Bulk high-Tc Superconductors Research Articles

Alternating Over-Current Characteristics of HTSC Tape Using DFT

The quench characteristics of high-TC superconducting (HTSC) tape for alternating over-current applications have been investigated at various amplitudes of the over-current and its quench developments have been analysed by applying discrete Fourier transform (DFT) for the voltage and the current waveforms of HTSC tape. Generally, the HTSC tape shows a broad superconducting and normal transition region above its critical current due to the metal sheath unlike HTSC thin film or HTSC bulk. Moreover, the voltage-current characteristics of HTSC tape are complicated because the quench and the recovery between the superconducting state and the normal state are repeated. In this paper, the alternating over-current experiments for the HTSC tape were performed to analyse its quench developments. The fundamental frequency components for the current and the voltage waveforms of the HTSC tape were abstracted by applying DFT. From those fundamental frequency components, the numerical formula for HTSC tape's resistance has been derived. By applying its numerical formula into circuit equation, the quench developments of HTSC tape dependent on alternating over-current have been estimated and the agreement with the experimental results has been found

Numerical evaluation of the magnetic field of aQMG® coil magnet

This paper reports the fabrication and properties of aQMG® coil magnet as a high field application of an oxide bulk highTc superconductor. QMG® is a high Jc bulk material, which consists of single crystalline 123 with finely dispersed 211. AQMG® coil with seven turns is made from a slice of Ag doped Gd–Ba–Cu–OQMG® by using precise machining techniques. The magnet is made from six layers of theQMG® coil with magnetic fields of 0.56 T at 340 A at 77 K and 1.18 T at 700 A at 63 K. In the presentstudy, the magnetic fields reported in experiments are numerically analysed with anaxisymmetric model, where the superconductive characteristic is evaluated using a criticalstate model. The numerical results for the magnetic field agree well with the experimentalresults.

Direct relationship between trapped magnetic flux characteristics and percolationtransition temperature in a practical sized bulk high-temperature superconductor

The trapped magnetic flux characteristics of a Gd-system bulkhigh-Tc superconductor (HTS) near the so-called percolation transition temperaturewas investigated experimentally. The percolation transition temperaturewas directly determined from the temperature dependence of theE–J characteristics with a bar-shaped HTS. Next, the disc-shaped bulk HTS sample wasmagnetized by means of a field-cooling process, and then the decay of the trapped magneticflux density was monitored for 30 min after the exciting current of the electromagnet wascut off. It was shown that the decay rate of trapped magnetic flux changed considerablyaround the percolation transition temperature. Furthermore, the trapped flux showedstepwise decay, and these phenomena could explain the percolation mechanism of thedepinning process. From our study, the percolation temperature may be utilized for thelimit parameter in order to design application systems with a practical sized bulk HTS.

Simulation of the current dynamics in a superconductor induced by a small permanentmagnet: application to the magnetoscan technique

The magnetoscan is a rather new technique for investigating local inhomogeneities of large bulkhigh-Tc superconductors. Local (shielding) currents, which are induced by a small permanentmagnet slightly above the sample surface, generate a magnetic field that is recorded by aHall probe. We report on a theoretical analysis of such a three-dimensional system usingnumerical simulations to calculate the current dynamics during a magnetoscan.The magnetoscan signal is analysed for different experimental setups and different criticalcurrent densities of the superconductor. We show that the shape of small defects(∼1 mm in size) is almost exactly displayed, even if the radius of the permanent magnet is muchlarger than the defect size. The critical current density of larger inhomogeneities can bedirectly determined at the sample surface, if all details of the experimental setup areknown. Different aspects of how to improve the experimental resolution are discussed.

Analytical Model of Superconducting to Normal Transition of Bulk High Tc Superconductor BSCCO-2212

A comprehensive theoretical expression for the superconducting-to-normal transition of high-temperature superconducting (HTS) materials is derived. The electric field and resistivity of bulk melt-cast processed BSCCO-2212 are derived from the superconducting, flux creep, flux flow, and resistive properties of high critical temperature superconductors (HTS). An asymptotic function is used to model the Bean and Ohmic limits of the E(B,T,J) characteristics. Comparison is made with the experimental results of Elschner et al. The new model may be used in simulating the operation of HTS power equipment, and may be applicable to other HTS materials

AC magnetic characteristics of bulk high-Tc superconductor with numerical analysis and evaluation of exothermic phenomenon

Since bulk high-Tc superconductor (HTS) characteristics are not homogeneous in some specimens due to grain boundaries and cracks, magnetic characteristics should be assessed under the static and AC magnetic field. Thus, static magnetic characteristics of HTS have been evaluated by measurements of the magnetic flux density on the specimen surface using a Hall element and numerical analyses using a finite element method. AC magnetic characteristics have been experimentally evaluated by measurements of the magnetic field using a Hall element and pickup coils. In this research, AC magnetic characteristics of HTS are evaluated by numerical analyses using a three-dimensional finite element method. Critical current densities of HTS under various external magnetic field strength are estimated by analysis and experiment results. Moreover, the heat generation of the superconductor is considered. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(4): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20134

