Department of Mechanical and Aerospace Engineering

Abstracts 2000 - 2001

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Fluid motion generated by phoretic transport mechanisms explains both the aggregation of particles on an electrode during electrophoretic deposition and bubble aggregation on a heated surface. The lateral interactions extend to several particle (bubble) radii and therefore cannot be attributed to colloidal forces. An externally imposed electrical or thermal field, interacting with each particle or bubble near a planar surface, drives flow of liquid that brings particles and bubbles together. The long-range lateral attraction observed in ensembles of solid particles and gas bubbles on surfaces can thus be understood as mutual entrainment in these micro-flows. Electroosmosis about each deposited particle is the source of the flow in the case of particle aggregation during electrophoretic deposition, while thermocapillary stresses generate a flow about each bubble. The flows in both cases are circulatory and the fluid motion is reversible upon reversal of the field. This unified hydrodynamic theory of aggregation on electrodes fits experimental observations qualitatively and quantitatively for the dc case. The subject of this seminar will be primarily the case of bubble aggregation, and the analogous case for aggregation of particles will be mentioned.

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We have been using electrodeposition to produce epitaxial films of metal oxide semiconductors like Bi₂O₃, Cu₂O, ZnO, Tl₂O₃, and Fe₃O₄ onto substrates such as single-crystal gold. A common theme in this work is that the epitaxial systems have high lattice mismatch. Although this is generally considered to be undesirable for epitaxial growth, strain relief in high mismatch systems (e.g., Ge on Si) is often used to produce ordered nanostructures. We have been exploring a new approach to nanostructure formation in which films undergo a transition from thermodynamic to kinetic control after a critical thickness. Nanostructures are produced at the critical thickness. The thermodynamic to kinetic transition will be discussed for the epitaxial electrodeposition of Cu₂O onto the three low index faces of gold. Very recent results on the electrodeposition of Cu₂O onto single-crystal silicon will also be presented.

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Gas turbine technology has benefited enormously from developments in materials, such as single crystal superalloys. These materials now operate at their maximum temperature capacity. To further enhance efficiency, as well as to control emissions, advanced cooling strategies have been used. The most recent innovations have involved the implementation of thermal barrier systems in conjunction with cooling. Such systems are used to provide both thermal and oxidation protection. They consist of three basic layers: (i) An inner layer comprising a NiAl-based alloy is designed to resist visco-plastic flow while forming (ii) a thin thermally grown oxide (TGO, typically a-alumina), that restricts oxygen ingress and protects the substrate from oxidation. (iii) An outer layer comprises a low thermal conductivity oxide, generally stabilized zirconia. This layer is referred to as a thermal barrier coating (TBC). It is designed to provide a temperature drop of about 150C subject to back-side air cooling. It must also have the strain tolerance to withstand the thermal expansion misfit with the substrate. The thermal barrier system is dynamic. The TGO thickens, the microchemistry and microstructure of the alloy layer change as the Al is depleted, and, in some scenarios, the TBC microstructure changes. The durability of the system is linked to these changes. The major constituent governing performance is the TGO layer, which is highly stressed and which, while thin (a few microns), still represents a high energy density domain. The means whereby this energy density couples into the TBC to cause failure are described. Based on this understanding, approaches for addressing durability are discussed.

Tuesday, December 5, 2000
479 EBU-II
3:30 p.m.

Dr. Stacey F. Bent
Department of Chemical Engineering
Stanford University

As the range of applications for semiconductor-based materials continues to expand, methods that can be used to tailor their surface properties become increasingly important.  Organic modification is one means of providing new functionality to the semiconductor surface, imparting properties useful for passivation, molecular recognition, lubrication, or biocompatibility.  This talk will focus on organic functionalization of Group IV surfaces.  Bifunctional and polyfunctional organic molecules have been used to form direct, covalent bonding at the surface in a dry processing environment.  A combination of experimental and theoretical methods has been used to identify the bonding and reactivity of the organic layers at the semiconductor surface.  On the Si(100)-2x1 surface, unsaturated molecules such as alkenes or dienes react by cycloaddition chemistry across the Si-Si surface dimers.  We show that the [4+2] cycloaddition (Diels-Alder reaction) occurs readily for a range of conjugated dienes at the (100)-2x1 surface of Si, and that the reaction occurs at the surfaces of Ge and C as well.  The reactivity of other functionalities, such as amine groups, with the semiconductor surface has also been explored as a potential means of surface modification.  The use of protecting groups to manipulate the surface reaction will be described, and the potential for these different classes of attachment reactions to impact future applications will be discussed.  An important aspect of these studies is in-situ analysis of the growth and functionalization processes.  I will also describe recent work on single photon ionization methods applied in our laboratory for detecting reactive gas-phase species in-situ.  As an example, the use of this technique for radical detection during hot-wire chemical vapor deposition will be discussed.

Dr. Stacey Bent is Assistant Professor at Stanford University.  Her research interests include semiconductor processing and reactivity, surface science, materials chemistry, chemical vapor deposition, etching, lithography, polymerization reactions under both vacuum and higher pressure conditions, semiconductor surface modification, development of in situ probes for materials processing, surface photochemistry, surface reaction dynamics, and laser spectroscopy.

Information: (858) 534-0113