We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor (the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n.

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

Coils are placed coaxially on either side of a slab of superconductor of thickness 0.05-1 mm. The coil diameters are less than the lateral dimentions of the slab. The voltage in the secondary coil is measured as a function of excitation current and frequency in the primary, and of the temperature and the steady magnetic field provided by an external solenoid. A YBa2Cu3O7 ceramic cooled in zero field screens almost completely, but the greater part of this screening is suppressed by low magnetic fields. The remaining part is shown to be due to isolated superconducting grains. When intergrain superconductivity is partly suppressed the condition is reasonably well described by the Bean critical-state model. Hysteresis of the screening coefficient reflects the known hysteresis of the transport critical current.

AC screening measurement for the characterization of oxide superconductors: I. Application to ceramics

Irreversibility and hysteretic aspects of granular YBCO

A superconductive element is described, comprising a rigid support made of a non-superconductive material, said support comprising at least one superconductive track formed by a groove containing a superconductive material having a density equal to at least 85% of the value of its theoretical density, and the process for producing said element. The present invention also relates to the possible uses of the superconductive elements, and also to superconductive devices comprising said superconductive elements.

Superconductive element and relative preparation process

Zn-substituted YBa2(Cu1-xZnx)3O7- delta superconductors have been synthesized, and characterized according to resistivity, AC susceptibility and DC magnetization measurements. DC magnetization measurements on randomly oriented powdered systems exhibit pronounced M(H) hysteresis loops, indicative of extensive flux pinning. The authors feel that this flux pinning arises due to the Zn acting as a pinning centre. The critical current density Jc has been deduced from hysteresis data.

http://iopscience.iop.org/article/10.1088/0953-2048/4/2/004/pdf

Enhanced flux pinning by Zn substitution in YBa2Cu3O7- delta

The authors have measured the magnetic field H' at the center of a hollow of sintered Y-Ba-Cu-O superconductor as a function of applied field H, temperature, tube length, and tube wall thickness. The maximum field that can be shielded H/sub sh/ agrees with the value calculated from the critical state model using the measured critical-current density J/sub c/. The maximum trapped field H/sub tr/ in the tube, on decreasing H to zero, exceeds H/sub sh/ by as much as a factor of two, and large enhancements in J/sub c/ are observed in the decreasing field. These phenomena are identified with intragranular flux pinning present only after H has exceeded H/sub c1/ of the grains. Finite tube length does not affect the H/sub tr//H/sub sh/ ratio appreciably. H/sub sh/ depends approximately on the square root of the wall thickness. >

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H. Sheinberg

Papers

Magnetic shielding by superconducting Y-Ba-Cu-O hollow cylinders