HOUSTON -- (Nov. 2, 2009) -- Rice University scientists today
unveiled a method for the industrial-scale processing of pure
carbon-nanotube fibers that could lead to revolutionary advances in
materials science, power distribution and nanoelectronics. The
result of a nine-year program, the method builds upon
tried-and-true processes that chemical firms have used for decades
to produce plastics. The research is available online in the
journal Nature Nanotechnology.
"Plastics is a $300 billion U.S. industry because of the massive
throughput that's possible with fluid processing," said Rice's
Matteo Pasquali, a paper co-author and professor in chemical and
biomolecular engineering and in chemistry. "The reason grocery
stores use plastic bags instead of paper and the reason polyester
shirts are cheaper than cotton is that polymers can be melted or
dissolved and processed as fluids by the train-car load. Processing
nanotubes as fluids opens up all of the fluid-processing technology
that has been developed for polymers."
The report was co-authored by an 18-member team of scientists
from Rice's Richard E. Smalley Institute for Nanoscale Science and
Technology, the University of Pennsylvania and the Technion-Israel
Institute of Technology. Co-authors include Smalley Institute
namesake Rick Smalley, the late Nobel laureate chemist who
developed the first high-throughput method for producing
high-quality carbon nanotubes, as well as Virginia Davis, a former
doctoral student of Pasquali's and Smalley's who is now a professor
at Auburn University, and Micah Green, a former postdoctoral
researcher of Pasquali's who is now a professor at Texas Tech
University.
The new process builds upon the 2003 Rice discovery of a way to
dissolve large amounts of pure nanotubes in strong acidic solvents
like sulfuric acid. The research team subsequently found that
nanotubes in these solutions aligned themselves, like spaghetti in
a package, to form liquid crystals that could be spun into
monofilament fibers about the size of a human hair.
"That research established an industrially relevant process for
nanotubes that was analogous to the methods used to create Kevlar
from rodlike polymers, except for the acid not being a true
solvent," said Wade Adams, director of the Smalley Institute and
co-author of the new paper. "The current research shows that we
have a true solvent for nanotubes -- chlorosulfonic acid -- which
is what we set out to find when we started this project nine years
ago."
Following the 2003 breakthrough with acid solvents, the team
methodically studied how nanotubes behaved in different types and
concentrations of acids. By comparing and contrasting the behavior
of nanotubes in acids with the literature on polymers and rodlike
colloids, the team developed both the theoretical and practical
tools that chemical firms will need to process nanotubes in
bulk.
"Ishi Talmon and his colleagues at Technion did the critical
work required to help get direct proof that nanotubes were
dissolving spontaneously in chlorosulfonic acid," Pasquali said.
"To do this, they had to develop new experimental techniques for
direct imaging of vitrified fast-frozen acid solutions."
Talmon said, "This was a very difficult study. Matteo's team not
only had to pioneer new experimental techniques to achieve this,
they also had to make significant extensions to the classical
theories that were used to describe solutions of rods. The Technion
team had to develop a new methodology to enable us to produce
high-resolution images of the nanotubes dispersed in chlorosulfonic
acid, a very corrosive fluid, by state-of-the-art electron
microscopy at cryogenic temperatures."
Co-author Nicholas Parra-Vasquez, a Rice graduate student
advised by Pasquali who is now working in France, said, "In looking
at the project when I started, I had no idea where it was going to
end up and how much work needed to be done. The project encompassed
many students and professors, as well as collaborations with other
schools. Because of this, it was a slow process but one that left
no avenue unchecked. Looking on it now, I can't believe how big it
became -- how much effort was put into every point found."
Few technological breakthroughs have been hyped as much as
carbon nanotubes. Since their discovery in 1991, nanotubes have
been touted as everything from a cure for cancer to a solution for
the world's energy crisis. The hype is all the more remarkable
given that nanotubes are notoriously difficult to work with and
that chemists worldwide struggled for years even to make them.
So why the hype? Put simply, carbon nanotubes are remarkable.
While they are roughly the same size and shape as some rodlike
polymer molecules, nanotubes can conduct electricity as well as
copper, and they can be either metals or semiconductors. They can
be tagged with antibodies to diagnose diseases or heated with radio
waves to destroy cancer. They've been used to make transistors far
smaller than those in today's finest microchips. Nanotubes also
weigh about one-sixth as much as steel but can be up to 100 times
stronger.
"Kevlar, the polymer fiber used in bulletproof vests, is about
five to 10 times stronger than our strongest nanotube fibers today,
but in principle we should be able to make our fibers about 100
times stronger," Pasquali said. "If we can realize even 20 percent
of our potential, we will have a great material, perhaps the
strongest ever known.
"The electrical conductivity is already pretty good," he said.
"It's about the same of the best-conducting carbon-carbon fibers,
and that could be improved 200 times if better production methods
for metallic nanotubes can be found."
The new research appears just as the Smalley Institute prepares
for a 10th anniversary celebration Nov. 5 of the creation of
Smalley's "HiPco" reactor, the first system capable of producing
high-quality nanotubes in bulk. HiPco, short for high-pressure
carbon monoxide process, broke the logjam on nanotube production
and cleared the way for more scientific study and for industry to
begin using them in some materials. Industrial nanotube reactors
today generate several tons of low-quality carbon nanotubes per
year, and the worldwide market for nanotubes is expected to top $2
billion annually within the next decade.
But a final breakthrough remains before the true potential of
high-quality carbon nanotubes can be realized. That's because HiPco
and all other methods of making high-end, "single-walled" nanotubes
generate a hodgepodge of nanotubes with different diameters,
lengths and molecular structures. Scientists worldwide are
scrambling to find a process that will generate just one kind of
nanotube in bulk, like the best-conducting metallic varieties, for
instance.
"One good thing about the process that we have right now is that
if anybody could give us one gram of pure metallic nanotubes, we
could give them one gram of fiber within a few days," Pasquali
said.
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