program abstracts
Friday, 2D-01, 3:10-3:20
Enhancement of Activity of Technical Oxide Buffers as a Pathway for Higher Performance of 2G Wires
Slowa Solovyov
Brookhaven National Laboratory, Upton, NY 11973
Tel: (631) 344-5437, FAX: (631) 344-4071, e-mail:solov@bnl.gov,
The second generation coated conductors are oriented structures which rely on epitaxial to achieve desirable texture. The substrate has a dual role: that of a template and a catalyst. The first role of a substrate is to provide desired orientation for the epitaxial layer. The substrate texture has been significantly improved in the last decade, probably reaching its limit. We will discuss the one important property of a substrate, its catalytic activity, which can be defined as an ability of a substrate to provide low energy nucleation sites.
The importance of the substrate activity becomes important as long-scale buffered tapes are made on increasingly faster rate to reduce cost of the 2G conductors. While the substrate orientation stays the same, the buffer activity is declining. The low activity hurts the conductor performance because secondary phases are allowed to nucleate along with YBCO. The secondary phase formation becomes especially acute as the YBCO layer thickness is increased. In YBCO layers deposited by meta-organic deposition the low buffer activity results in granularity. In granular samples YBCO nuclei are far apart and the YBCO grain has to grow laterally to make a continuous film. We demonstrate by using transmission electron microscopy how the non-superconducting secondary phases form on the grain periphery and block the super-current, thus reducing performance of the conductor.
In the case of metal-organic chemical vapor deposition the low substrate activity seems to allow nucleation of the copper-rich secondary phases along with YBCO. The grains of copper-rich phases grow faster than YBCO as the film gets thicker, eventually taking over high portion of the film cross-section as the film becomes thicker than 2 microns.
We discuss possible strategies for the buffer activities enhancement on example of 20 nm CeO2 film on buffers. It is concluded that well-defined, dislocation free (001) terminations are essential to achieve high activity in oxide buffers.
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