Composition gradient as a source of pinning in Nb-Ti and NbTa-Ti superconductors

Superconducting Nb-Ti and NbTa-Ti wires produced by solid-state diffusion were studied, correlating microstructural and transport properties. The most interesting result is that the diffusion layer sharp composition variation is an important source of pinning in these materials. Within this scenario, we suggest that the source of pinning centers in conventional NbTi wires should be re-examined, since the alpha-Ti precipitation creates a Nb gradient composition around it. We propose that in conventional superconducting wires there are actually two kinds of strong pinning centers. One is the well-known alpha-Ti normal interface and the new one now identified is the composition gradient around alpha-Ti precipitates.