The materials investigated in this study are those metals and alloys which tend spontaneously to form protective surface layers and become thereby susceptible to localized corrosion when the protective layers are disturbed. Special attention is given to transport-controlled flow of current between local anodic and cathodic regions on the corroding surface. Topics under current study include passivity breakdown in pits, crevices, and cracks.

A group project explores fundamental surface processes involved in electrochemical deposition of copper films, with specific emphasis on the role adsorbates play in mediating the film growth. The program involves comprehensive studies of the solid-liquid interface in conjunction with analytical modeling of the growth processes. Experimental techniques of study include atomic force microscopy of electrode surfaces, in situ IR spectroscopy of adsorbed organic additives, and confocal fluorescent microscopy.

The objective of the study is to examine whether gas processing can be used to control the particle shapes in supported metal catalysts for the first time. We have found that we can treat particles in reactive atmospheres to produce catalysts with novel properties. We are currently trying to correlate those novel morphologies to the reactivities of the catalysts.

The basic thesis of our research is that high pressure is an essential tool for understanding electronic phenomena in condensed systems. With increasing compression there is increased overlap among electronic orbitals. Different types of orbitals are perturbed to different degrees. A study of these perturbations permits one to characterize electronic states and excitations, to test theories, and, under some circumstances, to induce electronic transitions to new ground states. Our current work involves: (1) The tuning of triplet energy levels in molecules containing N or O atoms in rigid polymeric media. The stabilization of antibonding orbitals with respect to nonbonding orbitals brings about dramatic changes in physical and chemical properties important for applications in molecular electronic devices. (2) A comparison of the emission properties of molecules dissolved in polymers with those attached to a polymer chain.

Surface diffusion on semiconductors is important in several aspects of microelectronic device fabrication. We are making measurements of surface diffusion under real processing temperatures and pressures using our recently developed laser technique of second harmonic microscopy. Under such conditions, we find that the diffusion mechanism changes from simple site hopping to a previously unknown vacancy-mediated form. We are probing surface diffusion in a variety of adsorption systems to determine the precise nature of this mechanism.

Adsorption of molecular hydrogen on silicon surfaces has until now been assumed to be negligible. With the laser technique of surface second harmonic generation, we are showing that under conditions of microelectronics processing, the adsorption rate is quite large. Depending upon conditions, a direct or a surface diffusion-mediated mechanism dominates. The results are important for modeling the kinetic aspects of chemical vapor deposition and etching for electronic device fabrication.

Through the use of molecular dynamics simulations, we are investigating the various mechanisms of surface diffusion and the effects of such mechanisms in surface diffusion-limited chemical reactions. We have examined the transition from site-to-site hopping at low temperature to vacancy-mediated diffusion at high temperature in single-component systems, and we are trying to determine how the governing phenomena apply in heterodiffusion. We are also developing a general quantitative model relating surface diffusion coefficients to bimolecular surface reaction rate constants.

Deposition of titanium silicide is being investigated for metallizing cement and future generations of integrated circuits. Based on ultrahigh-vacuum kinetic studies, we have developed a quantitative predictive model for growth, and have confirmed potential growth conditions in real deposition experiments. This work represents the first such optimization performed based on fundamental kinetic surface studies in any adsorption system of practical interest. Work now focuses on bringing the process into suitable form for large-scale production.

The reaction network describing tropospheric pollution chemistry has until now been thought to involve only homogeneous gas-gas interactions, with solids serving merely as sinks for condensible species. We are showing quantitatively for the first time that in fact reactions of gases with suspended particulates can contribute substantially to this network. We are looking with laboratory reactors at the oxidation of simple ubiquitous alcohols to aldehydes on fly ash where the heterogeneous mechanism dominates the homogeneous, with implications for acid rain and atmospheric carcinogens.

I n this investigation we examine the flow properties of weakly flocculated suspensions. A model system has been chosen in which, by altering temperature, the suspension can be reversibly gelled. By mapping out a phase boundary in temperature volume fraction space, we are able to explore the relationship between flocculation in colloidal suspensions and sol-gel transitions observed in molecular systems. The mechanical properties of the gelled samples are of importance in determining porosity and suspension processibility. We are currently seeking general descriptions of yielding and flow in terms of the depth of the interparticle attractive potential.

The short-range interactions between colloidal particles become increasingly important as the suspension volume fraction is raised and the particle size shrinks. In this project we investigate the role of hydration forces that act between particles spaced on the order of several solvent diameters in controlling the processability of nanophase ceramic precursor powders. By varying the interaction potential through control of pH, ionic strength, and chemical potential of the continuous phase, dense suspensions of particles with diameters of 1 to 10 nm are prepared. The driving forces for densification, rheology of the suspensions, and properties of the fired ceramics are studied.

The structure and flow of dense suspensions of uniform particles is investigated with particular attention paid to how materials flow at high-volume fraction. Methods of achieving flowable suspensions at volume fractions above 0.6 are sought through the use of bimodel mixtures of particles. Effects of particle size ratio and number ratio are studied. Small angle neutron scattering studies are used to characterize microstructures at rest and under shear flow.

Dense suspensions often display a shear rate region where viscosity increases with shear rate. This phenomenon can be quite dramatic, changing low viscosity fluids at low rates of deformation into solids. In this investigation, we detail the role played by particle interaction potential and shape in shear thickening phenomena. Details of the particle interaction potential at small separations are determined with the atomic force microscope and linked to flow behavior seen in bulk suspensions.

The hydrolysis and condensation of metal alkoxides result in the formation of powders or space filling gels, depending on details of the reaction conditions. In this study the nucleation of the gel phase is investigated through optical scattering techniques. Particular attention is paid to events leading to the phase separation and growth of the gel phase from large molecular weight species that are thought to determine optical and structural properties of the resulting materials.