パウダー·イン·チューブ法MgB₂テープ線材における歪効果

—in situ法およびex situ法テープの比較—

抄録

We investigate the influence of lengthwise mechanical strain on the critical current (I_c), the strain effects, for three kinds of “powder-in-tube (PIT)" route MgB₂ conductors. We tested in situ monocore MgB₂/iron, ex situ monocore MgB₂/stainless steel, and ex situ 14-filament MgB₂/nickel tapes. I_c was measured for each sample as a function of lengthwise external strain in magnetic fields (B) at 4.2, 15, 20, and 25K using a U-shaped rig made of stainless steel. Even though the fabrication process and conductor configuration (number of filaments, width, and thickness) are different, the I_c-strain relations were essentially the same in all MgB₂ samples. By increasing the strain, large and irreversible degradation occurred after linear and reversible I_c-strain relation up to the maximum of I_c (I_cm). The slope of linear I_c-strain relation in the reversible regime, d(I_c/I_cm)/d(strain), depends on both temperature and magnetic field. We discuss the relations between d(I_c/I_cm)/d(strain) and magnetic fields normalized by the irreversibility field (B_irr.) We also examined the strain effect of ex situ PIT monocore in Nb₃Sn/stainless steel tape. The I_c-strain relation obtained is a parabolic one that has been reported so far for many kinds of multifilamentary Nb₃Sn conductors. This indicates that the linear strain dependence of I_c in MgB₂ conductors is an essential feature of MgB₂.