【作者】 孙立杰；

【导师】 姚忻；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2010， 博士

【摘要】 高温超导体(HTSC)自从被发现以来,其优异的性能和潜在的应用前景就一直受到科学界的广泛关注。由于结构上存在强烈各向异性,而且内部钉扎中心的存在也对超导性能有关键的影响。因此要得到具有高性能的高温超导材料,取向生长以及微结构控制的研究是必不可少的。通过控制高温超导材料的微观结构,可以有效改善内部磁通钉扎中心的尺寸与分布,减少弱连接的产生,提高临界电流密度。在众多种类的高温超导材料中,RE1Ba2Cu3O7 (REBCO, RE123,其中RE包括Y元素和Sm, Nd, Gd等稀土元素)系列高温超导块体材料因为具有生长方法简单,取向控制容易,临界电流密度大,冻结磁场强度高等特点而被广泛研究。在应用方面,REBCO块体材料具有潜在的实用价值。相比于YBCO, SmBCO和NdBCO具有更高的超导转变温度(Tc)以及强磁场下更高的临界电流密度值,因此更值得去开展研究。在块材的生长制备过程中,依然面临着一些困难：首先是缺乏合适的籽晶；其次是钉扎中心的改善；再者就是在生长中由于RE元素对Ba元素的替代而导致非超导固溶相的出现。本论文的主要科学意义在于从外延取向控制以及微观结构调整两方面,讨论了控制生长高性能SmBCO/NdBCO块材的手段及方法。在几年的研究学习过程中,本人主要取得了以下几项成果：一、研究NdBCO薄膜作为籽晶在块材生长中的过热特性以及应用。在块材生长中,YBCO薄膜作为籽晶显示出了过热的特性。NdBCO因为具有和YBCO相似的晶格结构及化学特性,有理由相信NdBCO薄膜应该也具有过热的特性。我们用NdBCO薄膜作为籽晶成功的用熔融织构法生长出NdBCO块体材料。经过分析,我们认为薄膜籽晶的高热稳定性可能和熔体内Nd元素的高饱和度,熔体中的Ba/Cu比例组分,以及薄膜表面晶粒在熔体中的粗化效应有关。研究中我们还发现,薄膜的结晶度对于薄膜籽晶的过热度有明显的影响,具有高结晶度的NdBCO薄膜由于界面能低,因此具有更高的热稳定性。考虑到NdBCO在整个REBCO系统中具有最高的包晶反应温度(Tp),因此NdBCO薄膜籽晶的使用,不仅解决了NdBCO/SmBCO块材生长中缺少合适籽晶的问题,更可以作为一种普适的籽晶选择来生长所有系列的REBCO块材。二、原位观察研究Sm2BaCu05(Sm211)晶须在高温下发生的的相外延关系利用高温金相显微镜(HTOM),我们用Sm211晶须的(010)面作为基板研究了Sm-Ba-Cu-0系统在不同热平衡温度下的相外延关系。在1100℃时,我们发现了一种微小针状相在Sm211晶须基板上的外延。通过XRD及EDS的分析,我们证实了该针状相为Sm2Ba4Cu2O9 (Sm242)相。我们认为富钡的Sm242相能够在晶须上保持亚稳态的存在,是因为外延界面的形成以及Sm242相的尺寸。当温度从高温缓降到1050℃时,我们观察到了Sm123晶粒开始在晶须表面上的外延形核。外延的Sm123和Sm211之间显示出了0°和45°两种不同的面内取向。当温度重新上升到1085℃时,发现0°晶粒在1055℃就出现熔化,而45°晶粒直到1085℃才熔化,说明45°晶粒和Sm211晶须有更好的晶格匹配,因此具有更高的热稳定性。经过晶格适配的计算以及前述的结果,我们进一步提出了Sm123与Sm242之间也可能存在外延关系。三、研究富钡Sm242相的添加对生长高质量Sm123块体材料的影响。组分控制是抑制SmBCO块材生长中Sm和Ba的替代,避免固溶体形成的有效方法。在本工作中,通过添加富钡的Sm242相,成功在空气中用熔融织构法生长出了具有高超导性能的Sm123超导块材。在生长过程中,我们使用了具有过热特性的SmBCO以及NdBCO薄膜作为籽晶来控制块材的外延取向。通过分析并优化Sm242相的添加量,我们获得了具有94K的高临界温度和小于1K的临界转变宽度(△Tc)的SmBCO块材。这是目前在空气中生长的同类样品中所获得的最高Tc值。在测量样品的临界电流密度时,我们观察到了一个相对较强的第二峰,这个第二峰的出现可能是由于Sm123内形成的组分涨落形成的。此外,由于Sm242相的添加,块材中Sm211非超导相和熔体中的界面能发生了改变,在块材内的尺寸和分布也有相当的改善。本论文的工作,为REBCO高温超导块体材料的取向控制和微结构控制提供了新的思路和方法,并希望在将来能够实现大尺寸高性能高温超导块材的制备,以适应实际应用中的需要更多还原

