D.A. Cardwell

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

Processing and properties of large grain (RE)BCO

Abstract The potential of high temperature superconductors to generate large magnetic fields and to carry current with low power dissipation at 77 K is particularly attractive for a variety of permanent magnet applications. As a result large grain bulk (RE)-Ba-Cu-O ((RE)BCO) materials have been developed by melt process techniques in an attempt to fabricate practical materials for use in high field devices. This review outlines the current state of the art in this field of processing, including seeding requirements for the controlled fabrication of these materials, the origin of striking growth features such as the formation of a facet plane around the seed, platelet boundaries and (RE)2BaCuO5 (RE-211) inclusions in the seeded melt grown microstructure. An observed variation in critical current density in large grain (RE)BCO samples is accounted for by Sm contamination of the material in the vicinity of the seed and with the development of a non-uniform growth morphology at ≈ 4 mm from the seed position. (RE)Ba2Cu3O7-gd (RE-123) dendrites are observed to form and broaden preferentially within the a/b plane of the lattice in this growth regime. Finally, trapped fields in excess of 3 T have been reported in irradiated U-doped YBCO and (RE)1+xBa2-xCu3Oy (RE = Sm, Nd) materials have been observed to carry transport current in fields of up to 10 T at 77 K. This underlines the potential of bulk (RE)BCO materials for practical permanent magnet type applications.

High intergranular critical currents in metallic MgB2 superconductor

Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning in superconducting polycrystalline MgB2 has been observed. The presence of strongly-coupled grain boundaries in this material has been confirmed by a dramatic collapse of the magnetic hysteresis loop when a bulk specimen is ground into a fine powder and re-measured under similar conditions. Further evidence for strong intergrain links in polycrystalline MgB2 is provided by the continuous variation of the remanent magnetic moment up to the full penetration field of a bulk sample. The absence of weak-link nature in this material has profound implications for its potential in a wide range of engineering applications.

Bulk high temperature superconductors for magnet applićations

A summary is given of the various techniques used to prepare melt processed YBCO. The use of these materials as permanent magnets is discussed. Since nearly all applications involve demagnetizing fields of some kind, these are compared with the equivalent ferromagnetic material. Superconductors behave in the opposite way to ferromagnets in that a reversed field can increase the magnetization, while a field parallel to the magnetization quickly reduces it to zero. Then ways of obtaining maximum repulsion and attraction without demagnetizing the sample are described. A summary of the forces and torques available is given. In particular, it is shown that YBCO can provide extremely high current densities in rotating machines.

Handbook of Superconducting Materials

VOLUME I: SUPERCONDUCTIVITY, MATERIALS AND PROCESSES Fundamentals of Superconductivity Introduction to superconductivity and superconducting materials Characteristic properties Elementary theory Critical currents of type II superconductors Processing Introduction to processing methods Bulk materials Wires and tapes Thick and thin films Superconductor contacts High Temperature Superconductors YBCO BSCCO TIBCCO Mercury superconductors Magnesium diboride VOLUME II: CHARACTERIZATION, APPLICATION AND CRYOGENICS Characterization Techniques Structure/microstructure Measurement and interpretation of electromagnetic properties Measurement of physical properties Applications High current applications Trapped flux devices High frequency devices Josephson junction devices Other devices Introduction to Refrigeration Methods Emerging Materials Chevrel phases Unconventional superconductivity in heavy fermion and ruthenate superconductors Organic superconductors Fullerene superconductors Future high Tc superconductors Appendices Manufacturer and supplier directory Hazards: environment and safety Teach yourself phase diagrams

Artificial flux pinning centers in large, single-grain (RE)-Ba-Cu-O superconductors

Second-phase, nanoscale inclusions of composition Y2Ba4CuMOy (M=U, Nb, Ta, W, Mo, and Re), which form artificial pinning centers, have been introduced into large, single-grain [rare-earth (RE)]-Ba-Cu-O superconductors. A significant improvement in critical current density is observed in these samples, due presumably to various combinations of normal conducting, paramagnetic, and geometrical properties of the Y2Ba4CuMOy particles in the superconducting (RE)Ba2Cu3O7−δ (RE-123) phase matrix. These Y2Ba4CuMOy phase particles are chemically stable in the Ba-Cu-O liquid during peritectic solidification, unlike Y2BaCuO5 (Y-211) phase particles in Y-Ba-Cu-O, which opens a processing window for the fabrication of nanostructured large, single-grain (RE)-Ba-Cu-O superconductors with enhanced flux pinning for high-field engineering applications.

Neutron irradiation of MgB2 bulk superconductors

Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

Fabrication of large grain YBCO by seeded peritectic solidification

The ability to process large grain, uniform high temperature superconducting ceramics that exhibit high critical current densities at 77 K is essential if the enormous potential of these materials for a range of permanent magnet-type applications is to be realized. We report a study of the fabrication of large grain YB{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} by seeded peritectic solidification in which key processing parameters such as the peritectic melting process, the seed-YBCO reaction, and the YBCO solidification kinetics are investigated in detail. Evolution of the sample microstructure during various stages of the growth process, in particular, has been studied extensively. The superconducting properties of specimens cut from different regions of large grain samples have been measured using vibrating sample magnetometry, and the results correlated with the microstructure of the materials. {copyright} {ital 1996 Materials Research Society.}

Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductors with very high critical current densities

Single domain Y?Ba?Cu?O (YBCO) composed of a YBa2Cu3Oy (Y-123) superconducting bulk matrix with discrete, non-superconducting Y2BaCuO5 (Y-211) phase inclusions has been fabricated by a seeded infiltration and growth (IG) technique in the form of cylindrical pellets up to 32?mm in diameter. Sample shrinkage in the radial direction for single domains prepared by this technique is relatively low at 5% and independent of sample size, in contrast to the shrinkage observed in samples grown by conventional melt processing, which increases significantly with increasing sample diameter. Furthermore, samples grown by the IG technique exhibit low porosity of typically 0.9% of the bulk volume fraction, compared with a corresponding value of around 4.9% observed in samples fabricated by conventional melt processing. Fine Y-211 particles were observed to be embedded within the Y-123 superconducting matrix for the IG processed samples, leading to a high critical current density, Jc, of over 100?000?A?cm?2 at 77.3?K in self-field. The distribution of Y-211 particles in the IG sample microstructure, however, was inhomogeneous (unlike in previous reports), which leads to a variation in the spatial distribution of Jc. The volume fraction of Y-211 in the vicinity of the seed crystal (i.e.?corresponding to the initial c-sector growth stage), in particular, is typically around 5%, compared with a value of up to 30% in the a growth sectors more distant from the seed crystal (which corresponds well to the theoretical value for the sample composition studied here). The volume fraction of Y-211 inclusions in the c growth sector more distant from the seed was around 22%. Finally, a trend of the variation in the distribution of Y-211 particles in the Y-123 matrix grown by the IG technique was similar to that in sample grown by conventional melt processing.

A trapped field of >3 T in bulk MgB2 fabricated by uniaxial hot pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetized stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot–Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.