Tape Length-Dependence of Critical Current and <i>n</i>-Value in Coated Conductor with a Local Crack

Abstract

Tape length-dependence of voltagecurrent curve, critical current and n-value in a coated conductor with a local crack were studied by modeling analysis. In calculation, the specimen length was varied in the range of 1.5 to 18 cm where the influence of cracking of superconducting layer is sharply reflected in the voltagecurrent curve. The following results were obtained, which can be utilized for analysis and interpretation of the experimental results under applied tensile stress in laboratory scale of specimen length. The existence of a crack changed the critical current and n-value through the decrease in current-transportable cross-sectional area of the superconducting layer and the current shunting in the cracked cross-section for any specimen length and any crack size, while the extent of the change was dependent on specimen length and crack size. When the crack was large, critical current increased slightly and n-value decreased significantly with increasing specimen length due to the enhanced shunting current. On the other hand, when the crack was small, critical current increased slightly with specimen length due to the enhanced shunting current similarly to the case of large crack, but n-value decreased with increasing specimen length due to the enhanced shunting current but then it increased due to the enhanced voltage-development at higher voltage in the non-cracked part. Also, the features of the dependence of the relation of n-value to critical current on specimen length were revealed; the decrease in n-value with decreasing critical current became sharper for longer specimen.

Effects of Heterogeneous Cracking of Superconducting Layer on Voltage-Current Curve, Critical Current, and <i>n</i>-Value in High-Temperature Superconducting Layer-Coated Conductor Tape

“…Moreover, we have used the model to describe the effects of specimen length on I c and n-value in a specimen where one crack is formed locally. 11) This model well describes the experimental observation that when cracking occurs locally, I c increases slightly and the n-value decreases sharply in longer specimens. 10) In the present work, the effects of heterogeneous cracking on I c and n-value were studied using a model specimen composed of three local sections.…”

“…In addition, voltage arises at cracks and in non-cracked region when the cracks are small, and hence I c /I c0 is not equal to ð1 À fÞðL 0 =sÞ 1=n 0 . However, ð1 À fÞðL 0 =sÞ 1=n 0 still approximates I c /I c0 fairly well, as we showed in our previous studies on a REBCO-coated conductor 4,5,11) and a BSCCO filamentary conductor.…”

“…(3)). 11) In practice, current shunting can be larger. In addition, voltage arises at cracks and in non-cracked region when the cracks are small, and hence I c /I c0 is not equal to ð1 À fÞðL 0 =sÞ 1=n 0 .…”

Relation between <i>n</i>-Value and Critical Current in Filamentary and Coated Superconducting Tapes with Tensile Stress-Induced Cracks

“…where L (= 1 cm) is the distance between the voltage taps, s is the current transfer length, 14) I c0 is the critical current of the non-cracked Bi2223 conductor, and n 0 is the n-value in the non-cracked state. Fang et al 10) used s as the opening displacement of the crack although it can be replaced by the current transfer length, from which eqs.…”

“…(2) has a physical meaning of I c /I c0 in a theoretical scenario where the crack is substantial and no current shunting occurs. 14) In practice, current shunting occurs, and thus the values of ð1 À fÞðL=sÞ 1=n 0 and R t are estimated from the measured V-I curve.…”