images give materials researchers new tool for developing
RIDGE, Tenn., April 15, 2004 — New atom-scale images
from the Department of Energy's Oak Ridge National Laboratory
promise to provide researchers the ability to predict
and model the properties and behavior of advanced ceramic
A paper published in the April 15, 2004, issue of the
scientific journal Nature describes research that would
represent a valuable advantage in the development of
strong and heat-resistant materials for a variety of
The work, by ORNL researchers Stephen Pennycook of the
Condensed Matter Sciences Division, Gayle Painter and
Paul Becher of the Metals and Ceramics Division and
visiting researcher Naoya Shibata, reveals, in world-record
0.7 angstrom resolution, the preferred location of atoms
within a silicon nitride ceramic.
Where specific atoms reside is key to the properties
of the materials. The atom-scale images match, almost
exactly, the positions predicted by theoretical calculations.
"With this new confidence in our theories, we will,
in the near future, model materials on a computer screen
and predict their properties without having to actually
fabricate and characterize a large number of samples,
which is very expensive and difficult," Pennycook
The images of silicon nitride were made with ORNL's
300-kilovolt Z-contrast scanning transmission electron
microscope (STEM), aided by an emerging technology called
aberration correction, which uses computer technology
to correct errors introduced to 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 with technology provided by
Pixon LLC of Setauket, N.Y.
Silicon nitride is of great interest to materials researchers
because it is strong and lightweight. However, it is
also intrinsically brittle, so researchers are constantly
searching for ways to make it tougher and less brittle
and thus more suitable for applications that require
strong, heat-resistant and light-weight components.
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 the whisker-like
grains in the silicon nitride by adding certain rare-earth
"doping" agents such as lanthanum oxide. However,
slight changes in the doping agents result in variations
in the properties of the materials. The ability to predict
and manipulate the structure of these materials at the
atomic level will aid researchers in developing the
ceramic materials with the most desirable properties.
Silicon nitride ceramics, like many ceramic materials,
are made by first compressing powders into a desired
shape, which still contains a large amount of pores.
To eliminate the pores, the material is sintered—essentially
baked—at very high temperature, which combined with
oxide powders produces a dense ceramic. The resulting
silicon nitride ceramic also contains a very thin amorphous,
glassy film, which surrounds all the silicon nitride
grains. The properties of the ceramic depend on how
the doping agents eventually situate themselves in the
silicon nitride ceramic. In the past, researchers seeking
the best properties have had to try different combinations
until they arrive at the best material.
"Rare-earth elements like lanthanum and lutetium
have quite different effects," said Becher. "You
get different looking microstructures with different
properties. Lanthanum will produce long, slender reinforcing
grains, while lutetium produces fatter grains. The real
question was, why do these elements cause these changes?
"The theoretical calculations led by Painter predicted
that these elements had different preferences for locating
themselves at the silicon nitride grain surfaces. Those
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, which allows the
grains to grow fatter.
"We know that the particular microstructure we
can obtain and the nature of the amorphous film strongly
affect the properties of the silicon nitride. So the
knowing ‘the why' is critical to the development of
new materials," Becher said.
But determining how accurate the theory was required
finding where specific elements like lanthanum resided.
Because of the presence of the amorphous films around
each silicon nitride grain, "it is very difficult
to see these dopant atoms in a microscope," Pennycook
said, adding that this was a "good problem"
for his world-record holding Z-contrast STEM. Shibata,
who arrived last April, proved to be up to the task.
Shibata's Pixon-enhanced images corresponded to the
theoretical predictions of ORNL's Painter so closely
that Pennycook and Becher, who are both ORNL corporate
fellows, believe researchers will, in the future, be
able to confidently design optimum materials by computer
using advances in theory and the understanding gained
by atomic scale analysis, significantly speeding the
development of new advanced ceramic materials.
"Now we know, at the atomic level, why things are
happening," Becher said. "This will allow
researchers to create materials that are much tougher
and stronger. And those materials will be found in tomorrow's
advanced microturbines and auxiliary power systems for
aircraft and trucks."
Co-authors on the paper are William A. Shelton of the
Computer Sciences and Mathematics Division and Tim Gosnell
of Pixon. The work is sponsored by the DOE Office of
Science, Basic Energy Sciences, Division of Materials
Sciences and Engineering.
ORNL is currently constructing an advanced materials
characterization laboratory that will further the application
of aberration-correction technologies to atom-scale
Oak Ridge National Laboratory is a multiprogram research
facility managed by UT-Battelle for the Department of
image of the thin glassy film between two silicon
nitride grains revealing individual lanthanum atoms
attached to the grain surfaces. The image was recorded
with the world-record 0.7 angstrom resolution scanning
transmission electron microscope at Oak Ridge National
Laboratory and processed with Pixon technology. The
derived atomic model is superimposed.