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Processing of single- and multi-domain YBa{sub 2}Cu{sub 3}O{sub x} bulk materials for levitation applications by Nd{sub 1+x}Ba{sub 2-x}Cu{sub 3}O{sub y} seeding

Description: Melt processing of YBa{sub 2}Cu{sub 3}O{sub x} (Y123) is a well established technique to produce materials for levitation applications. We report our new results on the melt-processing of Y123. To achieve materials with high levitation forces we utilize a seeding technique using Nd{sub 1+x}Ba{sub 2-x}Cu{sub 3}O{sub y-} seeds (Nd123). Two different kinds of Nd13-seeds, one plate-shaped and the other cube-shaped, were developed and used for this study. The microstructures of the levitators depended strongly on the type of the seeding material used and on the seeding geometry. We were able to produce large single domains as well as materials containing five domains with three different c-axis orientations. Levitation forces up to 8.2 N for samples with a diameter of about 18 mm were found for the multi-domain samples, which is well within the area of the levitation forces measured for the single domain materials.
Date: January 1, 1995
Creator: Todt, V. R.; Sengupta, S. & Miller, D. J.
Partner: UNT Libraries Government Documents Department

Fabrication of large domain YBa{sub 2}Cu{sub 3}O{sub x} for magnetic suspension applications

Description: Large domain YBa{sub 2}Cu{sub 3}O{sub x} levitators have been fabricated using a seeded melt processing technique. Depending upon the seed, either a single or five domained sample can be obtained. The grain boundaries separating each domains in the five domain levitator are found to be 90 degrees. Similar levitation forces can be observed for single and five domained samples. After thermal cycling, however, a small decrease in the levitation force of the five domain levitator was observed as a function of thermal cycles while nearly no change in force was observed in the single domain levitator. Finally it is shown that both, single and five domain YBCO, behave similarly as a function of sample thickness.
Date: December 31, 1995
Creator: Todt, V.R.; Zhang, X.; Miller, D.J.; Sengupta, S.; Corpus, J. & Gains, J.R. Jr.
Partner: UNT Libraries Government Documents Department

Nucleation and growth of single- and multiple-domain YBa{sub 2}Cu{sub 3}O{sub x} levitators: Influence of seed crystallography

Description: Large diameter YBa{sub 2}Cu{sub 3}O{sub x} levitators have been fabricated using seeded melt processing. Two types of levitators have been produced: single-domain, obtained using flat NdBa{sub 2}Cu{sub 3}O{sub x} seeds, and five-domain, obtained using cubic NdBa{sub 2}Cu{sub 3}O{sub x} seeds. The difference in these two types of levitators can be attributed to differences in nucleation and solidification processes. In particular, the nucleation of multiple (five) domain levitators may be related directly to the crystallography of the cubic seeding crystals which also exhibit a multiple-domain structure. Single-domain levitators are produced by seeding with plate-like seed crystals which are composed of a single domain. Subsequent growth of the levitator is dominated by anisotropic solidification. Using these types of seeds, the authors developed a dual-seeded melt textured growth process for the production of bulk bicrystals which are useful for studies of grain boundary transport behavior.
Date: December 31, 1996
Creator: Todt, V.R.; Zhang, X.F. & Miller, D.J.
Partner: UNT Libraries Government Documents Department

From meandering to straight grain boundaries: Improving the structures of artificially-induced grain boundaries in superconducting YBa{sub 2}Cu{sub 3}O{sub y} bicrystals

Description: This paper presents several key aspects of the successful approach to preparing artificially-induced [001] tilt YBa{sub 2}Cu{sub 3}O{sub y} (YBCO) grain boundaries (GBs) with uniform, well-defined structures. The authors have compared the structure of GBs produced in thin film bicrystals and bulk bicrystals, respectively. In the YBCO thin film bicrystals prepared by off-axis magnetron sputtering, meandering rather than planar GBs were generally formed due to the three-dimensional island-shaped nucleation and growth of the thin films. Experimentally, using a low film deposition rate has been demonstrated to reduce the magnitude of meander. However, complete elimination of the meandering configuration has not yet been accomplished due to the film growth mechanism. Thus, the authors have developed a dual-seeded-melt-texture process to produce uniform, planar GBs in [001] tilt YBCO bulk bicrystals. Transmission electron microscopy (TEM) studies revealed GBs with a remarkably planar configuration on both micro and nano-meter scales, demonstrating that simpler, meander-free GB microstructures with well defined [001] tilt angle have been successfully produced compared to those formed in bicrystal thin films. The high reproducibility, excellent stability and well controlled GB orientations have established the dual-seeded-melt-texture process as a reliable technique for engineering artificial GBs for the purpose of systematic studies of GB properties and allow for more insightful measurements of transport properties across individual GBs.
Date: August 1, 1997
Creator: Zhang, X.F.; Todt, V.R. & Miller, D.J.
Partner: UNT Libraries Government Documents Department

Melt-texturing of carbon containing YBa{sub 2}Cu{sub 3}O{sub 7-x}: Influence of processing parameters on microstructure and flux-pinning behavior

Description: A detailed study of the flux-pinning behavior of sintered and melt-textured YBa{sub 2}Cu{sub 3}O{sub 7-x} has been carried out by means of microstructural investigations (optical microscopy, SEM, TEM, EDS, DTA, and XRD) and magnetization measurements. It was found that both microstructure and magnetization behavior strongly depend on the starting material, the production method, and the maximum processing temperature. In our experiments, the critical current density, J{sub c}, increased with increasing processing temperature between 920{degrees}C and 1050{degrees}C (25 - 130 emu/cm{sup 3}), but those samples processed at temperatures just above the peritectic transformation point (1020 OC and 1030 OC) exhibited a decreased J{sub c}. The carbon content of the starting powder and the powder`s melting behavior seem to play an important role in the development of microstructure and flux-pinning behavior during melt-texturing. A comparison of our data with previously published results shows that an optimized melt-texturing process can result in materials with critical current densities comparable to those of samples produced by Quench-Melt Growth.
Date: January 1, 1994
Creator: Todt, V.R.; Sengupta, S.; Chen, Y.L.; Shi, Donglu; Poeppel, R.; McGinn, P.J. et al.
Partner: UNT Libraries Government Documents Department

Fabrication and characterization of melt-processed YBCO

Description: Large domain YBCO are fabricated by using a melt processing technique for magnetic levitation applications. A Nd{sub 1+x}Ba{sub 2{minus}x}Cu{sub 3}O{sub y} seed is used to initiate grain growth and to control the orientation of YBCO grains. Samples as large as 2 inch have been fabricated by utilizing this method. Microstructural studies reveals two distinct regions in these levitators due to different growth mechanism along a/b and c axis. Some initial results on the mass production of these levitators are also reported.
Date: November 1996
Creator: Sengupta, S.; Corpus, J.; Gaines, J. R., Jr.; Todt, V. R.; Zhang, X. F.; Miller, D. J. et al.
Partner: UNT Libraries Government Documents Department