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.
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