Development of high-Tc superconductor wires for magnet applications

Bi-2212/Ag φ 1-mm round untextured wires have been produced with critical current densities of 1200 A/cm² at 77 K, 0 T and up to about 2.0×10⁴ A/cm² 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 Bi₂(Sr,Ca)₂ Cu₁O_6+x. On cooling, the Bi₂(Sr,Ca)₂Cu₁O_6+x phase appears at 880°C, as revealed by in situ XRD (X-ray diffraction) measurements. Long-term annealing at 840°C leads to the transformation of this phase into the Bi₂Sr₂Ca₁ Cu₂O_8+x phase. The alkaline earth cuprate (Sr,Ca) ₁₄Cu₂₄O _≈40 and CuO occur as major extraneous phases. Helically shaped samples yield a lower j_c between 3×10³ and maximum 1.0×10⁴ A/cm² 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_c of 2.6×10⁴ A/cm² at 77 K, 0 T. Temperature-dependent measurements of j_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