ORNL has achieved a new world record in electron microscopy, attaining 0.6-angstrom (Å) resolution using a 300-kilovolt Z-contrast scanning transmission electron microscope (STEM) housed in a new $6 million facility. ORNL Corporate Fellow Steve Pennycook, Matt Chisolm, Albina Borisevich, and Andy Lupini, all of ORNL's Condensed Matter Sciences Division (CMSD), eclipsed the Laboratory's previous world record of 0.78 Å established in 1999, thanks to funding from ORNL's Laboratory Directed Research and Development Program.
Incredibly sharp, atom-scale images give researchers a leg up in predicting and modeling the properties and behavior of advanced ceramic materials. A paper in the journal Nature by Pennycook; Gayle Painter and ORNL Corporate Fellow Paul Becher, both of ORNL's Metals and Ceramics Division; and visiting researcher Naoya Shibata, illustrates the advantage the Z-contrast STEM gives to researchers seeking to develop strong, heat-resistant materials.
The work reveals the preferred location of "dopant" atoms—atoms added in small
amounts to influence the host's properties—within a silicon nitride ceramic.
Where specific atoms reside is key to the properties of the materials.
"With this new confidence in our theories, we will soon model materials on a computer screen and predict their properties," Pennycook says. "We will be able to minimize the dif- ficult and expensive task of fabricating and evaluating a large number of samples."
Images of atoms in silicon nitride, along with CMSD's record- setting images, were obtained with the help of an emerging technology from Nion Company called aberration correction, which uses computer technology to correct errors introduced into the images by imperfections in the electron lenses. Shibata, a fellow of the Japan Society for the Promotion of Science, produced the images, which were then refined using technology provided by Pixon LLC of Setauket, New York.
Silicon nitride could be useful for highly efficient energy production devices because it is strong, lightweight, and heat resistant. But it is also intrinsically brittle, so researchers are searching for ways to make it less likely to fracture.
One way to toughen the material is to induce the growth of whisker-like grains that act much like reinforcing rods in concrete. Researchers know how to form whisker-like grains by adding certain rare-earth "doping" agents such as lanthanum oxide. However, slight changes in how the doping agents eventually situate themselves in the silicon nitride ceramic affect the materials' properties. In the past, researchers seeking the best properties have had to try different combinations until they arrived at the best material.
"Rare-earth elements like lanthanum and lutetium have quite different effects," says Becher. "You get different looking microstructures with different properties. Our question was, 'why do these elements cause different changes?'
"Theoretical calculations led by Painter predicted that these elements had different preferences for locating themselves at the silicon nitride grain surfaces. Atoms like lanthanum were seen to want to go to the grain surfaces, causing long, thin grains to form. On the other hand, lutetium was predicted to be less likely to locate next to the grain surface, allowing the grains to grow fatter.
"We know that the particular microstructure we obtain and the nature of the amorphous film strongly affect silicon nitride's properties. So knowing 'the why' is critical to the development of new materials."
Because of the presence of amorphous films around each silicon nitride grain, "it is very difficult to see these dopant atoms in a microscope," Pennycook says, adding that this was a "good problem" for his world-record-holding Z-contrast STEM. Shibata's Pixon-enhanced images corresponded to Painter's theoretical predictions so closely that Pennycook and Becher believe future researchers will be able to confidently design materials by computer, significantly speeding up the development of new advanced ceramic materials.
"Now we know, at the atomic level, why things are happening," Becher says. "The world's most powerful microscope will enable the creation of materials that are tougher and stronger. Those materials will be found in advanced microturbines and auxiliary power systems for aircraft and trucks."—Bill Cabage
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