High Performance Bi-2212 Round Wires Made with Recent Powders.

Multifilamentary Bi2Sr2CaCu2Ox (Bi-2212) wire made by the powder-in-tube technique is the only high temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J C ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and a very promising conductor for very high field NMR and accelerator magnets. We studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J C (4.2 K, 15 T) = 6640 A/mm2 and J C (4.2 K, 30 T) = 4670 A/mm2, which correspond to engineering critical current densities J E (4.2 K, 15 T) = 1320 A/mm2 and J E (4.2 K, 30 T) = 930 A/mm2.

Experimental Study on Potential Heat Treatment Issues of Large Bi-2212 Coils.

The route to 30 T NMR endorsed by a recent National Academy report clearly still has many challenges to achieve high stability and homogeneous high temperature superconducting (HTS) magnet. As the only HTS conductor with round wire (RW) geometry, Bi2Sr2CaCu2O8-x (Bi-2212) RW conductor is very attractive for NMR magnet applications. At present, an NMR quality demonstration magnet with Bi-2212 RW wound insert coils is under development at the National High Magnetic Field Laboratory (NHMFL). The target of this demonstration magnet is to generate a total field of 23+ T with ppm level homogeneity. Since Bi-2212 coils require Wind-and-React (W&R) technology, our initial major concern was that large Bi-2212 coils might deform during the typical partial melt Bi-2212 heat treatment (HT) due to their large self-weight. To experimentally mimic the HT of large Bi-2212 coil, several small test coils were heat treated under deadweight loads. After 1 bar Bi-2212 full reaction, these coils were characterized in terms of coil geometry, transport critical current properties, oxygenation status and insulation performance. Coil geometry and individual wire shape was in fact not distorted, nor was transport properties degradation was induced by mechanical loading. Uniform oxygen equilibration was achieved in these coils even though they were coated with dense oxide insulation. However, although the TiO2-based insulation coating was well preserved on the wire surface, several coils developed electrical shorts.

Isotropic round-wire multifilament cuprate superconductor for generation of magnetic fields above 30 T.

Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, J(c). To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O(7-x) and (Bi, Pb)2Sr2Ca2Cu3O(10-x) are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O(8-x) (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology. An appealing attribute of this Bi-2212 conductor is that, being without macroscopic texture, it contains many high-angle grain boundaries but nevertheless attains a very high J(c) of 2,500 A mm(-2) at 20 T and 4.2 K. The large potential of the conductor has been demonstrated by building a small coil that generated almost 2.6 T in a 31 T background field. This demonstration that grain boundary limits to high Jc can be practically overcome underlines the value of a renewed focus on grain boundary properties in non-ideal geometries.

Process to densify Bi2Sr2CaCu2Ox round wire with overpressure before coil winding and final overpressure heat treatment.

Overpressure (OP) processing of wind-and-react Bi2Sr2CaCu2Ox (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4 % without change in wire length and substantially raising its J c . However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT). We found that about 80 % of the total wire diameter shrinkage occurs during the 50 atm OP-PD before melting. We successfully wound such pre-densified 1.2 mm diameter wires onto coil mandrels as small as 10 mm diameter for Ag-Mg-sheathed wire and 5 mm for Ag-sheathed wire, even though such small diameters impose plastic strains up to 12% on the conductor. A further ~20% shrinkage occurred during a standard OP-HT. No 2212 leakage was observed for coil diameters as small as 20 mm for Ag-Mg-sheathed wire and 10 mm for Ag-sheathed wire, and no J c degradation was observed on straight samples and 30 mm diameter coils.

Effect of sheath material and reaction overpressure on Ag extrusions into the TiO2 insulation coating of Bi-2212 round wire.

In order to develop a high current density in coils, Bi-2212 wires must be electrically discrete in tight winding packs. It is vital to use an insulating layer that is thin, fulfils the dielectric requirements, and can survive the heat treatment whose maximum temperature reaches 890 °C. A thin (20-30 µm) ceramic coating could be better as the insulating layer compared to alumino-silicate braided fiber insulation, which is about 100 µm thick and reacts with the Ag sheath during heat treatment, degrading the critical current density (Jc). At present, TiO2 seems to be the most viable ceramic material for such a thin insulation because it is chemically compatible with Ag and Bi-2212 and its sintering temperature is lower than the maximum temperature used for the Bi-2212 heat treatment. However, recent tests of a large Bi-2212 coil insulated only with TiO2 showed severe electrical shorting between the wires after over pressure heat treatment (OPHT). The origin of the shorting was frequent silver extrusions that penetrated the porous TiO2 layer and electrically connected adjacent Bi-2212 wires. To understand the mechanism of this unexpected behaviour, we investigated the effect of sheath material and hydrostatic pressure on the formation of Ag extrusions. We found that Ag extrusions occur only when TiO2-insulated Ag-0.2%Mg sheathed wire (Ag(Mg) wire) undergoes OPHT at 50 bar. No Ag extrusions were observed when the TiO2-insulated Ag(Mg) wire was processed at 1 bar. The TiO2-insulated wires sheathed with pure Ag that underwent 50 bar OPHT were also free from Ag extrusions. A key finding is that the Ag extrusions emanating from the Ag(Mg) sheath actually contain no MgO, suggesting that local depletion of MgO facilitates local, heterogeneous deformation of the sheath under hydrostatic overpressure. Our study also suggests that predensifying the Ag(Mg) wire before insulating it with TiO2 and doing the final OPHT can potentially prevent Ag extrusion.