Tools of the Trade
environmental chamber aids nano-studies of
CHAMBER – Pete
Baldo built the environmental chamber that allows
materials scientists to watch materials oxidize.
Ill. (April 7, 2006) — A new environmental chamber
constructed by Argonne's Materials
Science Division allows researchers to watch
materials as they grow step-by-step while interacting
in elevated-temperature, reactive-gas environments.
The first experiment in the new chamber revealed
intriguing information about how copper oxidizes
at the nano-level and established a new basic model
for understanding oxidation.
The initial study found that clean copper surfaces
are more resistant to oxidation than previously expected
when exposed to oxygen. These findings could lead
to improved electronic components. Industry is interested
in using copper in some devices that are processed
at high temperatures with oxygen present, but has
been concerned that the copper might oxidize, leading
to degraded electrical properties.
The chamber may also help researchers find better
ways to produce hydrogen from hydrocarbons.
Oxides can be protective, as when alumina forms
on aluminum surfaces, or damaging, as when iron rusts
and fails. Understanding these processes at the atomic
level will allow researchers to manipulate oxidation
to create better materials.
ISLANDS – With heat and time, small islands
oxidize on the copper substrate. As seen in this
transmission electron microscope image taken after
20 minutes at 350 degrees Celsius, the islands are
about 200 nanometers, or about 1,000th the diameter
of a human hair.
studies have traditionally been conducted on thick,
mature oxide layers on bulk materials. More recently,
transmission electron microscopy has revealed local
oxidation processes at the mesoscopic level – the
scale at which one can reasonably discuss the properties
of a material or phenomenon without having to discuss
the behavior of individual atoms. But the new environmental
chamber permits X-ray diffraction measurements at
Photon Source (APS) to reveal oxidation at the
atomic level – including chemical and microstructural
evolution – in a controlled environment over a sample's
"Synchrotron X-rays are excellent for that," explained
Jeff Eastman, MSD alloy oxidation group leader. "The
X-rays penetrate the layers where the oxygen is reacting
with the material's surface, and we can control the
angle of the X-rays to either penetrate deeply into
the material's surface or to provide exquisite sensitivity
to just the top few monolayers." A monolayer is a
one-atom thick structure.
The APS, source of the most brilliant research X-rays
in the Western Hemisphere, enables rapid measurements
while the processes are occurring.
first environmental chamber experiments focused
on copper. “We studied copper," Eastman said, "because
it is an important material whose oxidation is still
not fully understood. It is also attractive from
a research standpoint because both copper and its
most common oxide have simple, repeating cubic structures
that simplify data acquisition and analysis."
For this study, a thin film of copper was placed
in the chamber, and both temperature and gas environment
were controlled. As oxygen was added to the chamber,
it initially formed an ordered monolayer over the
GROWTH – Upper image: The first oxide monolayer
covers the copper sample. Lower image: With time
and heat some of the copper and oxygen mix to create
the oxygen level increased,” Eastman said, “it reacted
with the copper, and small ‘nano-islands' of copper
oxide appeared on the surface, resembling little
hockey pucks about 100 nanometers wide and just a
few nanometers thick.”
the islands formed, researchers varied the temperature
and oxygen levels to study the islands' growth.
Research revealed which conditions caused the islands
to grow or shrink. Scientists determined the phase
boundary – the dividing line that distinguishes
between growing or shrinking – for several temperatures.
“Compared to studies of bulk materials, the behavior
we observed for copper oxide nano-islands are strikingly
different,” Eastman said. “These oxide islands that
form at the beginning will actually shrink and disappear
under conditions that would cause larger, bulk copper
The research, which is funded by the U.S. Department
of Energy's Office of Basic
Energy Sciences , was carried out by a team of
MSD scientists including Eastman, Dillon Fong, Paul
Fuoss, Lynn Rehn, Guangwen Zhou, Pete Baldo, and
Loren Thompson. The results were recently published
Physics Letters ( 87, 051914,
The chamber has been used at the APS BESSRC/XOR sector
12 beamline. The APS' high-energy X-rays are ideal
for these studies since their penetrating nature
enabled the team to design the chamber with chemically
inert quartz walls that permit studying materials
in hostile environments. The chamber is currently
plumbed so that samples can be exposed to controlled
mixtures of oxygen, argon, hydrogen, methanol, carbon
monoxide, carbon dioxide or water vapor. The temperature
can be maintained from room temperature to 1,000
degrees Celsius, and the gas pressure in the chamber
can be adjusted over almost 10 orders-of-magnitude.
Future studies are not limited to metal and alloy
oxidation. For example, studies of the interaction
of hydrocarbons with metal or oxide materials could
help researchers develop more efficient ways to produce
and store hydrogen.
“With this new environmental chamber we also plan to
study catalysis and other gas/solid interactions," said
George Crabtree, director of Argonne's Materials Science
Division. — Evelyn Brown
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