Magnetic field orientation dependence of J/sub c/ in Bi-2212 round wires

We have performed measurements of the magnetic field dependence of the critical current density J/sub c/ of Bi-2212/Ag round wire produced by isothermal melt processing. In contrast to the case for flat tape, there is very little dependence of J/sub c/ on on the direction of the magnetic field as it is rotated normal to the wire axis which is the direction of the nominal current flow. However, when the angle of the magnetic field direction is rotated from normal to the wire axis to parallel to that axis, J/sub c/ at 64 K and 0.2 T increases by more than a factor of four. Again, this is in contrast to the results observed for Bi-2212/Ag and Bi-2223/Ag flat tapes, which show no anisotropy under similar experimental conditions. We can explain these differences in angular anisotropy by referring to the microstructure of these two conductor types, which have distinctly different types of grain alignment. We discuss the general behaviour of the dependence of J/sub c/ on the orientation of a magnetic field for high temperature superconductors.