G. Peluso

A new fabrication process of superconducting Nb tunnel junctions with ultralow leakage current for X-ray detection

A new fabrication process is presented to make Nb/Al/AlOx/Nb superconducting tunnel junctions with ultralow subgap leakage current at low temperatures below 4.2 K for X-ray detection. A Nb anodization process has been adopted to make electric insulation at the periphery of an ultrathin AlOx tunnel barrier through use in combination with the conventional SiO self-aligned insulation process. The temperature dependence of the subgap current of the junctions fabricated by the present process is in good agreement with that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, while junctions made by the conventional process show a weak temperature dependence of the subgap current. In addition, the Vm value of the junction fabricated by the present process was found to be more than 3000 mV at 1.2 K. It has been clarified that the anodization process introduces improvements to make the subgap leakage current extremely small in the temperature range from 4.2 K down to 1.2 K.

Analysis of low-velocity impact damage in reinforced carbon fiber composites by HTS-SQUID magnetometers

Composite materials are susceptible to damage which can be induced by service loads and accidental impacts. The detection of any signature produced by damage is critical to maintaining the integrity of aircraft parts during routine maintenance. A high critical temperature SQUID magnetometer has been successfully employed in the evaluation of the behavior of multi-ply carbon fibers reinforced composite panels for aeronautical applications under low-velocity impacts. Measurements of the induced magnetic field have been carried out above specimen damaged with energy impact from 1 to 40 J. A quasi-linear behavior in two different regimes between the SQUIDs response and the energy of the impact has been found. This suggests a correspondence to the detection of intrinsically different damage that occurs in the laminates at different energies.

Superconducting device with transistor-like properties including large current amplification

We have fabricated and studied a stacked superconducting double tunnel junction device with transistor-like properties. The intermediate electrode is a bilayer consisting of a Nb film together with an Al film that acts as a quasiparticle trap. Large current gains of more than 50 are observed at 4.2 K when the Al layer is normal. The operation is highly directional. Results are explained on the basis of trapping of quasiparticles from a superconductor into a normal metal, together with a conversion of relaxation energy into electronic excitations. Similar devices should have wide applications in low-temperature measurement and detection systems.

Nb-based Josephson junction devices for nuclear radiation detection: Design and preliminary experimental results

In this work we apply the Electrotechnical Laboratory Josephson junction fabrication technology to realize an integrated device for nuclear radiation detection. We have integrated in a single 5×5 mm2 chip 240 Josephson tunnel junctions (JTJs) having various geometrical configurations and interconnections, in order to investigate both the effects of various detection geometrical configurations and the fabrication process uniformity and reproducibility. The junction quality was extensively tested by measurements of the current‐voltage characteristics, down to T=1.2 K, showing an unexpected saturation of the Vm quality factor. Preliminary results concerning the response to α‐particle radiation of these samples have been obtained. Pulse height and rise time spectra of both ‘‘island’’ type and series connected JTJs are reported. Signals generated by nuclear radiation have been observed at temperatures up to 2 K, where a reduction of the collected charge occurs because of the enhancement of the recombination ra...

Non Destructive Evaluation of Advanced Composite Materials for Aerospace Application Using HTS SQUIDs

The main advantage of a HTS SQUID magnetometer in NDE applications is represented by its unrivalled magnetic flux sensitivity down to very low frequencies, which allows the detection of weak magnetic field variations due to defects also in materials characterized by a very low electrical conductivity. The imaging obtained by means of the magnetic flux variations can be an useful technique for an easier interpretation of SQUID magnetic responses getting along without post-processing algorithms and independent of the operator. Therefore, the SQUIDs NDE system output is compatible with the other conventional non destructive testing equipment for data fusion of aircraft inspections.

Experimental and numerical results of electromagnetic nondestructive testing with HTc SQUIDS

We present here recent results on detection of surface and subsurface artificial features in Al-Ti planar structures, to show current performance of our eddy-current nondestructive evaluation system based on HTc SQUIDs. The anomalous magnetic fields generated by flaws with known electromagnetic characteristics have been modeled by three-dimensional codes based on finite element method and volume integral formulation and developed for the investigated problem. Both numerical solutions have correctly predicted the shape of the complicated magnetic field response which is mainly the result of the shape of the defect, the geometry of the inducing coil and the characteristics of the SQUID gradiometer.

Magnetic field penetration in niobium- and vanadium-based Josephson junctions

Measurements on the temperature dependence of the magnetic field penetration in Nb-NbxOy-Pb and V-VxOy-Pb Josephson junctions have been performed. Results on the zero-temperature penetration depth in niobium films are far above the bulk values although consistent with other measurements on junctions reported in the literature. For vanadium junctions anomalously large penetration depth values are obtained at low temperatures. Nevertheless, the temperature dependence is in reasonable agreement with the local dirty limit model.

Structural health monitoring of materials by high critical temperature SQUID

Abstract Nowadays, tailoring the material properties is essential for advanced product design in engineering systems. The need to provide key information about micro- and macro-structural behaviour of materials, without destructively sectioning the sample, has spurred the development of nondestructive evaluation methodologies. These techniques are required during material production, quality testing of components during manufacturing, and in-service inspection of structural integrity. To ensure the highest possible operational safety along with an economic efficiency, it is necessary to carry out inspections with a high sensitivity and a proven reliability. Due to its unparalleled magnetic field sensitivity over a wide frequency range and large dynamic range, SQUID-based nondestructive evaluation has unique advantages for materials and structures characterization. We will present an overview of eddy-current nondestructive analysis utilizing high T c SQUIDs with an emphasis on examples relevant to the aeronautical industry. These include the detection of deep-lying defects in multi-layer structures of Al–Ti alloys and damage of extremely lightweight graphite/epoxy composites. Both of these can be successfully treated by this approach where conventional electromagnetic probes often fail. In addition some results based on the volume integral formulation, successfully developed to simulate the response of the system to different type of flaws in Al-alloy planar structures and to solve the inverse electromagnetic problem, will be shown.

Experimental Results in Eddy Current Non Destructive Testing Based on Superconductive and Conventional Electromagnetic Probes

This paper deals with the electromagentic nondestructive testing performed by SQUID magnetometry on stratified aluminum alloy plates as those commonly encountered in the aircraft industry. The anomalous magnetic fields generated by flaws with know electromagnetic characteristics have been modeled by a three-dimensional specific code based on a finite element formulation. The numerical solution has correctly predicted the shape of the complicated magnetic field response due to the defect. Once accuracy and reliability of experimental data taken by superconductive probe have been tested, a benchmark-like problem has been faced. Measurements performed by conventional probes like fluxgate and inductive coil have been compared with the ones taken by innovative device based on superconductive materials.

Influence of a NbN overlayer on Nb/Al–AlOx/Nb high quality Josephson tunnel junctions for x‐ray detection

Nb/Al–AlOx/Nb–NbN Josephson junctions for use as x‐ray detectors have been fabricated. The NbN overlayer has been used to realize the trapping of quasiparticles in the Nb top layer with an increase of the charge collection efficiency. The temperature dependence of quasiparticle and Josephson critical current have been investigated and compared with that without a NbN overlayer, showing low leakage currents, about 200 pA (V=0.5 mV, T=0.67 K, A=20×20 μm2), and high dynamical resistances in the subgap region. Preliminary measurements under 6 keV x‐ray irradiation have shown an increased value of the maximum collected charge.