A Hard Little Lesson: Squeezed nanospheres grow superstrong

Small is different. That’s a fact of life for scientists studying virus-size chunks of matter called nanoparticles.

Now, the first-ever experimental determinations of the hardness of individual silicon nanospheres reveal just how different mechanical properties can be. The nanospheres are up to four times as hard as bulk silicon, such as the silicon wafers from which computer chips are made, report William W. Gerberich of the University of Minnesota, Twin Cities and his coworkers in the June Journal of the Mechanics and Physics of Solids. The diameter of the spheres ranged from 40 to 100 nanometers.

Gerberich’s team, which includes researchers at Los Alamos (N.M.) National Laboratory, squished silicon nanospheres beneath a diamond point and caused atomic rearrangements to take place inside the spheres. From readings of the force on the spheres, as well as computer simulations of the squeezing process, the scientists calculated that the hardness of the silicon ranks between that of sapphire and diamond, two of the hardest materials known. Bulk silicon’s hardness isn’t in that ballpark.

If this hardness boost occurs in silicon when it’s formed into nanospheres, says Gerberich, perhaps materials that are already extremely hard could be recast into yet harder forms. “I would like to try sapphire and silicon carbide,” he says. The result could be new superhard materials for such uses as industrial polishing processes and making micromachines (SN: 7/22/00, p. 56: Available to subscribers at The Little Engines That Couldn’t).

Gerberich says that the surprising boost in hardness results from a familiar metallurgy process called work hardening. It’s normally achieved by operations such as hammering and rolling. However, unlike the ductile metals that are typically work hardened, bulk silicon is brittle, so it would shatter if subjected to those operations.

Besides hinting at practical payoffs, the new results “do a good job of extending our understanding of material behavior to a size range that has not been well studied before,” comments Richard P. Vinci of Lehigh University in Bethlehem, Pa.

Particularly intriguing, he says, is that the number of atoms in the actual nanospheres approaches the number that can be included in a computer simulation.

This near match suggests that researchers will soon be able to confidently predict from computer models how materials behave. “This . . . opens up a world of possibilities for virtual-materials design,” says Bob R. Keller of the National Institute of Standards and Technology in Boulder, Colo.

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