Formation and upper critical fields of the two distinct A15 phases in the subelements of Powder-In-Tube Nb3Sn wires

It is well known that the A15 layer in the subelements of a Powder-In-Tube (PIT) Nb3Sn wire exhibits two different grain morphologies: a region with fine grains (~200 nm in size) representing about 60% of the total A15 area and one with large grains (1-2 {\mu}m in size). By means of high field specific heat and magnetization measurements we have shown that these two A15 phases correspond to two distinctly different Tc distributions, the large grains region exhibiting a higher Tc and a lower Bc2, the fine grains region a lower Tc and a higher Bc2. We report here the values of the superconducting parameters (Tc, Bc2) of the two A15 phases, as determined from an original model to fit the experimental Tc distribution. After a prolonged reaction treatment (625{\deg}C/320h), an increase of the fine grain region was observed at the expenses of the large grain region, the Bc2(0K) value of the former being raised from 28.8 to 31.7 T. These changes explain the marked increase of Jc to 2'700 A/cm2 at 4.2K/12T, the highest value measured so far in PIT wires.