Magnetic shielding of open and semi-closed bulk superconductor tubes: the role of a cap

peer reviewedIn this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling

High-uniformity atomic layer deposition of superconducting niobium nitride thin films for quantum photonic integration

Atomic layer deposition (ALD) has been identified as a promising growth method for high-uniformity superconducting thin films for superconducting quantum photonic applications, offering superior uniformity, thickness control and conformality to techniques such as reactive sputtering. The potential scalability of ALD makes this method especially appealing for fabrication of superconducting nanowires and resonators across large areas. We report on the growth of highly uniform superconducting NbN thin films via plasma-enhanced atomic layer deposition (PEALD) with radio frequency (RF) substrate biasing, on a 200 mm (8-inch) Si wafer, specifically for superconducting nanowire single-photon detector (SNSPD) applications. Niobium nitride films were grown using (tert-butylimido)-tris(diethylamido)-niobium(V) (TBTDEN) precursor and a H2/Ar plasma. The superconducting properties of a variable thickness series of films (5 – 30 nm) show critical temperature (Tc) of 13.5 K approaching bulk thickness (30 nm) with low suppression down to the ultrathin regime (5 nm) with Tc > 11 K. Tc across the 200 mm wafer with 8 nm thick NbN, measured in 15 mm intervals, exhibits minimal variation (< 7%). Microbridge structures fabricated on 8 nm thick NbN films also exhibit high critical current densities (Jc), > 10 MA/cm2 at 2.6 K. PEALD could therefore be a pivotal technique in enabling large-scale fabrication of integrated quantum photonic devices across a variety of applications

Distribution du champ piégé au dessus d'un array de Halbach supraconducteur

peer reviewedAbstract In applications requiring a large magnetic force, permanent magnets with non-parallel magnetization directions can be assembled in a Halbach array to generate a large gradient of magnetic flux density. The saturation magnetization of permanent magnets, however, brings a fundamental limit on the performance of this configuration. In the present work, we investigate experimentally the assembly of cuboid bulk, large grain melt-textured YBa2Cu3O7−x superconductors (∼14×14×14 mm3) with orthogonal c-axes so as to form a basic unit of Halbach array. The experiments are carried out at 77 K. The experimental distribution of the magnetic flux density above the array of trapped-field superconductors is compared to a similar array made of permanent magnets. A simple analytical model is developed and is shown to accurately reproduce the main experimental observations. The results suggest that a redistribution occurs in the current flowing in the central sample when the distance between the superconductors is reduced, whereas the neighbouring superconductors are unaffected. It is shown that this current redistribution yields a reduced contribution of the central sample to the magnetic flux density above the centre of the array and a new negative contribution associated with stray fields to the magnetic flux density at this location. This interpretation is confirmed by modelling of the distribution of transport currents in the superconductor using a 3D finite element model

Improved trapped field performance of single grain Y‐Ba‐Cu‐O bulk superconductors containing artificial holes

Abstract: The intrinsic mechanical properties of single‐grain RE‐Ba‐Cu‐O bulk high‐temperature superconductors can be improved by employing a thin‐wall geometry. This is where the samples are melt‐processed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thin‐wall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thin‐wall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thin‐wall sample should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the sample. High‐field magnetization experiments were performed by applying magnetization fields of up to 11.5 T, specifically to break the samples in order to verify the effect of increased mechanical strength (and improved cooling) on the ability of bulk (RE)BCO to trap field successfully. The standard YBCO sample failed when it was magnetized with a field of 10 T at 35 K, suffering permanent damage. As a result, the standard sample could only trap a maximum surface field of 7.6 T without failure. On the other hand, the thin‐wall YBCO sample survived all magnetization cycles, including a maximum magnetization field of 11.5 T at 35 K, demonstrating a greater intrinsic ability to withstand significantly higher electromagnetic stresses. By subsequently field‐cooling the thin‐wall sample with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement

Trapped field evolution of two approaching bulk superconductors with parallel c-axes

Although the properties of individual trapped field magnets are well documented, there is little scientific information concerning their trapped field behaviour when several of them are brought in close vicinity. In this context, the translational approach of two cylindrical bulk YBCO superconductors (diameter 15.7 mm, height 8.8 mm) with parallel c-axes is experimentally investigated. The evolution of the trapped field at the surface of a fixed superconducting sample is measured while a second sample is approached. The following configurations are considered: (i) face to face (with anti-parallel trapped field orientation) approach and (ii) sideways (with parallel trapped field orientation) approach. The studied distance between the samples ranges from 120 mm, where the samples are observed to have no influence on each other, to contact (0 mm). The experiments are carried out at 77K and the samples are magnetised independently in field cooled under 1.2 T. The motion actuation is performed using a computer-controlled miniature linear actuator which is capable of overcoming the repulsive force arising between the trapped field magnets. Experimental trapped field data are compared to analytical calculations assuming fully magnetized superconductors. The trapped field of the fixed superconducting sample is measured to be altered by the second one when the separating distance is smaller than their diameters. If the distance is smaller than this limit, an irreversible diminution of the trapped field is measured when the samples are separated again. Repeated approach cycles showed that the irreversible loss of trapped field is more important for the first approach

