GdBa₂Cu₃O_7-δ高温超导体微结构与临界电流密度研究

【作者】 张红；

【导师】 李晓光；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2007， 博士

【摘要】 利用元素替代引入磁通钉扎中心是简单有效的方法，适合于实际应用。GdBa₂Cu₃O_7-δ（Gd123）超导体中由于Gd³⁺替代Ba²⁺形成的固溶体在磁场中可以作为有效的钉扎中心，从而显示了比YBa₂Cu₃O_7-δ超导体更高的捕获场和临界电流密度，并且Gd123单畴块材超导体的制备比YBa₂Cu₃O_7-δ相对简单，因此成为近代超导块材应用的主要候选者。研究Gd123超导体的微结构及其超导性能对优化其应用性能有着重要的指导意义。同时中子辐照方法可以人为的控制引入的钉扎中心的大小和密度，藉此可以研究高温超导体的钉扎磁通机制。因此我们首先使用粉末熔化生长法（PMP）制备了一系列的Gd123单畴块材超导体，接着研究了Gd123超导体单畴块材的微结构特性、磁通动力学和临界电流密度，以及中子辐照对Gd123超导体单畴块材的微结构和临界电流密度的影响。第一章：介绍了提高高温超导体临界电流密度的两种主要方法，即元素替代和辐照方法。重点介绍了REBa₂Cu₃O_7-δ（RE=Nd，Eu，Sm，Gd）超导体的RE³⁺对Ba²⁺替代形成的固溶体的形貌及其产生的特殊的磁通钉扎特点。另外介绍了辐照的主要反应方式，以及辐照对高温超导体的微结构和临界电流密度的影响。第二章：使用PMP法制备了不同名义组份的Gd_1+2xBa_2+xCu_3+xO_7-δ（x=0，0.1，0.2，0.3，0.4）单畴超导体块材，观察了其微结构特性以及Gd₂BaCuO₅（Gd211）相的添加量（即x值）对形成Gd_1+yBa_2-yCu₃O_7-δ固溶体的影响。块材的晶格c轴参数随x的增大几乎线形下降，表明即使添加40％Gd211也没有使样品中Gd³⁺对Ba²⁺的替代达到饱和。第三章：对不同名义组份的Gd_1+2xBa_2+xCu_3+xO_7-δ（x=0，0.1，0.2，0.3，0.4）单畴超导体块材进行了磁测量和输运测量，这种块材中存在Gd123(包含Gd_1+yBa_2+yCu₃O_7-δ固溶体)和Gd211相。Gd211相粒子贡献于低场下的磁通钉扎，因此常在制备过程中有目的的添加一些Gd211粒子。然而Gd³⁺对Ba²⁺替代形成的固溶体提供了高场下的钉扎中心。我们发现当x=0.2时Gd_1.4Ba_2.2Cu_3.2O_7-δ块材具有最高的捕获场。所有样品临界电流密度随磁场变化的曲线都出现显著的鱼尾峰，其中Gd_1.4Ba_2.2Cu_3.2O_7-δ样品具有最高的鱼尾峰值场。电阻随温度降低的曲线出现了第二个超导转变，这是由于弱超导性的Gd_1+yBa_2-yCu₃O_7-δ固溶体引起的。这些固溶体分散在Gd123基体中，在高的磁场中贡献于δT_c磁通钉扎，提高了高场下的临界电流密度，因此导致了鱼尾峰效应。x=0.2时的样品具有最佳的磁通钉扎特性。第四章：观察了中子辐照对名义组份Gd_1.4Ba_2.2Cu_3.2O_7-δ和Gd_1.8Ba_2.4Cu_3.4O_7-δ单畴块材超导体微结构及临界电流密度的影响。使用相同剂量的中子辐照不同Gd含量的超导样品，发现由于Gd_1.8Ba_2.4Cu_3.4O_7-δ高的Gd含量导致了严重的中子辐照损伤及相分解，证明了中子辐照过程中Gd与中子的反应发挥着重要的影响。辐照的Gd_1.4Ba_2.2Cu_3.2O_7-δ样品超导性能的严重退化是由于发生了相分解及大的微结构缺陷，即使经再退火其超导性能也不能恢复到辐照前的状态。第五章：研究了中子辐照对名义组份GdBa₂Cu₃O_7-δ单畴块材超导体的微结构及临界电流密度的影响。发现中子辐照抑制了磁滞回线中的鱼尾效应，然而在退火后显示了高的鱼尾峰值，且超过了辐照前样品的鱼尾峰值，说明辐照后退火使样品中产生了新的钉扎中心，提高了样品的磁通钉扎能力。更多还原

