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

块状高温超导体的超导性能，如俘获场，已经达到了很高的水平。然而，通常观察到，样品很容易被捕获通量的磁压或突然的热循环破坏。熔体加工后的显微组织。体超导体的断裂韧性受各相含量和制备条件的影响。因此，我们通过不同的成分和制备条件制备了样品，并检测了显微组织和断裂韧性。主要结果总结如下。我们首次成功制备了直径为30毫米的单粒、c轴排列的钕基块状超导体。捕获场比含有相同数量银的sm基样品的捕获场大。我们还研究了熔炼过程中气氛与气孔形成的关系，并成功地在氧气流动气氛或真空环境下对样品进行熔炼处理，消除了气孔。还证实，如果在生长过程中将大气切换为ar气体流动的大气，则样品显示出高性能。经球磨处理的211相粉末制备的样品中Sm211相的平均粒径为0.7 ^m，捕获场比未经球磨处理的211相粉末制备的样品增大。我们还研究了Sm和基于Sm的样品的氧化过程，以建立致密样品的高效氧处理方法。结果表明，影响氧扩散率的主要因素是微观结构的不同，而不是体系的不同；添加ag的样品由于孔隙较少，需要较长的氧处理时间；对于sm基样品，350aC以下的低温处理很重要。我们建立了直径为36 mm的Nd-Eu-Gd体系的熔炼工艺条件，其俘获场在液氮温度下为1.4 T，在20 K下为8.2 T。对于dy基样品，即使是直径为18 mm的小样品，在18 K时捕获场也达到5.7 T，而添加ceo2的18 mm基样品的捕获场达到6.2 Tat25K。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