Z. Yi

Local phase evolution in (Pb/Bi)2Sr2Ca2Cu3Ox tapes

Abstract The phase evolution has been measured locally from the core-sheath interface to the core-centre of (Pb/Bi)2223 tapes sintered for up to 350 h. The rate of the phase conversion is found to be much faster at the interface than at the centre. For example, in a tape sintered at 832°C in air for 30 h, the percentage of 2223 phase is ∼90% at the interface compared with ∼27% at the centre. Consequently, there is a 2223 phase gradient from the interface to the centre of the core, even after a long sintering time. There appears to be an associated gradient in the grain size and Vickers microhardness across the thickness of the core. The influence of the silver sheath on the phase conversion in half-peeled tapes has also been studied. The half-peeled tapes were made by removing one side of the silver sheath of the normal (Pb/Bi)2223 tape before sintering.

AC V-I characteristics of Ag sheathed PbBi2223 tapes up to 10 kHz: phenomena and interpretations

The self-field AC V-I characteristics of PB2223 silver sheathed superconducting tapes were measured at 9 different positions across the tape at 77 K and 65 K between 47 Hz and 10 kHz. The significant difference found between the centre and the edge was interpreted as the result of the complex geometry of tapes. The implication of this difference to the measurement of the total losses is discussed. Considerable contributions from eddy current losses in the silver sheath were also found for frequencies above 200 Hz.<<ETX>>

AC susceptibility of high-Tc superconductors: Hysteresis and intergranular critical state

Abstract AC susceptibility of granular YBCO samples with different grain sizes has been measured, both as a function of temperature and as a function of the applied DC field. Two types of systematic hysteresis were observed: (1) the significantly enhanced AC field screening ability in the field cooled (FC) virgin samples compared to the zero field cooled (ZFC) virgin sample; (2) enhanced AC field exclusion in both the ZFC and the FC samples compared to their virgin states, as the applied DC field was reduced. In addition, the grain size was found to have a strong influence on both the virgin and the hysteretic AC susceptibilities. The observed hysteresis is then explained by carefully analysing the intergranular local magnetic field (or the intergranular critical state), which is strongly modulated by the intragranular critical state, upon (1) cooling a sample with or without a DC field; (2) increasing or reducing the DC field applied to a virgin sample. Using the additional information from the grain size effect, a more detailed picture of the dynamic intergranular critical state is obtained.

Transport critical currents and flux pinning in melt-grown YBa2Cu3O7−δ

Abstract The critical current and flux pinning properties of melt grown YBa 2 Cu 3 O 7−δ bulk material were investigated by transport measurements and V-I characteristics. A J c of 16 000 A cm −2 in zero field and 3000 A cm −2 in 1 T at 77 K was obtained using a criterion of 3.3 μV cm −1 . No features of granularity could be detected as J c showed no hysteresis with increasing or decreasing field. By considering a distribution of activation energy U 0 together with the correlation volume of flux bundles V c and the pinning range r p , the measured V-I characteristics in the flux creep regime were found to be consistent with that of thermally activated flux flow (TAFF). The temperature and field dependence of the derived V c r p points to a collective pinning mechanism by point defects. The depinning current J c , one to tow orders of magnitude higher than the transport J c , was also obtained by using the U 0 and V c r p deduced from the V-I characteristics.

DC and AC transport current measurements up to 140 A in melt-grown YBCO superconductors

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.

Studies of self-field AC losses in PbBi-2223 Ag-sheathed tapes

Abstract Self-field AC losses in PbBi-2223 Ag sheathed tapes were measured at differenttemperatures and frequencies using a lock-in amplifier and a Rogowski coil. For J Jc, the E′(J) characteristics converge rapidly to a frequency independent function, as the losses become dominated by a resistive mechanism such as flux flow.

Anomalous magnetization of melt-grown SmBa2Cu3O7−δ

Abstract A single grain chosen from a melt-grown SmBa 2 Cu 3 O 7−δ sample was ground into a cylinder 4 mm long and 1.5 mm in diameter with the c -axis of the grain perpendicular to the axis of the cylinder. Magnetization curves show a peak at intermediate fields. This phenomenon has been attributed to field induced pinning by weak superconducting oxygen deficient regions. The temperature and angular dependence of the field, H max , at which a peak occurs in the M - H loops was measured in a vibrating sample magnetometer. H max shifts from 0.7 to 11 T at 55 K as the angle θ between the external field and the c -axis varies from 0 to 90°. As expected from the field induced pinning model, we found that the field at which all weak regions become normal has the same angular dependence as that for H c2 in anisotropic superconductors.

The critical state in YBa2Cu3O7-x high temperature superconductor

Abstract A practical method was devised for measurements of the diamagnetic response of high temperature superconductor specimens. The method is useful for the study of the temperature and field dependencies of the local magnetic induction and its two-dimensional visualisation B ( T , H a , x , y ) with a resolution of 0.1 gauss and spatial resolution of 0.1 mm in the plane of the specimen surface. The proposed method facilitates quite simple and quick measurements at liquid nitrogen temperature and in applied H a magnetic field in excess of 1 T. Cross-shaped miniature Hall probes (HPs) of thin film n -InSb on semi-insulating substrate i -GaAs with a small magnetically sensitive zone of 0.1 × 0.1 mm 2 were used as measuring transducers. The high degree of doping of n -InSb layers ( n ~ 10 18 cm −3 ) resulted in a sensitivity of 5–7 μV mT −1 (with nominal current up to 10 mA through HPs) and thermosensitivity of less than 0.03% K −1 , and made it possible to operate without the need for thermal stabilization of transducers. This method enabled us to study anisotropy of the critical current densities ( J c ) of elt-grown YBa 2 Cu 3 O 7 − x pellets. Also, we have been able to determine local cracks and grain connectivity in the YBa 2 Cu 3 O 7 − x pellet samples.

AC and DC transport current in melt-grown YBCO

It has been suggested that the transport J/sub c/ in multigrain samples of bulk YBCO is limited by the intergrain links. The authors report on preliminary measurements of intergrain currents. The intergrain critical currents in melt-grown YBCO do not appear to be as sensitive to the precise crystallographic alignment of adjacent grains as has been reported for thin films. The measured critical current of similar grain boundaries varies widely, between 15000 A/cm/sup 2/ and 200 A/cm/sup 2/ for adjacent boundaries in the same sample. Preliminary results support the conclusions of S.E. Babcock et al. (Nature, vol.347, p.167, 1990), who found weak-link-free behavior in some high-angle boundaries between bulk crystals.

Transverse distribution of the transport current density in (Bi, Pb) 2223 tapes

Abstract The distribution of the critical transport current density, J c , has been measured across the core thickness of silver sheathed (Bi, Pb)2223 tapes which contained silver addition of different particle sizes. We have found the J c of tapes without silver addition and with 8 wt% silver addition of small particle size decreases continuously with distance from the core-sheath interface. The J c of layers close to the silver sheath is up to four times higher than that of central layers, e.g. 3 × 10 4 A cm −2 near the sheath and 7 × 10 3 Acm −2 in the centre at 77 K and self field. The tape with addition of 50 wt% 80 μm silver particles shows a uniform J c of about 2 × 10 4 A cm −2 throughout the core thickness. The V − I characteristics of the baser core of the tape without silver addition have also been measured in magnetic fields.