CORRELATION OF TRANSPORT AND MAGNETIC CRITICAL CURRENTS IN MELT-PROCESSED YBA2CU3O7-DELTA THICK-FILMS

Transport critical current measurements have been carried out on melt- processed thick films of YBa2Cu3O7-delta on yttria-stabilized zirconia in fields of up to 8 T both within grains and across grain boundaries . These measurements yield J(c) values of approximately 3000 A cm-2 at 4.2 K and zero magnetic field and 400 A cm-2 at 77 K and zero magneti c field, taking the entire sample width as the definitive dimension. O ptical and scanning electron microscopy reveals that the thick-film gr ains consist typically of a central '' hub'' region approximately 50 m um in diameter, which is well connected to radial subgrains or ''spoke s'' which extend approximately 1 mm to define the complete grain struc ture. Attempts have been made to correlate the transport measurements of inter- and intra-hub-and-spoke (H-S) critical current with values o f this parameter derived previously from magnetization measurements. A nalysis of the transport measurements indicates that current flow thro ugh H-S grains is constrained to paths along the spokes via the grain hub. Taking the size of the hub as the definitive dimension yields an intra-H-S grain J(c) of approximately 60 000 A cm-2 at 4.2 K and 0 T, which is in reasonable agreement with the magnetization data. Experime nts in which the hub is removed from individual grains confirm that th is feature determines critically the J(c) of the film.