Research Projects

Mechanical properties at small length scales

		Large-scale atomistic simulations show dislocation behavior in pre-strained nanopillars.

We are currently examining dislocation behavior in strained nanopillars, to understand length scale effects on mechanical properties. The abilities to simulate large systems and to experimentally probe small volumes of materials have advanced sufficiently that experimental sizes are within the reach of simulations. Of particular interest is examining how the statistics of defects in small volumes affect the mechanical behavior, as experimentally probed both by pillar deformation and by nanoindentation.

Collaborators: Hongbin Bei, Yanfei Gao, Easo George, George Pharr

Poisson ratio effects in metallic glasses

	Simulating liquids and glasses with different potentials examine Poisson ratio effects in glass properties.

The Poisson ratio of a glass has recently been argued to correlate with a number of other properties of the glass and its associated liquid, including the ductility of the glass and the “fragility” of the supercooled liquid. However, there is little fundamental understanding of these correlations. We have constructed an atomistic model that allows us to tune the Poisson ratio, keeping the cohesive energy, bulk modulus, and lattice constant of the T=0 crystalline phase fixed. Our simulations show how changing the potential affects both thermodynamic properties and atomistic processes in the liquids and glasses.

Collaborators: Rachel Aga, Takeshi Egami, Valentin Levashov

Hydrogen storage in carbon systems

	Grand canonical simulations of hydrogen adsorption in carbon allow us to examine mechanisms of hydrogen storage.

The hydrogen storage capability of materials including activated carbon depends sensitively on the structure of the material. Through simulations, we can probe both the structure of the material, and the hydrogen storage capability. Present work includes work on both amorphous carbon and on expanded graphite lattices, in collaboration with experimental efforts examining the structure, kinetics, and storage capability of carbon systems.

Collaborators: Rachel Aga, Cristian Contescu, Takeshi Egami, Nidia Gallego, Lujian Peng

	Simulations of crystal nucleation and growth test the applicability of classical nucleation theory.

Crystal nucleation and growth is of fundamental importance both to theoretical and experimental studies. Using molecular dynamics, we can probe the nucleation process at the atomic scale, and also calculate thermodynamic parameters that control nucleation, including driving forces, diffusion rates, and solid-liquid interfacial free energies. By examining size effects, and correctly incorporating transient nucleation, we have accurately predicted simulation nucleation times from theory, without any fitting parameters, over a significant range of temperatures.

Collaborators: Rachel Aga, Lujian Peng