| Bi(Pb)SrCaCuO超导晶须的研究 | |
| 金华 | |
| Subtype | 硕士 |
| Thesis Advisor | 师昌绪 |
| 1992 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 中国科学院金属研究所 |
| Abstract | 本文研究了Bi(Pb)SrCaCuO晶须的制备工艺，晶体结构，显微组织，超导特性及生长条件对晶须的生长及其特性影响，并在大量的Bi系晶须生长实验的基础上，提出了Bi系超导晶须可能的生长机理。研究结果表明，将用快速凝固方法制得的 Bi(Pb)SrCaCuO 非晶薄片在 800－870 ℃ 于流通氧气气氛中长期加热后，可在薄片的上、下表面长出许许多多柔性超导晶须。晶须的长计为几个mm，最长的可达8mm，宽为几个至几十个μm，而厚度仅为1－10μm。晶须的显微组织均匀，在其轴向方向上没有晶界。每一根晶须上各点的成份都相同。而且在同一基底上生长出来的同胞晶须的成份也相同。晶须的晶体结构用X射线衍仪和电子衍射方法确定。晶须为正交相晶体结构，其晶格常数为a ～ 0.54 nm，b ～ 0.50nm，c ～ 3.06nm。本文还用透射Laue衍射实验确定了晶须的晶体学取向并估计了晶须的结晶完整程度。晶须的轴向平面为a-b平面，最短轴为c轴。晶须是由多根细长的片状单晶定向堆垛形成的层状结构，晶须中存在少量的镶嵌块结构。本文还计算了晶格常数为a = 0.545 nm，b ＝ 0.540 nm，c ＝ 3.045 nm的正交结构在小指数范围内的晶面夹角。晶须的超导特性用交流磁化率和电阻－温度曲线来测定。晶须的电流密度用传输电流法测量，其值为29000 A/cm~2 (63.4 K, 零场)。本文还研究了诸如加热时间，加热温度，Pb掺杂量，基材状态以及生长气氛等生长条件对Bi系晶须的生长及特性的影响。结构发现，晶须的长度并不是一味地随着加热时间的延长而增加，而是有一个饱和点；当初始原料的成分接近高温相（2223）相的理想化学配比时，晶须中的部分（2212）相将随着生长时间的延长而转变为（2223）相。适于晶须生长的温度范围很窄；适当含量的Pb元素掺杂及流通的氧气气氛对晶须的生长起促进作用。为了弄清Bi系晶须的生长机理，本文还系统地研究了Bi_2Pb_(0.5)Sr_2Ca_4Cu_5O_x非晶的晶化规律，提出了其晶化模型并建立了Bi_2Pb_(0.5)Sr_2Ca_4Cu_5O_x非晶晶化的温度－时间－相变（TTT）图，此外，本文还研究了基底在晶须生长前后的变化，在长出晶须以后，基底的显微组织变得疏松多孔。在大量的Bi系晶须生长实验的基础上，本文提出了晶须生长的机理。气相沉积，轴向螺位错和界面扩散在晶须生长过程中起了重要作用。 |
| Other Abstract | The preparation techniques, crystal structures, microstructures and superconducting properties of the Bi(Pb)SrCaCuO whiskers were studied in this thesis. It was found that a lot of flexible superconducting whiskers were grown out on the surface of an annealed melt-quenched plate which was heated at 800-870℃ for long periods in a steady stream of oxygen atmosphere. The lengths of the whiskers are several mm (the maximum length is 8 mm), their widths are several to several tens μm, and their thicknesses are 1-10μm. The microstructures of the whiskers are even and there are no crystal boundaries along the lengthwise of the whisker. The content of Bi, Sr, Ca, Cu, Pb at different sites of the whisker are very similar, and so are of the whiskers grown out of the same substrate. The crystal structures of the whiskers were determined by X-ray diffraction and electron diffraction method. The (Bi, Pb)_2(Sr, Ca)_3Cu_2O_x whiskers are of orthorhombic structures, their lattice parameters are a ～ 0.54 nm，b ～ 0.5 0nm，c ～ 3.06nm. The crystallographic orientation and the perfectness of the whisker were examined by the transmission Laue method. The axial direction of the whisker is the b-axis, the widthwise direction is the a-axis, and the c-axis is perpendicular to a- and b-axes. There exist a few mosaic structures in the whisker. The Bi-based superconducting whiskers are composed of several platelike single crystals stacked up into layered structures. The interplanar angles of the orthorhombic structure with the lattice parameter of a ＝ 0.545 nm，b ＝ 0.5400 nm and c ＝ 3.045 nm in small index range were also calculated in this thesis. The superconducting properties of the whiskers were measured by the ac susceptibility versus temperature and the R-T curve. Their critical current density value is 29000 A/cm~2 (63.4 K, zero magnetic field). The influence of growth conditions such as growth time, temperature, the Pb doping, the state of the substrate and the growth atmosphere on the whisker growth have been investigated. It was found that the maximum length of the whiskers is proportional to the growth time at the initial stage, however, after passing a certain growth time, the maximum length keeps constant. The high-Tc phase emerges when the growth time is prolonged for the whiskers which starting compositions are near the stoichiometry of the high-Tc phase. The optimum temperatures for the whiskers growth are in very narrow range. An appropriate amount of Pb doped in the starting materials and a steady stream of oxygen atmosphere play catalytic roles in the whisker growth. In order to clarify the growth mechanism of the Bi-based superconducting whiskers, the crystallization of the Bi_2Pb_(0.5)Sr_2Ca_4Cu_5O_x glassy plate was studied systematically. A crystallization model and a time-temperature-transformation (TTT) diagram of the Bi_2Pb_(0.5)Sr_2Ca_4Cu_5O_x glass were constructed. In addition, the change of the substrate before and after growing whiskers was studied, the microstructure of the whiskers was studied, the microstructure of the substrate after growing whiskers became porous. On the basis of a lot of the Bi-based superconducting whisker growth experiments, a growth mechanism of the whiskers was proposed. The vapor phase deposition, axial screw dislocation and surface diffusion play important roles in the whisker growth. |
| Pages | 177 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17570 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 金华. Bi(Pb)SrCaCuO超导晶须的研究[D]. 中国科学院金属研究所. 中国科学院金属研究所,1992. |
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