PINNING MECHANISM IN NB3(AL,SI,BI) SUPERCONDUCTORS PRODUCED BY THE INFILTRATION PROCESS

Nb3(Al,Si,Bi) superconductors were prepared using an infiltration process. The dependence of the microstructure and the superconducting properties on the Nb filament thickness and the reaction temperature, T(R), have been studied. The different phases occurring at various stages during the preparation were examined by optical microscopy, x-ray analysis and electron microprobe microanalysis. The critical temperature, T(c), ranged from 14 to 18 K. Critical current densities, J(c), of 10(5) A cm-2 and 10(4) A cm-2 at 4.2 K were observed at 1 T and 16 T, respectively. Upper critical fields, B(c2), up to 21 T were observed. To investigate the effective pinning mechanism, J(c) measurements in the temperature range between 10.5 and 16.5 K were carried out and compared with microstructure observations by using transmission electron microscopy (TEM). These results, as well as thermodynamic considerations, show that pinning in the high field region is dominated by Nb2Al-phase precipitates in the A15 Nb3Al grains.