The characterization effort within the STC supports the development of HTS composite conductors and new materials that have application within the context of the conductor program. The types of characterization methods used are scanning and transmission electron microscopy (SEM and TEM), x-ray diffraction (XRD), and automated electron backscattered diffraction in the SEM (EBSD). The types of conductors investigated include oxide-powder-in-tube (OPIT) tapes and wires which are used with the Bi(Pb)-Sr-Ca-Cu-O superconductors, and coated conductors (CC) which incorporate YBa2Cu3Oy (YBCO) as a coating on a strong and flexible substrate. The general areas of interest with respect to the characterization efforts are described below.
The superconducting materials are brittle ceramics and are not readily formed into monolithic, practical conductors. Therefore, the HTS materials are formed into composites in which the sheath or substrate provides the needed strength and flexibility. However, the types of materials that are compatible with HTS are limited and our characterization tools are used to determine the type and extent of such problems. The data gathered from these studies is then used for improving processing methods that overcome compatibility problems. Figure 1 is an example from TEM studies of the interfacial reactions that occur between a YBCO film and an underlying ceria buffer layer. The TEM work showed that interfacial reactions are greatly reduced and transport properties are optimized when the ceria buffer layer thickness is reduced below 300 angstroms.
Grain-to-grain alignment within the superconductor is an important consideration when trying to optimize the transport properties of the conductors. Quantification of the grain alignment within the conductor coupled with measurements of the superconducting transport is an important task within the characterization team. BI-axial texture is a necessary condition when assembling a coated conductor based on YBCO. Conditions in the BSCCO system are somewhat less demanding for producing high critical current conductors. In either case, it is important to understand the grain-to-grain alignment, quantify it, and then relate it to the transport properties. Examples of the types of measurements performed are shown in Figure 2 shows where EBSD has been to examine the local misorientations between colonies in a BSCCO OPIT tape used at the microscopic level. Conversely, as shown in Figure 2b, XRD pole figure analysis is used to quantify on a macroscopic scale the alignment of the superconducting material over a much larger area of the composite.
Figure 2 (right): An example of microscopic alignment measurements is shown in (a) where the apparent misorientations about the c-axis between adjoining grains were measured with EBSD. On a macroscopic scale, the XRD pole figures in (b) show that the misorientations about the c-axis within the tape are isotropic. There does not appear to be any bi-axial alignment of the superconductor in the Bi-2223 tapes.
Defects play an important role in determining the properties of HTS conductors. Macro defects, such as second phases, porosity, or cracks, may adversely affect the magnitude of the self-field transport critical currents. Conversely, microstructural defects such as nano-particles, dislocations, stacking faults, or columnar defects can be beneficial to both the self-field and in-field transport critical currents. Nanometer sized defects within the superconductor can serve as flux pinning centers. Some of these defects can pin the motion of vortices within the superconductor. This in turn increases the level at which disipative forces arise and allows the superconductor to carry more critical current. Examples of microscopic defects can be found in Figure 3.
Figure 3 (left): Examples of defects in the HTS conductors. In (a), intergrowths of related phases could be found in the YBCO grains. In (b), columnar defects can be seen in a BSCCO HTS conductor after irradiation with high-energy protons. The protons cause some of the bismuth atoms to fission. The daughter products are ejected in opposite directions and damage the material to form the columnar defects, which are highly efficient flux pinning defects.
The complex chemistry of the HTS materials is readily evident in just their chemical formulas. Local and global changes in the composition are likely to have significant impacts on the phase formation and phase assemblage. Likewise, it is important to be able to correlate composition with specific microstructural defects or structures within the conductors. The electron probes in the SEM, and especially in the TEM, are used for retrieving microscopic or local compositional information from the superconducting phase, around grain boundaries and from defects. The compositions or changes in compositions can be measured or inferred from a variety of techniques. Energy dispersive spectroscopy (EDS) or electron energy loss spectroscopy (EELS) can be used to directly probe the compositions of the material in question. Other techniques such as backscattered electron imaging (BEI) can be used to infer changes in the composition. A BEI micrograph is shown in Figure 4.
Figure 4 (right): Backscattered electron image of an Y-123 / Sm-123 multilayer film. The Sm-123 films appear as the bright bands in the micrograph due to the fact that the Sm atoms have a higher atomic number than the Y atoms.
The complexity of HTS materials requires continuing research into their fundamental properties and continuous evaluation of new materials for improved HTS composite conductors. Some of the work includes the synthesis, measurement of basic structural information such as lattice parameters or space group, phase diagram studies, and microstructural evaluations to look at structural uniformity, chemical uniformity, and compatibility issues. All of these areas rely heavily on the characterization tools discussed above.
HTS composite conductors are some of the most complex materials developed for practical applications. The enormous number of potential variables in processing, composition, structure, and design can be a roadblock to development if one cannot separate the effects of changes in these variables upon the superconducting properties of the conductors. The characterization tools employed allow us to quantify the structure and chemistry of the HTS conductors at the centimeter length scale down to the nanometer length scale. Coupled with the superconducting properties, we can determine the structure-composition-property relationships in the HTS conductors. Likewise, the structure-composition-processing relationships are determined when the processing history is correlated with the characterization results. The data collected from the various characterization tools has allowed researchers at Los Alamos, collaborators at other national laboratories and universities, and our CRADA partners to optimize the processing and properties of their HTS conductors in a fast, efficient, and cost-effective manner.
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