Sr1-xKxFe2As2超导材料的微结构调控与内耗谱研究

Sr1-xKxFe2As2 superconductor with very high upper critical field and low anisotropy has been regarded as a potential candidate material for high-field applications. Recently, the critical current density of the Sr1-xKxFe2As2 polycrystalline superconductor has reached higher than 105 A/cm2 at 4.2K and 10T, achieving the level for practical applications. However, the qualitative effect and regulatory mechanism of microstructural changes, such as grain growth, orientation and pinning defects, on the superconducting properties has been rarely researched. The sensitive instrument to study their kinetics and mechanism is also lacked. Thus, this project aims at control the microstructure of Sr1-xKxFe2As2 such as the gain size, orientation texture and microscopic defects through the complementary use of the high-energy ball milling, short-time hot pressing and nanoparticle doping technology. And the variation regulation of fabrication parameters and microstructural control on the inter-grain coupling, defect state, flux pinning and superconducting properties will be systematically investigated. Meanwhile, we also use the sensitive mean of internal friction measurement to characterize the internal friction and elastic modulus evolution of Sr1-xKxFe2As2. The kinetic parameters of the formation and evolution of the material defects will be obtained. Then combined with microscopic characterization and flux dynamic analysis, the intrinsic physical mechanism between the macroscopic physical properties and microstructure changes would be established, which could provide a scientific basis for construction of physical model and research of high Tc superconductivity mechanism.

Sr1-xKxFe2As2超导体具有优异的高场特性，在强磁场领域具有良好的应用前景。虽然目前Sr1-xKxFe2As2超导体的临界电流密度已经达到实用化水平，但是其微观结构变化如晶粒尺寸、排列取向和钉扎缺陷等对其超导性质的影响与调控机制仍然缺乏统一的理解和认识，亟需借助敏感手段来研究动力学过程和性能提高机制。为此本项目拟通过高能球磨、短时热压和纳米粒子掺杂相结合的方法调控Sr1-xKxFe2As2材料的晶粒尺寸、织构取向以及微观缺陷等微结构，揭示制备工艺参数和微结构调控对晶间耦合、缺陷状态、磁通钉扎能力以及超导性能的影响规律。并且通过内耗测量这一敏感手段来表征Sr1-xKxFe2As2的内耗和弹性模量演化过程，获得材料缺陷形成和演变的动力学参数，进而结合微观表征结果和磁通动力学分析，阐明样品宏观物性与微观结构变化之间的内在关联，为超导性能演变物理模型的构建和高温超导机理的研究提供参考。

122型铁基超导体具有优异的高场特性，在强磁场领域具有良好的应用前景；而高性能超导线带材的制备是铁基超导体走向实际工程应用的基础。我们采用热压法制备出高性能的Sr1-xKxFe2As2铁基超导带材，在10 T和4.2 K时的传输Jc达到~105 A/cm2；更有趣的是传输Jc存在各向异性反常的现象。接着系统分析了沿着B//c和B//ab双方向的带材电阻率、磁性参数、微观结构和微观缺陷。发现Sr1-xKxFe2As2带材具有优异的超导性能，其Tc高达36.2 K、Hc2大于70 T、同时钉扎势U0/kB对磁场的依赖性非常小，展示出在强磁场领域的巨大应用潜力。除了面钉扎外，观察到微观缺陷钉扎效应，从而产生较强的磁通钉扎力；磁化特征比Birr//c(0 K)/ Birr//ab(0 K)低至1.2，表明双方向的磁通钉扎力几乎接近各向同性。微观分析表明带材的反常钉扎行为起源于样品存在大量的微观尺度缺陷和沿两个方向的形貌分布差异性。这些结果表明可以通过调控微缺陷状态来调节铁基超导体的磁通钉扎特性，进而提高临界电流密度和抑制各向异性。另一方面，我们采用两步固相反应法制备Sr1-xKxFe2As2前驱粉末，随后采用该粉末制备出超导带材。研究表明两步固相反应法可以有效提高Sr1-xKxFe2As2前驱粉体的纯度和分散性，减小超导转变宽度，进而提高超导带材的载流性能。本课题的开展不仅对促进122型铁基超导材料的实际应用有重要的意义，还为进一步提高超导带材的传输Jc和减小各向异性提供了理论参考。