Abstract
Bulk single crystals are indispensable in the understanding of the physical properties of high-Tc superconductors having a common feature of a layered perovskite structure with CuO planes. For the study of physics and chemistry of condensed phases, it is crucial to obtain good quality single crystals of the material concerned, because in polycrystalline materials, the properties of the grain boundaries are often stronger than that of the material itself. Thus, at this current stage of research, significant efforts were invested in the growth of high-quality large single crystals to further improve our understanding of high-Tc superconductivity. This thesis presents the crystal growths of Bi2(SrxCa3-x)Cu2Oy, Bi2Sr2Ca1-xPrxCu2Oy and Bi2Sr2GdCu2Oy by using the traveling solvent floating zone method within an infrared radiation furnace. Crystals of Bi2(SrxCa3-x)Cu2Oy of various Sr/Ca ratios were obtained with a lowest value of approximately 1.32. A rotation rate of 30 rpm and a growth rate of 0.2 mm/h were established. Compositional analysis of the steady-state solvent zone indicated that a primary crystallization field (PCF) was present at the bismuth-rich and copper-deficient region relative to the 2212 stoichiometry, and that this compositional range approximately equal to that of Bi2.425Sr1.911Ca0.807Cu1.857Oy. At the same time, this PCF includes a corresponding tie-line relationship with a slightly copper-deficient 2212 single-phase region. A systematic study of Bi2Sr2Ca1-xPrxCu2Oy, where x = 0–1.0, was undertaken to determine growth parameters, crystal properties as well as the phase compositions. The typical dimension of single crystals with actual doping ratios x from 0.2 to 0.7 was 5×3×0.2 mm3. Moreover, large single crystals of Bi2Sr2PrCu2Oy (x = 1.0), with a typical size of 10×3´0.2 mm3, were successfully grown for the first time. These crystals contained no impurities and showed a clean X-ray diffraction pattern of the pure Bi-2212 phase. As the Pr doping level increased, the liquidus composition shifted pronouncedly to the corner of CuO as shown in the pseudo-ternary BiO1.5-(Sr,Ca,Pr)O-CuO phase diagram. As a result, the solidus composition, Bi1.965Sr2.046Pr1.096Cu1.894Oy, and liquidus composition, Bi2.17Sr1.92Pr0.31Cu2.60Oy, were obtained. Some physical measurements, such as determining the structural modulation, molar magnetic susceptibility and heat capacity on Pr-doped 2212 crystal were also performed. Crystals of Bi2Sr2GdCu2Oy were grown with the dimensions of approximately 6×2×0.2 mm3. As compared to the Bi-2212 solid solution region, the PCF of Bi2Sr2GdCu2Oy was approximately located at the BiO1.5-rich and CuO-rich side, as demonstrated in the pseudo-ternary BiO1.5-(Sr, Pr, Gd)O-CuO phase diagram. Such variation in solvent compositions may significantly influence the segregation behavior of each constituent so as to influence the maximum stable growth rates of each compound. The ratio of the temperature gradient G over the growth rate R, i.e. G/R, is in direct relation to the occurrence of constitutional supercooling. In our experiments, a planar interface was observed, indicating that a stable growth condition was established before zone quenching. The minimum critical values of G/R are 5.39×1011 K-s/m2 of Bi2Sr2GdCu2Oy, 4.5×1011 K-s/m2 of Bi2Sr2CaCu2Oy, and 2.94×1011 K-s/m2 of Bi2Sr2PrCu2Oy. These data obtained from fundamental estimations are consistent with experimental results, indicating a decreasing behavior of growth difficulty. Finally, Bi2+xSr2-yCuO6+δ (Bi-2201) crystals which compositions include Bi2.22Sr1.78CuOy and Bi2.3Sr1.7CuOy were grown. The typical size of the crystals is as large as 20×4 mm2. The transport properties (in-plane resistivity, out-of-plane resistivity and Hall coefficient) of some as-grown and post-annealed Bi-2201 crystals were reported.