Publications (75)75.73 Total impact
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ABSTRACT: AC loss measurements were carried out on MgB<sub>2</sub> wire and bulk conductors at temperatures between 25 K and 40 K in applied field up to 0.2 T transverse to conductor length. For bulk conductors in the shape of a flat plate (3 mm × 0.5 mm), loss behavior of a thin strip Q∝H<sub>0</sub><sup>4</sup> was observed. A reduction of width to 1 mm leads to a change in loss behavior similar to a round wire with Q∝H<sub>0</sub><sup>3</sup>. For the exsitu MgB<sub>2</sub> wire of Ni sheath, losses were totally dominated by the hysteresis of ferromagnetic Ni. While the losses in the superconducting core is greatly reduced by the strong magnetic shielding by the Ni sheath, a small increase in the Ni loss was observed upon superconducting transition in the core. This is attributed to the compression of magnetic flux due to the diamagnetic superconducting core. Such an interpretation is supported by measurements of PbBi2223 tape in a cavity of an iron block.IEEE Transactions on Applied Superconductivity 07/2005; DOI:10.1109/TASC.2005.848254 · 1.32 Impact Factor 
Article: Modelling of coupling between superconductors of finite length using an integral formulation
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ABSTRACT: An integral formulation based on the stream function of sheet currents is applied to finite length superconductors to model the coupling through a normal matrix. This formulation is an extension of Brandt's 2D formulation for modelling a 3D problem. Thin discs and infinite slabs were studied and the critical \dot {B}_{\mathrm {c}} was obtained as a function of applied field. While an excellent agreement for fully penetrated infinite slabs was found with existing theories, original results are presented for partially penetrated slabs as well as for thin discs in a wide range of applied fields.Superconductor Science and Technology 10/2004; DOI:10.1088/09532048/17/10/003 · 2.80 Impact Factor 
Article: Configuration and calibration of pickup coils for measurement of ac loss in long superconductors
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ABSTRACT: Pickup coils are widely used for measurements of magnetic susceptibility and hysteretic losses in magnetic and superconducting materials. A general formulation for the calibration of such pickup coils with different geometries is presented. Although the procedure described here is general and can be applied to any coil and sample, special emphasis has been placed on the calibration of saddlelike coils usually used to measure ac losses of long superconductors under ac fields perpendicular to their long dimension. Numerical simulations and experimental measurements have been carried out in order to determine the geometrical limitations of the pickup coil/sample configurations. This is especially important when simple coils of small dimensions and number of turns are used and/or samples of high demagnetization factor are measured. Superconductors of different geometry, in particular, wires and tapes under parallel and perpendicular fields, have been analyzed.Journal of Applied Physics 09/2004; DOI:10.1063/1.1766100 · 2.19 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using 3D finite element method (FEM) modelling, the present work investigates the coupling effect between two finite superconductors through a resistive matrix. This effect, related to the finite length of the conductors, is typically threedimensional and cannot be analyzed by the widely utilized 2D models. Superconductors are modelled with the non linear power law E=Ec(J/Jc)^n, which has been implemented in the FEM software. The main focus is to demonstrate the feasibility of such calculations and to establish the correlation between the coupling effect and the aspect ratio of the conductor crosssection, in order to extend the existing theory, which is only precise for superconductors of infinite slabs, fully penetrated by the magnetic field. The effect of other parameters such as the conductor length and the gap between the superconducting filaments is also considered. The latter is a parameter which does not feature in the approximate theories.Superconductor Science and Technology 10/2003; 16(10). DOI:10.1088/09532048/16/10/318 · 2.80 Impact Factor 
Article: 3D modeling of coupling between superconducting filaments via resistive matrix in AC magnetic field
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ABSTRACT: The ac loss of superconducting composite depends strongly on coupling between superconducting filaments via the resistive matrix. The established technique for loss reduction using twisted filaments relies on the decoupling of the filaments below a critical coupling field Bc, which increases with the reduction of the twist pitch and the matrix conductivity. Although the concept of Bc may be clearly demonstrated using two infinite slabs of finite length, further details on its correlation with the filament/conductor geometry are not yet available. The main obstacle is due to the fact that any accurate analysis of such a problem must be carried out in 3d. In this paper, we describe the initial results from 3d modeling using Cedrat's Flux3D, for which a superconductor module for handling powerlaw EJ characteristics was developed. Using a simple model of two rectangular superconductors