Experimental Superconductivity

Superconductors

Subtitle: Investigation on processing-structure-property relationships and failure mechanisms in superconducting materials and systems.

Summary:

In 1911, Kamerlingh Onnes observed a sudden loss of electrical resistance to the flow of DC current in mercury at 4.2 K, which marks the discovery of superconductivity. For decades, niobium-based low temperature superconductors (LTSCs) have been used most commonly in applications that require large magnetic fields, such as MRI devices and maglev systems. In 1986, Bednorz and Muller discovered high-temperature superconductivity (HTSC), which attracts many research activities were started all over the world. Besides the high transition temperatures Tc, high-temperature superconductors (HTSCs) are, in principle, have a very high upper critical field (up to 100T for the bismuth superconductors) and could thus carry high currents at much higher fields than the conventional LTSCs.

Our group is focusing on advanced science and technology in development of high temperature superconductors (HTSCs), including bismuth-strontium-calcium-copper oxides (BSCCO), yttrium-barium-copper oxides (YBCO), rare-earth-barium-copper-oxide (ReBCO) and magnesium diboride (MgB2).  Both processing-structure-property relationships and failure mechanisms are studied.

There are three best known compounds in the BSCCO system, Bi2Sr2CuO6+y (Bi2201), Bi2Sr2CaCu2O8+y (Bi2212) and Bi2Sr2Ca2Cu3O10+y (Bi2223). Particularly, Bi2212 is the only high temperature superconductor that can be made into isotropic round wire, which is the preferred configuration for solenoid magnets. Currently, Bi2212/Ag alloy wires are manufactured via the oxide-powder-in-tube (OPIT) route by filling Ag-tube with oxide precursors, deforming into wire, restacking (sometimes twice) and heat treating using partial-melt processing (PMP). PMP consists of heating the wire above its peritectic melt temperature (usually ~900 °C), cooling to a temperature below solidus, slow cooling and isothermal annealing for an extended period. But this processing method has several drawbacks, including low-tap density for the oxide precursors, formation of bubbles and non-superconducting secondary phases as well as limitations on winding process for high-field magnet. To address these problems, several approaches are under investigation in our group, including a new heat treatment process named as split-melt-processing (SMP), a new type of precursor using metallic powders, synthesis of high-quality Bi2212 oxide precursors and extensively research on materials properties and current-limiting mechanisms. Besides, in most high field applications, the superconductor materials are subjected to stresses that can reach substantial levels. There are many sources of mechanical loads on the superconductor, including stresses due to fabrication, winding, thermal contraction, stresses due to magnetic field during operation (Lorentz forces) and fault conditions. Thus, in addition to electrical behavior, the superconductor materials must have enough strength to sustain such condition. Our group is focusing on high-strength high-elastic-modulus dispersion strengthened (DS) silver aluminum (AgAl) for sheathing Bi2212 round wire. It is found that the AgAl solid wire shows high yield stress and ultimate tensile strength in the annealed condition at both room temperature and 4.0 K, as well as significant ductility at 4.0 K. Electrical transport measurements show that the Bi2212/AgAl wires perform as well or better than Bi2212/AgMg wires.

ReBCO (where Re=Y, Nd, Sm, Eu, Gd, etc) class of superconductors exhibits the highest irreversibility field over a wide temperature range and is promising for application at high currents and magnetic fields. The problem of granularity of ReBCO, however, limits its ability to carry current and hence to generate magnetic field. A field of only a few mT can reduce critical current density to zero. It is necessary to process ReBCO in the form of large, single grains to avoid the problem of grain boundaries. But, even large single ReBCO grains are not long enough and homogeneous enough for large-scale applications, such as high-field magnets. Thus, the joint techniques for this type of conductor are essential for their practical. Currently, our group is using a novel approach to developing ReBCO joints.

Apart from research on superconducting materials, the failure mechanism, which is important in large-scale application is also investigated by our group. quenching behaviors in LTSC magnet is now well understood, but the HTSC quench behavior is much different from that of LTSC, mainly due to the larger thermal margin in HTSC, i.e. the temperature difference from operation temperature (such as 77 K or 4.2 K), to the critical temperature (typically, >90 K). This difference results in a slow normal zone propagation velocity (NZPV) in HTSC magnets, which makes the quench detection more difficult in HTSCs. In our group, study on quench behavior on Bi2212 coils, YBCO conductors and MgB2 are carried out.  Recently, the two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K for Bi2212 coils.