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Special Seminar

Superconducting Magnets in Space

by: Dr. Stephen M. Harrison

Date: Tuesday October 23, 2007

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

A brief introduction to Scientific Cryomagnetics Company will be given, followed by a review of some of the earlier attempts to use superconducting magnets in space. Our present and past projects will be shown emphasizing design, construction and cryogenics of some of the superconducting magnets built (or still under fabrication) by our company for space applications: Alpha Magnetic Spectrometer (AMS), Variable Specific Impulse Magnetoplasma Rocket (VASIMR), and X-Ray Evolving Universe Spectroscopy (XEUS). Future possibilities for applied superconductivity in space will be presented, with project overviews and possibility to discuss details during and after the seminar.

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Special Seminar

New Properties in Old Materials: Doped Layered Dichalcogenides

by: Prof. Emilia Morosan

Date: Monday October 22, 2007

Time: 4:00 pm – 5:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Transition metal dichalcogenides MX2 (M is transition metal, X = S, Se, or Te) have long been known and explored. Due to their reduced dimensionality, such compounds sometimes display charge density wave (CDW) transitions, which are periodic modulations of the conduction electron density. In addition, the CDW state is believed to compete with a superconducting state (SC), both the CDW and the SC representing collective electron states induced by Fermi surface instabilities. Upon doping with various complexes, the transition metal dichalcogenides often reveal dramatic changes of their physical properties. I will discuss the effects of transition metal intercalation on the properties of two layered chalcogenide materials, TiSe2 and TaS2. Although TiSe2 is one of the first known CDW-bearing materials, the nature of its CDW transition remains controversial. Recently the interest in TiSe2 has been renewed by our discovery of the new superconducting state SC that emerges upon Cu doping. Thus CuxTiSe2 provides the first example of a system in which controlled chemical doping can be used to study the competition between the CDW and SC. I will also discuss experiments on FexTaS2 aimed at studying the sharp switching of the magnetization that we recently observed in this compound for x = 1/4. For this particular Fe content, FexTaS2 orders ferromagnetically below 160 K and displays very sharp hysteresis loops in the ordered state for H||c. This is indicative of a very rapid switch of the magnetization direction, and the time dependence of this magnetization switch reveals unexpected time dependence.

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Bi-Weekly Seminar

Oxygen Diffusion and Surface Exchange in Mixed Conducting Metal Oxides

Dr. Allan J. Jacobson

by: Dr. Allan J. Jacobson

Date: Friday September 28, 2007

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The performance of many energy conversion and storage devices depend on the properties of mixed electronic-ionic conducting materials. Mixed or ambipolar conductors simultaneously transport ions and electrons and provide the critical interface between chemical and electrical energy in devices such as fuel cells and batteries. Enhancements in storage capacity, reversibility, power density and life all require new materials and a better understanding of the fundamentals of ambipolar conductivity. In this presentation, I will describe some recent results for a remarkable new class of oxygen ion mixed conductors with potential applications in fuel cells and ion transport membranes.

We have shown that mixed-conducting non-stoichiometric perovskite oxides with ordered A site cations have remarkably high oxygen ion conductivity and surface reaction rates for oxygen exchange relative to conventional materials. Subsequent to our own studies, two other groups have demonstrated comparably high oxygen diffusion in similar compounds confirming that this class of compounds represents a significant enhancement in the achievable rates of oxygen diffusion in mixed conducting oxides.

In PrBaCo2O5+x (PBCO), a representative example of this class of compounds, the barium and praseodymium cations are located in planes that alternate along the c axis; oxygen vacancies occur only in the ab plane containing the Pr3+ cations. The oxygen diffusion coefficient measured in PrBaCo2O5+x as a function of temperature surpasses the diffusion coefficients of the compounds La0.5Sr0.5CoO3-x and La2NiO4+x which are among the highest of the known mixed conducting oxides.

The surface exchange coefficient for oxygen exchange has been measured on thin films of PrBaCo2O5+x by electrical conductivity relaxation and by oxygen-isotope exchange and depth profiling. Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The measurements reveal that the PBCO films have high electronic conductivity and more rapid surface exchange kinetics than those of other perovskites.

Reasons for the high oxide ion diffusion and surface exchange coefficients and the relation to the high electronic conductivity and diffusion pathways will be discussed together with the potential use of the compounds as electrodes for oxygen reduction in fuel cells and as membranes for oxygen separation.

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Bi-Weekly Seminar

Finding the Key to the High Tc Puzzle

by: Prof. Young Kim

Date: Friday September 14, 2007

Time: 11:00 am – 12:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

The high-Tc puzzle remains unsolved despite extensive experimental collection of the puzzle pieces over the past two decades. Granted, this could be due to the complexity of the problem, but it could also be very likely that some key building blocks might have been overlooked as they were hidden behind various experiments on different high temperature superconducting materials and, therefore, too subtle to be recognized. In order to solve this puzzle, we have re-searched the key pieces in hand and put them together to bring about a coherent picture that captures the essential physics of high Tc superconductivity.

Bi-Weekly Seminar

Quench Propagation Analysis in MgB2 Superconducting Magnets

by: Matteo Alessandrini

Date: Friday August 17, 2007

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Reliability and durability of high temperature superconducting magnets depends on our knowledge of their behavior during a quench. Simulation of quench propagation and voltage growth along composite MgB2 superconducting wires are presented by taking into account sharing current and temperature dependence of heat capacity, thermal conductivity and resistivity. A description will be given of our recently developed testing facility for quench propagation studies in MgB2 superconducting magnets. Finally some of the latest results on large bore solenoids will be presented and discussed.

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