Dimensional Changes of Nb₃Sn, Nb₃Al and Bi₂Sr₂CaCu₂O₈ Conductors During Heat Treatment and Their Implication for Coil Design

Abstract

This work has measured the dimensional changes of Nb₃Sn, Nb₃Al and Bi₂Sr₂CaCu₂O₈ 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 Nb₃Sn and Nb₃A1 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 Nb₃Sn 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 Bi₂Sr₂CaCu₂O₈ conductor. It expands continuously on heating without the contraction seen in internal-tin processed Nb₃Sn and jelly-roll processed Nb₃A1 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 Bi₂Sr₂CaCu₂O₈ melting-processing temperature the conductor behaves like a composite. The Bi₂Sr₂CaCu₂O₈ 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 Bi₂Sr₂CaCu₂O₈ is less than that of Ag.