ON THE ROLE OF VICKERS AND KNOOP MICROHARDNESS AS A GUIDE TO DEVELOPING HIGH CRITICAL-CURRENT DENSITY AG-CLAD BSCCO-2223 TAPES

Because there are many parameters influencing the critical current density (J(c)) of Ag sheathed (Bi, Pb)(2)Sr2Ca2Cu3Ox tapes, good and easily measured correlation variables to J(c) are highly desirable. Yamada et al. have recently shown that very high J(c) values can be obtained when the BSCCO core is rolled to a high Vickers microhardness (H-V) prior to heat treatment. The present experiment extends the correlation by showing that there is a linear relation between the H-V of reacted tapes and J(c). Measurement of the Knoop microhardness (H-K) shows that the hardness is highly anisotropic. H-K measured with the long indentor axis parallel to the a-b planes shows no dependence on reaction time or J(c), while H-K measured parallel to the c-axis is strongly correlated. A high microhardness appears to be directly correlated to a high-density BSCCO core with a small crack density. Such a core has good connectivity which directly translates to a high J(c) value.