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

Area of Science:

• Superconducting magnets
• Materials science
• Nuclear Magnetic Resonance (NMR)

Background:

• Achieving 30 T NMR magnets requires high stability and homogeneity in high-temperature superconducting (HTS) magnets.
• Bi2Sr2CaCu2O8-x (Bi-2212) round wire (RW) is a promising HTS conductor for NMR applications due to its unique geometry.
• A demonstration magnet targeting 23+ T with ppm homogeneity using Bi-2212 RW insert coils is under development at NHMFL.

Purpose of the Study:

• To investigate the potential deformation and property degradation of large Bi-2212 coils during the Wind-and-React (W&R) heat treatment (HT) process due to self-weight.
• To experimentally assess the impact of mechanical loading during HT on Bi-2212 coil geometry, critical current properties, oxygenation, and insulation.

Main Methods:

• Small Bi-2212 test coils were fabricated and subjected to heat treatment under deadweight loads to simulate the HT of large coils.
• Post-heat treatment characterization included assessment of coil geometry, transport critical current, oxygenation status, and insulation performance.

Main Results:

• Coil geometry and individual wire shape remained undistorted after heat treatment under load.
• No degradation in transport properties was observed due to mechanical loading.
• Uniform oxygen equilibration was achieved despite dense oxide insulation coatings.
• However, several coils developed electrical shorts, despite the TiO2-based insulation coating remaining intact.

Conclusions:

• Bi-2212 RW conductors can withstand mechanical loading during heat treatment without geometric distortion or critical current degradation, making them suitable for large NMR magnet applications.
• The development of electrical shorts in some coils indicates a need for further investigation and potential improvements in insulation strategies for Bi-2212 W&R coils.