Proximity effect depression of the critical temperature in two-phase Nb-Ti superconductors

The superconducting critical temperature of high critical current density Nb-Ti composites has been measured at various stages in the critical current optimization process. This process involves heat treatment steps which cause precipitation of normal α-Ti and make the matrix more Nb rich. T/sub c/ rises from 9.1 to 9.5 K during this stage. The final optimization stage involves extensive wire drawing during which the α-Ti precipitates are reduced to less than a coherence length in thickness. This does not result in a reduction in J/sub c/ but T/sub c/ is found to fall from 9.44 to 8.7 K during this step. The depression of T/sub c/ is found to be in fair agreement with the predicted proximity effect suppression of T/sub c/. Wires of optimum transport critical current density are seen to have T/sub c/ of around 9 K