【Abstract】 Since the high temperature superconductor (HTSC) was discovered in 1987, enormous efforts have been made to improve its properties because of its great potential in electric transmission and magnetic applications. Due to its large anisotropy and quantum flux penetration mechanism, the superconducting properties strongly depend on the crystal structure, especially the microstructure of HTSC sample. Hence, in order to obtain the HTSC materials with high performance, it is of importance to further understand the mechanism of growth orientation and microstructure control. By optimizing the microstructure, the size and distribution of flux pinning sites could be improved, the number of weak link will decrease and the critical current density be enhanced. Among all types of HTSC materials, the RE1Ba2Cu3O7 (REBCO,RE123, RE=Y,Sm, Nd, Gd etc.) bulk materials have attracted much attention because of its high critical current density under high magnetic field and the ability of trapping magnetic field. While scientific attempts are now concentrating to develop second generation superconducting wires or tapes, the bulk materials are meanwhile on the way to practical applications. On the other hand, there are still several problems to be solved in the growth of REBCO bulks, such as the lack of proper seed, control of flux pinning sites as well as the formation of solid solution inside the material. In this paper, attentions are mainly focused on both the growth orientation control and microstructure optimization of REBCO bulks. The main progresses are listed below:1. Study of thermal stability of NdBCO superheating film and its application as seed in bulk growthIn pervious study, a superheating phenomenon was realized on YBCO thin films. Since the NdBCO film has similar structure as YBCO, it is reasonable to suggest that NdBCO film could also exhibit superheating property. By using NdBCO film as seed, we have successfully grown the homogeneous NdBCO bulk via melt-texture method. During the experiment, NdBCO film seed maintained the crystal structure at a temperature higher than its Tp for 1.5h due to superheating property. It is also realized that the in-plane orientation also has influence on the superheating of film. NdBCO film with higher crystallinity possesses a better thermal stability in the MT process. The development of NdBCO film seed makes it possible to grow large SmBCO and NdBCO single grain bulks. What is more, since the Tp of NdBCO is higher than that of any other REBCO materials, it is further suggested that NdBCO film seed could be used in the growth of all kinds of REBCO bulks.2. In-situ observation of phase epitaxial relationships in Sm-Ba-Cu-O system.Phase epitaxial growth of Sm-Ba-Cu-O phases on the (010) surface of Sm2BaCu05 (Sm211) whiskers was in situ investigated under various thermal conditions by means of the high temperature optical microscope. At 1100℃, needle-shaped phase epitaxially grew on the Sm211 substrate. The EDS and XRD analyses identified this phase as Sm2Ba4Cu2O9 (Sm242). It is suggested that the metastable existence of Sm242 phase on Sm211 substrate is due to the formation of epitaxial interface and small size of Sm242. During a slow cooling down to 1050℃, two kinds of c-oriented Sm123 grains epitaxially grew on the Sm211 substrate. They were denoted as 0°grains and 45°grains. During a further reheating up to 1085℃, it was found that the 0°grains preferentially dissolved already at 1055℃, indicating that the 45°grains had a higher thermal stability than the 0°ones, mainly due to a better lattice matching. Finally, a possible epitaxial correlation between Sm123 and Sm242 phases was proposed and supported by lattice matching calculations. These studies help us to further understand the formation of epitaxial relationship under various thermal equilibrium conditions, which could be useful in the improvement of flux pinning sites inside the REBCO bulks.3. Influence of Ba-rich Sm242 phase addition in Sm123 bulk growth.By adding a novel Ba-rich Sm242 compound in the precursor material, high performance Sm123 bulk superconductors were successfully grown in air by melt-textured (MT) growth. Highly-c-oriented SmBCO and NdBCO thin films with a superheating property were effectively utilized as seeds to induce an epitaxy nucleation and growth. A primary study on the effect of the Sm242 addition on superconducting properties was carried out, proving that the critical transition temperature (Tc) can be substantially improved. It was found that, with the addition of 10 mol% Sm242, a high Tc of over 94 K with a sharp transition (△Tc less than 1 K) can be achieved in the air-processed Sm123 bulk, which is the higher than any other reported air-processed Sm123 bulks. Moreover, a relatively strong peak effect in the Jc-H curve is present at the intermediate field range, which could be explained by the formation of a compositional fluctuation within the Sm123 matrix. An improvement in the particle size and distribution of Sm211 in the Sm123 matrix was also observed.The research work presented here has provided some new ideas for the optimization of both the orientation and microstructure control of REBCO bulk. In the future, it is hoped that the growth of large-sized REBCO bulk with high performance could be achieved. More research work should be developed to meet the demand of various applications.更多还原