Researching in Bulk

Bulk materials can be synthesized to create nanoparticles that improve the materials' properties.

Swiss chocolates, cosmetics, and sunscreens—all these products contain nanocrystals that give them their appealing properties.

Toblerone chocolate.

Scientists classify such products as bulk nanostructured materials, a growing area of research at ORNL. The chief building block of these materials is often the nanocluster—a cluster of atoms or molecules measuring only a few nanometers in any direction.

A new phenomenon in which stable oxygen-enriched nanoclusters significantly strengthen ferritic steels is being investigated by David Hoelzer and collaborators in ORNL's Metals and Ceramics Division. The nanoclusters were discovered in a ferritic steel that was mechanically alloyed (MA) with yttrium oxide. Research shows that the MA ferritic steel containing the nanoclusters exhibits a dramatic improvement in high-temperature strength over that of any other iron-based alloy. A primary goal achieved by ORNL staff researching this phenomenon was determining the critical elements and processing conditions that favor the formation of nanoclusters in ferritic steels. The project's success was aided by advanced characterization tools, such as the atom probe microscopes available at ORNL.

ORNL research indicates that the MA ferritic steel may be suitable for high-temperature applications, such as advanced nuclear reactors, because of the high number density and uniform dispersion of the nanoclusters.

Silicon nitride grain boundary doped with lanthanum atoms.

These 2- to 5-nm nanoclusters containing yttrium, titanium, and oxygen have been found to have remarkable stability at temperatures as high as 1300°C. At 600 to 800°C, the temperatures expected in an advanced reactor, tests reveal that the nanocluster-strengthened ferritic steel is very resistant to deformation and rupture.

The reasons why nanoclusters are stable and significantly strengthen the ferritic steel at high temperatures are currently being investigated in a research project led by ORNL Senior Corporate Fellow C.T. Liu. The project's findings may allow the phenomenon of strengthening by nanoclusters to be applied not only to iron-based alloys, but also to other important alloy systems.

"Several ORNL research groups are forming crystalline nanostructures within bulk noncrystalline materials by aging, heating, or seeding the bulk material," says Linda Horton, director of ORNL's Center for Nanophase Materials Sciences. "At the center we have special tools, such as state-of-the-art atom probe microscopes, to help researchers determine whether crystalline nanoclusters are forming in a noncrystalline matrix."

Paul Becher is leading a project with ceramics researchers at Pennsylvania State University in which they use heat but no pressure to densify nanocrystalline zirconium oxide powders to at least 95% of theoretical density with grain sizes down to 30 nanometers. The researchers hope to determine whether these strong ceramic materials can be plastically deformed without breaking, as some scientists believe. Results thus far show such nanocrystalline oxide electrolytes have exceptionally high ionic conductivity, making them potentially usable in fuel cells.

An ORNL project led by Craig Blue involves heat-treating noncrystalline (amorphous) silicon on polymer substrates in a large-area format to incorporate evenly dispersed silicon nanocrystals, using the Laboratory's powerful Vortek infrared plasma lamp. This type of flexible electronics, in which the electronic properties of both nanocrystalline and amorphous silicon are exploited, could prove useful for photovoltaics. The lamp might be an economic technique for fabricating nanocrystalline and amorphous silicon thin-film composites for rooftop solar cells, which convert sunlight into electricity for buildings. Use of this type of silicon thin-film composite as solar cells could greatly boost the photon collection efficiency and reduce the cost of rooftop solar panels.

Yet another ORNL project focuses on fabricating membranes based on the materials used at the Oak Ridge Gaseous Diffusion Plant to enrich a gas in fissionable uranium. Researchers are striving to tailor membranes for such uses as fuel cells, water purification, catalyst supports, protein separation, and filters for homeland security.

One pioneer of research on the bulk behavior of nanostructured materials is Carl Koch, a former ORNL scientist now at North Carolina State University. Another expert on the mechanical properties of bulk nanostructured materials is ORNL Director Jeff Wadsworth. Both, as it turns out, like Swiss chocolates.

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