connected through a normal metal, we demonstrate the feasibility for quantitative modeling of their coupling behavior over a wide range of field sweep rates for different conductor geometries. Typical examples were given for cases not addressed by the existing approximate theory, as well as for the evolution of field profile for varying field sweep rate.IEEE Transactions on Applied Superconductivity 07/2003; DOI:10.1109/TASC.2003.812416 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In a twostep heat treatment with one intermediate rolling the final transport critical current, (I<sub>c</sub> at 77 K, self field, 1ı`V/cm), of 37 filament (Bi,Pb)2223/Ag tapes increases linearly with the heating ramp rate: 20 A to 40 A, (∼20,000 Acm<sup>2</sup>), for 20°C/hour to 400°C/hour. The principle benefit of the rapid ramp rate is likely to come from ramping fast though 800°C to 832°C in the 1st heat treatment. SEM and XRD analysis on the superconductorAg interface show a decrease in the amount of secondary phases, Cu<sub>2</sub>O, Bi<sub>2</sub>(Sr,Ca)<sub>2</sub>Cu<sub>1</sub>O<sub>x</sub>, Bi(Sr,Ca)O<sub>x</sub>, with increasing ramp rate and critical current.IEEE Transactions on Applied Superconductivity 07/2003; DOI:10.1109/TASC.2003.812062 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Bi2223 tapes with 37 filaments, Ag–Mg–Ni alloy outer sheath and pure Ag inner sheath were heat treated in 7.5% O2 at different temperatures for 40 h. XRD of etched samples for increasing sintering temperature from 798 to 847 °C showed an increase of eightfold in the absolute intensity of the Bi2223 (0010) peak, without substantial decrease in the intensity of the Bi2212 (008) peak. These changes were correlated with the amount of liquid phase obtained from the Bi2201 (006) peak. Optical microscopy also suggested an important role of the liquid phase by revealing significant changes in the morphology of Bi2223 grains with increasing sintering temperature.Physica C Superconductivity 08/2002; 372:931934. DOI:10.1016/S09214534(02)00936X · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The main obstacle to an effective loss reduction in standard PbBi2223 tapes is the low resistivity of the pure Ag sheath, which leads to a low critical coupling field and a high coupling current loss. To formulate an effective loss reduction strategy, it is important to assess the contribution of different loss mechanisms. The aim of the present work is to quantify the distribution of the coupling current by systematic removal of Ag using chemical etching. Our result showed that the majority of the coupling current flows transversely through the superconductorAg matrix, instead of the outer sheath enclosing the filaments. It was also found that the coupling field BC between the filaments and the inverse of the time constant of coupling current τ−1 increase linearly with the percentage of etching, while the matrix resistivity for the unetched area is expected to change very little. By completely uncoupling about a half of all the filaments, the reduction of the total loss was about 40–50% in the couplingcurrent dominated regime with filament uncoupled. The loss reduction for partially coupled regime is more than 80%.Physica C Superconductivity 08/2002; 372:17661770. DOI:10.1016/S09214534(02)01122X · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Long and thin Bi2212 rods textured by laser induced zone melting show inhomogeneous radial distribution of secondary phases and Bi concentration in transverse crosssections. The microstructure inhomogeneity, which depends on the growth conditions, is reflected in the physical properties of the samples. We have estimated the radial distribution of Jc (77 K) in different samples using destructive and nondestructive approaches, correlating these results with the microstructure variations. Using this knowledge, the conductor performance at 77 K has been optimised by controlling the distribution of secondary phases.Physica C Superconductivity 08/2002; 372:10551058. DOI:10.1016/S09214534(02)008419 · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The paper presents the main features of a 100 kVA high temperature superconducting (HTS) demonstrator generator, which is designed and being built at the University of Southampton. The generator is a 2pole synchronous machine with a conventional 3phase stator and a HTS rotor operating in the temperature range 57–77 K using either liquid nitrogen down to 65 K or liquid air down to 57 K. Liquid air has not been used before in the refrigeration of HTS devices but has recently been commercialised by BOC as a safe alternative to nitrogen for use in freezing of food. The generator will use an existing stator with a bore of 330 mm. The rotor is designed with a magnetic core (invar) to reduce the magnetising current and the field in the coils. For ease of manufacture, a hybrid salient pole construction is used, and the superconducting winding consists of twelve 50turn identical flat coils. Magnetic invar rings will be used between adjacent HTS coils of the winding to divert the normal component of the magnetic field away from the Bi2223 superconducting tapes. To avoid excessive eddycurrent losses in the rotor pole faces, a cold copper screen will be placed around the rotor core to exclude ac magnetic fields.Physica C Superconductivity 08/2002; DOI:10.1016/S09214534(02)010766 · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The heat treatment of (Bi,Pb)2223 tapes in a furnace with a temperature gradient allows the study of many heat treatment temperatures at the same time. 20 cm lengths of tape were given a typical first heat treatment of a period of 40 hours in a ~1°C/cm temperature gradient under either 7.5% partial pressure O<sub>2</sub>, or air. The maximum amount of (Bi,Pb)2223 occurred in a temperature window, (⩽2% change in the phase composition) of about 7°C under 7.5% partial pressure and 2°C under air. The relationship between the phase composition and the transport current, Ic, for both atmospheres is investigatedIEEE Transactions on Applied Superconductivity 04/2001; DOI:10.1109/77.919688 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: As shown by Parrell et al., a final reduced cooling rate can produce an improvement of the optimum critical current in field. In this paper we present results on the critical current density and magnetic field dependence of Agsheathed (Pb,Bi)2223 multifilamentary tapes as a function of the final sintering time for different cooling. Correlation between 2223 phase conversion, as determined by XRD, and the critical current density is also examined. For samples slow cooled to 800°C at 1°C/h followed by furnace cooling to room temperature, the critical current density remained almost constant at 20 KAcm<sup>2</sup> with different final sintering times between 10 to 150 hours. In contrast samples slowcooled to 730°C at 1°C/h showed a decreased J<sub>c</sub> of 7 KAcm<sup>2</sup> for a short final sintering time of 10 hours, rising gradually to 22 KAcm<sup>2</sup> at 200 hours. Corresponding to this, the samples cooled to 800°C show a phase composition of 90% apparent (Pb,Bi)2223 for all times, as opposed to a gradual increase of the (Pb,Bi)2223 phase from 65% to 90% with increasing sintering time for samples slow cooled to 730°C. As the conditions are exactly the same prior to the cooling below 800°C, the reduced conversion for the samples slow cooled to 730°C must be the result of 2212 precipitation from the apparent (Pb,Bi)2223 phase present at 800°CIEEE Transactions on Applied Superconductivity 07/1999; DOI:10.1109/77.785054 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: AC losses of textured polycrystalline Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> thin rods carrying AC transport currents have been measured in selffield and in DC magnetic fields at 77 K. Measurements of the first and third harmonic of the voltage are presented. The current amplitude and frequency dependence of losses as well as the ratio between the different harmonics of the voltage is analysed. The results are compared with the losses calculated numerically for a superconductor characterised by powerlaw currentvoltage dependenceIEEE Transactions on Applied Superconductivity 07/1999; DOI:10.1109/77.783419 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Magnetic interaction between two neighbouring superconducting tapes carrying a transport current can cause them to become coupled so that they react as one tape. One of the consequences of such coupling is the increase in the selffield loss per tape. The critical distance at which the tapes begin to couple is important in the assessment of the interactions among tapes in systems such as a power cable or coil. Experimental measurements of the selffield losses in two neighbouring Ag sheathed PbBi2223 tapes carrying the same transport current were carried out with the two tapes separated by various distances, while placed one on top of the other (stackconfiguration). This configuration is similar to that found in adjacent layers in a coil. The results indicate that the critical coupling distance for the topconfiguration is about 5 mm, where the increase in loss per tape is about 10%. Measurements are also undertaken for the more realistic situation where each of the tapes has a different critical currentIEEE Transactions on Applied Superconductivity 07/1999; DOI:10.1109/77.783409 · 1.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The effects on the phase formation and magnitude of the transport Jc of the addition of up to 15% excess (Pb,Bi)2212 stoichiometry to Pb0.34Bi1.84Sr1.91Ca2.03Cu3.06Ox has been studied. The excess (Pb,Bi)2212 was added at the solution stage of precursor preparation to ensure good mixing of constituents. The addition of the excess (Pb,Bi)2212 stoichiometry has been found to change the kinetics of formation of the (Pb,Bi)2223 phase in the early stages of sintering, t2×104 A cm−2 have been achieved for tapes with 10% excess (Pb,Bi)2212. The Jc–B characteristics (B≤0.5 T, Bab) of tapes with 10% excess (Pb,Bi)2212 was found to be superior to the Endo composition tapes.Physica C Superconductivity 06/1999; 319(1):5058. DOI:10.1016/S09214534(99)002750 · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A design feasibility study has been conducted for a 240 MVA hightemperature superconducting grid autotransformer. Conclusions are relevant to superconducting power transformers in general. It is argued that economic benefits may be predicted, subject to assumptions about achievable conductor properties, costs of components and power system operating requirements. Liquid nitrogen cryogenics is relatively cheap and simple, and refrigeration power demand is reduced by a factor of the order of 