Publications
Kinetic Monte Carlo simulation of the aging of nanoporous metals
Nanoporous metallic particles are of great interest for a range of applications including catalysis, gas storage, and electrical energy storage. In particular, recent work has shown that bulk powders of porous palladium can be synthesized in a scalable fashion. This material has pore sizes in the 2-5 nm range and has promise for use in hydrogen storage applications. However, because of the small pore size such materials are very susceptible to morphological evolution during aging, especially at elevated temperatures, leading to degradation of their storage properties. To better understand and predict the phenomena at work in nanoporous metal aging, we have developed a kinetic Monte Carlo (kMC) model for the simulation of atomic diffusion in a Pd lattice. The model is implemented in Sandia's parallelized kMC code SPPARKS. SPPARKS utilizes a spatial decomposition parallelization scheme, allowing large-scale simulations including millions of atoms. The diffusion model includes single-atom hops as well as Schwoebel barrier events that mimic concerted atom motions involving multiple lattice sites. Our simulations show that for statistically homogeneous nanoporous networks, coarsening at elevated temperature as measured by the surface area can be described by a scaling law that closely follows the L {approx} {sup 1/4} scaling predicted by continuum surface diffusion theory. This scaling holds despite the presence of surface faceting due to our simulations being run at temperatures below the roughening temperature of the material. Sensitivities of the rate of coarsening, the scaling exponent, and the amount of surface faceting to model parameters including temperature and event activation rates are explored. Because of the large spatial scales attainable in our computations, we are able to simulate nanoporous particle geometries similar to those synthesized in the laboratory, and compare directly to material aging experiments including porosimetry measurements and TEM images of particles.