Gd₂Ba₄CuNbO_y掺杂对单畴GdBCO超导块材性能的影响

【作者】 梁伟；

【导师】 杨万民；

【作者基本信息】 陕西师范大学 ， 凝聚态物理， 2011， 硕士

【摘要】 钇系超导体是高温超导材料中研究最多和应用最广泛的超导材料之一,受到全世界的普遍关注。其中,利用顶部籽晶熔融织构法(TSMTG)制备的单畴GdBCO超导体,因其具有较高的临界温度、较大的无阻载流能力和较强的磁通捕获能力,使它在超导磁悬浮列车、磁浮轴承、强磁场永久磁体、超导储能飞轮和超导电机等方面具有广泛的应用前景。为了进一步提高单畴GdBCO超导块材的性能,我们通过掺杂的方法在超导基体中引入有效的磁通钉扎中心,提高了超导块材的磁悬浮力和临界电流密度J。等性能。本文采用传统的固相烧结工艺制备出了较纯的初始粉体和新型第二相纳米粒子Gd2Ba4CuNbOy (GdNb2411)粉体。采用TSMTG工艺成功制备出了掺杂GdNb2411纳米粒子的单畴GdBCO超导块材,并对具有不同GdNb2411掺杂量样品的表面生长形貌、磁悬浮力和微观结构等进行了系统分析；在此基础上,进一步研究了不同粒径和不同夹层位置GdNb2411掺杂对单畴GdBCO超导块材性能的影响。在初始粉体制备方面,采用三次烧结和四次球磨相结合的方法获得了XRD较纯的GdBa2Cu3O7-δ(Gd123)、Gd2BaCuO5(Gd211)、YBa2Cu3O7-δ(Y123)和Y2BaCuO5(Y211)相初始粉体,它们对应的烧结温度分别为940℃、930℃、920℃和910℃。在新型第二相纳米粒子GdNb2411粉体制备方面,系统地研究了其物相成份、晶体结构、微观形貌、粒径尺寸和碳含量等,并确定了它的最佳烧结温度为1140℃C。研究了GdNb2411掺杂对TSMTG法制备的单畴GdBCO超导块材的生长形貌、磁悬浮力性能、微观结构以及生长前沿成份分布的影响。样品中GdNb2411按照摩尔比Gd123:Gd211: GdNb2411=1:(0.4-x):x的比例配制,从样品的表面生长形貌分析发现：样品的生长形貌与掺杂量x密切相关,当x≤0.06mol时,样品上表面光滑平整且四径分明,表现出典型的单畴形貌；当x≥0.08mol时,样品的单畴区域不再光滑和平整,表面出现褶皱,且随着掺杂量的增加,单畴区域变得越来越小。对样品的磁悬浮力测试结果表明：样品的磁悬浮力随着掺杂量x的增加先增大后减小,当x=0.06mol时,样品的磁悬浮力达到最大值,约25N。对样品的微观结构分析表明：随着掺杂量的增加,GdNb2411粒子的密度越来越大,粒子直径在100nm-300nm之间,但当掺杂量过大时,GdNb2411粒子会团聚成更大的粒子,这可能是导致样品的表面形貌发生变化和其磁悬浮力降低的主要原因。对样品的生长前沿成份分析表明：随着掺杂量的增加,大量的Gd211粒子和GdNb2411粒子被推出到Gd123相基体的外面,样品未生长区域的各元素含量严重偏离了Gd:Ba:Cu=1:2:3的比例,导致样品在熔融慢冷生长过程中,不能生成Gd123超导相,这可能是造成样品停止生长的主要原因。研究了不同粒径和不同夹层位置GdNb2411掺杂对单畴GdBCO超导块材的生长形貌、磁悬浮力性能和微观结构的影响。通过对样品的表面形貌观察,我们发现：不同粒径和不同夹层位置GdNb2411粉体掺杂的样品,均能生长出具有四个光滑扇区的典型单畴形貌。样品的磁悬浮力测试结果表明：在掺杂量均为x=0.06mol,采用过筛目数为360、240、120和80目的粉体进行掺杂的情况下,GdBCO超导块材的磁悬浮力随着掺杂GdNb2411粉体粒径的减小而逐渐增大,当过筛目数n=360目时,样品的磁悬浮力最大,约25N；当采用片层状GdNb2411与Gd123进行2-2复合时,结果发现：随着GdNb2411掺杂层位置的下移,样品的最大磁悬浮力呈现出逐渐增大的趋势,当夹层位于h=3/4处时,样品的磁悬浮力达到最大值,约为14N。另外,我们对样品的微观结构进行了观察和分析,结果表明：随着样品中掺杂GdNb2411粉体过筛目数的减小,样品中GdNb2411纳米粒子的粒径逐渐增大,结果导致样品的磁悬浮力性能降低；随着样品中掺杂GdNb2411夹层位置的降低,夹层中GdNb2411粒子尺寸并无明显变化,但都出现了粒子团聚现象,样品磁悬浮力的大小主要取决于夹层上部超导块的比例。最后,对单畴GdBCO超导块材的显微结构进行了系统的观察与分析,发现样品中存在大量Gd211粒子的团聚和长大现象以及气泡、孔洞和宏观裂纹等缺陷,它们的存在都会降低超导块材的性能,有待于进一步优化。更多还原

