The Effect of the Distribution of Ag in High T C Bi- Compound Superconductors
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THE EFFECT OF THE DISTRIBUTION OF Ag IN HIGH T. Bi- COMPOUND SUPERCONDUCTORS ALEXIS S. NASH*, K. C. GORETTA**, PHILIP NASH**, R. B. POEPPEL**, AND DONGLU SHI** University, Milwaukee, WI, 53233 National Laboratory, Argonne, IL 60439 '**Illinois Institute of Technology, Chicago, IL 60616
"Marquette "*Argonne
ABSTRACT A series of 4336 Bi-Sr-Ca-Cu oxide samples doped with 1 to 5% metallic Ag was prepared by solid state reaction. The distribution of Ag, the microstructure and the crystal structure of the samples were studied using energy dispersive spectrometry, EDS, scanning electron microscopy, SEM, and x-ray diffractometry, XRD. Addition of Ag leads to a marked increase in preferred orientation with (001) planes perpendicular to the pressing direction in sintered pellets. The resistivity-temperature data show an enhanced TC in Ag-doped samples under certain conditions. Energy dispersive spectrometry indicates that the dopant mostly segregates to the grain boundaries. INTRODUCTION The critical current density, J,, of bulk ceramic high-temperature superconductors is generally far lower than that of thin films of conventional metallic superconductors. Although grain boundaries may act to pin flux, they also act as weak links [1]. The weak link behavior of the grain boundaries has been attributed to segregation of impurities or the principal elements of the material, or to the presence of nonsuperconducting phases or microcracks. The superconducting transition temperature, Tc, of Bi compounds is influenced by the number of CuO 2 planes in the crystal structure and the oxygen content [2]. Doping also influences Tc. The most widely investigated example is addition of Pb, which stabilizes the 110 K phase. The dopant can be segregated, however, and an amorphous, Pb-rich phase has been observed at the grain boundaries of some specimens [3]. We have doped the Bi 2 Sr 2 CaCu2 Ox phase with Ag and have examined the distribution of the dopant and the effects of doping on T,, crystal structure, and microstructure. Experimental Procedure The undoped 2212 superconducting phase was synthesized by solid state reaction of oxide and carbonate precursors at the starting composition equivalent to Bi 4 Sr 3 Ca 3 Cu 6 Oy according to the procedure described previously [4]. Similarly processed 12.7 mm diameter pellets of Ag-doped samples of 4336 composition were compacted at a pressure of 175 MPa and fired on MgO single crystals. The firing schedules employed 50C/min heating and cooling rates. The electrical resistivity measurements were carried out using the standard four-probe method. Structural characterization was performed using EDS, SEM and XRD. Results and Discussion The resistivity-temperature (p-T) data for the Ag-doped samples have shown an increase in the onset transition temperature with firing temperature and time. Synthesis reactions occurred at lower temperatures for the doped than the undoped samples. The p-T data for the 2.5% Ag-doped Mat. Res. Soc. Symp. Proc. Vol. 169. -1990 Materials Research Society
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