PINHOLE DEFECTS IN AG SHEATH OF PIT BI-2212 TAPES

Pinhole defects have been observed in the silver sheath of powder-in-t ube (PIT) fabricated Bi-2212 tapes. Material from the oxide core exit through these pinholes. A relatively large area of (Sr, Ca)(x) oxide c ondensate was found to surround the pinholes on the outer surface of t he conductor following partial-melt-growth (PMG) heat treatment. The f ormation of these pinholes is attributed to the PIT mechanical deforma tion of the precursor powders, which contain a few relatively large (> 30 mu m) and hard (relative to silver) particles/aggregates. Results i ndicate that these pinholes, which are more pronounced in thin tapes ( <100 mu m), play a significant role in the mass transfer process betwe en the core and the ambient during annealing. This movement has a detr imental effect on the transport critical current l(c). This paper atte mpts to quantify the effects of these pinhole defects.