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

I Hossain,J Lu,U P Trociewitz,F Kametani,D Larbalestier,E Hellstrom,M Matras,J Jiang

doi:10.1088/1757-899x/279/1/012021

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 in oxygen. A thin (20-30 µm) ceramic coating could be better as the insulating layer compared to alumino-silicate braided fiber insulation, which is about 150 μm thick and reacts with the Ag sheathed Bi-2212 wire during heat treatment. 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 protrusions into the porous TiO2 layer that 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 Ag protrusions. We found that Ag protrusions occur only when TiO2-insulated Ag-0.2%Mg sheathed wire (Ag(Mg) wire) undergoes OPHT at 50 bar. No Ag protrusions 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 protrusions. A key finding is that the Ag protrusions 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 limit Ag protrusions.

Highlights

Bi2Sr2CaCu2Ox (Bi-2212) is the only high temperature superconductor available in a round wire form that has a very high critical current density (Jc is around 105 A cm−2 up to 45 T) and irreversibility field (>100 T) at 4.2 K [1,2]
The 1 bar heat treatments were done in pure O2 and the overpressure heat treatment (OPHT) were done in a Ar-2%O2 gas mixture at 50 bar total pressure, both of which maintained PO2 = 1 bar during the heat treatment. 8 cm long samples were used because earlier observations showed that Ag extrusions occurred in these short samples
The Ag extrusions were irregular in shape and size, and most of them did not penetrate the entire thickness of the insulation layer

Summary

Introduction

Bi2Sr2CaCu2Ox (Bi-2212) is the only high temperature superconductor available in a round wire form that has a very high critical current density (Jc is around 105 A cm−2 up to 45 T) and irreversibility field (>100 T) at 4.2 K [1,2]. This round wire geometry is preferred by magnet designers and builders as it can be twisted and cabled into desired shapes. Pure Ag is weak (yield strength 45 MPa at 4.2 K after OPHT), so the wire is strengthened by using a stronger outer sheath that is typically a dispersion strengthened Ag-0.2 wt% Mg (hereafter Ag(Mg)) [9]

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Conclusion