Flux Pinning in A15 Superconductors

Global pinning force is influenced greatly by microstructures like grain size and third elements, when Nb3 Sn and V3 Ga superconductors are prepared by various different techniques. The structure dependence of flux pinning has been investigated in details. The main flux pinning center was attributed to the grain boundary. Accordinng to the modified shear model proposed by Evetts and Plummer (1985), the global pinning force has been analysed. It was found to have the same magnitude for both NB3Sn and V3Ga. The third elements influenced the specific force as well as the upper critical field. In and Ga made the pinning force increase, while the pinning force decreased by the addition of Ti or Zr. Generally, increasing the critical field resulted in the decrease of specific pinning force. This apparent result is suggested to relate with the change of the thermodynamic critical field. The effect of strain on the pinning force was not direct, but appeared through the change of critical field.