THE EFFECT OF ANNEALING ON THE SUPERCONDUCTING PROPERTIES AND STRUCTURE OF BI-2212 SINGLE-CRYSTALS

Two types of Bi-2212 single crystals, one type having a low T-c and the other having a high T-c were annealed in air at different temperatures over the range 450 degrees C to 750 degrees C followed by furnace cooling. The transition temperature of the crystals with low T-c (similar to 56 K) increased with increase of annealing temperature, while the crystals with high T-c (similar to 91 K) had just the opposite behaviour under the same annealing conditions. It is indicated that the quality of as-grown crystal has a significant effect on the superconducting properties of the annealed crystal. The c-axis values for both types of crystals decreased with increase of annealing temperature. According to the relationship between c-axis values and the oxygen content, our results confirmed the existence of a dome-shaped curve of T-c with carrier concentration in Bi-2212 single crystal similar to that in Bi-2212 compound reported previously by Green and de Leeuw [Physica C 159 (1989) 417]. By studying the increase of annealing time at a fixed temperature of 650 degrees C, T-c and Delta T-c were found to change very little for both types of crystals. Thus, T-c and Delta T-c were more sensitive to annealing temperature than to annealing time. XRD, SEM and EDS revealed that a small quantity of powder phase appeared after annealing at temperatures above 500 degrees C for both types of crystals. This powder phase had the Bi-2212 composition, which indicated that the integrity of the Bi-2212 single crystals was significantly degraded by the annealing process.