Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes
01 Mar 1989-Journal of Applied Physics (American Institute of Physics)-Vol. 65, Iss: 5, pp 2142-2144
TL;DR: In this paper, high-Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube.
Abstract: High‐Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube. Relaxation phenomena with two different time dependencies are observed when the external magnetic field enters the tubes.
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TL;DR: In this article, the authors reviewed the stability predictions regarding cryostability, minimum propagating zones and quench protection with respect to the possible use of high T c superconductors (SC) in current carrying devices.
91 citations
TL;DR: In this article, the authors studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields, and systematically detail the spatial variation of the shielding factor along the axis of a high-temperature superconducting tube.
Abstract: 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.
71 citations
TL;DR: In this paper, the onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. The critical current (jc) has a magnetic field dependence given by j c = j 0 H 2 0 (H 2 +H 2 0 ) where j0 and H0 depend only on temperature.
Abstract: The onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. These measurements have been used to calculate the temperature dependence of the intergranular critical transport current employing three critical-state models. Good agreement was obtained between this calculated critical current and direct critical current measurements on rod-shaped samples of the same material. The critical-state magnetic field profiles were used to explain the observed voltage waveforms induced in a pick-up coil inside the bore of tubular samples when excited by an AC field. Best agreement was obtained using a model in which the critical current (jc) has a magnetic field dependence given by j c = j 0 H 2 0 (H 2 +H 2 0 ) where j0 and H0 depend only on temperature.
21 citations
TL;DR: In this article, a buffer-aided top seeded melt growth fabrication process was used to construct YBCO tubes of ∼10 mm diameter closed at one extremity, which can act as efficient 'dc' magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K.
Abstract: © 2002-2011 IEEE. YBCO tubes of ∼10 mm diameter closed at one extremity were engineered by a buffer-aided top seeded melt growth fabrication process. These tubes can act as efficient 'dc' magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding 'dc' stray fields generated by high temperature superconductor magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K), or liquid neon (27 K).
20 citations
Cites background from "Magnetic shielding and relaxation c..."
...In this paper we show that high field magnetic shielding can be achieved efficiently with bulk, large grain YBa2Cu3O7 (YBCO) closed tubes when they are cooled in the range 20-40 K....
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...This Jc value, however, is much higher than that obtained previously in polycrystalline YBCO [36] or BSCCO hollow cylinders (of the order of 350 A/cm² at 77 K and 20 mT [8, 19])....
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...In conclusion, we have fabricated a YBCO tube made by Buffer-aided Top Seeded Melt Growth and characterized its high-field magnetic shielding performances....
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...YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG)....
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...The value is smaller than that expected for single-grained YBCO samples in the form of disk pellets (of the order of 104 A/cm²), which can be ascribed to the fact that some parts of the tube are not perfectly textured especially those far away from the seed....
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TL;DR: In this article, the authors studied the shielding properties of high-temperature superconductors in DC and AC magnetic fields using Bean's critical state model and models, which account for the field dependence of the critical current density.
Abstract: The shielding properties of high-temperature superconductors in DC and AC magnetic fields are discussed using Bean's critical-state model and models, which account for the field dependence of the critical current density. Experimental work on the shielding properties of superconducting YBa 2 Cu 3 O 7−δ and conductive (aluminum and cooper) screens is presented. The maximum value of the shielded field for YBa 2 Cu 3 O 7−δ /Ag screens was measured at 3 mT. Shielding by superconducting screens was found independent of frequency over the range 80 Hz−10 5 Hz. The limited shielding capability of ceramic superconductors results from the magnetic field dependence and low values of the intergrain current density in bulk materials. There is a practical limit to the external field that can be shielded by a superconductor, characteristic of the material and size independent. Increasing of the shield thickness will not increase the maximum value of the field which can be shielded.
18 citations
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3,567 citations
TL;DR: In this paper, the authors show that the mechanism of flux creep is thermally activated motion of bundles of flux lines, aided by the Lorentz force, over free energy barriers coming from the pinning effect of inhomogeneities, strains, dislocations, or other physical defects.
Abstract: Previous investigations of the critical state of the hard superconductor have defined it in terms of constants alpha and B/sub O/; alpha (T) = J/sub cr/ (B/sub cr/ + B/sub 0/). If the current density J or the field B is increased beyond the critical values, a process called flux creep'' sets in, and flux leaks through the material and returns it to the critical state. alpha (T) is a structure-sensitive constant of the material. A theory is presented to show that this behavior of alpha can be explained by assuming that the mechanism of flux creep is thermally activated motion of bundles of flux lines, aided by the Lorentz force, over free energy barriers coming from the pinning effect of inhomogeneities, strains, dislocations, or other physical defects. The theory also explains B/sub 0/, and predicts time relaxation behavior similar to those of magnetic aftereffect and some forms of plastic creep. (H.D.R.)
1,403 citations
IBM1
TL;DR: Strong, anisotropic magnetic relaxation of the field-cooled and zero-field--cooled magnetization along the principal axes of an Y-Ba-Cu-O single crystal is reported and interpreted with a thermally activated flux-creep model.
Abstract: We report strong, anisotropic magnetic relaxation of the field-cooled and zero-field-cooled magnetization along the principal axes of an Y-Ba-Cu-O single crystal and interpret it with a thermally activated flux-creep model. A simple scaling argument shows that high thermal activation causes magnetic irreversibilities and critical currents to drop below the threshold of detectability at a reduced temperature difference $1\ensuremath{-}t$ proportional to ${H}^{\frac{2}{3}}$, a power frequently observed in experiment and in particular in our crystal.
1,358 citations
TL;DR: In this paper, the authors investigated persistent currents induced in hard superconductors (HSC) by measuring the resulting magnetic effects and found that the Lorentz force plays a crucial role in determining the critical current density and the critical persistent currents in decay with a measurable rate.
Abstract: Persistent currents induced in hard superconductors (HSC) were investigated by measuring the resulting magnetic effects. The tubular HSC examined were Nb powder, Nb/sub 3/Sn, and 3Nb- Zr samples of uniform wall thickness and length large compared to the diameter. Experimental evidence is presented that in HSC the Lorentz force plays a crucial role in determining the critical current density and the critical persistent currents in decay with a measurable rate. (H.D.R.)
973 citations
IBM1
TL;DR: In this article, the diamagnetism observed in the zero-field--cooled state is considerably larger than under field cooling, indicating the existence of a superconductive glass state.
Abstract: Susceptibility and magnetic-moment measurements from 1.9 to 35 K in magnetic fields up to 1.5 T in powder samples of La/sub 2/CuO/sub 4-//sub y/:Ba are reported. The diamagnetism observed in the zero-field--cooled state is considerably larger than under field cooling. The former is metastable like the magnetic moment induced after switching the field off. These observations indicate the existence of a superconductive glass state.
871 citations