A Simple Measurement Technique for Critical Current Density by Using a Permanent Magnet

Rapid and simple measurement of critical current density in HTS thin films using a permanent magnet method

We introduce a novel transmission-line method for determining the penetration depth in thin high-temperature superconducting (HTS) films. The method is based on the accurate measurement of the inductance per unit length of a superconducting coplanar-waveguide (CPW) transmission line fabricated on the HTS sample. Using the experimentally obtained inductance per unit length, we interpolate the penetration depth from a table of numerically determined values of inductance (per unit length) as a function of penetration depth, calculated from a coupled transmission-line model. A novelty of our procedure is the utilization of the multiline thru-reflect-line (TRL) method and the calibration-comparison method to accurately determine the inductance per unit length of the superconducting transmission line. By measuring different CPW geometries patterned onto the same thin-film sample, we can verify our penetration depth values. We demonstrate this technique by extracting the penetration depth for several different HTS films at 76 K. Estimated uncertainties in the extracted penetration depth are also provided.

While the global demand for energy continues to grow, the world’s depots of fossil fuels decrease. Logically, scientists have researched in different ways of creating sustainable energy. Dams, solar power and wind energy are all different ways of extracting power from nature. Having neither mountains nor many hours of annual sunshine, Denmark is, however, a flat country with many kilometers of coast line and calm weather, and therefore is perfectly suited for wind energy. Because of these facts, Denmark has always been a pioneer within the wind industry, building the world’s first off-shore wind energy park in 1991. In recent years, wind turbines have grown dramatically in size and capacity. Scientists predict that in order to make wind energy a more suitable alternative to fossil fuels, the wind turbines have to become even larger. The major problem with these super wind turbines, however, is the weight of the nacelle, of which a heavy part is the gear box. In order to avoid having a gear box in the wind turbine, a direct driven generator has been proposed, where superconductors play an important role. Energy losses due to Joule heating will be eliminated and high temperature superconductors are capable of working in high magnetic fields (several Teslas). The alternative to superconductors are permanent magnets, but the required amount would become expensive and inaccessible, while superconductors can be made from easily accessible elements. However, using superconductors also pose considerable challenges; low temperature cooling systems are required and the superconductors need to be of high quality. High temperature superconductors is a relatively new field of study, with the first discoveries made in the 1980s. Having found materials with superconducting properties at relatively high temperatures, the issue of optimizing other properties arose. A pinning center is an irregularity in the crystal, which destroys the superconducting properties. Here the magnetic field lines can penetrate without causing further damage to the superconductor. In 2005 Misko et al. found that a gradient in density of pinning centers could increase the critical current density (Jc) in a magnetic field. Pinning centers had been known to optimize superconductors, but with this new information, scientists have been equipped with a new tool to make superconductors more suitable for wind turbines. Since Misko et al. first predicted that different pinning densities throughout the superconductor could improve superconductors; many attempts have been made in order to show this property experimentally. Different methods to create the desired pinning patterns experimentally include Molecular Beam Epitaxy and E-beam Lithography. These methods are, however, both very expensive, making them less attractive for industrial production. This project investigates different options of creating simple gradients in the pinning center density.

The permanent magnet method for measuring the critical current density of the high-temperature superconducting (HTS) thin film is investigated numerically. The numerical code has been developed for analyzing the time evolution of the shielding current density in the HTS thin film. In the code, the initial-boundary-value problem of the shielding current density is solved numerically. After discretized by using the finite element method (FEM) and the backward Euler method, it is reduced to the problem in which the simultaneous nonlinear equations are solved at each time step. By using the numerical code, the maximum repulsive force is calculated as a function of the critical current density. The results of computations show an excellent proportional relationship between them. This is mainly because the spatial distribution of the shielding current density varies in proportion to the critical current density except for the neighborhood of the film edge.

