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Computer simulations demonstrate the necking and extension that occurs during the separation of both amorphous (top) and crystalline (bottom) nanoparticles.
The point at which a material will break on the macroscale has no bearing on how it will perform on the nanoscale, according to recent findings by researchers at the National Institute of Standards and Technology (NIST; College Park, MD; www.nist.gov). Using an atomic force microscope and computer simulation, the researchers discovered that silica—brittle in bulk form—exhibits ductility on the microscale.
As scientists have long known, materials such as gold or silver are characterized by ductility because their atoms can shift and cling together with greater resiliency than can the atoms of such breakable materials as silica. By imaging the surface of silica nanoparticles, however, researchers discovered that the distinction doesn’t apply on the nanoscale.
“We observed that the surface atoms of silica are mobile because they’re not surrounded by other atoms on all sides,” says Pradeep Namboodiri, NIST scientist. “This is important because surface atoms dominate on the nanoscale.” Because of the mobility of surface atoms, silica nanoparticles are stronger than the same material in bulk form.
Furthermore, researchers used computer simulation to explain the incongruity between silica in the observable world and in the microscopic world. They determined that the size of a material’s nanoparticles is a primary factor. Materials with smaller nanoparticles exhibited greater ductility and tensile strength on the macroscale than materials with larger nanoparticles. In addition, the morphology characteristics of the nanoparticles are important—amorphous structures hold up better on the macroscale than crystalline structures. Computer images of amorphous and crystalline structures on the nanoscale, however, showed little difference.
The findings could one day affect the design of microelectronic devices, particularly the manner in which potential materials are assessed, according to Namboodiri. For now, the researchers plan to expand their investigation. “So far, the experiments have taken place in a vacuum environment,” Namboodiri says. “Next, we plan to conduct experiments in ambient conditions and see how the behavior of the nanoparticles is impacted by the presence of water vapor.”