Project 2–5: Nanomechanics of Metal Plasticity in Thin Films and Cylinders

Principal Investigator: Wei Cai (Stanford University)

Electronic devices made using microscopically-sized parts are often limited by material behaviors that are unique to that size scale. Handheld spectrometers, inter-subband cascade lasers, next-generation solar cells, and high-resolution imaging satellites are among the devices that are currently constrained by low pixel operability and energy localization arising from dislocations (crystal defects) within a central detector element (usually a charge-coupled device).

Material behavior at small scales is an important predictor of the durability and useful lifetime of micro-electronic and micro-electromechanical devices (MEMS). Many of the mechanical properties of sub-micron-sized metal or semiconductor particles are size-dependent. These properties, including yield strength and resistance to fatigue, are not well predicted by macroscopic characterization.

Modeling and simulation tools guide materials scientists toward the best methods of producing durable, reliable nanoscale device components. Simulating such systems requires detailed models of materials with a high ratio of surface area to bulk material, the capability to simulate effects on a variety of size scales, and the inclusion of methods for simulating strain characteristics produced under varying stress rates.

ParaDiS (http://paradis.stanford.edu), a dislocation dynamics HPC code, is being developed to simulate the plastic deformation of metal thin films and cylinders under high rates of strain. Efficient algorithms have been designed to simulate strain hardening effects and stresses from free surfaces in films. At present, ParaDiS is the only code that can simulate the plastic deformation of a 10 cubic-micron bulk sample under 3D periodic boundary conditions up to a few percent plastic strain. This requires 512 CPUs running over several months. When free surfaces are added to the model, the calculation requires even more time.

The first direct comparison between the dislocation dynamics models and existing molecular dynamics models has been completed for dislocations in a free-standing thin film. When image stress (surface-related stress) is correctly accounted for, both types of models are in excellent agreement. The ParaDiS code was developed to model metals, but recent and upcoming work is focused on expanding it for use with semiconductor films as well.

The Stanford team is collaborating with a team at Lawrence Livermore National Laboratory to develop the ParaDiS software (which is now under version control), to exchange bug fixes, and to make joint public releases. A web-based developer forum is being set up to provide the Army Research Lab with frequent updates of new features and bug fixes.

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