STUDY OF THE EFFECT OF NA CU SUBSTITUTION IN BI4SR3CA3CU4O16 GLASS-CERAMIC SUPERCONDUCTOR/

The composition Bi4Sr3Ca3Cu4-xNaxO16 for x=0.2, 0.4, 0.6, 0.8 and 1.0 was examined, in order to study the effect of Na/Cu substitution on bo th the glass-forming ability as well as the superconducting properties of the glass-ceramic (GC) phase. Because the GC phase of the composit ion Bi4Sr3Ca3Cu4O16 (4334) showed superconducting properties below 78. 5 K, the crystalline phases formed after heat treatment were identifie d by X-ray diffraction. This suggests that Na+ substituted the Cut cat ions. The GC phases were studied by X-ray diffraction, differential th ermal analysis, infrared absorption, d.c. electrical conductivity and low-temperature a.c. magnetic susceptibility in the temperature range 77-300 K. The present results support the considerations that the addi tion of sodium enhances the crystallization of the 4334 phase while sa crificing the glass-forming ability. The crystalline phases precipitat ed from the rapidly quenched glasses in the Bi-Sr-Ca-Cu-Na-O system we re greatly dependent on the heat-treatment time as well as the treatme nt temperature. The stability and crystallization process of the glass differ greatly depending on the sodium content which acted as fluxing agent. Considering that the formation of the 4334 phase is largely en hanced in sodium-doped samples, it is concluded that the lowering of t he partial melting temperature is very important for the enhancement o f the formation of the 4334 phase, well as in raising the critical tra nsition temperature. Infrared spectroscopy measurements indicate that part of the ceramic phase is non-metallic. The two-probe method and th e standard four-point probe method electrical resistivity indicate tha t Bi-Sr-Ca-Cu-Na-O superconductor formation was greatly dependent on h eat-treatment time as well as treatment temperature. The superconducti ng crystalline phase, which grew upon heat treatment, was identified a s a quasi-tetragonal phase 4334. Electrical resistance measurements to gether with thermopower results indicate that the electrical propertie s move from a metal region to a semiconducting region according to the magnetic phase diagram of oxide superconductors. Superconducting 4334 phase with T-c=84 K could be successfully prepared by the GC techniqu es within 1-2 h thermal cycling, which renders a great saving in proce ssing costs and is a simple method of moulding superconducting article s.