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

大块高温超导体的超导特性，如俘获场等，已经达到了很高的水平。然而，它已被普遍观察到，样品很容易被破坏的磁压力的捕获通量或突然的热循环。熔体处理的大块超导体的显微结构受各相含量和制备条件的影响，而显微结构又影响断裂韧性。因此，我们通过改变成分和制备条件来制备样品，并检测其显微组织和断裂韧性。主要结果如下：首次成功制备出直径为30 mm的单晶粒c轴取向Nd基超导块材。捕获场大于含有相同量的Ag的基于Sm的样品的捕获场。并研究了熔融过程中的大气与成矿的关系。 关于我们 n的空隙，并通过在氧气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，如果在生长过程中将气氛切换到Ar气体流动的气氛，则样品显示出高性能。用球磨处理的211相粉末制备的样品中Sm 211相的平均粒径为0.7 μ m，与用相同条件下未球磨处理的211相粉末制备的样品相比，俘获场增加。结果表明，氧的扩散率主要受微观结构的影响，而与体系的不同无关;添加Ag的样品由于孔隙较少，需要较长的氧处理时间;我们建立了Nd-Eu-Gd系统的熔体加工条件，直径达36 mm，液氮温度下的俘获场为1.4T，20 K下的俘获场为8.2T。对于基于Dy的系统，即使使用直径为18 mm的小样品，在18 K下的捕获场也达到5.7 T，并且直径为18 mm的添加CeO 2的基于Y的样品的捕获场达到6.2 Tat 25 K。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