Relationship between grain connectivity, alignment, and critical current density in high-temperature superconducting bulk materials

Abstract

Uniaxial hydraulic pressure was successfully employed to partially reorient the c-axes of the highly anisotropic superconducting grains in bulk Tl 0.78Bi 0.22Sr 1.6Ba 0.4Ca 2Cu 3O 9−δ samples. The partial reorientation was confirmed by X-ray diffraction pattern, rocking curves, and SQUID magnetization measurements. Samples made from the superconducting powder with partial alignment in c-axis direction and samples made from reaction mixture (in-situ synthesis method) with random orientation should have a similar grain connectivity since both were made by the same fabrication procedure. Both types of samples were found to exhibit about the same J c value of 15 000–17 000 A/cm 2 at 77 K. By contrast, another pair of samples with the same c-axis alignment as shown by X-ray diffraction, rocking curve, and SEM measurements, but with different grain connectivity due to the different compression and heat treatment procedure used, showed a significant difference in J c. The just rolled tape had a J c of 13 300 A/cm 2, whereas the rolled and subsequently compressed tape had a J c of 19 500 A/cm 2 at 77 K. Based on the results of these and other related experiments, we have to conclude that grain connectivity, not lattice alignment, is the decisive factor in determining the J c of Tl based 1223 bulk superconducting materials and silver-sheathed superconducting tapes made by the powder-in-tube method.