Bending strain characteristics of critical current degradation behavior in externally-reinforced Bi-2223 tapes under pressurized liquid nitrogen

The critical current Ic degradation behaviors of externally-reinforced Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a ρ-shaped sample holder which could give a series of bending strains. Two kinds of externally-reinforced multifilamentary Bi-2223 superconducting tapes were used as samples. For both samples, the critical current at the unstrained state of the sample, Ic0, decreased with the increase of applied pressure, but the Ic degradation behavior with bending strain is similar although it slightly decreased. However, when depressurized to atmospheric pressure from 1 MPa, the Ic completely recovered to its initial values at atmospheric pressure. When the samples were warmed up to room temperature after pressurization tests, ballooning occurred and it is usually located at the straight or less bent regions of the samples. It was found out that ballooning caused a severe degradation of Ic. The occurrence of ballooning was suppressed at larger bending strain regions where the mandrel gives a structural constraint to the tape. For the externally-reinforced tapes without hermetic sealing,the immersion time in LN2 influenced on the Ic degradation due to the occurrence of ballooning in short-length Bi-2223 tapes, but the thermal cyclic test up to 10 cycles at atmospheric pressure did not cause any Ic degradation. The critical pressure level to initiate ballooning, Pc, was also obtained.