This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE?=?rare earth or Y) and MgB2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialization stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarized with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet?superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets. Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

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Modelling of bulk superconductor magnetization

Chapter headings: Old but new materials: "carbons". Highly orientated graphites. Isotropic carbons. Carbon fibers. Porous carbons. Intercalation compounds. Carbon composites. Concluding remarks.

New carbons : control of structure and functions

Options for the use of high temperature superconductor in tokamak fusion reactor designs

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

The preparation of Y?Ba?Cu-oxide (YBCO) powder by various techniques is critically reviewed and the synthesis of ultrafine superconducting YBa2Cu3Ox(123) powders is specially emphasized. The fine powder has assumed a large importance, requiring the minimization of processing parameters (time and temperature) during calcination, homogenization and sintering to avoid the problems of evaporation of the constituent oxides. From the voluminous research activities on YBCO it is known that by adopting any preparation technique the superconducting transition temperature (Tc) value of ?92?K could be achieved in the bulk samples, thus in this review the Tc values are not emphasized. Also, the other transport properties such as critical current densities (Jc) and magnetic properties are not discussed, since these properties are mainly controlled by many other parameters related to the weak link across the grain boundaries of the specimens, and a very little role is played by the synthetic procedure. The weak links across the grain boundaries are known to be the cause for the poor Jc values in bulk specimens and those weak links are observed due to the presence of impurities at the grain boundaries, misorientation of grains and oxygenation of the samples. By selecting the proper synthetic procedure, only the impurity problem can be solved, and to remove the other problems, a proper fabrication method needs to be adopted. This review will guide the new researcher to find the appropriate technique to synthesize good quality YBCO powder.

https://iopscience.iop.org/article/10.1088/0953-2048/18/9/R01/pdf

A review on the synthesis of Y?Ba?Cu-oxide powder

The critical current density (Jc) of high critical temperature (Tc) copper-oxide superconductors is an important parameter in magnetic shielding. However, it was determined that the value of Jc was not closely related to the degree of magnetic shielding. The degree of shielding can be improved by the use of a configuration of superimposed double cylinders constructed of a copper-oxide superconductor. Experimental results indicate that the characteristics of the double-cylinder system (such as the effect of the excitation magnetic field on the magnetic field within the cylinders, and the distribution of the magnetically shielded field along the axial direction of the cylinders) are improved over those of single superconducting cylinders.

http://iopscience.iop.org/article/10.1088/0022-3727/25/11/010/pdf

The improvement in magnetic shielding by use of a double-cylinder configuration of high-Tc oxide superconductors

During sintering, superconducting copper-oxide films commonly have a high chemical activity with substrate materials, which results in poor superconductivity. In this research, the substrate is prepared by coating multiple non-superconducting Y-Ba-Cu-O films on alumina. The superconducting Y-Ba-Cu-O thick films were prepared by mixing powders Y2O3, BaCO3 and CuO, making a paste by mixing this noncalcined mixture with solvent, coating it on to a film-coated substrate using a painting or printing method, and then sintering the specimen. The thick films have positive superconductive characteristics at high temperatures.

Preparation of superconducting Y−Ba−Cu−O thick film

Electrical properties of Y–Ba–Cu–O superconductors, such as the critical current density and the zero-resistance temperature, are systematically improved by changing the compression pressure during the formation process of the specimens. Also, by changing the silver powder content of the superconducting Y–Ba–Cu–O powder, the critical current density and the zero-resistance temperature are remarkably improved. The resistance-drop temperature is insensitive to changes in the compression pressure, silver content, and magnetic field, if the field is applied perpendicular to the specimens. The critical current density of the specimens with and without silver decreases exponentially with the perpendicular magnetic field. The critical current densities of specimens without silver showed much lower sensitivity to perpendicularly applied magnetic fields than those of the specimens having a silver content. Along with the above improvements, the present paper also examines several factors that affect the superconducting characteristics. These factors include impurities in the air and strong magnetic fields.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400014138

Influence of silver on critical current of the Y–Ba–Cu–O superconductor

Magnetic shielding was shown to be improved by superimposing a magnet cylinder over a superconducting cylinder. Examples of such configurations are soft iron and electromagnetic steel cylinders over a copper-oxide superconducting cylinder. The value of the magnetic shielded field for the superimposed cylinder was found to be several times more than that of single superconducting cylinders such as YBCO or a BPSCCO cylinder. Experimental results reveal good characteristics which include the effects of the excitation magnetic field on the inner magnetic field within both the single superconducting and superimposed cylinders, the temporal change of the trapped magnetic field within the superconducting cylinder, the distribution of the magnetic shielded field along the axial direction of the superimposed cylinders, and the relationship between the length of the magnetic cylinder and the shielding effect. >

Improvement in magnetic shielding by the superposition of a magnetic cylinder over a copper-oxide superconducting cylinder

High Tc copper oxide superconductors, such as Y-Ba-Cu-O, commonly have a high chemical activity during sintering. Y-Ba-Cu-O superconductors are influenced by impurities in air, which cause degradation of the superconducting properties, critical current density (Jc) and temperature of zero resistance (Tczero). XPS analysis of low Jc and Tczero materials shows a presence of carbon in a higher amount compared to high Jc and Tczero materials. This paper shows that Jc and Tczero are very susceptible to impure gases in air.

Mineo Itoh

Papers

The improvement in magnetic shielding by use of a double-cylinder configuration of high-Tc oxide superconductors

Preparation of superconducting Y−Ba−Cu−O thick film

Influence of silver on critical current of the Y–Ba–Cu–O superconductor

Improvement in magnetic shielding by the superposition of a magnetic cylinder over a copper-oxide superconducting cylinder

Influence of Carbon on Critical Temperatures of Superconducting Y-Ba-Cu-O