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This atomic-force Credit: NIST |
A closer look at a
promising nanotube coating that might one day improve solar cells has
turned up a few unexpected wrinkles, according to new research*
conducted at the National Institute of Standards and Technology (NIST)
and North Dakota State University (NDSU)—research that also may help
scientists iron out a solution.
The scientists have found that coatings made of
single-walled carbon nanotubes (SWCNTs) are not quite as deformable as
hoped, implying that they are not an easy answer to problems that other
materials present. Though films made of nanotubes possess many desirable
properties, the team’s findings reveal some issues that might need to
be addressed before the full potential of these coatings is realized.
“The irony of these nanotube coatings is that
they can change when they bend,” says Erik Hobbie, now the director of
the Materials and Nanotechnology program at NDSU. “Under modest strains,
these films can develop irreversible changes in nanotube arrangement
that reduce their conductivity. Our work is the first to suggest this,
and it opens up new approaches to engineering the films in ways that
minimize these effects.”
High on the wish list of the solar power
industry is a cheap, flexible, transparent coating that can conduct
electricity. If this combination of properties can somehow be realized
in a single material, solar cells might become far less expensive, and
manufacturers might be able to put them in unexpected places—such as
articles of clothing. Transparent conductive coatings can be made of
indium-tin oxide, but their rigidity and high cost make them less
practical for widespread use.
Carbon nanotubes are one possible solution.
Nanotubes, which resemble microscopic rolls of chicken wire, are
inexpensive, easy to produce, and can be formed en masse into
transparent conductive coatings whose weblike inner structure makes them
not only strong but deformable, like paper or fabric. However, the
team’s research found that some kinds of stretching cause microscopic
‘wrinkles’ in the coating that disrupt the random arrangement of the
nanotubes, which is what makes the coating conduct electricity.
“You want the nanotubes to stay randomly
arranged,” Hobbie says. “But when a nanotube coating wrinkles, it can
lose the connected network that gives it conductivity. Instead, the
nanotubes bundle irreversibly into ropelike formations.”
Hobbie says the study suggests a few ways to
address the problem, however. The films might be kept thin enough so the
wrinkling might be avoided in the first place, or designers could
engineer a second interpenetrating polymer network that would support
the nanotube network, to keep it from changing too much in response to
stress. “These approaches might allow us to make coatings of nanotubes
that could withstand large strains while retaining the traits we want,”
Hobbie says.
* E. K. Hobbie, D. O. Simien, J. A. Fagan, J.
Y. Huh, J. Y.Chung, S. D. Hudson, J. Obrzut, J. F. Douglas, and C. M.
Stafford. Wrinkling and Strain Softening in Single-Wall Carbon Nanotube
Membranes. Physical Review Letters, March 26, 2010, 104,
125505.
Media Contact: Chad Boutin, boutin@nist.gov, (301) 975-4261
