Sample-length dependence of the critical current of slightly and significantly bent-damaged Bi2223 superconducting composite tape

Abstract

The local critical current along a sample length is different from position to position in a long sample, especially when the sample is damaged by externally applied strain. In the present work, we attempted to reveal the relation of the distribution of the local critical current to overall critical current and the sample-length dependence of critical current for slightly and significantly damaged Bi2223 composite tape samples. In the experiment, 48 cm long Bi2223 composite tape samples, composed of 48 local elements with a length of 1 cm and 8 parts with a length 6 cm, were bent by 0.37 and 1.0% to cause slight and significant damage, respectively. The V-I curve, critical current (1 μV cm −1 criterion) and n value were measured for the overall sample as well as for the local elements and parts. It was found that the critical current distributions of the 1 cm elements at 0.37 and 1.0% bending strains are described by the three-parameter-and bimodal Weibull distribution functions, respectively. The critical current of a long sample at both bending strains could be described well by substituting the distributed critical current and n value of the short elements into the series circuit model for voltage generation. Also the measured relation of average critical current to sample length could be reproduced well in the computer by a Monte Carlo simulation method. It was shown that the critical current and n value decrease with increasing sample length at both bending strains. The extent of the decrease in critical current with sample length is dependent on the criterion of the critical current; the critical current decreases only slightly under the 1 μV cm −1 criterion which is not damage-sensitive, while it decreases greatly with increasing sample length under damage-sensitive criteria such as the 1 μV one.

Simulation on the relation of distribution of overall critical transport current to that of local one in bent-damaged Bi2223 superconducting composite tape

“…͑2͒ below͔. 23,24,32 This function has been also used to describe the critical current distribution at weak links for analysis of V-I curve under no applied strain. 35,36 On the other hand, when the applied strain is high or the damage amount is large, it has been shown that the critical current distribution can be described by the bimodal Weibull distribution function ͓given by Eq.…”

“…͑3͒ later͔. 24,37 The applicability of the three parameter and bimodal functions to the present experimental results ͑Fig. 3͒ was examined as follows.…”

“…͑2͒ with I c,min Ͼ 0͔, the plot of ln ln͑1−F͒ −1 against ln͑I c ͒ deviates from the linear relation and the ln ln͑1−F͒ −1 −ln͑I c ͒ curve shows a convex. 24 In the case of bimodal function ͓Eq. ͑3͔͒, the ln ln͑1−F͒ −1 −ln͑I c ͒ curve shows a concave.…”

“…͑3͔͒, the ln ln͑1−F͒ −1 −ln͑I c ͒ curve shows a concave. 24 Figure 4 shows the plots of ln ln͑1−F͒ −1 against ln͑I c ͒ for ͑a, b͒ the local I c ͑1 V / cm criterion͒, ͑aЈ ,bЈ͒ the overall I c ͑1 V / cm criterion͒, and ͑aЉ ,bЉ͒ the overall I c ͑1 V criterion͒ at B = ͑a,aЈ ,aЉ͒ 0.37% and ͑b,bЈ ,bЉ͒ 1.0%. The experimental results at B = 0.37% showed a convex but those at B = 1.0% showed a concave, suggesting that the results at B = 0.37% ͑a,aЈ ,aЉ͒ could be described by Eq.…”

“…1- 26 Also it has been known that the critical current is different from position to position in a specimen and from specimen to specimen. [19][20][21][22][23][24][25][26][27][28][29][30][31][32] Accordingly, when many specimens are tested, the local and overall critical currents are distributed, especially under the applied strain ͑Ͼ the irreversible strain͒ due to the inhomogeneous damage evolution. [19][20][21][22][23][24][25][26] It is needed to reveal the statistical features of the local and overall critical transport current distributions and the relation of overall critical current distribution to local one especially under various applied strains, which are important items for reliability and safe design.…”