MODELING THE CURRENT-VOLTAGE CHARACTERISTICS OF SILVER-SHEATHED BI-SR-CA-CU-O TAPES

Measurements of voltage-current (V-I) characteristics and critical currents of 2212 and 2223 Ag-clad Bi-Sr-Ca-Cu-O tapes at 0 < B < 7 T and 4.2 and 77 K are presented. We show that the V-I curves are nonlinear at I < I(c) and become linear above the critical current I > I(c) due to the effect of the Ag cladding. It is shown that the V-I curves can be well described by a simple universal formula, which enables one to extract I(c)(T, B) and the flux creep rate s(T, B) from the resistive measurements, taking into account the effects of voltage criterion, strong nonlinearity of V(I) and I < I(c) and the dependence of the resistivity of Ag upon T and B. For a 2212 tape, I(c)(B) is shown to decrease as B-alpha with alpha = 0.15 at 4.2 K, and exponentially at 77 K. For a 2223 tape, I(c)(B) displays a power law dependence with alpha = 0.1-0.3 at 4.2 K, whereas at 77 K I(c)(B) can be well described by the formula I0b-1/2(1-b), where b = B/B*, and B* is the irreversibility field. The results are interpreted in terms of strong bulk pinning modified by thermal fluctuations.