Tengming Shen

High Field Superconducting Solenoids Via High Temperature Superconductors

High-field superconducting solenoids have proven themselves to be of great value to scientific research in a number of fields, including chemistry, physics and biology. Present-day magnets take advantage of the high-field properties of Nb3Sn, but the high-field limits of this conductor are nearly reached and so a new conductor and magnet technology is necessary for superconducting magnets beyond 25 T. Twenty years after the initial discovery of superconductivity at high temperatures in complex oxides, a number of high temperature superconductor (HTS) based conductors are available in sufficient lengths to develop high-field superconducting magnets. In this paper, present day HTS conductor and magnet technologies are discussed. HTS conductors have demonstrated the ability to carry very large critical current densities at magnetic fields of 45 T, and two insert coil demonstrations have surpassed the 25 T barrier. There are, however, many challenges to the implementation of HTS conductors in high-field magnets, including coil manufacturing, electromechanical behavior and quench protection. These issues are discussed and a view to the future is provided.

Filament to filament bridging and its influence on developing high critical current density in multifilamentary Bi2Sr2CaCu2Ox, round wires

Increasing the critical current density (Jc) of the multifilamentary round wire Ag/Bi2Sr2CaCu2Ox(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- Jc inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of Jc without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high Jc in multifilamentary 2212 round wire.

Development of high critical current density in multifilamentary round-wire Bi2Sr2CaCu2O8+δ by strong overdoping

Bi2Sr2CaCu2O8+δ is the only cuprate superconductor that can be made into a round-wire conductor form with a high enough critical current density Jc for applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed filamentary wires can be doubled to more than 1.4×105 A/cm2 by low temperature oxygenation. Careful analysis shows that the improved performance is associated with a 12 K reduction in transition temperature Tc to 80 K, an increase in flux pinning, and particularly a significant enhancement in intergranular connectivity. In spite of the macroscopically untextured nature of the wire, overdoping is highly effective in producing high Jc values.

Heat treatment control of Ag–Bi2Sr2CaCu2Ox multifilamentary round wire: investigation of time in the melt

It is well known that the critical current density Jc of Ag-sheathed Bi2Sr2CaCu2Ox (2212) varies strongly with heat treatment details, particularly the maximum processing temperature Tmax, but the mechanism for such Jc variations and how the processing window can be widened remain unknown. We systematically measured the Jc and electromagnetic properties of a powder-in-tube Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox (2212) round wire processed with the maximum processing temperature Tmax ranging from 887 to 900 °C and the time at the maximum temperature tmax from 0 to 3 h using three representative heat treatment schedules. We found that Jc correlates weakly to Tmax, but it correlates strongly to the time in the melt tmelt, a processing parameter that has not been explicitly considered before. Jc is rather insensitive to Tmax in the temperature range 887–900 °C and the true cause of Jc declining with high Tmax appears to be the long tmelt that leads to collapse of filament structure. By tuning tmelt we were able to widen the Tmax window to 10 °C. The Jc–tmelt correlation, as well as quench studies, indicate that Jc is controlled by complex diffusion processes occurring in the melt (filament bonding, bubble agglomeration, and perhaps Cu loss). Our findings highlight tmelt as an important processing parameter for optimizing Jc and may serve as a general guide for heat treating 2212 coils.

React-Wind-Sinter Processing of High Superconductor Fraction Bi

Bi2Sr2CaCu2Ox (Bi2212) conductor technology has advanced significantly but the development of magnets is still hampered by difficulties associated with the partial-melt process (for wind&react magnets) and strain limitations (for react& wind magnets). To avoid these problems, the React-Wind-Sinter (RWS) approach has been proposed. Here we report on experiments that investigate three split processes that are based on the conventional partial-melt process within the RWS concept. The partial-melt process was interrupted at T1, T1 - 10degC and TS. After cooling to room temperature, the conductor is bent to a series of diameters (40 mm-100 mm), replicating magnet construction. The heat treatment process is then resumed on the bent samples from the split point and the heat treatment completed. The critical current is measured at 4.2 K in self-field using the four-probe method and the microstructure and phase composition of the Bi2212/AgMg wire are examined with scanning electron microscopy. For the split processes, the critical current after full heat treatment is as high as those from conventionally processed short samples, and in at least one case it is increased by 40% relative to conventional processing. These results show that a split process is a promising approach to improved Bi2212 conductors and magnets, and more broadly shows that conventional Bi2212 partial-melt processing is far from optimized.

_2

It has been shown previously that texture can be achieved in Bi2Sr2CaCu2Ox (Bi2212) bulk and tape conductors by applying a background magnetic field during the partial-melt stage. In this paper, we report on the texturing of low aspect ratio multifilamentary Bi2212/AgMg wires that were partial melt processed in a 12 T background magnetic field. To determine the effects of the applied magnetic field on the transport properties, the critical current was measured using the four point method. Transport measurements were carried out in magnetic fields up to 5 T with varying magnetic field angle. It was found that some texture is induced, resulting in electrical properties that vary with magnetic field orientation.

Sr

The NHMFL has had a long running program to develop Bi2Sr2CaCu2Ox (Bi2212) for high field magnets. The recent development of round wire Bi2212 (RW2212) has strengthened the effort to develop solenoid magnets with fields substantially greater than can be achieved with Nb3Sn. The present paper briefly summarizes some of the results obtained at the NHMFL in the past 12 months. It summarizes the talk given by David Larbalestier at WAMSDO on May 24, 2008. Much of the work is ongoing and will be reported in the normal peer reviewed literature in late 2008.