Annealing of YBa2Cu_Og(12 3) above the peritectic temperature, followed by slow cooling at 0.5 C/min, was found to produce a microstructure containing regions (d ‐ 200 microns) within which the grains were highly aligned. When these regions were used to “seed” 123 powder, an enhanced degree of texture was obtained, where the aligned regions were > 1mm in size.

(Y,Ho)‐Ba‐Cu‐0 superconductors with the high critical current densities (Jc) in an applied magnetic field have been synthesized by a unidirectional melt solidification method. Melt solidified bulk materials are composed of large plate‐shape (Y,Ho)Ba2Cu3Oy crystals which contain small (∼5 um) particles of (Y.Ho)2BaCuO5. The Jc value of the melt solidified (Y,Ho)‐Ba‐Cu‐0 sample is ∼10000 A/cm2 at 77K, H = IT and this value is much higher than the value of 4200 A/cm2 of the melt solidified Y‐Ba‐Cu‐O sample.

A liquid phase processing method has been developed to fabricate oriented grained YBa2Cu30x samples with high current density. The current density in these samples, however, is sensitive to the microstructure which is controlled by processing parameters. Cooling rate through the peritectic temperature is found to be the most crucial factor in controlling the extent of grain orientation and in turn the bulk current carrying capacity. The evolution of the microstructure during the slow cooling process is studied by quench experiments at different temperatures. From microstructural analysis, the suitable temperature profile for the slow cooling process is chosen. Current density measurements across grains show a Jc in excess of 104 A/cm2. This implies that the weak link behavior of grain boundaries in polycrystalline superconductors can be suppressed to a considerable extent.

Texture growth in incongruently melting ceramic superconductor materials, such as YBa2Cu3O7‐χ can be combined with zone refinement in thermal gradients to increase critical current densities. In gravity aided texture growth (GATEG), a vertical thermal gradient is used with downward motion of the partially molten specimen (relative to the furnace) so that the molten phase recrystallizes with grain growth. The oxidation features of Y‐Ba‐Cu‐0 and Bi‐Sr‐Ca‐Cu‐0 can be explained by Madelung potentials.

We have produced various high Tc superconductors by oxidation of rapidly quenched alloy precursors produced by planar flow casting. This method generally leads to reactive oxidized powders and improves homogeneity if there is a solubility domain in the liquid state for the alloy. The grain size distribution is often more uniform than that for specimens prepared by conventional methods from mixtures of constituent oxides and carbonates.

A miniaturized float zone technique using laser heating has been found to have potential for producing superconducting fibers of Bi2Sr2CaCu2O8 which can carry high currents. Growth stability and growth rate, however, need to be improved if this technique is to be useful for preparing long lengths of 30 μπι diameter high quality fibers for property evaluation and prototype devices. One of the first requirements toward achieving this goal is the preparation of dense, homogeneous starting material. Work on different strategies for starting material preparation are discussed.

Highly oriented Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting fibers have been prepared by the laser heated pedestal growth method. The annealed fiber showed both the high Tc (≃K) phase's and the low Tc (≃80K) phase's superconducting transition. The fiber grown at 2mm/h had the Jc value as high as l.0x103A/cm2. The misalignment of the high Tc phase's a‐b plane was measured by XRD less than ± 2°. It was found that the high Tc phase formed during the annealing was highly oriented as well as the low Tc phase.

Unidirectional solidification processing, including Floating Zone Melting, Vertical Bridgman, and Laser Zone Melting methods for making high Tc superconducting oxides has been investigated. The effects of processing parameters, growth rate (R) and temperature gradient (G), on texture, morphology, and structures of unidirectionally grown crystals were investigated. Two different crystallization mechanisms were recognized, one is non‐equilibrium phase transformation under high GR conditions, and the other is due to local equilibrium phase transformation under low GR conditions. Strongly aligned structures were obtained at high G/R ratios with low GR value. The interrelation between the superconducting characteristics and textured structure produced were also discussed.

Bi‐Pb‐Sr‐Ca‐Cu‐0 superconductors were prepared by the laser pedestal growth method. The high Tc phase was created only by post annealing. As the growing speed increased, the annealing time necessary to obtain a Tc above 100K decreased. When the low Tc phase was aligned with the growing direction, Tc was under 100K, but a Jc of 15B0A/cm2(77.3K, OT) was obtained. In this case, magnetic field dependence below 300 gauss was superior to that of Pb‐free materials.

Ceramic rods of Bi2Sr2CaCu2O8 (2212) were directionally solidified by laser‐heated float‐zone crystal growth [1]. The microstructure of the resulting material can be controlled through variation of growth rate. The 2212 phase is Sr‐deficient, having an actual composition Bi2.5Sr2CaCu2.2Oy. At slow growth rates (0.2 cm/h), plane front growth produced highly textured samples of almost phase pure 2212, and pole figures show that the solidification front proceeds along the (200) axis of the 2212 grains. Faster growth rates produced dendritic growth and polyphase microstructures. At the fastest growth rate studied (16 cm/h), no 2212 phase was observed. Observations indicate that Bi2.5Sr2CaCu2.2Oy melts peritectically into (Sr1‐x.Cax.)14Cu24Oy and a Bi‐rich liquid.

