TcSUH
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Bi-Weekly Seminar
Optical and Electronic Properties of Zn4Sb3 Thermoelectrics
by: Dr. Alexander P. Litvinchuk
Date: Friday July 20, 2007
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We will report on an experimental study of optical and electronic properties of Zn4Sb3, an excellent thermoelectric material, across the structural ß-α transition using optical techniques. The phase transition from highly disordered high temperature ß phase into low symmetry ordered &alpha phase is shown to be accompanied by an increase of thermoelectric power and electrical conductivity, and also significant rise in the free charge carrier density. Concomitant with these electronic changes is an unexpected increase in the carrier scattering rate, which exhibits features of localization.
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Bi-Weekly Seminar
Electromagnetic Probes of Enzymatic Activity in Live Organisms
Date: Friday May 25, 2007
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We report on measurements of the harmonics generated by live cells, mitochondria, photosynthetic organelles, and whole organisms in response to sinusoidal electric fields. The frequency and amplitude dependence of the induced harmonics correlate with physiological processes occurring in various enzyme complexes. The motion of charged residues and ions leads to a nonsinusoidal response to an applied sinusoidal field, leading to the generation of harmonics. For example, H+-ATPase, a proton pump that plays an important role in yeast (S. cerevisiae) generates harmonics that are affected by suitable inhibitors or substrates, such as vanadate or glucose. A high-Tc SQUID is used to measure its response at low frequencies (< 1 kHz). At higher frequencies the field capacitively couples through the plasma membrane and probes complexes within internal organelles. Some features in the frequency-dependent harmonics produced by whole cells appear to correlate with those seen in isolated mitochondria, which can be increased by adding substrates that activate the electron transport chain, or suppressed by inhibitors such as rotenone or antimycin A. Finally, both the linear and harmonic responses of whole leaves and thylakoid membrane (chloroplast) suspensions, responsible for photosynthesis in plants, are strongly affected by the presence or absence of light.
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Bi-Weekly Seminar
Some New Insights on the Chemistry of Intermetallics and Metal-Rich Phases
Date: Friday April 13, 2007
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Polar intermetallic phases formed between transition and post-transition metals with one or more of the electropositive alkali, alkaline-earth, and rare-earth metals exhibit a rich variety of complex structures. More importantly, the smooth transition of electronic properties from semiconducting intermetallics to classical metallic compounds along the Zintl border provides a fertile area to search for materials with novel electronic properties. This also offers unique opportunities in investigating structure-bonding-property relationships among materials at the border between metals and nonmetals. We have used the Zintl concept in rationalizing the synthesis, stoichiometry, and chemical bonding of novel ternary and quaternary “electron-poor†Zintl phases that exhibit unusual π-bonding. Recently, our exploratory work near the Zintl border has also resulted in a number of novel polar intermetallic structures. These compounds and other newly discovered polar intermetallics add new insight to the structural and chemical bonding peculiarities of Zintl phases and the less polar intermetallics. In addition, results of exploratory forays into the yet unexplored chemical reactivity of Zintl phases will be presented. The mild reduction and oxidation of the “salt-like” intermetallics provides a novel route to new materials with interesting properties and structures.
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Bi-Weekly Seminar
Nanoscale Self-Assembly: A Theoretical Analysis
by: Gemunu Gunaratne
Date: Friday November 10, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
We use linear and nonlinear stability analysis on a paradigmatic model to extract general characteristics of nano-scale self-assembly. In particular, we identify the order of occurrence of hexagonal and striped arrays, and show that square arrays cannot form when the elastic forces between the substrate and the monolayer are isotropic. In addition, we introduce a method that can be used to estimate hard-to-extract material properties of the monolayer using characteristics of the self-assembled patterns. Finally, we will discuss a technique that can be used to help generate patterns with long-range order.
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Bi-Weekly Seminar
Lattice-strained Nanoparticle Electrocatalysts for PEMFC Cathodes — From Combinatorial Discovery to Structure-property Relationships
by: Dr. Peter Strasser
Date: Friday September 29, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The cell voltage and performance of Polymer-Electrolyte-Membrane Fuel Cells (PEMFCs) deviate strongly from their theoretical values due to severe kinetic overpotentials at the cathode where oxygen is electroreduced to water. The overpotentials are a manifestation of the sluggish rate of adsorption and reaction of molecular oxygen on Pt cathode electrocatalysts. The identification of more active, cost-effective and corrosion stable electrocatalysts for the oxygen reduction reaction (ORR) therefore continues to be a scientific priority in Fuel Cell catalysis research.
We report the combinatorial discovery, bulk synthesis and physico-chemical characterization of a new structural class of Pt electrocatalysts for use for the ORR in PEM fuel cell cathodes. The catalysts exhibit outstanding performance characteristics in terms of their Pt mass based as well as their Pt surface specific activity for the ORR, meeting Department of Energy performance targets for 2010.
Electrochemical Rotating Disk Electrode (RDE) measurements and physico-chemical characterization - including synchrotron X-ray diffraction (XRD) and synchrotron Small Angle X-ray Scattering (SAXS) - show that rapid de-alloying and corrosion processes of base metal rich alloy nanoparticles of a catalyst precursor compound result in the formation of Pt particle lattices with unusually high lattice strain. The data suggests that the formation of strained Pt lattices is correlated with the favorable catalytic activity. SAXS results further show how the electrochemical treatment affects the particle size and metal composition distributions of the catalytic particles inside their ionomer-carbon matrix. Our synchrotron studies allow us to formulate relationships between synthetic conditions, structural characteristics and electrochemical activity. Experimental observations are compared to DFT computational predictions as to the impact of strain on the ORR activity of Pt lattices.
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