Laboratory for Applied Superconductivity and Ceramic Processing

The Laboratory for Applied Superconductivity and Ceramic Processing (LASCP) is involved in experimental and computational research aimed at advancing applications of ceramic materials. Run by Professors Justin Schwartz and Simone C. Peterson, the primary research focus is currently on high temperature superconducting (HTS) materials. LASCP activities are an interdisciplinary attack on the materials processing, composite mechanical behavior, and applied physics of these emerging technical superconductors.

A number of promising HTS material systems exist, but at present the most technologically developed is the Bi-Sr-Ca-Cu-O system. These materials, when deformed into wire or tape within an Ag sheathing, have demonstrated very large critical current density (J_c) at high magnetic field. LASCP research is investigating a number of the key obstacles to implementing Ag and Ag-alloy clad Bi₂Sr₂CaCu₂O_x into superconducting magnets. For example, very small quantities of chemical impurities within the starting powder (most importantly carbon containing compounds) lead to dimensional instabilities that destroy the superconducting properties. We are mitigating this problem via high pressure sintering processes to counterbalance the internal pressure, and with chemical additions that trap the carbon in a solid phase, thereby preventing the formation of CO₂ gas. While dimensional instabilities represent an extrinsic limit to the superconducting properties, weak magnetic flux pinning represents an intrinsic limit to the superconducting properties of Bi₂Sr₂CaCu₂O_x. Thus, to improve the material, we are attempting to improve the flux pinning at the fundamental level of the superconductivity: the nanometer length scale that governs the magnetic behavior. By incorporating very fine inclusions directly into the superconducting grains, flux pinning centers are created and the superconducting properties improved. We are also developing techniques to investigate the magnetic properties of the superconductors, and in p articular are using thermal conductivity measurements at very high magnetic fields (available at the National High Magnetic Field Laboratory). These measurements provide unique insight into the intragranular magnetic behavior and the conduction behavior across grain boundaries, while also providing important engineering data for magnet design. Lastly, we are integrating the superconducting material processing with the superconducting magnet manufacturing techniques. This reduces the mechanical strain in the conductor induced during the coil winding process while reducing the number of steps, and the cost, of coil manufacture.

Although not nearly as developed as the Bi-Sr-Ca-Cu-O system from an engineering point of view, the HgBa₂Ca_n-1Cu_nO_x compounds have the highest critical temperatures (Tc) of all known superconductors and thus warrant significant attention. LASCP research is focused on the key issues that will determine if these materials have technological significance. Therefore, we are also investigating the synthesis and processing of polycrystalline materials. By adding Re to the starting mixture, the highest T_c phases become chemically stable, overcoming the first major obstacle. We are investigating other additives that reduce the sintering temperature (and thus the Hg vapor pressure during processing) while accelerating the growth kinetics of the superconducting phases relative to competing non-superconducting phases. As these materials, like all HTS materials, are brittle ceramics requiring a ductile matrix for wire forming, we are studying chemical interactions with high purity metals, considering the effects of the metal on the synthesis of the superconducting phase and on aligned grain growth (texturing). Furthermore, we are investigating the magnetic behavior of these compounds, including improving magnetic properties via irradiation, and the relationships between applied pressure, T_c, and magnetic properties.

Computational research in the LASCP and is motivated by motivates and the experimental research. By investigating the design implications of HTS materials, we better understand how these materials may be integrated into engineering systems, while also better understanding the required material properties for technological viability. We are studying a unique winding configuration for toroidal superconducting magnets that takes advantage of the anisotropic superconducting properties in HTS materials while minimizing the Lorentz forces and thus the mechanical strain. Furthermore, we are studying the magnetothermal stability of HTS conductors and coils under a variety of potential operating modes. Although the expanded temperature range implies significantly greater coil stability, the intrinsic anisotropy of the HTS compounds and their very low thermal conductivity, particularly in the c-direction, implies very slow normal zone propagation and potentially very large hot-spot temperatures. This research is aimed at defining the stable operating limits and potential magnet quenching modes for HTS systems.

HTS materials are brittle ceramics and their superconducting properties are very sensitive to mechanical strain. To understand the fundamental operating limits of these materials in magnet systems, we are investigating the mechanical properties of Ag-alloy clad Bi₂Sr₂CaCu₂O_x conductors. Individual constituent properties, composite properties, and the effects of stress and strain on the Jc are being measured. Furthermore, as even small values of strains may degrade the superconducting properties, we are studying the effects of low-cycle and high-cycle fatigue. This research not only provides very important insight into the mechanical behavior of these materials from an engineering point of view, but also may elucidate an understanding of the fundamental limits to intergranular transport in Bi₂Sr₂CaCu₂O_x composite conductors. This research is collaborative with the Advanced Mechanics and Materials Laboratory (AMML) within the Department of Mechanical Engineering.

LASCP receives research funding from a variety of sources including the Advanced Research Projects Agency, the Department of Energy (through the Argonne National Laboratory) the National Science Foundation, the National High Magnetic Field Laboratory, and the Small Business Innovation Research program (in cooperation with Intermagnetics General Corporation).