Applied Superconductivity Center

During 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.

What is "LTS"?

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 coolant).

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.

	False 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.

Research Focus

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 T_c (~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 J_c in the A15 layer can go. We are also establishing the extent to which the specific grain boundary pinning force, Q_gb, can vary.

(ii). Understanding the magnetic field temperature phase diagram, that is H*, H_c2, T_c, 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-J_c internal Sn composites are failing to optimize their primary properties (T_c and H_c2). It seems likely that this is contributing to significant loss of J_c(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 strand.

(iii). Detailed studies of high J_c internal Sn and powder-in-tube (PIT) conductors. Our studies of high-J_c PIT and internal-Sn conductors have shown that the both conductors have a J_c, T_c and H_c2 performance that is remarkably sensitive to composition and compositional uniformity.

(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 T_c, H*, H_c2, grain size, D(%Sn)/Dr and Q_gb.

(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 properties.

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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