Microstructure Characterization of Superconducting Materials

We report on atomic-scale analyses of the microstructure of an Nb₃Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy, electron backscatter diffraction and first-principles calculations. Epitaxial growth of Nb₃Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb₃Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb₃Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb₃Sn $(1\bar{2}0)$//Nb $(\bar{1}11)$ and Nb₃Sn $(002)$//Nb $(0\bar{1}1).$ The Nb₃Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb $(0\bar{1}1)$ and Nb₃Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb₃Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb₃Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb₃Sn/Nb interface provides important information about the formation of imperfections in Nb₃Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb₃Sn SRF cavities for accelerators.

CaKFe₄As₄ is a new member of the 1144-type iron-based superconductors (IBSs) which is expected to show remarkable superconducting properties because its crystal structure is similar to that of 122-type IBSs. Recently, substantial anisotropy between the in-plane and out-of-plane critical current densities (J_C), with peculiar temperature dependence of in-plane J_C at high fields, has been reported in CaKFe₄As₄ single crystals. The anisotropy of J_C was attributed to the presence of planar defects, which were shown to be confined to the ab plane. However, the origin of these defects remains unclear. To elucidate the origin of these planar defects, we herein present our results on atomic-scale microstructure analysis of high-quality CaKFe₄As₄ single crystals. Using high-resolution scanning transmission electron microscopy (STEM), along with electron energy loss spectroscopy, we have demonstrated that these planar defects consist of one- or two-layer KFe₂As₂ step network, which is spread over the periodically ordered KFe₂As₂ and CaFe₂As₂ monolayers. Further, we report that the dark contrast regions observed via conventional STEM are local strain distributions, which originate from the substitution of Ca by K in a unit cell.

Conduction in disordered media has been the subject of interest in applications and basic research. Percolation theory is used to study conduction, and mean-field theory has been used to describe macroscopic conduction. However, such an approximation theory cannot provide specific insights into the local limiting factors of conduction. In this study, a finite element simulation based on the site percolation model is used to investigate the effect of microstructural defects in materials on transport current. Statistical analysis of the local current obtained by the simulation experiment shows that the meandering of the transport current because of the disturbance of the current path increases as the concentration of defects increases, thus observing a dimensional crossover of the transport current from 1D to 3D. Λ, which represents the degree of current meandering, has a value of 5.3 in the vicinity of the singularity of the percolation threshold (P_c). The governing mechanisms of the transport current differ depending on the concentration and correlation of defects. The main limiting factors of the transport current are the following: 'shadow effect' derived from isolated defects in the quasi-1D region where the defects are dilute; the meandering of the current path including the reverse current in the 3D region where the defects are highly concentrated and begin to correlate. These results can be used to understand the current limiting mechanisms in 3D polycrystalline superconducting materials and are expected to provide guidelines for controlling complex microstructures in iron-based superconductors, MgB₂, and BSCCO.

This paper reports a diagnostic method for clarifying performance bottlenecks in superconducting wires and tapes for their further performance enhancements. In particular, our achievements by scanning Hall-probe microscopy (SHPM), which worked well for selecting positions for microstructure observation by SEM, TEM, and x-ray CT, are introduced. This hybrid microscopy offers the information of the direct relationship between the performance of a practical-scale superconducting wire or tape and its origin in micro-scale or nano-scale structure. As such examples, characterization results for an MgB₂ multi-filamentary wire, a commercially available long RE-123 coated conductor, and a striated multi-filamentary coated conductor are reported in this paper through reviewing our past studies.

High-temperature superconducting (HTS) maglev has great potential in the field of high-speed transportation due to its capability for passive stabilization. The levitation force between the bulk HTSs and the permanent magnet guideway is a significant parameter relating to operational safety and comfort. This force has an obvious hysteresis nonlinear characteristic, which can be represented by nonlinear stiffness and damping. The stiffness and the damping are functions of vertical displacement and velocity, respectively. The vibration velocity of a HTS maglev vehicle can at times exceed 100 mm s⁻¹, but the existing levitation force test methods are almost quasi-static. These methods are unable to accurately measure the damping characteristic of the maglev system. In this paper, a viscoelasticity model is introduced to describe the dynamic force. The parameters in the model are identified using the least square method based on the vibration response of the HTS maglev system. Meanwhile, the effectiveness of the model and identification method are tested by numerical simulations. The hysteresis loops derived from the motion theory coincide with the practical ones. Finally, the method is applied to identify the parameters of hysteresis nonlinear levitation force in a previous experiment with dampers. Based on the established hysteretic model, the dynamic characteristics of the HTS maglev system can be well presented.

Accelerator magnets that can reach magnetic fields well beyond the Nb-Ti performance limits are presently being built and developed, using Nb₃Sn superconductors. This technology requires reaction heat treatment (RHT) of the magnet coils, during which Nb₃Sn is formed from its ductile precursor materials (a "wind and react" approach). The Nb₃Sn microstructure and microchemistry are strongly influenced by the conductor fabrication route, and by the phase changes during RHT. By combining in situ differential scanning calorimetry, high energy synchrotron x-ray diffraction, and micro-tomography experiments, we have acquired a unique data set that describes in great detail the phase and microstructure changes that take place during the processing of restacked rod process (RRP), powder-in-tube (PIT), and internal tin (IT) Nb₃Sn wires. At temperatures below 450 °C the phase evolutions in the three wire types are similar, with respectively solid state interdiffusion of Cu and Sn, Cu₆Sn₅ formation, and Cu₆Sn₅ peritectic transformation. Distinct differences in phase evolutions in the wires are found when temperatures exceed 450 °C. The volume changes of the conductor during RHT are a difficulty in the production of Nb₃Sn accelerator magnets. We compare the wire diameter changes measured in situ by dilatometry with the phase and void volume evolution of the three types of Nb₃Sn wire. Unlike the Nb₃Sn wire length changes, the wire diameter evolution is characteristic for each Nb₃Sn wire type. The strongest volume increase, of about 5%, is observed in the RRP wire, where the main diameter increase occurs above 600 °C upon Nb₃Sn formation.

We characterize nanostructures of two types of EuBa₂Cu₃O_y (EuBCO) layers containing 3.5 mol%BaHfO₃ (BHO) nanorods fabricated by pulsed laser deposition (PLD) process using scanning electron microscopy (SEM), transmission electron microscopy and three-dimensional reconstructions that used a series of cross-sectional SEM images taken in a focused ion beam (FIB)-SEM system. One type of PLD layer was grown in a vapor–solid (V–S) mode, and the other was grown in a vapor–liquid–solid (V–L–S) mode. Relatively large polycrystalline of CuO and Ba–Cu–O phases were found on the surface of EuBCO layer grown in the V–L–S mode, consistent with precipitation from liquid phase. Misoriented EuBCO grains with different orientations to those of the matrix grains nucleated on CuO grains (or clusters of CuO/Ba–Cu–O grains) in both EuBCO layers. Both the self-field and in-field critical current values of the EuBCO layers containing BHO nanorods depend on the volume fraction of the misoriented grains, and thus the V–L–S mode layer exhibits better superconductive properties. The volume fraction of the misoriented EuBCO grains in the matrix of c-axis oriented EuBCO grains grown in the V–L–S mode was smaller than that of those grown in the V–S mode.