Superconducting fibers of [Bi,Pb(Sb)]2Sr2Ca2Cu3O10 + x high Tc superconducting materials have been prepared by means of the laser-heated pedestal growth method with Tc 0= 110K and Jc>103 A/cm2. Structure analysis showed that the a-b plane of the material prefers to align along the growth direction, though the post-growth heat treatment, which is still necessary at present stage, may destroy the texture structure in some extent.The existence of Pb is vital for stabilizing the 110K phase of the Bi-based fibers.

Unidirectional crystallization of melt processed 2122 and 2223 composition in the Bi-Ca-Sr-Cu-O system has been carried out. Crystallization was controlled by heating the glass or cooling the melt using a moving furnace and a stationary sample. Fairly large oriented crystalline specimens of 2122 phases were obtained by cooling from the melt in a shallow gradient furnace. Glassy samples processed under steep gradients showed better crystallinity than those processed under shallow gradients.

Fibrous crystals containing the high-Tc phase (2223 phase) of the Bi(Pb)-Sr-Ca-Cu-O superconductor have been prepared by heating a glassy melt-quenched plate in a stream of oxygen gas. Each fiber is composed of several platelike single crystals which are stacked in a layered structure. The fibers show two stepdrops in resistance at 105K and 73K, and a zero-resistance state around 70K. From the results of SQUID measurements, the volume fraction of the high-T phase is found to be about 0.6%. The high-Tc phase is also detected with a microarea X-ray diffractometer, and it is stacked in the interlayer between the low-Tc platelike crystals. For the fibrous Bi-Sr-Ca-Cu-Li-O crystals, the volume fraction of the high-Tc phase increases to 1.5% and a zero resistance state is achieved at 81K. The Jc value of the Li doped fiber is 30,000A/cm2 at 77K and 300,000A/cm2 at 66K in a zero magnetic field.

Materials in the system Bi-Ca-Sr-Cu-O were prepared by rapid solidification. Emphasis was placed on optimizing processing conditions for enhancement of the high Tc (2223) phase. A number of starting compositions and heat-treatments were investigated. The effect of dopants (such as, Pb, Sb, Ge, Sn) on the stabilization of the high Tc phase was evaluated. Small amounts of Pb and Sb were effective in significantly increasing the 2223 yield. Improvements in superconducting properties as a function of 2223 evolution are discussed with reference to SEM, TEM, XRD and SQUID results.

The effect of adding of Bi1.80Pb1.33Sr1.87Ca2Cu3O2 seeds(the 37Å phase) to the polycrystalline and glassy Bi-(Pb)-Sr-Ca-Cu-O materials was studied using XRD and SEM. The seeding effect was observed at 835 °C, 855 °C, and 870 °C in several compositions. Adding pre-synthesized 37Å phase powder to a mixture of unreacted starting materials accelerates the formation of additional 37Å phase.

Superconductivity in Bi compounds is examined in samples prepared from CuO and Cu2O powders, and also in samples doped with Ag. In BiSrCaCuxO (x=2,3,4,5) prepared from CuO powder, Tc decreases with an increase of Cu content, while in samples prepared from Cu2O the opposite trend, an increase of Tc with Cu content, is observed. From Ag doping, BiSrCaCu2Oy+Agx (x=0, 1.0, 1.5, 2.0), the volume ratio of high to low Tc phase is found to be a maximum at a Ag content between 1.5 and 2.0. From these experiments, a strong correlation between the high Tc phase and Cu+ valence state is suggested.

Ceramics of the phase Bi2Sr2Ca2Cu3O10 (2223) without Pb are synthesized by a modified conventional ceramic process. The influence of the absence of Pb is observed in the lattice constant.

To explore high Tc in Bi-Sr-Ca-Cu-O system, many different nominal compositions have been prepared by using the traditional solid state reactions and using the method of crystallizion from glass samples. The better results are as follows: single high-Tc phase and essentially single high-Tc phase have been obtained from Bi1.8Pb0.35Sr2Ca2Cu3O and Bi2Sr2Ca6Cu7O respectively, by solid reaction. Basically single hign-T phase material with nominal composition Bi2Sr2Ca3Cu4Oy has been produced by the method of crystallizion from glass samples. The relationship between the starting composition heat treatment, preparation methods and high Tce is briefly discussed.

The effect of oxygen partial pressure on stability of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system has been investigated using x-ray diffraction, resistivity and a.c. susceptibility measurements. It was found that part of the high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system was transformed into the low-Tc phase (80 K) and insulating phases by heating in the temperature range 600 C to 880°C in O2 of one atmosphere. The original high-Tc phase was restored upon retreatment in a low oxygen partial pressure of 0.1 atmosphere. The reversible superconducting phase transformation is discussed in conjunction with the formation mechanism of the high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system.