Microstructures and flux pinning properties in Nb3Al tapes by ohmic-heating method

Two kinds of Nb3Al superconducting tapes with monocores in Nb sheaths have been fabricated from either Nb/Nb2Al(sigma-phase) powders or Nb/Al powders. Tape samples 3 mm wide, 0.3 mm thick, and 40 - 50 mm long are ohmically-heated at maximum temperatures which range from 1650 to 2000 degrees C in vacuum, to obtain the stoichiometric composition of the A15 phase. Critical currents up to 23 T were measured by a four probe method. Microstructures were studied using an X-ray diffractometer, TEM, and EDX. The Nb/sigma powder sample showed a maximum T-c at an ohmically-heated temperature 200 degrees C lower than the Nb/Al powder sample. Production of A15 is influenced by the differences in diffusion length related to different starting materials. Nb3Al grain sizes were 1-2 mu m in both the Nb/Al sample and the Nb/sigma sample. Also, sigma -phase was observed only in Nb/Al sample. The pinning force densities in the Nb/Al sample and the Nb/sigma sample showed a large peak at high magnetic field (above 20 T), and a peak at low magnetic field, respectively. From the relation between microstructure and flux pinning properties, it is supposed that the sigma -phase acts as an effective pinning center at high magnetic fields in Nb3Al.