YBa2Cu3O7-x thick films on silver tubes for magnetic shielding applications

YBa2Cu3O7-x (YBCO) thick films are investigated for magnetic shielding applications at low frequencies (< 1 kHz). This requires deposition of YBCO on curved substrates such as tubes, half-tubes or even more complex shapes. Electrophoretic Deposition (EPD) is an efficient method to achieve this goal: positively-charged YBCO particles suspended in a non aqueous medium drift towards the substrate, used as negative electrode for the application of the electric field. A crucial point is then to optimize the heat treatment of the as-deposited layers in order to achieve suitable superconducting properties. In the present work, we have developed a new suspension formulation in butanol, using a suitable surfactant to stabilize the suspension of YBCO powder (grain size < 2 µm). The EPD parameters (deposition voltage, deposition time, number of layers,...) have been selected to provide uniform layers of YBCO on silver substrates of various shapes. In the proposed communication, we shall discuss in detail the optimization of the heat treatments to achieve densification, peritectic recombination and oxygenation of the YBCO thick films deposited on silver. Scanning electron microscopy coupled with energy dispersive analysis was extensively used to characterize the (i) density / porosity, (ii) presence of macro-cracks, (iii) thickness uniformity, (iv) secondary phase content. We shall then present the characterization of the superconducting properties of the best films. Typically, a uniformly coated 55μm-thick YBCO film on a curved Ag substrate shows a magnetic Tc onset at 92.2 K and sharp resistive transition (< 1K)

Magnetic Shielding Above 1 T at 20 K With Bulk, Large Grain YBCO Tubes Made by Buffer-Aided Top Seeded Melt Growth

YBCO tubes of ∼10 mm diameter closed at one extremity were engineered by a buffer-aided top seeded melt growth fabrication process. These tubes can act as efficient “dc” magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding “dc” stray fields generated by high temperature superconductor magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K), or liquid neon (27 K)

Research data supporting "An improved top seeded infiltration growth method for the fabrication of Y–Ba–Cu–O bulk superconductors"

This data, in the form of both raw text and an origin file is the raw hall scan data supporting figure 14 in the associated paper. Sample 510 is the TSMG sample, sample 511 is the TSIG sample. The data is tab delimited and contains the magnitude in Tesla of the measured magnetic field at various positions across the sample denoted in both X and Y coordinates as well as polar coordinates.This work was supported by the Engineering and Physical Research Council [grant number EP/K02910X/1] and KACST

YBCO superconducting thick films: electrophoretic deposition (EPD) on non-planar silver substrates

A new formulation of a stable YBa2Cu3O7-δ (YBCO) suspension is proposed in which butanol is preferred to the commonly-used acetone as the suspension medium. Appropriate surfactant has been used to develop the superficial charge on the YBCO particles in order to promote migration during the process of electrophoresis. YBCO thick films were deposited on silver tubes and half-tubes by electrophoretic deposition (EPD). The EPD parameters (deposition voltage, deposition time, number of layers etc.) were optimized with respect to the microstructural properties of the YBCO layers after an intermediate heat treatment at 920°C. An essential criterion is the minimization of macrocracks after the 920°C heat treatment, since it was found to favour good superconducting properties after the final heat treatment. This final heat treatment involves (i) partial melting above the peritectic temperature, (ii) peritectic recombination at lower temperature and (iii) reoxygenation at 500°C. Finally, the superconducting properties of the best films are discussed. A uniformly coated 55μm-thick YBCO film on a curved Ag substrate showed excellent superconducting properties with the onset of critical temperature at 92.2 K and an associated sharp resistive transition with transition width < 1 K

Distribution du champ piégé au dessus d'un array de Halbach supraconducteur

peer reviewedA compact system generating a large gradient of the magnetic flux density is often required in applications involving a magnetic force. One well-documented way to compactly generate such magnetic gradients is to assemble permanent magnets in a Halbach array. However, the saturation magnetization of the magnets involved in the array is a fundamental limitation impeding the performance of this configuration. The use of trapped field superconducting magnets instead of permanent magnets in a Halbach array could potentially allow the generation of larger field gradients as they do not suffer from the same limitation. In this context, the assembly of cuboid bulk, large grain melt-textured YBCO superconductors (~14x14x14 mm3) with orthogonal c-axes is investigated experimentally. The experiments are carried out at 77K and the samples are independently magnetized in field cooled configuration under 1.2 T. The assembly is performed by maintaining a central magnetized superconductor stationary while approaching additional trapped field magnets from left and right with their magnetization axes perpendicular to theirs neighbours. The flux density over the surface of the stationary sample is measured during the assembly of the array. A flux creep period of 45 minutes is then allowed before performing several scans of the magnetic flux density generated by the array of superconductors above the surface of the whole configuration. The experimental distribution of the flux density above the array of trapped field superconductors is compared to a similar array made of permanent magnets. The results suggest that a reorganisation of the current density distribution occurs in the central sample when the distance between the superconductors is reduced. In spite of this phenomenon, the array of superconductors is shown to produce a field gradient as high as 10 Tm-1 at a distance of 1 cm away from the array