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Guest Writer - Gastautor - Gast Schrijver


Morinobu ENDO

Department of Electrical and Electronic Engineering,
Faculty of Engineering, Shinshu University

Opening an era of carbon nanotubes
through large-scale production

- An old but new material -


"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.)

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