[Show abstract][Hide abstract] ABSTRACT: Bi-2223 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 Bi-2223 (0010) peak, without substantial decrease in the intensity of the Bi-2212 (008) peak. These changes were correlated with the amount of liquid phase obtained from the Bi-2201 (006) peak. Optical microscopy also suggested an important role of the liquid phase by revealing significant changes in the morphology of Bi-2223 grains with increasing sintering temperature.
Physica C Superconductivity 01/2002; 372:931-934. · 0.72 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 superconductor-Ag 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 un-etched 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 coupling-current dominated regime with filament uncoupled. The loss reduction for partially coupled regime is more than 80%.
Physica C Superconductivity 01/2002; 372:1766-1770. · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Long and thin Bi-2212 rods textured by laser induced zone melting show inhomogeneous radial distribution of secondary phases and Bi concentration in transverse cross-sections. 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 non-destructive 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 01/2002; 372:1055-1058. · 0.72 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 2-pole synchronous machine with a conventional 3-phase 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 50-turn 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 eddy-current 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 01/2002; · 0.72 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 investigated
IEEE Transactions on Applied Superconductivity 04/2001; · 1.20 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 self-field 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 self-field 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 (stack-configuration). This configuration
is similar to that found in adjacent layers in a coil. The results
indicate that the critical coupling distance for the top-configuration
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 current
IEEE Transactions on Applied Superconductivity 07/1999; · 1.20 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 self-field 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
power-law current-voltage dependence
IEEE Transactions on Applied Superconductivity 07/1999; · 1.20 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 Ag-sheathed (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 slow-cooled 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°C
IEEE Transactions on Applied Superconductivity 07/1999; · 1.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A design feasibility study has been conducted for a 240 MVA
high-temperature superconducting grid auto-transformer. 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 low-temperature 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 high-temperature
superconductors for power applications
IEE Proceedings - Electric Power Applications 02/1999; · 0.55 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 01/1999; 319(1):50-58. · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Measurements have been made of the a.c. self-field 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 half-width) 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 thin-rectangle 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. · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transport currents of up to 140 A have been injected into melt-grown YBCO to measure AC and DC critical currents. Values of critical current densities of 16000 A cm-2 (zero field), 2000 A cm-2 (1 T) and 550 A cm-2 (5 T) were obtained at 77 K. The 50 Hz AC critical current densities are also reported, with values of 23000 A cm-2 and 12000 A cm-2 (RMS) in applied fields of 1 and 5 T respectively at 77 K. Silver contact pads enabled steady-state 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 cm-1 detection level for flux creep was applied.
Superconductor Science and Technology 12/1998; 3(6):282. · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetic interaction between two neighbouring superconducting tapes has significant influences on their self-field 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 self-field 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 self-field 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 (side-configuration) or one on top of the other (top-configuration). The results indicates that the critical coupling distance for the top-configuration is about 5 mm, where the increase in loss per tape is about 10%. The critical length for the side-configuration is found to be of the same order of magnitude.
Physica C Superconductivity 12/1998; · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A round-robin of self-field ac loss measurements on Bi-2223 tapes has been performed for the first time, under the Brite EuRam research programme SACPA. Excellent agreement on loss data has been obtained by the four measurement partners. This further confirms the validity of the electrical technique employed and allows reliable comparisons between the loss data of the individual partners in the project. The different measurement systems and sample mounting procedure are described. For accurate measurement of the loss voltage to the required sensitivity, only a lock-in technique was suitable. The phase of the applied current was measured with a Rogowski coil (with or without compensation) or a non-inductive resistor.
Superconductor Science and Technology 07/1998; 11:675-679. · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A series of electrical AC loss measurements on Bi-2223 tapes have been performed under the Brite Euram project SACPA. This included, for the first time, a round-robin of independent self-field AC loss measurements between four laboratories. The very close agreement of data demonstrates the validity of the electrical technique and lays the basis for a measurement standard. Other preliminary measurements in SACPA showing the variation of losses with frequency, temperature and applied DC field are also reported.
Physica C Superconductivity 01/1998; 310:67-70. · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-field AC losses of polycrystalline Bi-2212 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 1.6-2 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm2 in the self-field which decreases to about 1.5 kA/cm2 in fields of 0.02 T applied perpendicular to the rod axis. The self-field 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 resistive-like 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 01/1998; 310:71-75. · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-field AC losses of polycrystalline Bi-2212 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 1.6–2 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm2 in the self-field which decreases to about 1.5 kA/cm2 in fields of 0.02 T applied perpendicular to the rod axis. The self-field 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 resistive-like 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 and Its Applications - PHYSICA C. 01/1998; 310(1):71-75.
[Show abstract][Hide abstract] ABSTRACT: A substantial increase in flux pinning has been reported in Li doped YBCO grains following thermal neutron irradiation. This effect observed in sintered YBCO was attributed to the increased number of defects in the crystal lattice produced by the particles emitted from the fission of 6Li. In this study we report preliminary measurements on the effect of irradiating Li doped melt grown YBCO with thermal neutrons. A single grain was divided into two samples, one of which was then irradiated with ≈ 1017 neutrons. VSM measurements have revealed that the irradiated sample is far superior at low fields (at best a factor of five increase in Jc at 77 K), but at high fields the magnetic moments become very similar and the irreversibility field remains the same. The absence of fishtail irradiation suggests the introduction of an additional low field pinning mechanism.
Materials Science and Engineering B-advanced Functional Solid-state Materials. 01/1998; 53(1):182-184.
[Show abstract][Hide abstract] ABSTRACT: Self-field ac losses in Ag sheathed PbBi-2223 tapes were measured
using a thermometric method, which determines the losses by measuring
the temperature profile of a vacuum insulated sample, with both ends at
a fixed temperature. In practice, the samples were placed in a vacuum
capsule immersed in LIN bath. By minimizing the bath superheat, thermal
emf and heating at the current contacts, a loss induced temperature
increase as low as 2 mK was measured using a Si diode thermometer. With
a typical sample length of 600 mm, self-held losses between 7×10
<sup>-6</sup> W/m and 4×10<sup>-3</sup> W/m were measured at
different frequencies. The results are in good agreement with both the
electric measurement and theoretical calculation. This provides the
first independent confirmation that electric measurement with carefully
placed voltage loops can give the true losses of the sample
IEEE Transactions on Applied Superconductivity 07/1997; · 1.20 Impact Factor