【Abstract】 In the viewpoint of practical application, element substitution is a simple approach to introduce the flux pinning centers. For GdBa₂Cu₃O_7-δ （Gd123） superconductor, the Gd_1+yBa_2-yCu₃O_7-δ solid solution due to the substitution of Ba²⁺ by Gd³⁺ can act as strong flux pinning centers and are highly effective in increasing the flux pinning capability. And consequently Gd123 superconductor exhibits higher trapped fields and higher critical current density compared with those of YBa₂Cu₃O_7-δ superconductor. Moreover, the preparation process for Gd123 superconductor is simpler than that for YBa₂Cu₃O_7-δ superconductor. So Gd123 superconductor is very attractive for various industrial applications.In addition, neutron irradiation is a controlled way for the introduction of flux pinning centers and provides a feasible route to investigate the flux pinning mechanisms in superconductor. In this dissertation, firstly, we have grown Gd123 single domain bulks by a powder melt process （PMP）. Secondly we investigated their microstructure, flux pinning properties and critical current density with the aim of optimizing the applied properties. Finally, we studied the effect of neutron irradiation on microstructure and critical current density of Gd123 superconductor.In chapter one, two methods, element substitution and neutron irradiation, for the enhancement of critical current density were introduced. The microstructure of the solid solution by RE （RE = Nd, Eu, Sm, Gd） substitution for Ba in the REBa₂Cu₃O_7-δ superconductor and the flux pinning characteristic of the REBa₂Cu₃O_7-δ superconductor were summarized. The main interactions between neutron and the atoms of the high T_c superconductor （HTS） and the effects of the neutron irradiation on the microstructure and the critical current density of the HTS were also introduced.In chapter two, a series of the nominal Gd_1+2xBa_2+xCu_3+xO_7-δ （x = 0, 0.1, 0.2, 0.3, 0.4） single domain superconductors were synthesized by PMP. The microstructures of the single domains were observed and the effects of the Gd211 addition on the Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions were investigated. The steady reduction of c of single domain bulks with increasing x means that the occupation of Gd³⁺ ions in Ba²⁺ sites dose not reach the maximum amount even Gd211 addition high up to 40 mol%, i.e. x = 0.4.In chapter three, the superconducting properties of the nominal Gd_1+2x,Ba_2+xCu_3+xO_7-δ （x = 0, 0.1, 0.2, 0.3, 0.4） single domain bulks which consist of Gd123, Gd₂BaCuO₅ （Gd211） and Gd_1+y,Ba_2-yCu₃O_7-δ, have been investigated through magnetization and resistivity measurements. The Gd211 particles in the bulk sample can pin simultaneously several vortices and usually are added to increase pinning in low magnetic fields. However, the Gd_1+y,Ba_2-yCu₃O_7-δ solid solution due to the substitution of Ba²⁺ by Gd³⁺ can act as strong flux pinning centers and are highly effective in increasing the flux pinning capability in high magnetic fields. With increasing Gd211 content x, the trapped magnetic field reaches a maximum for the sample x = 0.2. The second peak in the critical current density J_c vs magnetic field B curve shifts toward high magnetic regime for the sample x = 0.2 compared with that for the sample x = 0, but decreases to low magnetic field for the samples x > 0.2. Two superconducting transitions in resistivityρ_ab indicate that the low-T_c clusters, i.e. Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions, intersperse in GdBa₂Cu₃O_7-δ superconducting matrix. TheδT_c-pinning resulting from the compositional fluctuations should be responsible for the enhancement of J_c and trapped field for the sample x = 0.2.In chapter four, the effects of neutron irradiation on the microstructure and critical current density of the nominal Gd_1.4,Ba_2.2Cu₃O_7-δ and Gd_1.8,Ba_2.4Cu₃O_7-δ single domain superconductors were investigated. It is found that the more serious irradiation damage was produced by the interactions of the neutrons and the Gd atoms in Gd_1.8,Ba_2.4Cu_3.4O_7-δ sample than in Gd_1.4,Ba_2.2Cu_3.2O_7-δ. For the nominal Gd_1.4,Ba_2.2Cu_3.2O_7-δ superconductor, the fishtail can not reappear after annealing the irradiated sample due to the large irradiation damage and the dissolution of Gd123 (containing Gd_1+y,Ba_2-yCu₃O_7-δ solid solutions) by irradiation. In chapter five, the effects of neutron irradiation on the microstructure and critical current density of the nominal GdBa₂Cu₃O_7-δ single domain superconductor were studied. It is found that the fishtail in magnetization curve was repressed in the irradiated sample. However, the fishtail was enhanced in annealed irradiated sample and the J_c value was higher than that of nonirradiated sample.更多还原