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How size affects particles in the nanoworld

Researchers discover that brittle substances to the naked eye are surprisingly ductile at the nanoscale.

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For ultrasmall particles, size is as important as chemical composition. The mobility of atoms on a particle's surface can make a brittle material ductile (malleable). Particles of identical composition but that are slightly different in size can have distinctly different physical and chemical properties. Scientists exploring the size-properties connection are finding that familiar materials they think they understand can behave in unexpected ways.

Such is the nanoworld, where things are measured by the nanometer. That's one billionth of a meter. It's about the length of six carbon atoms bonded together. Strange things can happen with particles that measure between a few to a couple of dozen nanometers in at least one dimension.

Researchers with the National Institute of Standards and Technology (NIST) found this out as they studied silica, a brittle material. As a NIST announcement explained last month, experiments had shown that silica became "as ductile as gold at the nanoscale." Subsequent computer simulations of nanoparticle aggregates now illustrate how this happens. They also suggest that the smaller the particles in the material aggregate, the more ductile the material becomes. Crystalline structures, in particular, should withstand stress well beyond the critical point seen in macroscopic samples of the material.

A material is better able to withstand stress without cracking when its atoms can move around and maintain cohesion. Brittle materials have structural flaws that limit that mobility and act as failure points. Atoms on a nanoparticle's surface are less constrained than they are in bulk material. This dominates the particles' physical properties and makes a normally brittle material ductile. Also the nanoparticles don't have the bulk materials' structural flaws.

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