program abstracts
Saturday, 3G-06, 12:10-12:15
Critical Current Densities of Isolated Grains and Grain Boundaries in Coated Conductors
M. Weigand
J. H. Durrell, N. A. Rutter
Dept. of Materials Science and Metallurgy, University of Cambridge, Pembroke St, Cambridge CB2 3QZ, U.K.
Tel.: ++44 / (0)1223 767919, Fax: ++44 / (0)1223 334373, Email: mw404@cam.ac.uk
S. Speller, G. M. Hughes, C. Grovenor
Dept. of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.
In cuprate superconductors the critical current density Jc across grain boundaries (GBs) shows an exponential decrease with misorientation angle between the adjacent grains. Coated conductors (2nd generation high temperature superconductors) overcome this detrimental effect by epitaxial growth of the superconducting layer on a strongly textured substrate and buffer, thus offering a significantly higher critical current density compared to tapes of the 1st generation. As a consequence, in certain magnetic field and temperature ranges the grains, rather than the grain boundaries, limit Jc. The best method to ascertain what governs Jc is the electromagnetic characterisation of grains and grain boundaries isolated in coated conductor samples.
We have used Electron Backscatter Diffraction (EBSD) and a Focused Ion Beam microscope (FIB) to achieve this goal. This technique allowed us to isolate single grains and GBs on coated conductors manufactured by RABiTS (Rolling Assisted Bi-axially Textured Substrates). Our approach offers higher resolution and enables us to target specific areas of the samples more precisely. Consequently, unlike photolithographic techniques such as the one developed by Feldmann et al., it can be extended to systems with smaller grain sizes, down to about 1 μm.
Critical current densities were obtained by a four-point measurement for magnetic fields applied both perpendicular to the film surface (H||c) and swept in-plane (H||ab). A single GB of the RABiTS conductor showed a hysteretic dependence of Jc on field for H||c, due to flux trapped in the grains. For fields in-plane the critical current density was limited either by the grains or the GBs, depending on the angle between applied field and macroscopic current direction. We explain this result by the advantageous properties of meandering GBs, as are found in samples produced by chemical routes. Our findings should allow manufacturers to assess how growth parameters affect properties of grains and grain boundaries, enabling them to optimise their production processes to obtain higher critical currents.
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