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The
science-fiction fantasy of nanotechnology — building
novel structures, devices, and materials at the atomic
or molecular scale — is becoming a reality. For the
great potential of nanoscience and nanotechnology to be
fully realized, however, research efforts must cross many
disciplines, from electrical engineering, mechanical engineering,
materials science, and computer science to bioengineering,
chemistry, and physics.
Nowhere
is this cross-disciplinary approach fostered more than
at UC Berkeley. Each month, Lab Notes is proud
to present the work of nanotechnology researchers from
the College of Engineering and our collaborators across
the campus.
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Considering Corrosion
by David Pescovitz
Professor
Devine adjusts
equipment used to identify molecules resting on specific
kinds of metals. The spectra, once analyzed, act as fingerprints
of the particles, some of which can help prevent corrosion.
(Click for larger image.)
Peg Skorpinski photo
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The full impact
of Thomas M. Devine Jr.'s research into the nanoscale properties
of certain metals may not be fully realized for millennia. At the
same time, his laboratory results could have a positive impact on
the computer industry immediately.
"Events on the scale of atoms are dictating the performance
and lifetime of structures" ranging from the magnetic media
in hard disks to piping in nuclear power plants to stainless steel
canisters that must safely contain radioactive waste for thousands
of years, says Devine, also the chair of Berkeley's Department of
Materials Science and Engineering.
One of Devine's most recent efforts is to understand what causes
corrosion in computer disk drives. In today's hard drives, a disk
of magnetic material is covered in a non-magnetic chromium-rich
alloy that keeps the magnetic grains isolated. Because the magnetic
alloy is prone to corrosion and wear, it's coated with a diamond-like
carbon layer that protects it. To a certain point.
As the storage density of hard drives increases – currently
at a rate of 100 percent per year – the read/write heads have
to be moved closer to the disk itself. This means that the carbon
layer has to become increasingly thinner. A film of carbon thicker
than 10 nm protects the disk by acting as a chemically-inert, electrical
insulator. The problem is that at about sit to ten nanometers, the
carbon layer's protectant properties fundamentally change. This
is a consequence of the carbon layer becoming so thin that its properties
are less dependent on its bulk characteristics and more dependent
on the nature of the interfaces that it forms with, on one side,
the metal substrate and, on the other side, the atmosphere.
Two effects result from decreasing the carbon's thickness below
10 nm. First, the films no longer block charge, as an insulator
does. This can actually increase the corrosion of the magnetic alloy
exposed to the atmosphere through holes in the carbon. Second, the
limited chemical reactivity of carbon becomes significant.
"Although it's considered a nonreactant, the layer forms what's
analogous to rust," Devine explains. "When the coating
is relatively thick, that doesn't matter. But if the layer is only
four or five atoms, that corrosion can penetrate right through."
Through laboratory experiments using nanotechnological tools like
atomic force microscopy, Devine and his students discovered that
the surface corrosion is caused by specific contaminants in the
atmosphere. The next step is to determine why these specific oxidizers
affect the material and how to prevent the corrosion.
"We hope to use the information we're collecting to not only
determine what kind of contaminants must be avoided by also how
to improve quality-control testing during the manufacturing process,"
he says.
Devine's scientific understanding of naturally-occurring nanoscale
films also has applications in the nuclear power industry. In order
for nuclear power plants to be granted operating license extensions,
the plant operators "must show that they understand what could
cause their plants to fail," Devine says. This failure often
occurs as a result of corrosion at the molecular level.
Take the snaking pipes that carry high-pressure coolant water past
the fuel of the nuclear reactor. The pipes are made from stainless
steel, a metal that grows its own anti-corrosive thin film when
exposed to water and oxygen. The film, Devine says, behaves similarly
to a protective paint. But when the stainless steel is stressed,
at welded joints for example, the thin film cracks, leading to "stress-corrosion."
Eventually, the pipe itself will also crack allowing radioactive
water to leak and putting the plant at risk of a meltdown. The problem
is that these cracks can take five to ten years to become large
enough to be detected by traditionally methods like X-Rays, at which
point they need to be rapidly repaired.
To solve this detection limitation, Devine and his team propose
placing containers of pre-stressed bits of stainless steel samples
(each sample is only 50 nanometers thick) in specific locations
inside the cooling system. Plant operators can keep a constant look-out
for a cracked sample, Devine says, so they can know immediately
"that the conditions inside the plant are appropriate for cracking
to occur on a larger scale."
"The operators will realize right away that whatever they just
did (a certain power cycling procedure, for example) was not a good
thing."
If the cracks are detected when they're only nanometers in size,
the plant engineers can schedule maintenance well in advance instead
of having to shut down the plant shortly after a crack has grown
large enough to be detected using traditional means.
Devine hopes the information he gleans about these naturally-occurring
shields against corrosion will also help engineers determine the
reliability of storage systems for buried nuclear waste.
"In the past, our corrosion resistant structures like airplanes
and oil wells have only needed to last for forty years or so,"
he says. "How do you make sure these films that last for forty
years will maintain their properties for 10,000 years?"
Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking
research underway today at the College of Engineering that will dramatically change our lives tomorrow.
Editor, Director of Public Affairs: Teresa Moore
Writer, Researcher: David Pescovitz
Designer: Robyn Altman
Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu.
© 2002 UC Regents.
Updated 7/25/02.
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