The influence of intermediate roll characteristics on the residual crack density and critical current density in multifilamentary (Bi, Pb)₂Sr₂Ca₂Cu₃O_xtapes

Abstract

We studied the influence of the intermediate rolling deformation on the residual crack network and critical current density in multifilamentary (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes. Although an intermediate rolling step between heat treatment is generally beneficial in raising the critical current density, it also has the negative effect of causing cracking. Cracking was evaluated both by magneto optical imaging and by ultrasonic fracture of extracted filaments. We found that the characteristic fragment size was sensitive to both roll diameter and reduction percentage, decreasing from ~210 µm for a 10% reduction to ~115 µm for a ~20% reduction with 152 mm diameter rolls. At a constant 10% reduction, the fragment size decreased from ~210 µm to ~140 µm when changing from 152 mm to 38 mm diameter rolls. We conclude that intermediate deformation is positive in so far as it decreases the filament porosity, but that it cannot reduce this too much without introducing more cracking than can be healed in subsequent heat treatment. The critical current density thus maximizes at an intermediate deformation that counterbalances these two oppositely directed influences on the filament connectivity.

Magneto-optical studies of current distributions in high-Tcsuperconductors

“…Whereas in the pressed samples the cracks are oriented parallel to the tape axis, the rolled samples exhibit a transverse crack structure which disturbs more seriously the current flow compared to the longitudinal cracks. These experiments also give insight into one of the central optimization problems for the Bi-2223 tapes (Anderson et al 1999): the deformation process exerts a beneficial effect on the grain alignment and the density of the core; however, it is responsible for cracking, and a healing of the cracks during the heat treatment requires a sufficient amount of liquid phase in the core. For the rolled sample, the second deformation step was of little value because the more j c -detrimental transverse cracks (compared to the longitudinal cracks produced by pressing) were not healed.…”

Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Oxtapes

“…As noted in the data of figures 2, 4, 5 and 6, it seems unlikely, perhaps due to the great basal-plane plasticity of the BSCCO compounds, that Bi-2223 tapes ever fracture into single-grain arrays. Thus the correct value of R grain is not at all clear and is probably a little higher than proposed in [24]. In a recent attempt to apply this method to a monofilament composite with J c (0 T, 77 K) = 35 kA cm −2 , we have deduced a value of C = 0.2 [39].…”

“…As noted earlier, we believe that a good first approximation is to treat the magnitude of J c (H, T ) as having two independent components, the upper limit being due to flux pinning which defines the local, intragranular current density which provides the upper limit to J c , the second being the connectivity which determines the fractional area that conducts the long-range transport current [11,12]. Müller et al [24] have recently estimated this fraction to be only 3% for a tape with J c (0 T, 77 K) = 20 kA cm −2 , although uncertainty in various geometrical factors makes the estimate unsure to a factor of order two to three. An important finding of the present work is that a tape representative of a much higher critical current density (J c (0 T, 77 K) = 50 kA cm −2 ) also shows abundant evidence for much less than full connectivity.…”

“…For example the density does not improve between the high-temperature dwell of HT2 and HT2, yet J c (0 T, 77 K) more than doubles. The MO images of figure 4(d) suggest some improvement in fine-scale connectivity, without however suggesting that the large-scale damage introduced by the intermediate deformation is fully healed [24]. Important elements of HT2 remain to be better understood.…”

“…Both MO and local critical current measurements on stateof-the-art tapes show that the cross section contributing to the current transport is much smaller than the total, a result ascribed to the presence of partially healed ('incipient') cracks, porosity and secondary phases [11,15,24]. Müller et al [25] have recently used magnetization curve analysis on intact and fragmented tapes to quantitatively estimate the connected cross section.…”