the 1980s the key microstructure-property relationships for
Nb-Ti were determined at The University of Wisconsin-Madison.
The role of alpha-Ti precipitate, the influence of multiple
heat treatments on precipitate volume and size, the relationship
between prestrain and precipitate morphology and alloy composition,
and the relationship between critical current density and
volume percent of precipitate. Simultaneous with these developments
in Nb-Ti, work continued on Nb3Sn, developing a
quantitative understanding of microstructural and micro-chemical
variations in the A15 phase.
LTS stands for "low temperature superconductor",
which typically refers to the Nb based alloy (most commonly
Nb-47wt.%Ti) and A15 (Nb3Sn and Nb3Al)
superconductors in use prior to the discovery of "high
temperature" oxide superconductors in 1986. "Temperature"
here refers to the temperature below which the superconductor
must be cooled in order for it to become superconducting.
For LTS superconductors that temperature is usually well below
20 K (-253 °C). Nb-47wt.%Ti alloy has become the
dominant commercial superconductor because it can be economically
manufactured in a ductile form with the prerequisite nano-structure
needed for high critical current. Similarly Nb3Sn
based strand, although based on a brittle A15 superconducting
phase, can be manufactured into strong composites in km lengths
and microstructures that promote high critical current densities.
These superconductors are often termed "technical superconductors"
because of their applicability to engineering tasks. All these
conductors require cooling to 4 K (liquid He is the most common
LTS and The University of Wisconsin-Madison
Our studies at the University of Wisconsin-Madison are centered
on the improvement in the properties of these conductors for
application to High Energy Physics and Fusion devices. These
fields have traditionally provided the main drivers for the
improvement in superconducting strand properties. These improvements
have benefited the full range of LTS superconductor applications,
including MRI and NMR devices.
A key feature of the work at The Applied Superconductivity
Center has been the combination of microstructural and physical
property characterization. The center has built up an extensive
facility for the superconducting property measurements which
are combined with the center's own metallographic laboratory
and the state-of-the-art TEM, FESEM and SAM instruments of
the University of Wisconsin-Madison Integrated Microscopy
resource. In addition the center has a unique industrial quality
wire fabrication facility that includes a hydrostatic extrusion
press. The result has been a continuous output of landmark
papers and important developments in our understanding of
low temperature superconductors.
colored FESEM image of outer edge of high critical current
OI-ST Nb3Sn sub-element showing extent of
A15 phase (yellow) formation after 60 °C per hour
ramp to 650 °C. Trapped Cu-Sn phase is in pink and
remainder Nb barrier and Nb(0.8wt.%Ti) in gray.
In addition to recent external international collaborations
on Nb3Al (National Institute for Materials Science
in Tsukuba Japan) and Nb-Ti (Kharkov Institute, Ukraine) our
primary focus is the Development and Understanding of Nb3Sn
for high field application.Nb3Sn remains the most
likely superconductor choice for the next generation of accelerator
and fusion magnets. Our focus is on:
(i). Understanding what controls the A15 composition
and microstructure and how this controls flux pinning.
Being produced by a diffusion process that does not proceed
to equilibrium, Nb3Sn must have a range of composition
(~18-25 at.%Sn) and a range of Tc (~6-18 K).
Nb3Sn composites are longitudinally uniform but
radially non-uniform. Thus micro-chemistry, micro-structure
and micro-superconducting properties must be studied simultaneously.
We have developed new techniques to monitor compositional
changes in the sub-micron range and advanced our microstructural
quantification techniques. Key issues are control of grain
size and how high Jc in the A15 layer
can go. We are also establishing the extent to which the specific
grain boundary pinning force, Qgb, can
(ii). Understanding the magnetic field temperature
phase diagram, that is H*, Hc2,
Tc, and its variation with composition
of the A15 phase, reaction conditions, prestrain, and alloying
with Ti, Ta or other X elements that can enhance the conductor
properties. Recent work of our graduate students on internal
Sn MJR and SMI PIT composites has made it clear that several
present high-Jc internal Sn composites
are failing to optimize their primary properties (Tc
and Hc2). It seems likely that this is
contributing to significant loss of Jc(12
T) in conductors. These results help explain the remarkable
recent progress in HEP Nb3Sn strand, that now exceeds
the previous design goal of 3000 A/mm² (12 T, 4.2 K).
We seek to exploit this understanding by working with industry
(see (iv) below) to further improve the properties of the
(iii). Detailed studies of high Jc
internal Sn and powder-in-tube (PIT) conductors.
Our studies of high-Jc PIT and internal-Sn
conductors have shown that the both conductors have a Jc,
Tc and Hc2 performance
that is remarkably sensitive to composition and compositional
(iv). Monitor and support industrial developments.
There have been remarkable developments from industry in recent
years and this has been bolstered by the new US-DOE HEP Conductor
Development Program. We support these developments by using
our unique range of facilities to provide measurements of
Tc, H*, Hc2,
grain size, D(%Sn)/Dr and Qgb.
(v). Model A15 composite fabrication. Our
integrated facilities for superconductor fabrication and characterization
are amongst the most advanced and comprehensive of any university
facility. We are using our HIP'ing and extrusion facilities
to produce micro-chemically homogeneous A15 in order to study
directly the effects of composition on primary superconducting
We also have a small ongoing collaboration with Fermi National
Accelerator Laboratory, investigating the materials properties
and underlying theory of superconducting RF cavities.
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