Improvement of the fracture toughness and trapped field of bulk high temperature superconductors by controlling the microstructure

块体高温超导体的超导特性，如囚禁场，已经达到了很高的水平。然而，人们普遍观察到，样品很容易被捕获的磁通的磁压或突然的热循环打破。块体超导体的微观结构受各相含量和制备条件的影响，而后者又影响断裂韧性。因此，我们通过改变样品的成分和制备条件来制备样品，并对样品的显微组织和断裂韧性进行了测试。主要研究结果如下：首次成功地制备了直径为30 mm的单晶c轴取向的Nd基大块超导体。俘获场大于含相同量银的Sm基样品的俘获场。研究了熔融过程中的气氛与…形成的关系通过在氧气-气体流动气氛或真空中熔融处理样品，成功地消除了空隙。还证实了在生长过程中，如果将气氛切换到Ar气体流动的气氛中，样品表现出高性能。经球磨处理的211相粉末制备的样品中SM211相的平均粒径为0.7m，与未经球磨处理的211相粉末制备的样品相比，俘获场增大。此外，我们还研究了Sm和Y基样品的氧化过程，为致密样品建立了有效的氧气处理方法。结果表明，氧的扩散率受显微组织的差异而不是体系的差异所影响，添加银的样品由于空洞数量较少而需要较长的氧气处理时间，而350 aC以下的低温处理对Sm基样品是重要的。确定了直径为36 mm的Nd-Eu-Gd系的熔体加工条件，其中在液氮温度下的俘获场为1.4T，在20K时的俘获场为8.2T。对于Dy系，即使是直径为18 mm的小样品，18K时的俘获场也达到了5.7T，而添加了直径为18 mm的CeO2的Y基样品的俘获场在25K时达到了6.2 Tat。较少

The superconducting properties of bulk high temperature superconductors, such as the trapped field, had already reached a very high level. However, it has been commonly observed that the samples are easily broken by the magnetic pressure of the trapped fluxes or by an abrupt heat cycling. The microstructure of melt processed.bulk superconductors is influenced by the contents of each phase as well as by the preparation condition, which in turn affects the fracture toughness. Therefore, we prepared samples by varying the composition and preparation condition, and examined the microstructure and the fracture toughness. The major results are summarized in the following.We succeeded in the preparation of a single-grain, c-axis aligned, Nd-based bulk superconductor that is 30 mm in diameter for the first time. The trapped field was larger than that of a Sm-based sample that contains the same amount of Ag. We also studied the relation between the atmosphere during the melting and the formatio … More n of voids, and succeeded in the elimination of voids by melt-processing the samples in an oxygen-gas flowing atmosphere or vacuum. It was also confirmed that the samples show high performance if the atmosphere were switched to an Ar-gas flowing one during the growth process. The average particle size of the Sm211 phase in a sample prepared with 211-phase powders that were treated with ball-milling was 0.7 ^m, and the trapped field increased compared to samples prepared with the same condition using 211-phase powders that were not ball-milling treated.We also studied the oxygenation process of Sm- and Y-based samples to establish an efficient oxygen treatment for dense samples. It was found that the diffusivity of oxygen is influenced by the difference in the microstructure rather than the difference of the system, Ag-added samples require a longer oxygen treatment because of the less amount of voids, and a low temperature treatment below 350aC is important for Sm-based samples. We established the melt-processing conditions of the Nd-Eu-Gd system up to 36 mm in diameter, for which the trapped field was 1.4 T at liquid nitrogen temperature and 8.2 T at 20 K. For the Dy-based system, the trapped field reached 5.7 T at 18 K even with a small sample whose diameter was 18 mm, and the trapped field of a CeO2-added Y-based sample that is 18 mm in diameter reached 6.2 Tat25K. Less