Coated Conductors

We address the key underlying scientific and engineering issues of YBa₂Cu₃O_7-x Coated Conductors (CCs). We characterize forefront samples using magneto-optical imaging, SEM and J_c(H) measurements for the purpose of understanding and resolving key performance issues. Areas of concentration for us include substrates, buffer layers, the superconducting over layer and their complex interactions. CCs are polycrystalline by nature, consisting of multiple GBs and intragrain regions, and we also seek to understand the consequences of this granularity.

Macroscale

Magneto-optical image of RABiTS-type CC. The CC was cooled in the absence of a magnetic field to 77K, and then a field of 60mT was applied.

Current does not flow through a CC uniformly, like in a single crystal, but instead percolates due to local variations in J_c. These J_c variations are (among other things) a result of the multiple GBs and intra-grain regions in the conductor. This is illustrated at right, where a granular flux pattern can be seen in a YBa₂Cu₃O_7-x RABiTS-type CC. Our previous work has shown that this grain structure is replicated directly from the underlying substrate. Most GBs introduced by the substrate have a J_c less than that of the intragrain, and flux flows more easily along these boundaries.

Current percolation may also be observed under an applied transport current.

Single Grain/Single Boundary Scale

It is also very useful to have knowledge of J_c and current flow at the single grain/single boundary level. Most of our understanding of thin film YBa₂Cu₃O_7-x comes from films grown on STO single crystal or bicrystal substrates. In RABiTS-type CCs, each substrate grain acts as its own single crystal template, all with different vicinal angles and potentially different surface qualities. GBs in CC substrates are generally neither pure tilt nor pure twist (unlike most bicrystals). It is useful to know what effect these differences have on the YBa₂Cu₃O_7-x.

Using optical photolithography, transport links have been placed across individual GBs and within individual grains in RABiTS-type CCs. This allows us to directly measure the J_c(H) properties of the intra-grain and GBs regions. These transport links are very small (~30μm long) and voltage-current measurements with sub-nanovolt voltage resolution ensure that J_c is evaluated at the 1uV/cm criterion.

Recent Publications

High-T_c superconducting materials for electric power applications
This Nature Insight article reviews the choice of superconducting materials for electric power applications with the emphasis on key mechanisms, which limit current-carrying capability of HTS conductors. Advantages and disadvantages of YBCO coated conductors, Bi-2223 multifilamentary composites and the newly discovered MgB2 are discussed and compared with those of low-Tc superconductors like NbTi and A-15 compounds.

Thermal instability near planar defects in superconductors
Local Joule heating due to planar defects can cause thermal instabilities, which limit current-carrying capability of YBCO coated conductors. Based on exact solutions that describe distributions of electric field in superconductors, instability criteria are obtained for a planar defect in a film and for a grain boundary. Instabilities can be triggered by low-angle grain boundaries or small planar defects, which block only a small fraction of the sample cross-section. Hot spots near defects and overheating of grain boundaries can result in significant variations of critical currents along HTS coated conductors.

Inter- and intragrain transport measurements in YBa₂Cu₃O_7-xdeformation
In this paper, current flow is examined at the level of the substrate grain, ~20-100 µm. Transport measurements of the YBCO were made within single substrate grains and across single substrate grain boundaries. Results show current percolation, intra-grain Jc values exceeding 5 MA/cm2, and a Jc dependence consistent with thin film bicrystal studies.

Nonlinear current flow in superconductors with restricted geometries
We obtained exact solutions for distributions of transport current and electric field near planar defects in superconductors of different geometries, taking into account the highly nonlinear E-J characteristics. These solutions reveal novel features of nonlinear percolative current flow, such as current domain walls of variable width and extended domains of strong electric field near defects. It is shown that even small defects can effectively block current flow, causing highly inhomogeneous distribution of E(x,y) and localized hotspots, which significantly limit the current-carrying capability of HTS polycrystals.

For more information contact David Larbalestier at larbalestier@asc.magnet.fsu.edu.