铁硒基超导线材的制备及其输运性能优化研究

Since the first report of the superconductivity of LaFeAsOF by Hosono et al in 2008, the preparation and researches of Fe based superconductors have attracted great attentions. Within all the Fe based superconductors, iron selenide has the simplist structures and promising superconducting properties, such as low anisotropy factor (2~3), relatively high upper critical field (Hc2~50 T), applicable critical current density (10^5 Acm-2@4.2 K) and slow decrease of Jc under high magnetic filed. Meanwhile, the low toxicity, rich reserves and less sensitivity to atomosphere of starting materials guaranteed the low fabrication cost. Therefore, FeSe based superconducting wires show great potentials in the practical applications. Meanwhile, the newly discovered iron selenide of KxFe2Se2 offered more opportunities for practical applications with higher critical temperature of over 33 K. However, the study of iron selenide wires fabrication is still under the beginning level, with the weak intergrain connections greatly restrict the improvement of the transport properties. In our study, different densification process, including hot extrusion and high pressure heat treatment, will be adopted for the optimization of both FeSe and KxFe2Se2 superconducting wires based on powder in tube process. The relationship among structure, composition and superconducting properties will be systematically discussed. The kinetics model of the phase transition between hexagonal and tetragonal FeSe phase will be built to analyse the influences of different heat treatment conditions. During the optimization process, the chemical composition and microstructures of grain boundaries will be examined to obtain the type and dimension of grain boundaries and discuss the influences of these grain boundaries on the current transport process. The feasibility of grain boundary engineering on tuning the chemical composition and function of grain boundaries will be discussed in order to obtain a new type of grain boundaries which can both offer carrier concentrations during current transport and act as flux pinning centers. The study of this project will not only obtain high performance superconducting wires for practical applications, but also built the theoretical and experimental basis for the further developments of other Fe based superconducting wires.

铁基超导体体系众多，其中铁硒基超导体具有最简单的晶格结构，并且其各向异性小，上临界场高，载流性能在磁场中衰减较慢。同时，原料毒性小，储量丰富，因此具有一定的实用化潜力。特别是新型超导体KxFe2Se2的发现，提高了铁硒基超导体的临界温度，为铁硒基线材的发展提供了新的机遇。目前实用化铁硒基线材的制备尚处于起步阶段，晶间弱连接是制约线材载流性能的重要因素。本项目将一系列致密化方法优化FeSe和KxFe2Se2超导线材制备工艺，建立两个体系中密度等结构因素、元素比例等成分因素与涡旋钉扎性能和输运性能之间的对应关系，并通过建立相演变模型，阐明相变过程中的位相关系、成分变化等，并阐述热处理条件对超导相相变动力学的影响，分析芯丝的晶界类型、晶界尺度及其对载流性能的影响机理，探讨晶界改性的可行性，从而提高线材载流性能。本项目的研究将为实用化铁硒基超导线材的开发和应用奠定坚实的理论和实验基础。

本课题以FeSe基超导块体和超导线带材为研究对象，通过对Fe-Se二元体系及Fe-Se-Te三元体系在烧结过程中的相演变机理进行分析，开发出高能球磨辅助烧结工艺，从而获得了超导相含量的显著提高。同时通过调整球磨和烧结过程中的重点工艺参数，对该工艺进行了系统的优化。在此基础上，通过对FeSe块体进行元素掺杂，对FeSe体系的晶格结构、相组成、微结构、超导性能等重点参数进行调控，获得了体系超导性能的进一步优化，并对不同元素的掺杂机理进行了分析。最终结合块体制备工艺和粉末装管工艺，开发出高能球磨辅助装管法，结合具体工艺参数的优化，选择和适当的包套材料和前驱体粉末，从而制备出具有一定载流性能的FeSe超导线材。其传输法的载流性能在4.2 K，自场条件下达到350 Acm-2，略高于其他课题组报道的结果。另一方面，在课题研究过程中还分别开发出用于制备FeSe纳米粉体的低温溶剂法和用于制备FeSe薄膜的化学气相输运法。在Fe(Se,Te)三元体系的制备过程中，同样引入了高能球磨工艺，并对该工艺过程中的元素比例、球磨成分和烧结工艺进行了系统的优化，最终也获得了体系超导性能的显著提升。