Partners and International Organizations
(French)
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Universität Göttingen (D) , Université de Madrid, (E), INPG, Grenoble (F), Imperial College, London (GB), Schneider (F) , ABB (CH), Solvay, Hannover (D)
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Abstract
(French)
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The main task of the Geneva group within the SUPERTEXT Project (in the frame of the Brite-EuRam III Program) was to develop biaxially textured, mechanically reinforced Ag substrates as a substrate material for the deposition of Y(123) superconducting layers with high critical current densities. A way was found how to fabricate Ag substrates with the appropriate microstructure and texture. Not only the texture had to be obtained, but it had to survive deposition and subsequent processing at rather high temperatures (850°C). The appropriate texture, {110}<011> or {110}<112>, was achieved by mechanical deformation followed by annealing; excellent degrees of textures have been obtained. Ag itself is too weak, especially after the heat treatment, and had to be reinforced. We have explored two approaches: a: 1% Mg was added to the Ag ribbons produced by cold-rolling and annealing. After recrystallization at 800°C, a weak {110}<112> texture was observed, and the microhardness of the textured ribbons was increased by a factor of 4. The annealing in flowing oxygen produced MgO particles of about 1-2 mm, which induced a dispersion strength. The {110}<112> texture was maintained. An improved texture was obtained with a pure Ag foil bonded to the Ag0.99Mg0.01 layer by a thin Cu foil: the pure Ag gives the texture, and the Ag0.99Mg0.01 contributes to the strength. However, both ways had a very stringent limitation: indeed, it was not possible to maintain the biaxial texture in Ag ribbons of thicknesses below 250 mm thickness. This problem was solved by a second, original approach: b: The Ag ribbon is reinforced by forming a Ag-Ni composite ribbon, the bonding between these two elements being ensured by a very thin Cu coated layer. Indeed, Cu forms an eutectic with Ag at 780°C, while the bonding with Ni is obtained by Cu-Ni diffusion. The bonding was of strong enough to allow deformation of the whole composite by rolling. This procedure has led for the first time to high strength ribbons with a promising {110}<011> texture. A somewhat unexpected, but very important result is that biaxial texturing in these reinforced Ag-Ni ribbons can be obtained for thicknesses being one order of magnitude lower than for unreinforced Ag ribbons: Biaxially textured Ag-Ni ribbons with overall thickness of 30 mm were prepared, the Ag layer being less than 10 mm thick. The Ag content in the reinforced ribbon is as low as 30%, which is an important economical factor for future industrial applications As a conclusion, we have succeeded in fabricating reinforced, {110}<011> biaxially textured Ag-Ni ribbons by a simple method allowing to envisage the production of long lengths, in view of the production of Y(123)/Ag coated conductors. Ag substrates do not require the use of buffer layers, in contrast to all other known substrate materials. The present method allows to replace up to 70% of Ag by Ni and is thus extremely cost effective. . The next step will be to enhance the degree of texturing, i.e to reduce the misalignment angle in the plane from 10 to 4 degrees, in order to compete with buffered Ni substrates.
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