Preparation and High Field Properties of Superconducting NbN Tapes

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In order to evaluate the high field properties of NbN with B1 structure thin films of NbN have been investigated on their superconducting critical temperature T_C, upper critical field H_iC and field dependent critical current density J_C. NbN has been formed on quartz and sapphire substrates heated up to a temperature between 200 and 700゜C in an argon-nitrogen atmosphere by rf sputtering from a Nb disk target. The thickness of obtained films ranges from 1500 to 6000<Ang>. The T_C of films is strongly influenced by one of deposition parameters, the nitrogen partial pressure P_<N2>. The maximum T_C exceeding 16K is obtained around P_<N2>=4mTorr. Most films deposited at P_<N2> higher than 4mTorr show a single B1 phase and exhibit a strong anisotropy of H_<C2>. Upper critical fields perpendicular to the film plane (H_<C2> ) are larger than those parallel(H_<C2> ). Films show an increasing tendency of H_<C2> with <rho>n(normal state resistivity). High values of H_<C2> are obtained in films with <rho>n above 500<micrn><OMEGA>cm and with a T_iC of 14-15K. NbN films with <rho>n's of 500-1000<micrn><OMEGA>cm show high J_C in fields above 10Tesla. Under the optimum deposition conditions above obtained NbN tapes with metal substrates of Hatelloy-B and SUS304 have been produced. These tapes show H_<C2>'s as high as 25Tesla and exhibit no dependence of J_C on strain up to above 1%.