On the stabilization of ion sputtered surfaces

The classical theory of ion beam sputtering predicts the instability of a flat surface to uniform ion irradiation at any incidence angle. We relax the assumption of the classical theory that the average surface erosion rate is determined by a Gaussian response function representing the effect of the collision cascade and consider the surface dynamics for other physically-motivated response functions. We show that although instability of flat surfaces at any beam angle results from all Gaussian and a wide class of non-Gaussian erosive response functions, there exist classes of modifications to the response that can have a dramatic effect. In contrast to the classical theory, these types of response render the flat surface linearly stable, while imperceptibly modifying the predicted sputter yield vs. incidence angle. We discuss the possibility that such corrections underlie recent reports of a ``window of stability'' of ion-bombarded surfaces at a range of beam angles for certain ion and surface types, and describe some characteristic aspects of pattern evolution near the transition from unstable to stable dynamics. We point out that careful analysis of the transition regime may provide valuable tests for the consistency of any theory of pattern formation on ion sputtered surfaces

Development of Knife-Edge Ridges on Ion-Bombarded Surfaces

We demonstrate in both laboratory and numerical experiments that ion bombardment of a modestly sloped surface can create knife-edge like ridges with extremely high slopes. Small pre-fabricated pits expand under ion bombardment, and the collision of two such pits creates knife-edge ridges. Both laboratory and numerical experiments show that the pit propagation speed and the precise shape of the knife edge ridges are universal, independent of initial conditions, as has been predicted theoretically. These observations suggest a novel method of fabrication in which a surface is pre-patterned so that it dynamically evolves to a desired target pattern made of knife-edge ridges.Comment: 5 pages, 4 figure

Evolution of Nanoporosity in Dealloying

Dealloying is a common corrosion process during which an alloy is "parted" by the selective dissolution of the electrochemically more active elements. This process results in the formation of a nanoporous sponge composed almost entirely of the more noble alloy constituents . Even though this morphology evolution problem has attracted considerable attention, the physics responsible for porosity evolution have remained a mystery . Here we show by experiment, lattice computer simulation, and a continuum model, that nanoporosity is due to an intrinsic dynamical pattern formation process - pores form because the more noble atoms are chemically driven to aggregate into two-dimensional clusters via a spinodal decomposition process at the solid-electrolyte interface. At the same time, the surface area continuously increases due to etching. Together, these processes evolve a characteristic length scale predicted by our continuum model. The applications potential of nanoporous metals is enormous. For instance, the high surface area of nanoporous gold made by dealloying Ag-Au can be chemically tailored, making it suitable for sensor applications, particularly in biomaterials contexts.Comment: 13 pages, PDF, incl. 4 figures. avi movies of simulations available at http://www.deas.harvard.edu/matsci/downdata/downdata.htm

Electricity Storage for Intermittent Renewable Sources

With sufficient electricity storage capacity, any power production profile may be mapped onto any desired supply profile. We present a framework to determine the required storage power as a function of time for any power production profile, supply profile, and targeted system efficiency, given the loss characteristics of the storage system. We apply the framework to the electrochemical storage of intermittent renewable power, employing a simplifying linear response approximation that permits the entire efficiency behavior of the system to be described by a single scalar figure of merit—the discharge power capacity. We consider three exemplary grid supply scenarios: constant, grid-minus-baseload, and square wave; and two different production scenarios: wind with a capacity factor 32.5%, and solar photovoltaic (PV) with a capacity factor of 14%. For each of these six combinations of scenarios, the storage energy and discharge power capacity requirements are found for a range of system efficiencies. Significantly diminishing efficiency returns are found on increasing the discharge power capacity. Solid-electrode batteries are shown to have two orders of magnitude too little energy to power ratio to be well suited to the storage of intermittent renewable power.Engineering and Applied Science

Performance Model of a Regenerative Hydrogen Bromine Fuel Cell for Grid-Scale Energy Storage

We develop a performance model for a polymer electrolyte membrane based regenerative hydrogen-bromine fuel cell (rHBFC). The model includes four voltage loss mechanisms: ohmic loss through the membrane, hydrogen electrode activation, bromine electrode activation, and bromine electrode mass transport. We explore a large parameter space by looking at the dependences of each of these losses as a function of two “operating parameters”, acid concentration and temperature; and five “engineering parameters”, bromine electrode exchange current density, hydrogen electrode exchange current density, membrane thickness, diffusion layer thickness, and hydrogen gas pressure. The relative importance of each of the losses is explored as both the engineering parameters and operating parameters are varied. The model is also compared to published experimental results on the performance of a hydrogen-bromine cell. By varying engineering parameters and operating parameters within plausible ranges, we project that, with further research, a cell of this design could be developed that operates at greater than 90% voltage efficiency at current densities 700 mA cm­-2 in both electrolytic and galvanic modes and that has a peak galvanic power density of 2760 mW cm-2

Interface Stability During Rapid Directional Solidification

Engineering and Applied Science

Unified Kinetic Model of Dopant Segregation During Vapor-Phase Growth

We develop a unified kinetic model for surface segregation during vapor phase growth that concisely and quantitatively describes the observed behavior in silicon-based systems. A simple analytic function for the segregation length is derived by treating terrace-mediated and step-edge-mediated mechanisms in parallel. The predicted behavior of this parameter is examined through its temperature, flux, and terrace length dependence. Six distinct temperature regimes are predicted for the segregation length that depend on the relative segregation energies and activation barriers of the two mechanisms. The model is compared to reported behavior of Sb and P in Si(001) and excellent agreement is obtained using realistic energies and preexponential factors. The model accounts for the experimentally observed anomalous low-temperature segregation of Sb as a consequence of the competition between step-edge-mediated segregation, dominant at low temperatures, and terrace-mediated segregation, dominant at higher temperatures. The generalized treatment of segregation mechanisms in the model makes it applicable to other segregating systems, including metals and III-V semiconductors.Engineering and Applied Science

On the Temperature Dependence of Point-Defect-Mediated Luminescence in Silicon

We present a model of the temperature dependence of point-defect-mediated luminescence in silicon derived from basic kinetics and semiconductor physics and based on the kinetics of bound exciton formation. The model provides a good fit to data for W line electroluminescence and G line photoluminescence in silicon. Strategies are discussed for extending luminescence to room temperature.Engineering and Applied Science