Metastable Metallic Superconductors with High Transition Temperatures and Composite Superconductors.

摘要

Chemical vapor deposition (CVD) was found to be a practical method for preparing large quantities of Nb-Ge. The fabrication of Nb-Ge multifilamentary and tape conductors by this method was shown to be feasible. CVD Nb-Ge had critical temperatures and upper critical fields comparable to the sputtered material. Critical-current densities were lower. Second-phase doping was shown effective toward improving current-carrying capacity. A new Nb-Ge growth method (reactive sputtering of niobium in Argon/Germane) was demonstrated and its applicability for use in a magnetron sputtering system evaluated. A study of the effect of impurities (oxygen, nitrogen and silicon) on sputtered Nb-Ge films indicated that each of these impurities can stabilize the high-critical temperature metastable A15 phase. An investigation of the bronze diffusion process illustrated why A15 Nb-Ge cannot readily be made by this method. The formation of A15 Nb-Ga by bronze diffusion was achieved. The preparation of superconducting niobium and A15 structure Nb-Sn was accomplished by liquid sodium reduction of the halides. The anomalously high upper critical field of sputtered NbN film was shown to be due to a surface critical field. (Author)