D. Dew-Hughes

The role of grain boundaries in determining J c in high-field high-current superconductors

Abstract The scaling laws that describe flux pinning in the two most common commercial superconductors, NbTi and Nb3Sn made by the bronze process, are totally different. This is despite the fact the microstructural features that are responsible for flux pinning, sub-boundaries in NbTi and grain boundaries in Nb3Sn, in both cases have concentrations of what in the bulk would be non-superconducting material. The only significant differences between the microstructures of the two materials is the morphology of the boundary structure. It is shown how this can lead to the two distinct pinning behaviours.

Computer modelling of type II superconductors in applications

A numerical method for solving for the current and field distributions inside devices containing type II superconductors is described. The two-dimensional solution technique accommodates the effects of surrounding media including iron and can handle systems with an arbitrary number of type II superconductors and conventional materials. The technique is based on the finite element method, the method of moments and the critical state model. The impetus behind this work is the study of rotating electrical machines which contain superconducting artefacts placed alongside magnetic materials and current sources. In this paper, a reluctance machine consisting of an iron rotor flanked with two HTS pieces is chosen to demonstrate the method.

Effects of MgO addition on phase evolution and flux pinning of Bi-2212/Ag tapes fabricated by electrophoretic deposition and partial-melting processing

MgO or particulates were introduced into Bi-2212/Ag tapes during the fabrication process with the intention of producing flux pinning centres. It was found that (i) MgO or normal particles added to Bi-2212 superconductor do not react chemically with or corrupt the Bi-2212 phase, but MgO may alter the phase evolution during the partial melting and the solidification of Bi-2212 and reduce the second-phase inclusions, (ii) MgO addition may enhance the flux pinning potential in a certain region of temperature and magnetic field (the contributions of MgO addition to the flux pinning potential are still not clear) and (iii) addition appears to have no effect on the properties of finished samples.

The addition of Mn and Al to the hydriding compound FeTi: Range of homogeneity and lattice parameters

Isothermal sections, at 1000 °C, of the Ti-rich corners of the Mn-Fe-Ti and Al-Fe-Ti ternary phase diagrams have been determined. Particular attention has been paid to the FeTi hydriding compound. Mn and Al both substitute for Fe in the compound; the former up to 27 at. pct, the latter up to 24 at. pct. Analysis of the lattice parameter data suggest that effective atomic radii, in the B2 structure, are Al: 1.38Å, Fe: 1.18Å, Mn: 1.22Å, and Ti: 1.40Å.

Finite difference modelling of bulk high temperature superconducting cylindrical hysteresis machines

A mathematical model of the critical state based on averaged fluxon motion has been implemented to solve for the current and field distributions inside a high temperature superconducting hysteresis machine. The machine consists of a rotor made from a solid cylindrical single domain HTS placed in a perpendicular rotating field. The solution technique uses the finite difference approximation for a two-dimensional domain, discretized in cylindrical polar co-ordinates. The torque generated or equivalently the hysteresis loss in such a machine has been investigated using the model. It was found that to maximize the efficiency, the field needs to penetrate the rotor such that B 0 /?0 Jc R = 0.56, where B 0 is the applied field amplitude, Jc is the critical current density and R is the rotor radius. This corresponds to a penetration that is 27% greater than that which reaches the centre of the rotor. An examination of the torque density distributions across the rotor reveal that for situations where the field is less than optimal, a significant increase in the performance can be achieved by removing an inner cylinder from the rotor.

Highly reproducible high critical current density in partial-melt Bi2Sr2CaCu2Oy/Ag tapes fabricated by electrophoretic deposition

Highly c-axis-oriented Bi2Sr2CaCu2Oy/Ag tapes with high critical current density (Jc) were prepared by electrophoretic deposition followed by partial-melt processing. Slow cooling at 0.2 degrees C min-1 from the partial-melt temperature and a subsequent prolonged annealing at 825-830 degrees C resulted in an almost phase-pure highly textured microstructure in the samples fabricated from partly reacted precursor powder. The transport Jc can be improved to 1.7*104 A cm-2 at 77 K, and, at 4.2 K, 3.0*105 A cm-2 (Ic=470 A for a 6 mm wide sample) and 1.3*105 A cm-2 with zero and 15 T applied magnetic induction, respectively. Values of Jc were reproducible to within 10% for samples that experienced the same heat treatment and were of similar thickness.

Torque from hysteresis machines with type-II superconducting segmented rotors

Electrical machines having rotors constructed from different numbers of type-II superconducting segments have been modelled numerically. The hollow cylindrical rotors are subjected to a rotating applied field that induces currents in the superconducting pieces and the resulting torque is evaluated. The mathematical technique used is based on the critical state model and solves for the current and field distributions within the superconducting pieces using the finite element method. In a two-pole rotating field, a rotor made from two half-ringed pieces is found to behave like a reluctance machine, having a larger peak torque than hysteresis type rotors constructed from other numbers of segments. In general, splitting the rotor up into more segments is advantageous when the flux penetration is small since this increases the hysteresis loss and equivalently, the torque. The modelling results indicate the number of segments which will result in the largest torque for a given rotor size, material critical current density and applied field amplitude. Such information is valuable when considering the performance optimisation of such machines.

Microstrip disk resonators for filters fabricated from TBCCO thin films

Microstrip disk type resonators have been designed and tested. These have been fabricated from TBCCO 2212 thin films deposited by DC sputtering onto 2 cm square LaAlO/sub 3/ substrates. The R/sub s/ of such large area films has been measured at 40 GHz using a copper end wall replacement cavity and shown to be less than 200 /spl mu//spl Omega/ scaled to 10 GHz and at 80 K. Q values of 2 to 6 GHz disk resonators have demonstrated considerable improvements when compared to both linear HTS microstrip resonators and comparable copper disk resonators. Additionally, preliminary results for the performance of such disk resonators as a function of microwave power will be presented for application in high power filters.

Transmission characteristics of HTS microstrip resonators with a ferrite component

Microwave transmission characteristics of HTS microstrip resonators containing a monocrystalline ferrite layer magnetized in an arbitrary direction were calculated using a RCL approach. Dependencies of the resonance frequencies on the external magnetic field strength and the field orientation angles were analyzed and the results of the calculations were compared with the experiment.

Low‐temperature processing of superconducting Tl2Ba2Ca1Cu2Ox films on CeO2 buffered sapphire

Tl2Ba2Ca1Cu2Ox thin films with excellent alignment have been grown on CeO2 buffered R‐plane sapphire using an ex situ anneal step in argon atmospheres at temperatures of 720–740 °C. With this low‐temperature process we have overcome the serious problem of reaction layers being formed at the CeO2/Tl2Ba2Ca1Cu2Ox interface, which can degrade film properties. The presence of a sharp CeO2/Tl2Ba2Ca1Cu2Ox interface has been confirmed by x‐ray diffraction, transmission electron microscopy, and high resolution electron microscopy observations. Films have well connected morphologies, with critical temperature (Tc) values of up to 101.6 K and critical current density (Jc) values of up to 1.25×105 A cm−2.