提高SmFeAsO1-xFx超导体晶间连接性的研究

SmFeAsO1-xFx possesses the highest superconducting transition temperature (Tc) among the iron-based superconductors, with a very high upper critical field, critical current density (Jc) and low anisotropy. Especially, the weak linking at the grain boundary for iron-based superconductors is visibly small than that in cuprate superconductors. A transport Jc as large as 22 kA/cm2 at 4.2 K and self field has been achieved in Sn-added SmFeAsO1-xFx specimens by the applicant, indicating a potential application in the future. However, there are still some problems, such as serious fluorine losses, many impurity phases in grain boundaries, relationship between intergranular connectivity and grain size, and how to enhance intergranular coupling by Sn addition, in preparing high-performance SmFeAsO1-xFx superconductors.. This study aims to resolve the weak-link problems in SmFeAsO1-xFx superconductors through novel methods such as high-energy ball milling, microwave synthesis, and adding Sn element. In order to improve intergranular connectivity and enhance flux pinning force, we will prepare samples possessing very clean and high density of grain boundaries by refining grain size and enhancing phase purity. Furthermore, we will illuminate the influencing factors of intergranular connectivity by studying the relationship between the macroscopic superconducting properties and microstructures of SmFeAsO1-xFx superconductors. At last, the superconducting properties of SmFeAsO1-xFx will be further improved based on the optimized process.

SmFeAsO1-xFx是已知的临界转变温度最高的铁基超导体，其具有临界电流密度大、上临界场高、各向异性小等优点，且研究表明它的晶间弱连接性远小于铜氧化物超导体。目前申请者通过锡掺杂将SmFeAsO1-xFx在自场下的传输临界电流密度提高到22 kA/cm2以上，展现出较好的应用前景；然而SmFeAsO1-xFx材料中仍然存在着氟元素难以控制、样品中含有较多杂相以及锡掺杂是如何强化晶间耦合等诸多问题，必须进行更为深入的研究。本项目旨在通过高能球磨、微波合成等工艺，达到有效控制样品中氟元素含量、提高样品相纯度的目的，进而制备出不同晶粒大小SmFeAsO1-xFx前驱体，在此基础上利用先位法进行掺杂改性，系统研究晶界杂相、晶粒尺寸、金属锡掺杂等因素对其晶间连接性影响的内在规律，以及样品宏观超导性能和微观结构之间的联系，并在此基础上获得性能优良SmFeAsO1-xFx超导材料。

铁基超导体具有较高临界转变温度、临界电流密度和非常高的上临界场，因此在高场和强电流密度方面有着重要的应用前景。本项目针对SmFeAsO1-xFx超导体中存在的弱连接问题展开研究，我们首先优化了SmFeAsO1-xFx超导体的制备工艺，研究了F元素添加量、烧结温度、保温时间等参数对其超导电性的影响，得到了制备SmFeAsO1-xFx先驱粉的最佳工艺。在此基础上研究了Sn元素掺杂对SmFeAsO1-xFx超导性能的影响，发现Sn元素可以大量的扩散到铁包套材料中，因此我们推断Sn元素可能与包套材料形成固溶体，因此可减少F元素的扩散损失，从而使其超导性能获得提高。此外，我们还制备了Li1-xFexOHFeSe单晶，并系统的研究了其超导电性能。发现Li1-xFexOHFeSe其Tc可达42K，在2K自场条件下其Jc可达2.5×105 A/cm2，其具有非常高的上临界场和大的各向异性，并且其各向异性与温度呈现出线性相关性，根据涡旋玻璃理论与热激活磁通蠕动理论，计算得到(Li1–xFex)OHFeSe单晶的混合态涡旋相图，发现其具有较宽的涡旋液态区。动态磁弛豫测量计算结果表明磁弛豫率较大，并随着磁场和温度的升高而单调增大，这主要源于弱钉扎造成的磁通热激活。