Experimental Study of Potential Heat Treatment Issues of Large Bi-2212 Coils
The route to 30 T NMR endorsed by a recent National Academy report [1] clearly still has many challenges to achieve a high stability and a homogeneous high-temperature superconducting (HTS) insert magnetic field. As the only HTS conductor with round wire (RW) geometry, Bi2Sr2CaCu2 O8-x (Bi-2212) RW conductor is very attractive for nuclear magnetic resonance (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. 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 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 (Ic) properties, oxygenation status, and insulation performance. Coil geometry and individual wire shape were in fact not distorted, nor was transport property degradation 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.