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Special Seminar

Molecule and Crystal Engineering of High-Tc Structures and Devices

by: Prof. John T. McDevitt

Date: Friday September 25, 1998

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In an attempt to foster more rapid developments in the areas of high-[Tc] superconductivity and macromolecular electronic devices, our group has recently initiated a series of studies designed to explore fundamental issues related to the chemistry of high-[Tc] compounds. Knowledge in this regard is particularly important because the poor reactivity problems and materials processing limitations have hampered developments in this area. From our studies, we have identified lattice stress and strain as important variables which influence the environmental reactivity of the most popular high-[Tc] superconductor compound, YBa2Cu3O7. Using solid-state substitution reactions, we have tailored the high-[Tc] lattice so as to relieve the internal stress and strain. Consequently, formulations such as Y0.6Ca0.4Ba1.6La0.4Cu3O7 have been prepared that exhibit both excellent stability and good superconducting properties. Armed with detailed fundamental knowledge of the surface and interface properties of the high-[Tc] materials, our group has completed the initial studies in a new area of research which involves the study of superconductor elements that are derivatized with molecular compounds. Consequently, we have designed, fabricated and tested the first two classes of molecule/superconductor devices. Through the combination of carefully chosen photoactive and electroactive molecular assemblies with high-[Tc] electronic devices, we have shown that it is possible to engineer the molecular layer so that a number of structures can be prepared to serve a variety of applications.

Special Seminar

Fabrication and Characterization of Superconducting Bi-2223/Ag Tapes with High Critical Current Densities in km Lengths

by: Dr. Zhenghe Han

Date: Monday September 21, 1998

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

High critical current density, long-length Bi-2223/Ag tapes are needed for large-scale applications. I will describe our recent results reaching critical current densities of 23 kA/cm2 and engineering critical current densities of 5.2 kA/cm2 at 77 K over the whole length of 1250 m Bi-2223/Ag tapes. To our knowledge, this is the highest critical current density reported for Bi-2223/Ag tapes longer than 1 km. Detailed measurements of the critical current over the 1250 m tape by both the conventional four-probe method and a specially developed continuous measurement of the remanent field by Hall probes will be reported. Various Ag alloy sheathed Bi-2223 tapes have been made by our standard production line. The approach for improving the homogeneity and considerations to be made in processing and handling the long tape will be presented. Electrical, mechanical, and thermal properties of our Ag alloy sheathed tapes, including Ag-Au alloy sheathed tapes, will also be described.

Special Seminar

Al-Cu Approximates in the Al3Cu4 Alloy

by: Prof. Chuang Dong

Date: Thursday September 03, 1998

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In the binary alloy systems constituting the ternary Al-Cu-TM (transition metal) quasicrystalline systems, many metastable quasicrystals have been found in Al-rich Al-TM systems, but little has been done in the Al-Cu system. The latter system has been addressed only recently because the only Al-rich Al-Cu phase is the non-approximate Al2Cu phase. The present talk will report on an investigation of the Al3Cu4 alloy, with an e/a ratio of 1.86, close to ternary Al-Cu-TM quasicrystals, in the search for Al-Cu approximates. Principal results include: 1) two new phases, a face-centered orthorhombic oF-Al43.2Cu56.8 (a = 0.8166, b = 1.4149, c = 0.9995 nm) and a body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a = 0.4083, b = 0.7074, c = 0.9995 nm), were found; 2) the twinning modes of these phases suggest that they are the decomposition products of a high-temperature parent phase e2-Al2Cu3, the atomic structure of which contains pentagonal atomic arrangements; and 3) the chemical-twinning mode of their basic structure B2 is 180[deg]/<111>, which is responsible for the pentagonal atomic configuration in the Al-Cu approximates as well as for the pseudo 5-fold B2 twinning.

Some preliminary friction tests on such B2-based approximates show that these phases have friction behaviors similar to those of quasicrystals.

Special Seminar

Normal and Superconducting Properties of Fermi System with Non-Degenerate Component: A Model for High-[Tc] Cuprates

by: Dr. George A. Levin

Date: Wednesday August 19, 1998

Time: 10:30 am – 11:30 am

Location: Houston Science Center – Building 593 — Room 102

Overview

Many of the anomalies of the normal and superconducting state properties of the cuprates and their evolution with doping, from underdoped to optimally to overdoped regime, can be understood within the framework of a two-component model, one of which is conventional degenerate and the other, non-degenerate. The non-degenerate component appears when the Fermi surface of a conventional band is brought by tuning the stoichiometry in the vicinity of the top of another, submerged below the Fermi level, quasi-two-dimensional band. In the underdoped regime the excitations in the submerged band manifest themselves as a pseudogap, evident in the spin susceptibility, specific heat, tunneling conductances, resistivity, and Hall coefficient. The pseudogap vanishes with increasing doping, but the presence of the submerged band strongly affects the properties of the optimally and overdoped systems as well.

Special Seminar

Large Increases in Jc of Textured HTS via Radiation Damage

Dr. Roy  Weinstein

by: Dr. Roy Weinstein

Date: Friday July 31, 1998

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Jc is limited by motion of magnetic fluxoids. Pinning these fluxoids in place greatly increases Jc. Successful pinning centers have been produced by chemical, mechanical, and radiation techniques. The radiation methods which have been used include high Z ion beams, protons, neutrons, fusion of 6Li with neutrons followed by 7Li decay, and fission of U by neutrons (the U/n method).

I will address the U/n method and results to date. A detailed study of YBCO is reported, plus introductory studies of NdBCO, SmBCO, and BiSCCO. In YBCO increases in Jc by factors of 15-40 are obtained. In BiSCCO, increases in Jc by a factor of 20 are seen (in the high field regime), the irreversibility is approximately doubled, and the anisotropy is decreased by a factor of 8.

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