Rf Surface Resistance Measurements in the Pb-Bi-Sr-Ca-Cu-O System
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RF SURFACE RESISTANCE MEASUREMENTS IN THE Pb-Bi-Sr-Ca-Cu-O SYSTEM M. T. Lanagan, C. L. Bohn, J. R. Delayen, M. C. Einloth, R. N. Vogt, and U. Balachandran, Argonne National Laboratory, Argonne, IL 60439 ABSTRACT Wires were fabricated from the high-Tc superconductor Bio.Pb0. 7 3 SrCaCu1.8Ox (Pb-BSCCO) by an extrusion process. The specimens were sintered for periods up to 14 days, and density was found to decrease with longer sintering times. Rf surface resistance data revealed that the onset of superconductivity occurred at 105 K and that the transition was very broad. INTRODUCTION It has been shown previously that the rf surface resistance (Rs) of bulk YBa 2 Cu 3 O7 -. (YBCO) is below that of copper at frequencies between 100 MHz and 1 GHz. However, Rs degrades as the material is subjected to large rf magnetic fields. 1 The Rs of high-quality YBCO single crystals is significantly lower than 2 that of bulk YBCO, owing to the absence of Josephson junctions. The rf properties of Bi2Sr2CaCu2O. (BSCCO) thick films were measured from 3 to 30 GHz at temperatures between 4 and 140 K. A large decrease in Rs was observed as temperature decreased below 80 K.3 The surface resistance of the film was higher than that of bulk YBCO, although the BSCCO films exhibited a large degree of crystal orientation. The high Rs was attributed to inhomogeneities in the BSCCO thick film. PROCEDURE Precursor powder was made from constituent oxides, nitrates, and carbonates mixed in stoichiometric proportions and calcined in air at 800-830°C for 30 h. Wires with diameters of 0.6 and 1.1 mm in diameter were fabricated by a plastic extrusion process; the details may be found elsewhere. 4 The Pb-BSCCO wires were sintered between 845 and 8601C in air for times up to 14 days. Specimens were slowly cooled to 5001C and quenched to room temperature. Temperatures were kept below the melting temperature, thus allowing the material to be sintered without a substrate. Critical current density (Jc) was measured by a four-point probe technique with an electric field criterion of ljiV/cm, and all measurements were carried out in liquid nitrogen. Wire lengths were about 3 cm for Jc measurement. Longer wires (about 15 cm) were made for the rf surface resistance measurement. The wire was the center conductor of a coaxial resonant cavity, which was operated in the fundamental TEM mode. The cavity was cooled by liquid helium to 4.2 K and data were taken as the specimen was heated to room temperature.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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RESULTS AND DISCUSSION The best Jc was found in samples sintered at 855°C, and specimens were subsequently sintered at 855°C for 6 and 14 days. Data shown in Table 1 are averages .of two measurements. The geometric densities for all specimens sintered for 14 days were lower than the densities of specimens sintered for 6 days; this has been attributed to a highly anisotropic grain growth that leads to a retrograde densification process. 5 Scanning electron microscopy revealed that grain size
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