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

MgB_2超导体T_c高达39K，为了使其实际应用，有必要改善其在外加磁场中的特性。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思想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材料的性能奠定基础。在制粉过程中，系统地考察了起始粉、气氛、制粉条件、热处理条件和纳米级颗粒添加量对制粉效果的影响。通过优化这些参数，J_c对磁场的依赖性得到了改善，这可能是由于晶粒中引入了应变。我们还通过PIT工艺制作磁带，其中研磨粉末被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，可能比传统的准分子激光有几个优点；成本低，操作方便，稳定性高，占用空间小。用该体系合成的MgB_2薄膜显示出良好的H_<c2>(0)值为41T，这可能是过量c掺杂到B位的非平衡状态所致。本文还开发了一种基于Fe-Mg合金与B层复合材料界面扩散制备MgB_2线的新工艺。MgB_2层结构致密，不含MgO夹杂物，这是传统PIT工艺难以达到的。我们还发现了Li_2Pd_3B的8K超导性，这是第一个以Li为成分的硼化物超导体。

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.