The magnetic shielding properties of YBa 2 Cu 3 O 7− x coatings, sprayed on silver cylinders with the continuous detonation spray (CDS) technique, have been investigated in weak magnetic fields. Extended films 40–80 μm thick showed flat-sided sharp-cornered hysteresis loops. Critical penetration field values, when scaled for thickness according to Bean's model were 1.6–3.7 mTmm -1 at 77.3 K and 21–35 mTmm -1 at 4.2 K. In the actual samples, J c was calculated to be 0.95−1.4×10 3 A cm -2 at 77.3 K and 10−12.5×10 3 A cm -2 at 4.2 K. Relaxation rates S = 11−23 ×10 -3 were measured for the flux creep at 77.3 K, and were found to be much lower at 4.2 K. The CDS technique allowed one to have precise control of the film thickness, a high coating productivity and a relatively low (∼ 1000°C) processing temperature.

Magnetic shielding properties of YBa2Cu3O7-x thick films deposited on silver cylinders with the continuous detonation spray technique

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.

Shielding effects in ceramic superconductors

A local magnetic characterization of disk-shaped MgB2 bulk samples synthesized by the Spark-Plasma-Sintering technique was carried out aimed at studying the material magnetic shielding properties. This growth technique allows one to produce samples in different shapes and sizes as required for shielding applications. The analysis employed a linear array of Hall probes mounted on a custom-designed stage able to move perpendicular to the sample surface with micrometric resolution. Spatial distributions of the axial component of the shielding magnetic-induction field generated by the superconductor were evaluated in the temperature range 20-36 K and in applied magnetic field up to 1.5 T. By the analysis of these shielding field profiles in the framework of the critical state model, critical current densities, Jc, higher than 1010 A/m2 were found at T=20 K. On the basis of the found Jc values a preliminary estimation of the full penetration field inside a tube-shaped shield component was carried out. It indicates that the material under test is a good candidate for passive magnetic shields.

Local Magnetic Investigations of ${\rm MgB}_{2}$ Bulk Samples for Magnetic Shielding Applications

Magnetic shielding and noise spectrum measurements of YBaCuO, BiSrCaCuO and (Bi,Pb)SrCaCuO superconducting tubes

Studies of the propagation of an AC magnetic field through plasma sprayed superconducting Y1Ba2Cu3O7-x coatings have been carried out in the incident field frequency and intensity ranges 100 Hz to 100 kHz and 0.01 to 3.5 mT respectively. Results show that the shielding effectiveness increases exponentially with decreasing field intensity. Shielding effectiveness increases with coating thickness and its density. Coatings thinner than 100 mu m do not show any shielding. A non-linear dependence of critical magnetic field H* on both thickness and critical current density Jc of the specimen has been observed.

Study of the propagation of an AC magnetic field through superconducting thick films

Tubes fabricated of polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-x/ are characterized, and details of the fabrication procedure are discussed. The microstructure of the tubes determined by scanning electron microscopy and X-ray diffractometry is described. Resistive measurements of T/sub c/ and Delta T/sub c/ have been made. The tubes have also been characterized by means of SQUID (superconducting quantum interference device) magnetometry. The temperature dependence of magnetic fields trapped axially in the tubes has been measured and estimates of penetration depth are given. Moreover, measurements of the transverse shielding effectiveness of the tubes have been made and are compared with theoretical predictions. Studies on flux penetration into the tubes are described. Findings from the microstructure studies are correlated with the observed superconductivity properties. The results on the high-temperature materials are compared to results obtained previously on tubes made from conventional superconductors. >

T.R. Clem

Papers

The fabrication and characterization of high temperature superconducting magnetic shields