발표제목: Magnetic hysteresis properties and local magnetic properties of high-temperature superconducting strips
Over the past two decades, technologies related to high-temperature superconduc- tors (HTS) have made remarkable progress. Notably, a tape called second-generation high-temperature superconductor coated conductor (2G HTS CC or tape) is expected to be a useful technology for high-temperature superconductor applications. 2G HTS coated conductors are based on rare earth elements and include YBCO, GdBCO, and SmBCO. The advantage of HTS in large-scale applications is that they can be made smaller and lighter in size compared to other materials to conduct high currents. Ac- cording to the expected demand, it is necessary to study the properties of the high- temperature superconducting thin films for various aspects.
The most severe problem in the application of HTS is AC loss caused by an alter- nating current or magnetic field. Therefore, HTS should be studied for efficient use. Many experimental and theoretical models have been studied by several researchers and groups. The critical state model (CSM) shows that the AC loss is related to the width of the superconductor. It can thus be concluded that the 2G HTS tape can be used in a form divided into several filaments for efficient use.
In this study, GdBCO, a 2G HTS coated conductor, was etched with a multiple strip structure with various types of strips, and an experiment was conducted to ana- lyze the magnetic hysteresis characteristics. Experiments were performed for the case where an external magnetic field was applied in the out-of-plane direction (c-axis) to all these samples. The local magnetic field distributions of the sample in the applied field were measured using a low-temperature scanning probe microscope (LTSHPM) to analyze the magnetic hysteresis properties of each sample.
First, to understand the fundamental properties of high-temperature superconduct- ing wires, experiments were carried out on single strip type samples, which are the purest form. For two temperatures, an external magnetic field was applied perpendic- ular to the sample to obtain the local magnetic fields. Local hysteresis loops were obtained from the measured magnetic field distributions, and the local hysteresis losses were defined from each loop. For a theoretical understanding of these local magnetic properties, I used the CSM assuming field-independent current density. In this model, the relation between global remanent magnetization and hysteresis loss can be obtained analytically. In addition, the remanent magnetic field and local hys- teresis loss correspond with each other, and the two could be theoretically linear. In the case of CSM with field-dependent critical current density, this relation cannot be obtained analytically, so it must be calculated numerically. Therefore, the CSM is simulated for situations similar to the characteristics of the samples obtained from the experiment, and the results are compared. As a result, the relation between the rema- nent field and the local hysteresis loss was observed to be still valid.
Next, based on the analytical method performed for the single strip sample, the hysteresis loss study was performed for several strips that were parallel to each other on the same plane. This configuration was one of the proposed methods to reduce AC loss of superconducting wires. Experiments were performed on the uncoupled double strips with single strips of half the width and uncoupled triple strips with 1/3 width single strips. The local magnetic field was obtained for each sample for the same experimental condition as the single strip. From these results, the global magnetiza- tions of each sample was calculated and compared. To calculate the global magneti- zation from the measured local magnetic field, the demagnetizing field effect is large in the strip, so a constant 𝑘 should be applied unlike the slab model. Therefore, be- fore comparing the hysteresis loss of each sample, I discussed the theoretical impli- cations of the factor 𝑘 and then the method of obtaining it from the experimental measurement. As a result, it was found that the constant 𝑘 depends on the geometry of the sample. In the results of SHPM, local magnetic properties can be used to ana- lyze the hysteresis loss considering not only the geometrical effect but also the dif- ference in the critical current density characteristics of each strip. Then, it was con- firmed that the detailed analysis from the measurement results of the whole sample was possible from the local magnetic properties.
Next, the coupled double strips were studied. In this case, numerical modeling was needed rather than a CSM based on analytical formulas. Therefore, prior to the ex- perimental analysis, numerical modeling was developed, and the features of the cou- pled double strip were discussed. In this study, the numerical modeling based on magnetic energy minimization method was used.
Because of the model calculation for the coupled double strips, I observed an un- usual current distribution in the region close to the gap. Therefore, the accuracy of the inversion calculation was essential for the experimental data. I discussed the cor- rection method considering some experimental conditions with the conventional in- version method.
From the results of the numerical modeling, SHPM results for the coupled double strips were analyzed after improving the inverse transformation method for experi- ments. The measured experimental data corresponded to the results of a single strip. In the case of coupled double strips, since the strips were separated from each other, but the ends are connected to each other, so flux is trapped in the gaps surrounded by the strips so that the magnetization value is relatively increased. This effect was qual- itatively understood using the CSM for coupled double slabs. The experimental re- sults thus show that although the uncoupled multiple strips help to reduce the AC loss, the losses can be several times higher when the strip contacts occur. Therefore, to reduce the risk of this loss when using striated HTS tapes, it would be effective to decrease the gaps between the strips.
Thesis Advisor: Prof. Hyeong-Cheol Ri