パルス通電加熱法により作製したNb3Al超電導線材のピン止め特性

概要

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Two kinds of superconducting Nb_3Al wire have been ohmically-heated at controlled maximum temperatures ranging from 1600 to 2050℃ in vacuum, in order to investigate the properties of pinning force densities at high magnetic fields. Nb_3Al tape, which is 3 mm wide and 0.3 mm thick, with monocore in Nb sheath has been fabricated from Nb powders and Al powders. Jelly-rolled Nb_3Al wire with a Nb matrix is 1.34 mm diameter and 50 mm long. The maximum temperature at the sample-surface was measured by a photodiode. Critical currents were measured up to 23 T by a four probe method. In the tape samples, pinning force densities were changed by the number of ohmic-heating cycles. To make the inside of the superconducting core react more uniformly, ohmic-heating with 5 cycles at 1950℃ is needed. In the tape samples heated at the approximate melting point of Nb_3Al, the pinning force densities decrease by the increase of the number of ohmic-heating cycles. On the other hand, the Jelly-rolled samples heated at maximum temperatures less than the melting point of Nb_3Al have layer structures consisting of Nb-rich regions and A15 phase regions in longitudinal cross- sections. In the sample heated up to melting point of Nb_3Al, these layers are not observed. This sample shows a remarkable peak in the pinning force density at high magnetic fields, over 20 T.
山口大学工学部の論文