【Abstract】 Y-based superconductors is one of the most promising materials for high temperature superconducting materials, which can be widely used for various applications, so they are concerned all over the world.The single domain GdBCO superconductors can be prepared by the top-seeded melt texture growth process (TSMTG). They have significant potential for practical applications, such as maglev, strong magnetic field of permanent magnets, superconducting flywheel energy storage systems and superconducting motors, etc, since they have high critical temperature(Tc), high critical current density(Jc) and high trapped magnetic fields. In order to further promote the levitation force and critical current density of single domain GdBCO bulks. The effective flux pinning centers have to be introduced into the superconducting matrix by doping methods.In this paper, the precusor powders and the new second-phase nanoparticles Gd2Ba4CuNbOy(GdNb2411) powders were prepared by the traditional solid-state reaction method. Single domain GdBCO bulk superconductors doped with GdNb2411 nanoparticles were prepared by the TSMTG, meanwhile, their growth morphology, magnetic levitation force and microstructure were analyzed systematically. In addition, the effect of different size and layer position of GdNb2411 nanoparticles on the physical properties of the single domain GdBCO bulk superconductors have also been investigated.The initial powders of GdBa2Cu307-δ(Gdl23), Gd2BaCuO5(Gd211), YBa2Cu307-δ(Y123) and Y2BaCuO5(Y211) were prepared by solid state reaction method with three times sintering and four times ball milling process, the sintering temperature is 940℃,930℃,920℃and 910℃respectively. Meanwhile, the phase composition, cystal structure, microstructure, particle size and carbon content of GdNb2411 powders were investigated systematically, then the optimal sintering temperature of the powders were defined to be 1140℃.The effect of GdNb2411 dopant on the growth morphology, magnetic levitation force, microstructure and chemical component at the cystal growth front of the single domain GdBCO bulk superconductors have been investigated. The samples with compositions of Gd123:Gd211: GdNb2411=1:(0.4-x):x. The surface morphologies of the samples are closely related to the doping content x. It is of a smooth surface with four fam sections for the samples when x≤0.06 mol, but it is of a wrinkle surface for the samples when x≥0.08 mol and the single domain area is reducing with the increasing of x. The levitation force of samples increases first and then decreases with the increasing of x, the largest levitation force of 25N is obtained in the sample with x=0.06mol. The microstructure of the samples shows that the density of GdNb2411 particles is increasing with the increasing of x and the particles size in the range of 100nm to 300nm. The pyramidal surface morphologies and the smaller levitation force of the samples may be due to the aggregation of GdNb2411 particles when x>0.06mol. Additionally, the chemical composition at growth front of the samples shows that a large number of Gd211 and GdNb2411 particles are remaining in the liquid phase matrix, the reason for the stopping growth of GdBCO cystal is because of serious deviation from the elements in Gd:Ba:Cu=1:2:3 ratio.The effect of different size and layer position of GdNb2411 nanoparticles on the growth morphology, levitation force and microstructure of the single domain GdBCO bulk superconductors have been investigated. The results shows that the surface of the samples are of four fan sections. The levitation force measurement shows that the levitation force is increasing with the decreasing of particle size and the lowering of the GdNb2411 layer position in the samples. The largest levitation force of 25N and 14N are obtained for the samples with GdNb2411 particle sifted with n=360 mesh seive, and the layer position h=3/4 at the lower part of Gd123 sample. Additionally, the microstructure of the samples shows that the GdNb2411 nanoparticles are uniformly distributed in the Gd123 matrix for all the samples with different GdNb2411 particle size, but the decreasement of the levitation force is because of the reduction of the total GdNb2411/Gd123 interface in the samples. The GdNb2411 particles size is nearly the same, no matter where the GdNb2411 layer placed in the sample, but the levitation force of the samples is dependent on the volume of bulk superconductor above the GdNb2411 layer.Finally, it is found that there are still some problems need to be solved, such as the Gd211 particles agglomeration, voids and macro-cracks in the GdBCO samples, solving the problems will help us to improve the quality GdBCO bulk superconductor.更多还原