Dr. Toshiya Okazaki of the National Institute of Advanced Industrial Science and Technology (AIST) in Japan is conducting research that contributes to the supply of stable quality and production volume for the practical use of carbon nanotubes. This article is based on an interview with him.
National Institute of Advanced Industrial Science and Technology (AIST) CARBON NANOTUBES -Application Research Center Toshiya Okazaki, Deputy Director
Carbon nanotubes are attracting attention as a new dream material.
It is about half as light as aluminum, about 20 times stronger than steel, and more than 1,000 times more current density resistant than copper.
Carbon nanotubes, which are made of 100 percent carbon, have the structure of a sheet of carbon atoms arranged in a plane and rolled into a cylindrical shape (tube) with a diameter as small as one nanometer (one billionth of a meter). A single layer of cylindrical tubes is called single-walled carbon nanotubes, while multiple cylinders of different diameters are layered together to form multi-walled carbon nanotubes. Depending on how the sheets are rounded, single-walled carbon nanotubes can have either "metallic" or "semiconducting" properties. (1)
For example, "indium," one of the rare metals, is a material with limited production, but is currently used for LCD screens and touch panels as a "transparent conductive film.” Not only is indium rare, but it is also difficult to handle the stretching and bending required for future IoT device components. Therefore, the development of materials including carbon nanotubes is underway as an alternative to indium. (2)
In addition, silicon is currently the main material used for semiconductors, but it is expected that silicon will eventually reach its limits in the field of semiconductors, where miniaturization is progressing. Research is underway to create high-performance CPUs and memories using carbon nanotubes instead of silicon. (3)
Furthermore, new materials with superior functionality are being created by combining existing industrial materials with carbon nanotubes. (4)
Although carbon nanotubes are still not widely seen in our daily lives, research is being conducted in various fields toward their practical application.
To achieve the required properties with high yield, the establishment of a dispersion method, including quality evaluation, is an issue in the fabrication of materials using carbon nanotubes.
The dispersion slurry before processing is a black liquid because it is derived from carbon. When observed under magnification, carbon nanotubes exist in the liquid one by one in a tangled state, but it is difficult to understand under what conditions they are mixed together.
When carbon nanotubes mixture conditions changed in the liquid, even though it was the same dispersion liquid, it product completed different materials properties.
For example, the performance of the same product carbon nanotubes slurry bought last year may differ from that bought this year. Perhaps the carbon nanotubes diameter distribution is different or the aggregation/dispersion state is different, but neither the manufacturer nor the user knows this. The reality is that the development of materials using Carbon nanotubes is proceeding under such circumstances, and quality control for each batch has not yet been established.
Carbon nanotubes are fibers with a diameter of a few nanometers (nm) , but it is rarely used as single fiber. Instead, it is mostly used in an aggregated state, such as when mixed with composite materials. How aggregated they are and how each fiber is connected in a higher-order structure greatly affects the physical properties of the final material.
Therefore, how to create and use such an aggregated state is important.
In other words, it is essential for practical use to accurately analyze and evaluate the state of carbon nanotubes and the size of the carbon nanotubes that are entangled in the aggregate. We are researching these evaluation methods using a variety of equipment.
Conventional dynamic light scattering and laser diffraction particle size analyzers have difficulty in accurately measuring a wide range of particle sizes, but we found that centrifugal sedimentation particle size analyzers could do the job.
In the case of dynamic light scattering and laser diffraction, the signal from larger particles is stronger, and the weak signal from smaller particles is hidden by the signal from larger particles. At the same time, scattered light from many particles of different sizes is superimposed and detected, making it difficult to obtain a high resolution. With the centrifugal sedimentation method, particles are separated by size and then measured, so the faint signal is not hidden, and high resolution can be obtained over a wide range.
So I consulted with the HORIBA salesperson who was in charge of AIST and knew that the centrifugal sedimentation principal analyzer (CAPA series) was no longer available. Unable to give up, I approached HORIBA to ask if they could develop an analyzer based on this principle once again. After making direct appeals to the then HORIBA R&D Division General Manager and other executives, the development of the centrifugal nanoparticle analyzer Partica CENTRIFUGE was finally launched.
After beginning, Dr. Tetsuji Yamaguchi, the Partica CENTRIFUGE project leader who enthusiastically led the development, took a strong interest in the project as his own and advanced the development beyond our expectations. It was also a great pleasure to see that many HORIBA employees are interested in the project and embracing it.
Partica CENTRIFUGE has a wide range of particle sizes that can be measured and can stably measure relatively large particles, making it attractive for measuring samples with a wide size distribution, such as carbon nanotubes, at once. There are many methods for measuring a limited particle size range, but it is difficult to find a method that can measure particles of two or three orders of magnitude differences in size at once with high resolution. Partica CENTRIFUGE, a centrifugal sedimentation method, can classify particles by size, allowing measurement of a wide range of samples from 10 nanometers to more than 10 micrometers.
Carbon nanotubes particle size distribution in a commercial multi-walled Carbon nanotubes slurry measured with Partica CENTRIFUGE. It can be seen that there is a mixture of isolated dispersed carbon nanotubes observed at about 30 nm and aggregates observed at about 200 nm.
Carbon nanotubes are already widely used as conductive auxiliaries in electrodes of lithium-ion batteries and as a filler*1 in composite materials. More familiarly, carbon nanotubes are also blended in rubber gaskets, contributing to improved heat resistance. Personally, I think the next target is the field of electronics that takes advantage of quantum phenomena associated with the nanometer size of carbon nanotubes.
1 Filler: Materials to be mixed into composites to improve their properties
(Interview conducted in June 2021)
*All the names of organizations, affiliations, and positions mentioned in the text are current as of the time of the interview.
Toshiya Okazaki
Prime Senior Researcher, Nano Carbon Device Research Center, National Institute of Advanced Industrial Science and Technology (AIST)
After completing his doctoral studies at the Graduate School of Science, Kyoto University, he worked as a Research Fellow of the Japan Society for the Promotion of Science (JSPS) and as an Assistant Professor at the Graduate School of Science (Chemistry), Nagoya University before joining the Carbon Nanotubes-Application Research Center at AIST in 2004. He has been in his current position since 2015.
Awards: AIST President's Award (2018), Paper Award of the Japan Society of Applied Physics, Subcommittee on Thin Film and Surface Physics (2020).
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