Although often overshadowed by their single- and multiwalled siblings, double-walled carbon nanotubes (DWNTs) are predicted to have superior mechanical properties, thermal conductivity, and structural stability. However, scientists have found it difficult to create DWNTs in the pure, highly crystalline form needed to study the tubes' properties in depth.
Now, an international team led by Morinobu Endo, an engineering professor at Shinshu University in Nagano, Japan, has developed a process for making highly pure DWNTs [Nature, 433, 476 (2005)]. They isolate the material in sheets as a tough "buckypaper" that is flexible enough to fold into a tiny paper airplane. Endo and coworkers envision more high-tech applications for the material in the future, such as nanotube bi-cables and electronic devices.
Endo's group makes DWNTs using a standard chemical vapor deposition procedure. They employ a molybdenum conditioning catalyst to favor formation of double-walled tubes over single-walled ones. Next, they purify the DWNTs using HCl, followed by air oxidation.
The researchers have demonstrated their ability to produce nanotubes with uniform diameters and to effectively remove impurities in the form of single-walled carbon nanotubes, amorphous carbon, catalyst metals, and support materials, notes Chuck Hassen, vice president of business development for Tucson-based Tailored Materials Corp. The firm also makes high-purity DWNTs using an unpublished, proprietary process.