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Correlating pit initiation in aluminum with passive oxide defect structure

ECS Transactions

Zavadil, Kevin R.

Microelectrochemical methods are combined with scanning electron microscopy to explore passive oxide breakdown and pit initiation on Al(111) thin films. Anodic galvanostatic polarization is conducted in a microcapillary cell to limit the available current and to restrict the analysis area for subsequent microscopic evaluation. An ability to drive a single pit initiation event is demonstrated using this approach. Subsequent microscopy shows that pore cluster formation on the off-(111) axis facets that are emergent from the grain boundaries is responsible for pit initiation. Early stage, fully formed pits possess oxide membranes that contain pore clusters. Pores evolve from voids that form at the oxide-Al interface, establishing a link between these interfacial voids and pit initiation. Localized oxygen vacancy saturation and the anion-cation vacancy annihilation reaction during anodic polarization drive void growth, the void-to-pore transition, and pit initiation at off-(111) axis facets in this system. ©The Electrochemical Society.

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Hierarchical electrode architectures for electrical energy storage & conversion

Zavadil, Kevin R.; Missert, Nancy A.; Van Swol, Frank

The integration and stability of electrocatalytic nanostructures, which represent one level of porosity in a hierarchical structural scheme when combined with a three-dimensional support scaffold, has been studied using a combination of synthetic processes, characterization techniques, and computational methods. Dendritic platinum nanostructures have been covalently linked to common electrode surfaces using a newly developed chemical route; a chemical route equally applicable to a range of metals, oxides, and semiconductive materials. Characterization of the resulting bound nanostructure system confirms successful binding, while electrochemistry and microscopy demonstrate the viability of these electroactive particles. Scanning tunneling microscopy has been used to image and validate the short-term stability of several electrode-bound platinum dendritic sheet structures toward Oswald ripening. Kinetic Monte Carlo methods have been applied to develop an understanding of the stability of the basic nano-scale porous platinum sheets as they transform from an initial dendrite to hole containing sheets. Alternate synthetic strategies were pursued to grow dendritic platinum structures directly onto subunits (graphitic particles) of the electrode scaffold. A two-step photocatalytic seeding process proved successful at generating desirable nano-scale porous structures. Growth in-place is an alternate strategy to the covalent linking of the electrocatalytic nanostructures.

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Activation of erbium films for hydrogen storage

Journal of Applied Physics

Brumbach, Michael T.; Ohlhausen, J.A.; Zavadil, Kevin R.; Snow, Clark S.

Hydriding of metals can be routinely performed at high temperature in a rich hydrogen atmosphere. Prior to the hydrogen loading process, a thermal activation procedure is required to promote facile hydrogen sorption into the metal. Despite the wide spread utilization of this activation procedure, little is known about the chemical and electronic changes that occur during activation and how this thermal pretreatment leads to increased rates of hydrogen uptake. This study utilized variable kinetic energy X-ray photoelectron spectroscopy to interrogate the changes during in situ thermal annealing of erbium films, with results confirmed by time-of-flight secondary ion mass spectrometry and low energy ion scattering. Activation can be identified by a large increase in photoemission between the valence band edge and the Fermi level and appears to occur over a two stage process. The first stage involves desorption of contaminants and recrystallization of the oxide, initially impeding hydrogen loading. Further heating overcomes the first stage and leads to degradation of the passive surface oxide leading to a bulk film more accessible for hydrogen loading. © 2011 American Institute of Physics.

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Results 151–175 of 231
Results 151–175 of 231