Computer Simulation of Nanoparticle Aggregate Fracture
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Computer Simulation of Nanoparticle Aggregate Fracture Takumi Hawa1,2, Brian Henz3, and Michael Zachariah1,2 1 National Institute of Standards and Technology, Gaithersburg, MD, 20899 2 University of Maryland, College Park, MD, 20742 3 U.S. Army Research Laboratory, APG, MD, 21005 ABSTRACT Nanoparticle aggregates have been found to possess unique mechanical properties. Aggregates of metal nanoparticles can be strained up to 100% before failure, and even typically brittle materials are observed to have a ductile failure mode. In this effort two materials; namely silver and silicon, were chosen to represent ductile and brittle materials, respectively. Aggregates with 2 to 6 particles were simulated using the molecular dynamics (MD) algorithm to determine the stress-strain behavior of the aggregate. Many interesting observations are made including the negligible affect of strain rate on ultimate tensile strength, and the direct relationship between Young’s modulus and nanoparticle size. INTRODUCTION This effort is motivated by trying to gain an understanding of an AFM (Atomic Force Microscope) experiment [1]. In the experiment an AFM tip with a microsphere (radius ~ 12.5µm) of gold or silica attached was used to form a contact with a flat surface of the same material. Using a two nanoparticle aggregate of 5 nm diameter particles we have been able to simulate a system similar to this AFM experiment and measure the force required to separate the AFM tip from the surface after contact has occurred. We have found the largest limitation of the computer simulations for aggregate systems to be the actual versus simulation strain rates. We are currently studying the effect of strain rate on the stress versus strain behavior of the nanoparticle aggregate. These results will provide a relationship for comparing simulation results at a high strain rate to experimental results at a much lower strain rate. SIMULATION RESULTS Using MD simulations we are able to compare results with an AFM experiment that collected force versus displacement data for an AFM tip bonded to a surface. This experiment is modeled as the straining of two sintered nanoparticles of like material. We then extended this model to investigate longer nanoparticle aggregates of up to 6 nanoparticles. These results along with some strain rate sensitivity investigations are presented in the following sections. We also investigate the affect of another important aggregate parameter in this work, namely the primary particle diameter. The nanoparticle diameter affects many of the physical properties of the nanoparticle aggregate. Some of the parameters affected are the contact area, nanoparticle melting temperature [2], sintering time, and Young’s modulus. Through MD
simulations we have been able to determine the relationship between nanoparticle diameter and contact area, a major determining factor in the maximum force for failure of a nanoparticle aggregate. AFM Tip Experiment In this experiment an AFM tip is brought into contact with a flat surface. A
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