Growth mechanism of Ag-foil-based artificially superconducting joints of YBa 2 Cu 3 O 7 monoliths
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A new method was developed allowing large superconducting YBa2Cu3O7 (YBCO) monoliths having complex shapes to be obtained. This method consists of joining two or more YBCO monoliths, and it is based on the interfacial melting induced by metallic Ag thin foils inserted between YBCO pellets. Studies of the microstructure and the superconducting properties of the joints obtained by using this technology have shown that a perfect interface can be obtained without agglomerations of non-superconducting phases and with a critical current density as high as that of the original blocks. No evidence of Ag precipitates was detected either at the interface or into the YBCO solid matrix, suggesting a migration of Ag. For a better understanding of the interface growth mechanism, we studied the influence of the cooling rate. The knowledge on the Ag diffusion process has enabled us to propose a model for the growth mechanism of the YBCO/Ag/YBCO interfaces.
I. INTRODUCTION
The achievement of high critical current YBCO superconducting ceramics is a big issue for the development of practical power applications based on these materials. Melt processes have been found to be effective for obtaining highly aligned REBa2Cu3O7 (REBCO, where RE ⳱ rare earth) crystals and thus large critical current densities. Additionally, for practical applications, superconducting pieces with large dimensions and different geometries are necessary. Thus, much effort is being put into developing new methodologies allowing large superconducting pieces with complex shapes to be obtained. The idea of welding two or more YBCO superconducting oxides has been considered by several authors as a promising alternative to the production of large-scale monoliths with complex shapes.1–12 The methodologies described up to now have been based on the use of welding agents based on REBCO such as TmBCO, YbBCO, or ErBCO and on melt-textured YBCO/Ag composites. In general, the decomposition temperature of a solder material must be lower than that of the mother blocks. In addition, the superconducting properties should be similar or superior to those of the material to be joined. If the solder material has poor superconducting properties, supercurrent flow will be interrupted at the joined region.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0306 2534
J. Mater. Res., Vol. 21, No. 10, Oct 2006
In all cases, welding is achieved through a slow cooling process of the molten solder, which solidifies epitaxially on the adjacent REBCO single domains. Unfortunately, these welding techniques present some practical limitations. With low melting point REBCO oxides used as welding agents, some microstructural imperfections have been detected at the interface: some phase segregation, such as BaCuO2 and RE2BaCuO5 (RE211; i.e, Yb211, Tm211, Y211, or Er211) and residual porosity. Moreover, excessive mismatch between lattice parameters of the matrix and the joining material can lead to the formation of micro- and macrocrack
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