To fabricate high-field magnets, flexible tapes, having the advantage of a small distance from the brittle A15 compound to the neutral phase in bending direction, have been used successfully. The large area-to-thickness ratio of the A15 layer leads to instability (flux jumps), especially if magnets have a rapid ramping rate.
The solid-state diffusion process, as used for bronze conductors of Nb3Sn and V3Ga (31), has solved this problem, by dividing the core material into plenty of fine filaments. The formation of A15 layers is principally limited by the amount of Sn and Ga in the bronze.
The solubility of Sn in Cu is 8.5 at.% and for Ga in Cu is 20 at.%. Bronze with about 7.5 at.% Sn or about 18 at.% Ga has been used. The diffusion process forms the A15 layer until the equilibrium for a given temperature is established.
At a temperature of 620#C to 700#C, the diffusion for Ga ends at a remaining concentration at 14 at.% to 15 at.% Ga in the matrix. Sn diffusion from the bronze proceeds at approximately 700#C to 850#C, leaving a Sn concentration in the bronze of approximately at 3% to 4 at.%;
The heat-treatment time and temperature has to be controlled in such a way as to receive an optimum layer thickness, but without increasing too much the grain size. Especially for a long heat treatment of #200 h, the matrix volume has to be increased, in order to provide enough Sn or Ga.
Small distances between the filaments seem to be desirable, due to reduced bending strain, but the space between the filaments acts as a diffusion path for the B ions from the conductor periphery, too. Those diffusion paths are reduced in their effective width by the Kirkendall voids caused by the diffusion mechanism during heat treatment.
At a given temperature and a constant concentration gap, the quantity of B ions diffusing through a cross section in a given time is proportional to the area of this cross section (Fick’s first law). From this follows that the cross section of the cores of Nb or V should be divided in as many portions as feasible, to increase the in layer between the bronze and the core material.
This leads to an increase of the total amount of A15 material, even with reduced heat treatment time. Optimization studies of diffusion treatment versus layer thickness have shown that filament diameters should be in the range of 3 #m to 5 #m. For conductors with a diameter of 1.5 mm, and taking into account the cross section needed for stabilization and diffusion barrier, approximately 15,000 filaments are necessary.
Workability of the component is an essential request to arrive with technically and commercially usable conductors. While the basic components, Nb or V, electron beam- or arc-melted, are high-purity materials of excellent ductility, they are sensitive to imbrittlement by interstitials of oxygen (O), nitrogen (N), or C.
This is especially true for the V, but the more problematic part is the bronze. For many years, the technically attainable Sn content was limited to about 8 at.%. Newer processes made homogeneous bronze at 8.5 at.% Sn available.