Determination of irreversibility field variations in mono- and multifilamentary (Bi,Pb)2Sr2Ca2Cu3Ox tapes by transport current methods

Abstract

The irreversibility field, H*, has been measured for a variety of mono-and multifilamentary ͑Bi,Pb͒ 2 Sr 2 Ca 2 Cu 3 O x ͑2223͒ tapes using two different transport current techniques. It is common to characterize the quality of 2223 tapes by their zero-field, 77 K critical current density ͓J c (0 T, 77 K͔͒, even though this ignores the fact that significant self-fields depress J c (0 T, 77 K) and the possibility that the in-field J c (B) characteristics may be optimized independently of the J c (0 T, 77 K) value. To provide more useful information, we propose a second characterization, that of the irreversibility field, H*. Having both H* and J c (0 T, 77 K) information helps in separating the two independent contributions that better connectivity and stronger flux pinning can make to the J c of a tape. We illustrate this point with results from a variety of mono-and multifilamentary Bi-2223/Ag tapes in damaged and undamaged conditions, which show that H* ͑77 K͒ can vary from ϳ100 to ϳ200 mT and not directly correlate with J c (0 T, 77 K). The two proposed protocols for H* measurement are robust and compatible with common transport measurement procedures. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00352-5͔Both the zero-field, J c (0), and the in-field, J c (B), critical current densities of ͑Bi,Pb͒ 2 Sr 2 Ca 2 Cu 3 O x ͑2223͒ tapes must be improved to make applications economically feasible. J c is defined experimentally as J c ϭI c /A where I c is the critical current and A is the area of the whole ceramic cross section. However, this averaged, overall J c is most certainly an understatement of the maximum local J c because the current percolates through only a fraction of the cross-sectional area, A active . J c (0) contains significant contributions from current paths that include weakly linked grain boundaries, which are decoupled by small magnetic fields, and it is thus at least partially limited by self-fields generated by the transport current. However, J c (B) is strongly influenced by the strength of the flux pinning. At high fields, either intragranular 1-6 or intergranular 7,8 flux pinning becomes the main J c -limiting mechanism. Because different mechanisms determine J c (0) and J c (B), this leads us to believe that J c (B) may be optimized independently of J c (0). The irreversibility field, H*, is an increasingly common parameter used to measure flux pinning because it is strongly influenced by flux pinning. H* can be measured by several magnetization and transport current techniques. Previous work [8][9][10] has shown that individual composites often show a direct correlation between H* and J c (0). However, as we have accumulated a larger database of H* and J c (0) characterizations, we have found that there is no universal correlation and we, therefore, conclude that it is vital to characterize both the flux pinning and zero-field properties, if the highest performance is to be developed in BSCCO-2223 tapes.When H* is measured by magnetization, the magnetization signal is averaged ov...

The use of the in-field critical current density, Jc(0.1 T), as a better descriptor of (Bi, Pb)2Sr2Ca2Cu3Ox/Ag tape performance

“…There is a large range in J c (0) values, from 12 to 63 kA/cm 2 . J c (H) also differs from tape to tape, stronger flux-pinning tapes having a smaller rate of decrease of J c with H. In agreement with the results of Anderson et al, 11 there is no direct correlation between J c (0) and in-field performance, here parametrized by H p ; for example, the tape with the highest J c (0) value of 63 kA/cm 2 ͑tape B͒, is not the same as the tape with the least slope of J c (H) ͑and, hence, the largest value of H p ͒, ͑tape I͒. To isolate the infield performance, we normalized the J c (H) curves to J c (0.1 T); the resulting plots are shown in Fig.…”

“…Contrary to earlier work, 11 we here propose H p rather than H* to parametrize in-field performance, because the exponential decay of J c (H) can be evaluated at the experimentally easily accessible 1 V/cm criterion. Table I indicates a strong, though not yet universal, correlation between H p and H*: for example, tape B has the lowest value of H p ͑136 mT͒ and the lowest value of H* ͑163 mT͒, and tape K has the highest value of H p ͑204 mT͒ and one of the higher values of H* ͑328 mT͒.…”

“…J c (H) is controlled by flux pinning, the strength of which can be characterized by the irreversibility field H*. Anderson et al have recently shown that there is no universal correlation between J c (0) and H*, 11 thus suggesting that J c (0) and H* can be optimized independently 7 and that a useful description of 2223 tape performance must include both parameters, J c (0) and H*. In this work, we show that more extensive electromagnetic characterizations of 2223 tapes can indeed yield additional vital information about their performance and current-limiting mechanisms.…”

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

“…The voltage was measured between contacts spaced ∼6 mm apart with a Keithley 1801 nanovolt preamplifier feeding a Keithley 2001 multimeter, which produced a typical noise floor of 10 nV or less. The irreversibility field H * was found by determining the field at which the curvature of the V -I curves changed from positive to negative [12,26,27]. The critical current I c was extracted from the V -I curves using a criterion of 1 µV cm −1 and J c was defined as I c /S, where S is the average of two or three Bi-2223 cross section measurements.…”

Equivalent scheme for theV-Icharacteristics of Bi-2223/Ag tapes at 77 K

“…This indicates the existence of a finite dissipationless critical current below which the voltage appears to be truly zero [2][3][4][5][6][7]. Somewhat different behaviour referred to in [8][9][10] seems to have been attributed rather to some mechanical damage (e.g. microcracks) than to principal physical reasons.…”