DEVELOPMENT OF HIGH FIELD MgB_2 SUPERCONDUCTOR BY USING NON-EQUILIBRIUM REACTION

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For the practical applications of MgB_2 superconductor, which has very high T_c of 39K, it is necessary to improve the characteristics in applied magnetic field. This would be achieved by the improvement of present fabrication processes based on nanometer size structural control or the development of new processes based on different idea. The purpose of this work is to establish the basis for the improvement of properties by applying non-equilibrium processes such as mechanical milling and laser deposition technique. For the milling process, we investigated systematically the influence of starting powder, atmosphere, milling condition, heat treatment condition and nanometer-size particles addition. By optimizing these parameters, the J_c dependence on magnetic field was improved probably due to the introduction of strain into the grain. We also fabricated tapes by PIT process in which the milled powders were encased in a metal sheath and cold worked into a final size. For laser deposition, we developed a new pulsed vapor deposition system using Nd-YAG laser which might have several advantages over conventional excimer laser ; low cost, easy operation, high stability and reduced space. The MgB_2 film synthesized by using this system showed excellent H_<c2>(0) value of 41T, which might be caused by non-equilibrium state of excess C-doping into B site. In this work, we also developed a new process for the fabrication of MgB_2 wire based on interface diffusion in the composite of Fe-Mg alloy and B layers. The MgB_2 layer has dense structure free from MgO inclusion, which is difficult to achieve by the conventional PIT process. We also discovered 8K superconductivity in Li_2Pd_3B which is the first boride superconductor with Li as a constituent.