Improvement in the magnetic shielding property of Bi(Pb)-Sr-Ca-Cu-O superconducting tubes

Abstract

A remarkable improvement in the magnetic shielding property of Bi (Pb)-Sr-Ca-Cu-O superconducting tubes has been achieved by applying an intermediate cold isostatic pressing between the sintering treatments. The shielding magnetic flux density, at which the flux starts to penetrate into the tube centre, was increased from 2.1 to 17.6 mT by this modified process. This improvement might be due to the increase in the density of the tube. Furthermore, the shielding magnetic flux density was proportional to the critical current density. Magnetic shielding may be regarded as one of the important applications of high-T c superconductors. The magnetic shielding by high-Tc superconductors has been studied by several groups, both for discs [1-3] and for tubes [4-11]. However, a specimen prepared by the conventional sintering process showed a low magnetic shielding property because of the low relative density [11]. In order to overcome the drawbacks of the sintering process, intermediate pressing [12, 13] and hot pressing [14] have been reported. However, these processes are difficult to apply to tubes with curved surfaces. In this work an improvement in the magnetic shielding property of a Bi(Pb)-Sr-Ca-Cu-O tube was achieved by applying an intermediate cold isostatic pressing between the sintering treatments. The Bi(Pb)-Sr-Ca-Cu-O superconducing tube was fabricated as follows. The prescribed amounts of powders of Bi203, PbO, SrCO 3, CaCO3 and CuO were mixed in the molecular ratio Bii.sPb03Srl.9Ca2.0Cu3.0Ox and calcined at 800 °C for 10 h. The calcined powder was pressed into a cylindrical specimen. Some of the specimen was sintered at 850 °C for 150 h (NON-CIP specimen). The rest of the specimen was sintered at 850 °C for 100 h and cold isostatically pressed at a pressure of 150 MPa, then sintered at 850 °C for 50 h (CIP specimen). Subsequently, tubes were obtained to bore in the sintered specimen. The tubes had an inner diameter of 0.8 cm, a wall thickness of 0.8 cm and a length of 5.4 cm. The critical current density, Jc, was measured using a rod-like sample into which the tube was cut. The rod had a width of 0.1 cm, a thickness of 0.1 cm and a length of 3 cm. The critical current density was measured at 77.3 K by the four-probe method and was evaluated from the current that induced 1/xV across a 1 cm length of the rod. Fig. 1 is a schematic diagram of the measuring system for the magnetic shielding property of the tube. The measurement was carried out at 77.3 K. An external magnetic field (the external magnetic flux density Bex ) was applied parallel to the tube axis, and the internal magnetic flux density Bin at the tube centre was measured with a sensitive cryogenic Hall probe (Bell model BHA-921). The sensitivity of the probe was 8.27/xV mT -1 . Fig. 2 shows scanning electron micrographs taken of the fractured cross-section of the NON-CIP tube and the CIP tube. The NON-CIP tube shows a very porous microstructure in which large plate-like grains formed in random directions, whereas the CIP tube shows a dense microstructure. However, the plate-like grains were not observed to be aligned in the same direction as shown in the specimen prepared by the intermediate pressing [13]. The density of the tube was increased from 3.2 to 4.8 gcm -3. Fig. 3 shows the external magnetic flux density, Bex, dependence of the internal magnetic flux density Bin of the NON-CIP and CIP tubes. In the case of the NON-CIP tube the Bin remained zero up