The LHC superconducting cables are submitted to a 200°C heat-treatment in air in order to increase the resistance between the crossing strands (R C ) within the cable. During this treatment the as-applied Sn-Ag alloy strand coating is transformed into a CuSn intermetallic compound layer. The microstructure, the surface topography and the surface chemistry of the non-reacted and reacted coatings have been characterised by different techniques, notably focused ion beam (FIB), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Based on the results obtained by these techniques the different influences that the intermetallics have on R C are discussed. The desired R C is obtained only when a continuous Cu 3 Sn layer is formed, i.e. a sufficient wetting of the Cu substrate by the tinning alloy is crucial. Among other effects the formation of the comparatively hard intermetallics roughens the surface and, thus, reduces the true contact area and it strongly affects the oxide growth on the strand surface. The oxide formed on the fully reacted coatings, which may contain essentially Cu-oxides, appears to be more stable, both mechanically and thermally, as compared to the oxide formed on the tinning alloy.
SYNOPSISThe role of steric effects in the adsorption of chemically similar acidic probes to the model basic surface of poly (methyl methacrylate) (PMMA) was examined using the technique of inverse gas chromatography (IGC). The results were expressed in terms of the acidbase contribution to the Gibbs free-energy change upon adsorption. The acid-base component of the adsorption free energy, computed on the basis of theoretical modes of adduct formation, was found to depend strongly on the molecular structure of the adsorbate, suggesting the importance of steric effects.
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