A Molecular Dynamics Study of Short Nanoparticle Chains under Mechanical Strain: Linear and Kinked Configurations
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A Molecular Dynamics Study of Short Nanoparticle Chains under Mechanical Strain: Linear and Kinked Configurations Adamos Dalis and Sheldon K. Friedlander Department of Chemical Engineering, University of California in Los Angeles Los Angeles, CA 90095-1592
ABSTRACT Previous studies in our laboratory have shown that individual nanoparticle chain aggregates (NCA) exhibit remarkable mechanical behavior when under strain inside the transmission electron microscope. NCA made of various materials (e.g. carbon, metal oxides, metals, etc.) were strained by as much as 100% when tension was applied to them. After breaking, the NCA rapidly contracted to form more compact structures. In this study, molecular dynamics (MD) computer simulations are employed to investigate, at the atomic scale, the behavior of short nanoparticle chains under strain and to obtain quantitative information of the forces involved in chain straining and fracturing. The interaction potential used is that of copper obtained with the embedded atom method (EAM). Although the methodology is generally applicable, copper was selected as a test material because reliable interatomic potentials are available. Seven singlecrystal nanoparticles, each 2.452 nm in diameter, are placed in contact in two chain configurations, linear and kinked. The structures are initially relaxed adiabatically with MD steps for 225 ps, at a starting temperature of 300 K. The bonding energy between any two nanoparticles in contact ranges from about 20 eV to 30 eV at 0 K. The two relaxed chain configurations are strained along their longest dimension, to the breaking point, at strain rates spanning from 0.3 m/s to 10 m/s. We identify mechanisms of stress accommodation that lead to plastic deformation and eventually fracture for both chain configurations, linear and kinked, and we construct the corresponding stress-strain curves. The two chain configurations exhibit different mechanical behavior. Applications of our experimental and simulation studies on NCA are to the behavior of nanocomposite materials, including carbon black reinforced rubber, sampling of aggregates by high speed impactors and the formation of flexible coatings of nanoparticles.
INTRODUCTION Previous studies in our laboratory have shown that individual nanoparticle chain aggregates (NCA) exhibit remarkable elastic behavior when under strain inside the transmission electron microscope (TEM) [1-3]. NCA made of various materials (e.g. carbon, metal oxides, metals, etc.) were strained by as much as 100% when tension was applied to them at strain rates from 0.5 nm/s to 300 nm/s. Much of this strain was probably associated with rotation and grain boundary sliding of the particles that led to chain straightening. After breaking, the straightened NCA rapidly contracted to form smaller compact structures. The purpose of the current work is to obtain atomic scale understanding of straining a nanoparticle chain to its breaking point. Within the time and length scales attainable by molecular dynamics (MD), this is a di
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