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Research Activities > Programs > Nonequilibrium Interface and Surface Dynamics 2007

Using Impurities to Tailor Mesoporous Metals: A KMC Study of Dealloying

CSIC Building (#406), Seminar Room 4122.
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Using Impurities to Tailor Mesoporous Metals: A KMC Study of Dealloying

Professor Jonah Erlebacher

Johns Hopkins

Abstract:   Dealloying, the selective dissolution of one or more component from a multi-component alloy, was originally studied in the context of corrosion, where it can lead to undesirable materials failure (e.g, dissolution of Zn from CuZn brass can lead to stress-corrosion cracking). More recently, dealloying has been used to form beautiful and functional mesoporous metals; for instance, dealloying of Ag from Ag/Au yields a nanoporous gold with average pore size ~15 nm in diameter. For some applications, 15 nm pores is good enough, but to really produce metals with extraordinary surface/volume ratios, one would prefer smaller pore sizes, like 2-5 nm. We have been studying the evolution of (nano)porosity during dealloying using Kinetic Monte Carlo simulation. A crystallographically appropriate model incorporating surface diffusion in competition with potential-dependent dissolution rate of silver from an fcc gold/silver lattice is a minimal model that reproduces the important experimental aspects of dealloying. These include the observation of compositional parting limits, critical potentials, dealloying and porosity evolution at sufficiently high potential, and passivation at sufficiently low potential. The model also explains that the ~15 nm pores in silver/gold is a lower limit in this system, due to the intrinsically fast mobility of Au in acidic environments, which leads to a coarsened microstructure. Here we show that the KMC model shows that pore size and critical potential are also highly dependent on step edge mobility, and this can be slowed significantly by the presence of impurities. This effect explains the corrosion protection afforded to brass by the addition of ~1% As to the alloy. It also predicts that the addition of small amounts of lower-mobility Pt (<5%) to Ag/Au alloys should lead not only to smaller pore sizes, but Pt segregation to step edges during dissolution will also lead to a ?core/shell? mesoporous metal ? a kinetically stabilized mesporous metal with ~2 nm pores. We will introduce experiments showing this is indeed the case.

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