Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters $T_c$ and $I_c$ of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7$I_c$) in liquid nitrogen (77 K) is studied. Voltage-current characteristics $U(I)$, as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by $\sim$30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.