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Phase-pure, large grained Nb/sub 3/Sn films on sapphire substrates have been prepared by a two-step process. The average grain size increased with the film thickness. Transport properties like the penetration depth, mean free path, and critical current density have been investigated in relation to the microstructure of the films. Measurements of the DC- and HF-magnetic field dependent surface impedance were performed. Nonlinear surface resistance occurred at field levels above B/sub s/=25 mT, and was related to extrinsic mechanisms. The relevance of weak coupling at grain boundaries decreased with increasing grain size. Thus heating at local defects was concluded to be the dominant limiting mechanism in large grained films. An intrinsic field limit of B/sub cl/=140 mT was found. The polycrystalline films showed a much lower residual resistance and comparable power handling compared to high-quality epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//-films.
Date of Publication: June 1999