Compact electron beam ion source with high-TC bulk superconductors

We have developed an electron beam ion source (EBIS) assembling three rings made of high-TC superconductor as a solenoid coil, which enables us to construct a “table-top” EBIS operated at the liquid N2 temperature with a strong magnetic field. Optimizing a pulse field magnetization procedure, the assembly yielded a magnetic field as high as 0.8 T under a persistent mode, which stably lasted more than two days. An electron beam of 12 keV-50 mA was compressed to a current density of about 300 A/cm2 and guided by the magnetic field along the axis of the drift tube. After exiting the drift tube, the electron beam was decelerated to 2 keV and collected with an electron collector with an efficiency of more than 99%. We have succeeded in extracting highly charged ions such as Ar17+ and Xe42+.

Repulsed magnetization of a bulk superconductor by long and low field with temperature control

Pulsed field magnetization is important technique to magnetize a bulk high T c superconductor (HTS), especially for practical applications of bulk superconducting magnets. Several important points for the pulsed field magnetization are reported from many experimental results. There is appropriate applied field for the maximum magnetization because of heat generation, and repulsed technique is also useful to obtain the maximum magnetization. In this study, pulsed field magnetizations of a Y–Ba–Cu–O bulk HTS with long (320 ms) and low (137 mT) pulsed field are evaluated with axisymmetric numerical simulation code. Shielding currents are obtained based on critical state mode, and Kim model is applied to its dependence on the magnetic field. Numerical results agree well with the experimental field measurements.

Surface flux density distribution characteristics of bulk high-Tc superconductor in external magnetic field

Surface flux density distribution characteristics of bulk high-Tc superconductor in external magnetic field

バルク高温超電導体の交流磁気特性解析と発熱現象の評価

Since bulk high Tc superconductor (HTS) characteristics are not homogeneous in some specimens due to grain boundaries and cracks, magnetic characteristics should be assessed under the static and AC magnetic field. Thus static magnetic characteristics of the HTS have been evaluated by measurements of the magnetic flux density on the specimen surface using a hall element and numerical analyses using a finite element method. AC magnetic characteristics have been experimentally evaluated by measurements of the magnetic field using a hall element and pick up coils. In this research, AC magnetic characteristics of HTS are evaluated by numerical analyses using a three-dimensional finite element method. Critical current densities of the HTS under various external magnetic field strength are estimated by analysis and experiment results. Moreover, the heat generation of the superconductor is considered.

Evaluation by numerical analysis of magnetic shielding characteristics of bulk high‐Tc superconductor containing weak links

AbstractHigh‐Tc superconductors (HTS), which have the characteristics of critical current density over 3 × 104 A/cm2 at liquid nitrogen temperature (77 K) and 1 T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, magnetic levitation, flywheel, and magnetic shielding. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a Hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a Hall element is difficult regarding the distribution of the magnetic flux density on the actual surface and inside of the HTS sample. In this research, we conducted a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under a static magnetic field with the three‐dimensional finite element method analysis. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 9–17, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10209

Low Field Second Harmonic Response and AC Susceptibility of (Bi,Pb)-2223 Pellet in a Generalized Critical State Model

The AC susceptibility and second harmonic response of bulk high Tc superconductor (Bi,Pb) -2223 is measured in presence of small AC field excitation and DC magnetic field superimposed on it. This response increases with the increase in DC field whereas decreases at a higher excitation amplitude. Critical state models are used to explain the nonlinear magnetic response in HTSCs. With a generalized field dependence of critical current Jc : Jc(B) = J0/(1 + B/B0)n, the fundamental AC susceptibility and low field second harmonics response are calculated for different n values. It is found that the theoretical consideration adopted here explains the observed AC susceptibility and low field second harmonic response qualitatively reasonably well within the Anderson–Kim regime (n = 1).

Study on a mobile-type magnetic separator applying high-Tc bulk superconductors

Abstract We have developed a new water-treatment system that consists of a membrane separator and a magnetic separator that uses long high-Tc bulk superconductors (SCs) as permanent magnets. Basic tests were performed using a new prototype water-treatment system to verify the separator’s ability to remove water bloom. Based on the test results, we design the mobile-type magnetic separator system. The designed magnetic separator system consists of three components; a pre-process unit, a rotating membrane separator and a magnetic separator. The capacity of the magnetic separator is 100 ton/day. Removal rate is 90% or more. It is thought that the system is compact and high efficiency.

