Development of high-T/sub c/ superconductor wires for magnet applications

Bi-2212/Ag phi 1-mm round untextured wires have been produced with critical current densities of 1200 A/cm/sup 2/ at 77 K, 0 T and up to about 2.0*10/sup 4/ A/cm/sup 2/ at 4.2 K in magnetic fields beyond 20 T. In order to achieve this, a two-step annealing procedure is necessary with a partial melting of the wire core, the melt having a composition near Bi/sub 2/(Sr,Ca)/sub 2/Cu/sub 1/O/sub 6+x/. On cooling, the Bi/sub 2/(Sr,Ca)/sub 2/Cu/sub 1/O/sub 6+x/ phase appears at 880 degrees C, as revealed by in situ XRD (X-ray diffraction) measurements. Long-term annealing at 840 degrees C leads to the transformation of this phase into the Bi/sub 2/Sr/sub 2/Ca/sub 1/ Cu/sub 2/O/sub 8+x/ phase. The alkaline earth cuprate (Sr,Ca) /sub 14/Cu/sub 24/O/sub approximately=40/ and CuO occur as major extraneous phases. Helically shaped samples yield a lower j/sub c/ between 3*10/sup 3/ and maximum 1.0*10/sup 4/ A/cm/sup 2/ at 4.2 K, 10 T. Cracks due to thermal expansion mismatch with the sample holder and remaining inhomogeneities along the wire are responsible for these lower values. From I-V curves measured at 4.2 K, n-values were determined to be 20 to 25 in the interesting very-high-field region beyond 20 T. Bi-2223/Ag highly textured thin tapes yield an appreciably higher j/sub c/ of 2.6*10/sup 4/ A/cm/sup 2/ at 77 K, 0 T. Temperature-dependent measurements of j/sub c/ as a function of magnetic field B yielded onset of significant flux creep above 20 K, limiting the range of application in magnet technology to the temperature range up to 20 K unless an additional more effective pinning mechanism is introduced.