New high-rc superconductors have exceedingly high critical fields,
as apparent in Table 8.7, that do
not seem to necessarily translate to high critical current densities. The critical current
density in Type II superconductors depends not only on the temperature and the
applied magnetic field but also on the preparation and hence the microstructure (e.g. ,
polycrystallinity) of the superconductor material . Critical current densities in new
high-rc superconductors vary widely with preparation conditions. For example, in
Y-Ba-Cu-O
,
Jc may be greater than 1 07 A cm-2 in some carefully prepared thini lms
and single crystal s but around 1 03-1 06 A cm-2 in some of the polycrystalline bulk
material (e.g. , sintered bulk samples). In NbsSn, used in superconducting solenoid
magnets, on the other hand, Jc i s close to 1 07 A cm-2 at near 0 K.