Thermal and mechanical properties of high Tc bulk superconductors and their applications

The Iwate CREATE (Collaboration of Regional Entities for the Advancement of Technological Excellence) Project, “Advanced Applications of Magnetic Field”, started since October 1999, aims a creation of a COE and a new industrial area in the field of magnetic field applications and magnetic measurements. The basic study group in this project is making extensive measurements on mechanical properties (strength, toughness and so on) and thermal properties (thermal conductivity, thermal diffusivity, Seebeck effect, and so on) of melt-textured high T c bulk superconductors in order to build up a database and feed it back to the higher performance bulk material processing technique in the future. This group also studies the magnetizing technique for these materials and realization of various magnetic field environment for the application to the magnetic separation, highly oriented polymer film and so on.

Magnetization Method for Long High-Tc Bulk Superconductors. Study on Swing-Field-Cooling Method.

We have developed a new water-treatment system that consists of a membrane separator and a magnetic separator using long (>200mm) high-Tc bulk superconductors (SC's) as permanent magnets. For the magnetization of the long bulk SC's, we developed a new magnetization method, i. e., a swing-field-cooling method. This method involves applying a field, and cooling the bulk SC's below their transition temperature (Tc). The bulk SC's are then moved inside and outside of the applied field repeatedly. As a result, the bulk SC's experience high and low fields under the gradually cooled ambient, and the trapped field strength gradually increases. The advantage of the method is that it can magnetize bulk SC's with larger area than that of the applied field magnet. To evaluate the method, we fabricated a bulk magnet system more than 200 mm long, and magnetized it by split solenoid superconducting magnets with a diameter of approximately 100 mm. We then perfored field measurements and found that a wider area could be magnetized by the present method. In addition, we also developed a simple model-based on the Bean model-of the magnetization process. Taking account of the temperature dependency of critical current density (Jc), the model could successfully simulate the resulting magnetic field produced by the present method.

Copper Defects and Conductivity in Bi—Pb—Sr—Ca—Cu—O Glasses

Glass ceramics of the composition (Bi0.8Pb0.2)4Sr3Ca3Cu4O8 prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu2☎ paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-Tc superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu2☎ paramagnetic ions and the exchange coupled clusters.

A compact electron beam ion source equipped with a bulk high-Tc superconductor solenoid

We have been developing an electron beam ion source equipped with an assembly of bulk high-Tc superconductors (YBCO) as a solenoid, which fulfil the following practical requirements: high magnetic field, compact, operation at liquid nitrogen temperature. The energy and current of the electron beam are designed to be 30 keV and 100 mA at the maximum, which are sufficient to produce highly charged ions such as Ne-like U82+. Recently, we have succeeded in producing a stable solenoidal field of 0.8 T, and in injecting an electron beam of 10 keV – 40 mA keeping beam collection efficiency of ∼99% at the electron collector. The details of the design and the results of extraction tests are presented.

Examination of the method for analyzing inverse problem to determine the internal current of bulk high-Tc superconductor

This paper reports about the validity of the numerical model for presuming the internal current of the superconductor that is difficult to measure. The validity of the numerical method is confirmed by the simple analysis model. This model is constituted applying boundary element method and the difficulty of measuring the internal current is solved by the inverse problem from the exterior flux density distribution. Since the application of this model draws the solution of an irreversible magnetization phenomenon not clarified, the apparatus that uses the superconductor will develop.

Application of electron backscatter diffraction to bulk high-Tc superconductors

We have successfully obtained electron backscatter diffraction (EBSD) Kikuchi patterns on various bulk high-Tc superconductors (polycrystalline KClO3-doped and undoped YBa2Cu3O7−δ (YBCO), single crystalline YBCO and NdBa2Cu3O7−y) and on YBCO and NdBCO thin films. We have established a sample polishing procedure to obtain intense Kikuchi patterns without an additional carbon coating of the ceramic superconductors. We present Kikuchi patterns, pole figures and orientation maps of the samples. On the polycrystalline KClO3-doped YBCO samples we find two major orientations (001) and (100) normal to the sample surface, while the undoped sample does not exhibit any orientation maxima, which is ascribed to the effect of a liquid phase due to the KClO3-additive during the preparation process. Furthermore, we discuss the future possibilities for the application of the EBSD technique for growing larger, bulk melt-textured samples for applications.

Characteristics of the magnetic gear using a bulk high-Tc superconductor

A magnetic gear which can transmit force without contact has operational benefits of requiring neither vibration, noise, nor lubrication. Still, its transmission force is considerably smaller than a mechanical gear. Strong magnets are needed to improve magnetic gear transmission force. For this purpose, a magnet formed by the pinning effect in a bulk superconductor might have magnetic flux density several times stronger than rare-earth permanent magnets. We propose a noncontact magnetic gear using a bulk high-Tc superconductor. In this paper, a three-dimensional electromagnetic field analysis program has been developed to analyze characteristics of the proposed gear. Two superconducting magnetic gears with different magnet configuration are manufactured, and their characteristics are measured. The analysis and the experiment explore the possibility of a magnetic gear using the pinning magnet formed in the superconductor.