BERKELEY, CA Because they are riddled with defects, bulk crystalline materials never achieve their ideal strength; nanocrystals, on the other hand, are so small there's no room for defects. ("Nano" is short for nanometer, a billionth of a meter.) Yet while nanocrystalline materials may approach ideal strength in their resistance to stress, most nanostructures have shown only a limited ability to withstand large internal strains before they fail. Overcoming this limitation could lead to great advances in engineered materials on all scales.
Scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley have used in situ transmission electron microscopy to measure hollow spherical nanoparticles that withstand extreme stress and deform without losing strength. The geometry of the novel nanospheres can be engineered to approach the theoretical ideal shear strength of the material from which they are made, in this case cadmium sulfide. The researchers report their results in the November, 2008 issue of Nature Materials, available to subscribers in advanced online publication.
"To understand what happens to the individual nanoparticles when stress is applied, you need to actually see the deformation as it is happening," says Andrew Minor of the National Center for Electron Microscopy (NCEM) in Berkeley Lab's Materials Sciences Division (MSD); Minor is also a member of UC Berkeley's Department of Materials Science and Engineering (MSE). "We put the nanospheres on a flat silicon substrate inside NCEM's In Situ Microscope sample chamber and compressed them with a flat-faced diamond punch until they fractured. Using videos of the compression tests, we could determine exactly when they fractured, and then measure the force applied at that exact moment."
Minor refers to one video in which the graph of applied pressure drops suddenly when the compressed nanosphere squ
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory