ELECTRICAL RESISTIVITY OF A HIGH-T_c SUPERCONDUCTOR FOR ELECTRIC CURRENT HIGHER THAN I_c

摘要

The electrical resistivity of a high-Tc superconductor for I > I_c is not clarified until now as a function of electric current through the conductor, I, and temperature of the conductor, T, because T varies with I due to joule heating. An estimation method of the electrical resistvity based on non-boiling heat transfer coefficients for exponentially increased heat inputs to the high-Tc test sample in LN2 was developed: exponential heat inputs, Q = Q_0e~(t/T), with the periods, τ, ranging from 10 ms to 10 s were given to a silver sheathed BiPbSrCaCuO tape in LN2, and electrical current I through the sheathed tape and the terminal voltage V between potential taps on the tape during the transient heating were measured. An empirical equation for the electrical resistance R. of the sheathed tape was obtained as a function of I and T: two constants in the equation were fitted iteratively so that the value of T at each time obtained from the measured values of I and V at the time by using the equation of R may agree with the theoretical solution of nonboiling heat transfer for the exponential period. The distribution ratio of the electric current through the superconductor and the Ag sheath, I_N/I, and the flow resistivity of the superconductor itself were estimated by using this method.