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Special Seminar

In-Situ Formation of Nanoscaled Al2O3 in Al-ZnO System by Friction Stir Technique

by: Prof. New-Jin Ho

Date: Monday February 05, 2007

Time: 12:00 pm – 12:45 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In past years, we have successfully blended nano-Al2O3 into aluminum alloys with excellent dispersion by friction stir processing. The strength and ductility of this composite is superior to those made by sintering and casting. In addition to successful blending of nano particles in the stir zone, we also found ultra-fine grains ranging from several microns to one hundred nanometers. Nevertheless, it was difficult to make clustering and grain sizes smaller even with adding a large amount of nano-particles unless an in-situ formation of nano oxides occurs.

It was well understood that aluminum can redox most of metal oxides to produce Al2O3 during sintering process at high temperature. An Al-ZnO system was first chosen because zinc is soluble in aluminum at high temperature to form stable alloys at room temperature. The preliminary results indicate that Al2O3 as small as 2 nm~10 nm without nano-clustering can be made in the Al-Zn matrix. According to Orowon strengthening mechanism, the shear strength can be as high as G/20, or 1.5GPa.

Further investigations will be carried out to (1) understand how a large amount of ultra-fine nano Al2O3 were formed at temperatures lower than 600°C, and within a short period (<1 sec) of the friction stir process, (2) understand the crystallographic change in 2nm~10nm Al2O3 from amorphous state to gamma- and to alpha-phase, and at what size is the change irreversible comparing to constraint-free nano-Al2O3, and (3) understand the effect of such fine nano-Al2O3 in excellent dispersion condition on the mechanical properties—such as strength, ductility, low cycle fatigue, fatigue crack propagation, and fracture toughness.

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Special Seminar

Structural Kinetics, Electronic and Optical Properties of Organic Molecules Adsorbed on Solid Surfaces

by: Prof. Vladimir I. Gavrilenko

Date: Friday January 12, 2007

Time: 2:00 pm – 3:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Structural kinetics of molecular aggregates and adsorption of organic molecules on metallic and semiconductor surfaces are studied by first principle methods within Density Functional Theory (DFT). Equilibrium geometries of molecular aggregates and molecules adsorbed on solid surfaces are determined through the total energy minimization method. Electron energy structure and optical functions of solid surfaces and effects of the adsorption of organic molecules are studied by first principle evaluation of eigen-value and eigen-vector problems using ab initio pseudopotentials (PP). Intermolecular interaction is shown to be responsible for substantial modifications of optical spectra of molecular aggregates. It is demonstrated that adsorption of water on transition metal surfaces (silver, gold) could be monitored through differential linear optics (e.g. differential reflectance anisotropy). Electronic structure and optical properties of complex organic dye molecules (Rhodamine 6G) strongly depends on the molecular aggregation. Predicted electron energy structure modifications of silicon surfaces due to the adsorption of ethanol explain observed enhancement of sum frequency generation intensity from nanostructured silicon. Theoretical results are discussed in comparison with optical spectroscopy data (luminescence, optical absorption, reflectance, etc).

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