Strain effect in Bi-based oxide/Ag superconducting tapes

The influence of mechanical strain on the critical current (I,) is investig ated for ( Bi,Pb)(2)Sr2Ca2Cu3Ox(Bi-2223)/ Ag-0.2wt%Mg-0.3wt%Sb superconduct ing tapes at 77 K, The tensile axial strain along tape length is successful ly induced to the sample by using a U-shape holder. Continuous change of th e axial strain can be obtained by changing the distance between both ends o f the holder. The U-shape holders made of Ti, SUS304, or brass are used to examine the effect of thermal strain due to the contraction caused by the c ooling. For example, the results for the samples glued to Ti or brass holde r are as follows, Strain dependence of normalized I-c (I-c/I-c (at as-coole d state)) is affected by holder material. A steep decrease of I, occurs whe n we apply 0.1% and 0.3% strain for Ti and brass holder, respectively. The different thermal expansion (-0.15% for Ti and -0.37% for brass from 300 K to 77 K) explains it. All the results for normalized I-c vs. strain relatio n fall on a master curve with taking into account of the effect of the ther mal strain. Thermal expansion of the tape from 300 K to 77 K is measured to be -035% by using a strain gage. The critical strain (c) where a steep dec rease of I-c occurs is evaluated to be 0.27% from self-contracted state of the tape.