Synthetic Method for Double-Walled Carbon Nanotube Production

08 March 2002

Toray Industries Inc., in cooperation with Professor Hisanori Shinohara of Nagoya University, has developed a revolutionary low-cost synthetic method for producing double-walled carbon nanotubes (DWNTs).

Up until now, synthesized single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) have been possible by employing a catalyst-supported chemical vapor deposition (CCVD) method, one of chemical vapor deposition (CVD) methods that utilize zeolite supports for preparing metal catalysts. By combining Professor Shinohara's CCVD method with Toray's zeolite technology, selective synthesis of DWNTs has been realized. This method can easily be applied to mass-production, with improved yield lowering manufacturing cost to as much as $100/kg.

DWNTs have many advantages over other types of carbon nanotubes (CNTs). They are smaller in diameter (1-3nm), stronger and more durable, and their inner tubes maintain the inherent SWNT character after outer tube modification. In field emission display (FED) applications, for example, DWNTs can offer both excellent emitting properties and high durability because of their small diameter and double-walled features. And by modifying their outer tubes, DWNTs can serve as good nanocomposite filler, again due to their small diameter as well as their affinity towards matrix materials.

Conventional CNT production employs arc discharge, laser abrasion or CVD synthetic methods. Recent breakthroughs in the arc discharge method have led to selective synthesis of DWNTs. However, this method is not suitable for mass-production, and existing CVD methods have several drawbacks. First, it is difficult to synthesize thin, fine-quality carbon nanotubes, such as SWNTs and DWNTs; second, CNTs produced via conventional methods have many defects and do not display their inherent properties; and third, the large amount of catalyst metal required limits improvements in purity and cost.

With Toray's new method, DWNTs are synthesized by a catalytic decomposition of simple hydrocarbons over well-dispersed metal particles embedded in zeolites at temperatures between 600 degrees centigrade and 900 degrees centigrade. This results in high-quality DWNTs measuring 1-3nm in diameter that are highly graphitized without heat treatment and free of metal catalysts. While the current DWNT production yield is 10%, this figure is expected to increase by more than 50% in the near future.

Toray plans to begin commercializing DWNTs in two years.

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