Ultrasonic Dispersing of Carbon Nanotubes (CNT)

Carbonnanotubes are strong and flexible but very cohesive. They are difficult to disperse into liquids, such as water, ethanol, oil, polymer or epoxy resin. Ultrasound is an effective method to obtain discrete - single-dispersed - carbonnanotubes.

Carbonnanotubes (CNT) are used in adhesives, coatings and polymers and as electrically conductive fillers in plastics to dissipate static charges in electrical equipment and in electrostatically paintable automobile body panels. By the use of nanotubes, polymers can be made more resistant against temperatures, harsh chemicals, corrosive environments, extreme pressures and abrasion. There are two categories of carbon nanotubes: Single-wall nanotubes (SWNT) and multi-wall nanotubes (MWNT).

Carbonnanotubes are generally available as dry material, e.g. from companies, such as SES Research or CNT Co., Ltd. A simple, reliable and scalable process for deagglomeration is needed, in order to utilize the nanotubes to their maximum potential. For liquids of up to 100,000cP ultrasound is a very effective technology for the dispersing of nanotubes in water, oil or polymers at low or high concentrations. The liquid jet streams resulting from ultrasonic cavitation, overcome the bonding forces between the nanotubes, and separate the tubes. Because of the ultrasonically generated shear forces and micro turbulences ultrasound can assist in the surface coating and chemical reaction of nanotubes with other materials, too.

Generally, a coarse nanotube-dispersion is first premixed by a standard stirrer and then homogenized in the ultrasonic flow cell reactor. The video to the right (Click image to start!) shows a lab trial (batch sonication using a UP400S) dispersing multiwall carbonnanotubes in water at low concentration. Because of the chemical nature of carbon the dispersing behavior of nanotubes in water is rather difficult. As shown in the video, it can be easily demonstrated that ultrasonication is capable to disperse nanotubes effectively.

Dispersion of Individual SWNTs of High Length

A major problem for processing and manipulation of SWNTs is the inherent insolubility of the tubes in common organic solvents and water. Functionalization of the nanotube side wall or open ends to create an appropriate interface between the SWNTs and the solvent mostly lead to partial exfoliation of the SWNT ropes, only. As a result, the SWNTs are typically dispersed as bundles rather than fully isolated individual objects. When too harsh conditions are employed during dispersion, the SWNTs are shortened to lengths between 80 and 200nm. Although this is useful for certain tests, this length is too small for most practical applications, such as semiconducting or reinforcing SWNTs. Controlled, mild ultrasonic treatment (e.g. by UP200S with 40mm sonotrode) is a effective procedure to prepare aqueous dispersions of long individual SWNTs. Sequences of mild ultrasonication minimize the shortening and allow maximal preservation of structural and electronic properties.

Purification of SWNT by Polymer-Assisted Ultrasonication

Ultrasonically assisted purification of single-wall carbon nanotubes.

image: Koshio 2001

It is difficult to study the chemical modification of SWNTs at the molecular level, because is difficult to obtain pure SWNTs. As-grown SWNTs contain many impurities, such as metal particles and amorphous carbons. Ultrasonication of SWNTs in a monochlorobenzene (MCB) solution of poly(methyl methacrylate) PMMA followed by filtration is an effective way to purify SWNTs. This polymer-assisted purification method allows to remove impurities from as-grown SWNTs effectively. (Yudasaka et al.) Accurate control of the ultrasonication amplitude allows to limit damages to the SWNTs.

Hielscher a range of ultrasonic devices and accessories for the efficient dispersing of nanotubes.

Compact laboratory devices of up to 400 watts power for the dispersing into smaller volumes of up to 2 liters
500 and 1,000 and 2,000 watts ultrasonic processors like the UIP1000hd can process larger volumes.
Ultrasonic systems of 2, 4, 10 and 16kW and more for processing at commercial level.

Literature

Koshio, A., Yudasaka, M., Zhang, M., Iijima, S. (2001): A Simple Way to Chemically React Single-Wall Crabon Nanotubes with Organic Materials Using Ultrasonication; in Nano Letters, Vol. 1, No. 7, 2001, p. 361-363.

Yudasaka, M.; Zhang, M.; Jabs, C.; Iijima, S. (2000): Appl. Phys. A 2000, 71, 449.

Paredes, J. I., Burghard, M. (2004): Dispersions of Individual Single-Walled Carbon Nanotubes of High Length, in: Langmuir, Vol. 20, No. 12, 2004, 5149-5152, American Chemical Society.