At the nanoscale, these structural flaws do not exist, and hence the materials are nearly “perfect.” In addition, these objects are so small that most of the atoms that comprise them reside on the surface. According to Namboodiri and Kim, the properties of the surface atoms, which are more mobile because they are not bounded on all sides, dominate at the nanoscale. This dominance gives an otherwise brittle material such as silica its counterintuitive fracture characteristics.
“The terms ‘brittle’ and ‘ductile’ are macroscopic terminology,” Kim says. “It seems that these terms don’t apply at the nanoscale.”
Using an atomic force microscope (AFM), Kim and Namboodiri were able to look more closely at interfacial fracture than had been done before at the nanoscale. They found that the silica will stretch as much as gold or silver and will continue to deform beyond the point that would be predicted using its bulk-scale properties.
Hawa, Henz and Zachariah’s simulations reaffirmed their study and added some additional details. They showed that both nanoparticle size and morphology—whether the material is basically crystalline or amorphous, for example—have an effect on the observed ductility and tensile strength because those factors influence the mobility of surface atoms. In the simulations, the smaller the particles in the aggregate the more ductile the material behaved. Crystalline structures exhibited greater strength when stressed and deformed long after the critical yield point observed macroscopically.
Namboodiri explained that although the work is very basic, these findings might one day inform the design of microelectronic mechanical devices.
Mark Esser | EurekAlert!
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