Pinhole defects in Ag sheath of PIT Bi-2212 tapes

Pinhole defects have been observed in the silver sheath of powder-in-tube (PIT) fabricated Bi-2212 tapes. Material from the oxide core exit through these pinholes. A relatively large area of (Sr, Ca)_x oxide condensate was found to surround the pinholes on the outer surface of the conductor following partial-melt-growth (PMG) heat treatment. The formation of these pinholes is attributed to the PIT mechanical deformation of the precursor powders, which contain a few relatively large (> 30 μm) and hard (relative to silver) particles/aggregates. Results indicate that these pinholes, which are more pronounced in thin tapes (< 100 μm), play a significant role in the mass transfer process between the core and the ambient during annealing. This movement has a detrimental effect on the transport critical current I_c. This paper attempts to quantify the effects of these pinhole defects.