| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 92 === Abstract
REBCO high temperature superconductors have significant potential for high field engineering applications due to their inherent peak effect which exhibit high Jc values in a high magnetic field.
The magnitude of trapped field within bulk superconductors is proportional to the critical current density. In addition, Jc is related to both the type and number of pinning centers in the superconductor. Therefore, the key to enhancing Jc is an understanding of the relation between the pinning mechanism and microstructure.
In this study, bulk SmBCO was melt-processed in air using the cold seeding technique with MgO single crystal as the seed. In order to increase the number of pinning centers, �慆-sized additives, ex. Pd、Pt、CeO2, and nm-sized Y2O3、Al2O3、SiO2、Sm2BaCuO5 (“Sm211”) and Nd4Ba2Cu2O (“Nd422”) were used. Also, Sm210 was used to increase the Ba/Cu ratio of the precursor powders.
The results show that except for the Pd-doped samples, Jc of SmBCO with �慆-sized additives was enhanced by refining the 211-particles. Notably, for the Pd-doped samples, the shape of the 211-particles changed to rectangular, and thus induced a higher density of defects in the 123/211 interface. It was also found that Jc could be further enhanced by the combined addition of two types of additives.
As for the nm-sized additions, it was shown that the reactions between different nano-scale additives and the matrix can be classified into three types: one dose not react with the Sm123 (Sm1Ba2Cu3Oy) matrix (e.g. Y2O3), one becomes a compositional fluctuation region (e.g. Sm211 and Y2O3), and the other reacts with the matrix, which results in the suppression of superconductivity (e.g. Al2O3 and SiO2).
As for the effect of Sm210 addition, the results indicate that the substitution of Sm-Ba was suppressed and thus Tc was enhanced. In order to further enhance Jc for samples with Sm210 addition, different types of additives were also used. The results for the �慆-sized additives were similar to the Sm211 samples, in which 211-particles were refined. But the effect of nm-sized additives on Jc in the Sm210 and Sm211 samples was different.
In this study, scaling analysis found that the pinning mechanism for samples with �慆-sized additives were similar to the un-doped samples, in which defects and 123/211 interfaces contributed to pinning. As for samples with nm-sized addition, in which Jc is superior in high field regions, the dominant pinning becomes a combination of ���� pinning and normal pinning.
Finally, the current transport behavior for samples with different pinning mechanisms was examined. The poorer superconducting phase was found in samples with ���� pinning centers, ex. Sm210, nm211 doped samples. As for the control and Pt/CeO2 doped samples, there was only one type of superconducting phase. In addition, it was also found that the difference between the main superconducting phase and the poorer superconducting phase results in various Jc-H behaviors.
Notably, the Jc value for samples with Sm210/Pt/CeO2/nmSm211 addition was 3.6�e104 A/cm2 at 77K at 0T, and 1.8�e104 A/cm2 at 77K at 2T. The above results were the highest values for SmBCO samples grown in air.
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