| 其他摘要 | 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. |
修改评论