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A new appreciation for grain boundaries

This illustration of a complexion phase, supplied to <em>Science</em> by Lehigh research scientist Animesh Kundu, shows the blue and orange crystal planes of the interface. The green atoms in the complexion come from dopant elements introduced during the material processing. The complexion phase can be transformed into another complexion state to change the material’s properties.

The boundaries between the bonded crystals, or grains, in metals and ceramics play a “decisive role” in determining the properties and processing of engineering materials, says Martin Harmer.

New characterization techniques are yielding new insights into grain boundaries, says Harmer. This will help overcome abnormal grain growth, embrittlement and other problems associated with materials engineering, and could lead to new applications in semiconductors, biomaterials and clean energy storage and conversion.

Harmer, the director of Lehigh’s Center for Advanced Materials and Nanotechnology (CAMN), recently wrote an article on grain boundaries for the Perspective section of Science, the nation’s leading science journal. Titled “The Phase Behavior of Interfaces,” the article was published in the April 8 issue of the weekly journal.

Because of their intrinsic instability, Harmer wrote, “the different types of structures that can form at grain boundaries have generally not been described as thermodynamic phases.”

Harmer has proposed the term “complexion” to represent “an equilibrium state of matter at a crystalline interface, which is neither amorphous nor crystalline.” His group has identified six grain-boundary complexions in the ceramic alumina, each characterized by a distinct rate of grain growth. The complexions can be controlled—and the material properties of  the ceramic fine-tuned—by making changes, often subtle, in chemistry and temperature.

New classes of materials, with distinctive properties

In his Science article, Harmer reviewed efforts by a research group led by Mor Baram of Technion-Israel Institute of Technology in Haifa, to confirm the existence of complexions in materials with conventional high-resolution transmission electron microscopy (TEM).

“Despite [the] demonstration by Baram et al. for phase behavior by nanofilm complexions, direct evidence for some of the thinner complexions (especially the bilayers and trilayers) is sparse and controversial.

“Such studies push the limits of resolution for conventional high-resolution TEM. Fortunately, that is no longer such a limitation with the availability of the new generation of aberration-corrected scanning transmission electron microscopes of much higher atomic resolution.”

Thus, Harmer wrote, the work by Baram’s group could lead to “a rational and robust unifying scientific framework for the understanding of grain boundaries.

“[This] may enable the design of entirely new classes of materials with distinctive combinations of properties, as well as the optimization of the performance of existing materials in applications such as clean energy storage and conversion.”

 

Story by Kurt Pfitzer

Posted on Wednesday, April 27, 2011

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