"Carbon
is a material that has contributed greatly to human lives
and has been involved in scientific breakthroughs. It is
'An old but new material'. Research concerning carbon always
results in new discoveries," says Prof. Endo who developed
a large-scale method for the production of carbon nanotubes.
Now, he is promoting the practical applications of carbon
nanotubes for a new 'carbon age'.
The
fabrication of carbon nanotubes by chemical vapor deposition
was discovered by accident. At the time, he was researching
carbon fibers, which are prepared by flowing hydrogen gas
and benzene into an electric furnace containing a substrate.
After the experiment, the substrate which has been blackened
with carbon, was taken out, washed and then heated in air
for reuse. The process took two and a half days.
He did not want to take his time doing this. He tried to
clean the substrate with sand paper normally used for woodwork,
and to his surprise, he found a large amount of carbon fibers
in the electric furnace. Like a charm, he always got good
results when he used sand paper, but when he used a new
black one, he did not get the same results. He says, "I
thought about the differences between the two sand papers
and noticed that the sand paper I first used was brown.
So, I took out a piece of brown sand paper from my desk
drawer and used it, and then I got lots of carbon fibers."
He was not sure how it worked, and all he knew was that
the brown sand paper was made of iron oxide and the black
one was made of silicon carbide. In 1974, while he was doing
research in France, he discovered fine tubes in the middle
of aggregates of carbon fibers with iron particles at the
tips of tubes.
From this observation, he confirmed that the catalyst was
iron oxide.
Those tubes are now called carbon nanotubes.
Prof.
Endo developed not only a seeding method that makes carbon
fibers by dispersing small metal particles which act as
a catalyst to decompose hydrocarbons onto a substrate but
also a floating reagent method that makes carbon fibers
by introducing the catalytic particles and hydrocarbon gas
into the reactor and decomposing the hydrocarbon.
In 1988, this method made serial production possible and
led to commercialization of multi-walled carbon nanotubes
of 10 nm to 100 nm in diameter, or Endo fibers. Endo fibers
have been used in lithium-ion batteries and lead-acid batteries
to prolong the lifetime of these batteries. He says, "Nanomaterials
are used not only for developing novel devices but also
for improving the performance of existing devices.
He
has been working to develop further the floating reactant
method into a catalytic chemical vapor deposition (CCVD)
method by incorporating the catalytic particles into cavities
of zeolites, which absorb the particles based on size. The
use of zeolites has made it possible to fabricate selectively
different types of carbon nanotubes such as single-walled
carbon nanotubes and multi-walled carbon nanotubes. They
have been trying to determine the relationship between the
size and type of iron particle catalyst and the type of
carbon nanotube. He has succeeded in growing isolated single-walled
carbon nanotubes with a diameter of 0.43 nm, which have
been recognized as the world's smallest as of 2003. These
carbon nanotubes are about the size of polymer molecules,
and so, they have the possibility of being used to connect
molecules and make composite materials at the molecular
level. The diameter of carbon nanotubes of this size determines
their chirality and electrical properties. Prof. Endo's
carbon nanotubes, which have a diameter of 0.43 nm, have
been indexed to be semiconductors. Carbon nanotubes with
a diameter of 0.41 nm are metallic and the ones with a diameter
of 0.42 nm are semiconducting.
Since he has been able to control the sizes of the catalytic
particles, he has been able to control the diameters of
carbon nanotubes. This technique has enabled him to make
different types of carbon nanotubes.
Prof.
Endo says, "'See what is really there, not what you
would like to see.' This is what I learned from my supervisor,
Prof. Agnes Oberlin, when I was in France. Researchers tend
to only see what they are looking for. If you approach your
research objective with an open mind, you may find something
you have never imagined before."
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/036a.html
