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

MgB_2超导体具有很高的T_c（39 K），为了使其应用于实际，必须改善其外磁场特性。这将通过基于纳米尺寸结构控制的现有制造工艺的改进或基于不同理念的新工艺的开发来实现。本工作的目的是为应用非平衡工艺如机械铣削和激光沉积技术来改善性能奠定基础。在球磨过程中，我们系统地研究了原料、气氛、球磨条件、热处理条件以及纳米颗粒添加量对球磨过程的影响。通过优化这些参数，改善了J_c对磁场的依赖性，这可能是由于在晶粒中引入了应变。我们还通过PIT工艺制造了带材，其中将研磨的粉末包裹在金属护套中并冷加工成最终尺寸。对于激光沉积，我们开发了一种新的脉冲气相沉积系统，使用Nd-YAG激光器，它可能有几个优点比传统的准分子激光器：成本低，操作简单，高稳定性和减少空间。采用该体系制备的MgB_2薄膜具有<c2>41 T的优良H_0值，这可能是由于B位过量C掺杂的非平衡态所致。本文还提出了一种基于Fe-Mg合金与B层复合界面扩散的MgB_2丝制备新工艺。MgB_2层具有致密的结构，没有MgO夹杂物，这是常规PIT工艺难以实现的。我们还在第一个以Li为组分的硼化物超导体Li_2Pd_3B中发现了8 K超导电性。

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.