Effect of sheath material and reaction overpressure on Ag protrusions into the TiO₂insulation coating of Bi-2212 round wire

Abstract

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.

Process to densify Bi2Sr2CaCu2O X round wire with overpressure before coil winding and final overpressure heat treatment

“…The fact that Ag wire showed a very homogeneous decrease in diameter, whereas the Ag-Mg wire showed slight variations in the decrease in wire diameter along its length, indicates a nonuniformly densified wire. This may indicate an inhomogeneous sheath hardness in the Ag-Mg wire caused by an inhomogeneous distribution of the MgO particles that occurs as the MgO grains form during the heat treatments, as shown in [10]. The oxidation of Mg during the OP-PD may be homogeneous, but as the MgO particles grow during the OP-PD they may become inhomogeneously distributed in the sheath, especially at Ag grain boundaries [11], leading to variations in hardness from place to place within Ag-Mg sheath.…”

Tripled critical current in racetrack coils made of Bi-2212 Rutherford cables with overpressure processing and leakage control

“…RC3 is identical to RC2 except that its cable was brush painted with several coats of TiO 2 -polymer slurry before being insulated with the same mullite sleeve used for the cables and coil winding of RC1 and RC2. The TiO 2 slurry was prepared with a recipe [21,22] used for single strand solenoids with properties already reported [23][24][25]. Figure 1 shows cross-sections of the wires and the insulated Rutherford cable.…”

First demonstration of high current canted-cosine-theta coils with Bi-2212 Rutherford cables

“…It allows us to intercept the Lorentz force at each single turn and transfer it to the mandrel structure, avoiding stress accumulation on the conductor [7][8][9]. The wind-and-react method, on the other hand, poses additional technical problems because the mandrel material must withstand the Bi-2212 heat treatment cycle that reaches a maximum temperature of 890 • C and pressures up to 50 bar in a flowing O 2 /Ar gas mixture environment [10]. The mandrel material must not experience significant oxidation, otherwise the oxygen partial pressure (pO 2 ) in the furnace during heat treatment (usually, the selected pO 2 = 1 bar) may not be sufficient to ensure the proper phase sequence and Bi-2212 melting temperature for achieving a high critical current density.…”