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.

中文翻译：

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大型 Bi-2212 卷材潜在热处理问题的实验研究

美国国家科学院最近的一份报告 [1] 所认可的 30 T NMR 路线显然在实现高稳定性和均匀的高温超导 (HTS) 插入磁场方面仍然面临许多挑战。作为唯一具有圆线 (RW) 几何形状的高温超导导体，Bi2Sr2CaCu2 O8-x (Bi-2212) RW 导体对于核磁共振 (NMR) 磁体应用非常有吸引力。目前，国家强磁场实验室正在开发采用 Bi-2212 RW 绕线插入线圈的 NMR 质量演示磁体。该演示磁体的目标是产生 23+ T 的总磁场，且具有 ppm 级均匀性。由于 Bi-2212 线圈需要缠绕反应技术，我们最初主要担心的是，大型 Bi-2212 线圈可能会由于其较大的自重而在典型的部分熔化 Bi-2212 热处理 (HT) 过程中变形。为了通过实验模拟大型 Bi-2212 线圈的 HT，几个小型测试线圈在自重负载下进行了热处理。在 1 bar Bi-2212 完全反应后，对这些线圈的几何形状、传输临界电流 (Ic) 特性、氧化状态和绝缘性能进行了表征。事实上，线圈的几何形状和单个线材的形状没有变形，机械负载也没有引起传输性能下降。即使这些线圈涂有致密的氧化物绝缘层，也能实现均匀的氧平衡。然而，尽管基于 TiO2 的绝缘涂层在电线表面得到了很好的保存，但仍有几个线圈出现了电气短路。