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August 27, 2006

Materials with nearly zero structural imperfections possible with controlled grain size

A scientist at North Carolina State University has discovered that the tiny grains comprising many bulk materials can potentially contain nearly zero structural imperfections when the grains are smaller than a certain critical size, typically a few to several nanometers.

Materials created with grains of the right (optimal) size could be structurally flawless. Not only would these materials possess exceptional strength and durablity, but their optical, electrical, and magnetic properties could be vastly improved as well. The number of potential applications, such as smart sensors and ultra-efficient “solid-state” lighting, as well as entire industries impacted, such as automobiles and defense, is staggering.

Narayan discusses the many types of defects that can be present in a material and shows, via theoretical arguments and a couple of specific examples, how controlling grain size may be able to prevent them.

One type of defect is a dislocation, an irregularity in the repetitive pattern of a crystalline material that compromises strength. It is nearly impossible to eliminate dislocations, Narayan says, but reducing the grain size can theoretically restrict the movement of a dislocation so that it can't propagate through the rest of the material. For copper, he calculates that this will occur at the critical grain size of about 7.5 nanometers (nm).

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