Pulsed laser deposition (PLD) has been used to deposit high quality thin films of Ni81Fe19/Au/YBa2Cu3O7- (delta ) onto (100) oriented substrates of MgO and SrTiO3 for the purpose of fabricating a novel high temperature superconducting three terminal device. The ferromagnet-normal metal-superconductor (F-N-S) structure is currently being investigated to determine the effect of the injection of a spin-polarized current on the order parameter of a high temperature superconducting thin film. High quality films with sharp interfaces, free of defects, are required in order to maximize the spin-injection effect. The surface morphology and transport properties of the YBa2Cu3O7-(delta ) films have been investigated using scanning electron microscopy and ac susceptibility measurements, respectively, as a function of increasing laser fluence. Deposition at 2.0 - 2.4 J/cm2, 790 degrees Celsius and 320 m Torr O2 produces films with a sharp superconducting transition and a smooth surface. The growth of Au on YBCO under different PLD conditions has been observed by atomic force microscopy. Surface clustering of Au occurs at elevated temperatures and is attributed to increased surface mobility. The presence or absence of a background gas influences the cluster size. These results are discussed within the framework of the role of excess energy of PLD adatoms with changing laser fluence and background gas.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A receiver with an rf band of 34.6 GHz to 35.4 GHz, an integral local oscillator with a frequency of 36.5 GHz, and an IF frequency of 1.1 GHz to 1.9 GHz was designed and fabricated. The rf filter was a three pole distributed design fabricated with High Temperature Superconductor (HTS) films. The IF filter was a 9 pole lumped element design fabricated with HTS films. The local oscillator was a parallel feedback type design using a sapphire resonator shielded by two HTS ground planes. The rest of the elements used normal conductors or semiconductors operated at 77 K. The entire subassembly was integrated into a liquid nitrogen dewar and its performance is reported in the text, with the important figures of merit being low noise figure and low phase noise.

ABSTRACTFor type II superconductors (SC), magnetic field can be trapped, or pinned due to persistent internal current. Upon magnetization, SC samples behave in some ways similar to a metallic permanent magnet. The trapped field is high and quasi-persistent, and we refer to it as a “magnet replica”. So far, nearly 1T @ 65 K, and over 0.4 T @ 77 K have been measured within small (about 1 × 1 × 0.6 cm3) melt-textured Yba2Cu3Ox (MT-Y123) samples. Based on our theoretical studies, extrapolation to larger scale magnets indicates that 2–4 Tesla in liquid Nitrogen (and even larger field at lower temperatures) is achievable with our high temperature superconductor (HTS) material. Using this effect, magnets with dipole, quadrupole, or more complicated configurations can be made of existing MT-Y123 material, thus bypassing the need for HTS wires. Two types of motors have been successfully constructed, using the trapped field in MT-Y123 samples.The spatial distribution of the trapped magnetic field on MT-Y123 materials has been studied. A phenomenological model has been developed to account for the trapped field intensity and profile in HTS samples. General features of magnet replicas by HTS will also be discussed.

A numerical code has been developed for analyzing the shielding current density in a High-Temperature Superconducting (HTS) thin film. By using the code, the inductive method has been successfully reproduced even when the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> -dependence of the critical current density is taken into account. The results of computations show that the outer radius of the coil must be nearly equal to or less than the sample radius to ensure the sufficient accuracy of the inductive method. Furthermore, Mawatari's conjecture on the third-harmonic voltage generation is justified by use of the numerical simulation.

The inductive method for measuring the critical current density in a high-temperature superconducting (HTS) sample has been investigated numerically. In the method, the magnetic field is generated by a small coil placed just above an HTS sample. While applying an ac current I(t)=I0sin2πft in the coil, the third-harmonic voltage V3sin(6πft+θ3) induced in the coil is measured. For the purpose of numerically reproducing the inductive method, a numerical code has been developed on the basis of the finite element method and the backward Euler method. By means of the code, I0-V3 curves are determined for an HTS bulk or for double-sided HTS films on a substrate. The results of computations show that, for an HTS bulk, the accuracy of the inductive method is drastically affected by the coil position, the coil radius and the frequency. In addition, it is also found that, for double-sided films, the accuracy is significantly influenced by a substrate thickness.

The current carrying ability of the high temperature superconducting (HTS) material would be seriously influenced by the additional complex magnetic field. Because of this effect, the HTS material cannot be adapted in the field of high speed motors that work with high frequency current and strong magnetic field density. Therefore, this paper presents a novel HTS high speed motor system with high efficiency, high power density and high speed that could be adopted in the field of flywheel energy storage and high efficiency propulsion system. The one armature winding of the novel HTS high speed motor system is the superconducting coils, and the other armature winding is copper winding. Under this configuration, the motor with two armature windings is the doubly-fed motor. The two kinds of windings should operate in synchronous state. In this paper, a 2kW, 12000r/min high speed flywheel energy storage motor is designed. Surface magnetic field distribution of the superconducting winding and the characteristic of this type motor are detailed analyzed based on the current carrying ability of the HTS material. The new configuration of our novel HTS motor broaden the application of HTS materials in various electrical fields.

(Bi,Pb)-2223 HTSs (high temperature superconductors) were synthesized from nominally pure (reference) and BN-added Bi1.7Pb0.3Ca2Sr2Cu3O y (BN) x precursors (x=0,0.10,0.15, and 0.20) by the solid state reaction method using alumina crucibles. The influence of boron nitride addition on the phase formation kinetics and transport properties of (Bi,Pb)-2223 HTSs was studied using X-ray diffraction (XRD), resistivity and critical current density measurements. BN-added compounds reveal a significant enhancement in both the high-T c 2223 phase formation and critical current density compared to the reference specimen.

Superconducting quantum interference devices (SQUIDs), which are the most ultrasensitive magnetic sensors, are generally made by microfabrication of superconducting thin films. The bicrystal junction, which is often used in high-temperature superconducting (HTS) Josephson junctions (JJs), has problems expensive bicrystal substrates, and the fact that JJs can only be arranged in a fixed straight line [1-2]. Because the HTS thin film is a type-II superconductor, magnetic fluxes penetrates the HTS thin film and moves for the Lorentz force, which increases the flux noise at low frequencies.In this study, we investigated a method to fabricate low-noise SQUID magnetic sensors by Ga focused ion beam (FIB) irradiation of high-temperature superconducting thin films. When HTS thin film is irradiated with FIB, point defects are introduced into the HTS crystal structure, which leads to changing into normal conduction. The dependence of the critical current change ratio IC/IC0 on the dose was shown in Fig.1. The dependence of the normal resistance RN are also depicted. The thickness of the YBa2Cu3O7-δ (YBCO) film was 50 nm and it was protected by a thin Au layer. IC/IC0 decreased with irradiation fluence and changed into normal conduction at 2.0×1017 ions/cm2; after this point RN increased steeply.HTS nanobridge JJ, were fabricated by FIB irradiation. Figure 2 shows the V-I characteristics of a nanobridge with an HTS film thickness of 50 nm and a bridge width of 120 nm. The critical current was reduced to 264.5 μA and shows a clear Shapiro-steps under microwave irradiation with frequency of 10 GHz. As a method to reduce the magnetic flux noise, we also investigated the introducing of normal conducting pinning sites (anti-dots) into HTS thin films by FIB irradiation. We have confirmed that anti-dots with diameters of several hundred nm can be formed by point irradiation of HTS thin films with FIB. In the future, we will confirm SQUID operation at a nanobridge JJ with an YBCO film thickness of 50 nm and investigate the improvement of SQUID noise characteristics by introducing anti-dots.  Fig. 1. Dependence of IC/IC0 and RN on FIB irradiation fluence.  Fig. 2. V-I characteristics of the HTS nanobridge with a width of 120 nm.

We have derived general equations to calculate intermodulation distortion in resonators with high temperature superconducting (HTS) films which are not restricted to a specific resonator shape and may be used whenever thefields in a resonator generate currents on the surface of one or more HTS films. These equations are applied to rutile-loaded cavities with one or two 10 x 10 mm 2 Y-Ba 2 -Cu 3 -O 7 - δ endplates and are used to extract parameters characterizing the nonlinearities of these films from intermodulation measurements. Even though the films have similar small-signal performance, we have found large variation in the strength of their nonlinearities.

Complex approach to the investigation of microstrip resonators and filters based on High-Temperature Superconductor (HTS) films is described, which includes modeling of the electrodynamic parameters of HTS films, designing of microstrip resonators and filters, their manufacturing, and testing. Test samples were prepared using YBCO films on 0.5-mm-thick lanthanum aluminate substrates. The resonators and filter structures were patterned using ion-beam photolithography. Experimental data were used to determine the parameters of the model of surface impedance of the YBCO film and the film thickness and permittivity. Adequacy of the model and reliability of the model parameters, which were used for the synthesis and design of a 4th-order filter, were confirmed by coincidence of the experimental and calculated characteristics.

High quality thin films of high temperature superconductors (HTS) have been fabricated using pulsed or cw lasers as physical vapor deposition sources. There is current interest in scaling the pulsed laser deposition (PLD) technique to coat 3-in. wafers with HTS. Process control and diagnostics relevant to achieving the required scaling and reproducibility will be reviewed.