20 compared to the lowtemperature case. Attention is drawn to the importance of AC losses in the superconductor and the difficulty of keeping these sufficiently low. Various technical problem areas, and their likely influence on the overall design concept, are reviewed. Three particularly important influences are identified: insulating properties of liquid nitrogen coolant; required transformer performance in the through fault condition; and mechanical strength to withstand electromagnetic forces. Design proposals are detailed, and recommendations made for future development of hightemperature superconductors for power applicationsIEE Proceedings  Electric Power Applications 02/1999; 146(1146):41  52. DOI:10.1049/ipepa:19990192 · 1.26 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Transport currents of up to 140 A have been injected into meltgrown YBCO to measure AC and DC critical currents. Values of critical current densities of 16000 A cm2 (zero field), 2000 A cm2 (1 T) and 550 A cm2 (5 T) were obtained at 77 K. The 50 Hz AC critical current densities are also reported, with values of 23000 A cm2 and 12000 A cm2 (RMS) in applied fields of 1 and 5 T respectively at 77 K. Silver contact pads enabled steadystate currents of up to 7000 A per cm2 of contact area to be injected into the sample without any observable heating effects in applied fields up to 5 T. It should be noted that the 50 Hz results are believed to correspond to the onset of flux depinning in contrast with the DC measurements where a 3 mu V cm1 detection level for flux creep was applied.Superconductor Science and Technology 12/1998; 3(6):282. DOI:10.1088/09532048/3/6/002 · 2.80 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Measurements have been made of the a.c. selffield losses in PbBi2223 tapes with different core/sheath configurations. Three different tapes have been measured, two monocore tapes with core thicknesses of and and a multifilamentary tape of overall thickness . The a.c. losses have been obtained by measurement of the loss electric field at a sufficient distance (three times the tape halfwidth) from the tape axis. Measurements of the spatial dependence of the loss and inductive electric fields and have also been made. The results indicate that the `thin' monocore tape can be adequately described by the thinrectangle geometry whilst the behaviour of the `thick' monocore tape lies in between those of the thin rectangle and thin ellipse. The behaviour of the multifilamentary tape follows closely that of a monocore ellipse tape indicating that there is a strong interaction between the filaments.Superconductor Science and Technology 12/1998; 9(9):801. DOI:10.1088/09532048/9/9/016 · 2.80 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Selffield AC losses of polycrystalline Bi2212 thin rods textured by a Laser Floating Zone (LFZ) melting technique have been measured at 77 K. With the optimal processing parameters, these rods, of 1.62 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm2 in the selffield which decreases to about 1.5 kA/cm2 in fields of 0.02 T applied perpendicular to the rod axis. The selffield AC losses have been measured in DC magnetic fields up to 0.03 T. The measurements in zero field show that for a large current range the losses are dominated by hysteresis losses as described by the Critical State Model for a cylinder. For the measurements in DC fields the losses show an increasingly resistivelike dependence with current, while the hysteretic component expected from the CSM becomes less important. Measurements at different frequencies also indicated that the loss per cycle in fields is strongly frequency dependent.Physica C Superconductivity 12/1998; 310:7175. DOI:10.1016/S09214534(98)004365 · 1.11 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The magnetic interaction between two neighbouring superconducting tapes has significant influences on their selffield AC losses. While the two tapes are independent of each other when separated far apart, they are expected to be fully coupled and behave as a single tape when placed very near. One of the consequences of such coupling is the increase in the selffield loss per tape, which is double of that for an uncoupled tape carrying the same current. In order to assess the interactions among tapes in systems such as a power cable, a better understanding is necessary on the critical distance where the coupling becomes important. Experimental measurements on the selffield losses in two neighbouring Ag sheathed PbBi2223 tapes were carried out with the two tapes separated by various distances, while placed either side by side (sideconfiguration) or one on top of the other (topconfiguration). The results indicates that the critical coupling distance for the topconfiguration is about 5 mm, where the increase in loss per tape is about 10%. The critical length for the sideconfiguration is found to be of the same order of magnitude.Physica C Superconductivity 12/1998; DOI:10.1016/S09214534(98)004596 · 1.11 Impact Factor
Publication Stats
407  Citations  
75.73  Total Impact Points  
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Institutions

1989–2013

University of Southampton
 • Faculty of Engineering and the Environment
 • Division of Chemistry
Southampton, England, United Kingdom


1990

University of Oxford
 Department of Materials
Oxford, ENG, United Kingdom
