DIMENSIONAL CHANGES OF Nb_3Sn, Nb_3Al AND Bi_2Sr_2CaCu_2O_8 CONDUCTORS DURING HEAT TREATMENT AND THEIR IMPLICATION FOR COIL DESIGN

摘要

This work has measured the dimensional changes of Nb_3Sn, Nb_3Al and Bi_2Sr_2CaCu_2O_8 conductor, and other coil materials during heat treatment to determine the strains associated with these changes. The question is: can a conductor be damaged due to thermal expansion and contraction differences of the coil components during the heat treatment thermal cycle? Two conclusions regarding coil fabrication can be drawn from the dilatometry results. One is that Nb_3Sn and Nb_3Al conductors produced by an internal-tin process and by a jelly-roll process, respectively, are placed in tension on heating since they initially contract and then expand at a lower rate than the coil form material (i.e. Al-bronze or stainless steel). Both of these conductors have a contraction near 200-250°C which is due to relaxation of the Nb or Nb-Al filaments. Also, the post-heat-treatment contraction of these conductors is greater the larger the Cu and bronze fraction. A bronze-processed Nb_3Sn conductor does not have the contraction at 200-250°C. The second conclusion is that the conductor is not placed in compression on cooling from the reaction temperature (650-800°C) to room temperature if the coil components (i.e. cable and end pieces) are unconstrained, since the Al-bronze pole piece contracts more than the cable. This should also be the case for conductor wound on a stainless steel coil form. The opposite seems to be the case for powder-in-tube Bi_2Sr_2CaCu_2O_8 conductor. It expands continuously on heating without the contraction seen in internal-tin processed Nb_3Sn and jelly-roll processed Nb_3Al conductor at 200-250°C. No residual stress develops in this conductor since the filaments have no tensile strength and are not bonded to the silver matrix. However, on cooling from above the Bi_2Sr_2CaCu_2O_8 melting-processing temperature the conductor behaves like a composite. The Bi_2Sr_2CaCu_2O_8 powder has now sintered or fused forming a rigid filament that has bonded to the Ag matrix. This decreases the contraction on cooling suggesting that the thermal expansion coefficient of Bi_2Sr_2CaCu_2O_8 is less than that of Ag.