshielded environment is also involved in this thesis. For the optimized first-order planar gradiometer with the outer antenna size of 9mm×9 mm, the baseline length b is about 6mm. When the central SQUID hole is 4μm×110μm and the SQUID line width is 5 μm, the parameter bAeff for the planar gradiometer has the maximum value of 0.7mm³. The first-order planar gradiometer is fabricated on 10×10mm2 substrate and its characteristics are also tested. The testing results show that the flux sensitivity of the fabricated high-Tc dc-SQUID first-order gradiometer is about 15μΦ0/Hz1/2 at white noise zone and the corresponding magnetic field gradient resolution is 443 fT/cmHz^1/2. A nondestructive evaluation (NDE) system with the fabricated high-Tc dc-SQUID magnetometer has been built up based on eddy current NDE. The NDE experimental results for several aluminium plates show that the sensible high-Tc dc-SQUID NDE can be effectively applied in nondestructive evaluation. When flaws are over ten and even several tens millimeters below the sample surface, the additive signals from the flaws can be clearly identified. By mapping the electric field distribution, we can locate the flaw position. Up to now, however, no appropriate model can quantitatively describe the relationship between additive signals and the real size of the flaw. Therefore, we cannot determine the corresponding numeric relationship between observed additive signals and flaw shape.