Single-walled nanotubes are a family of more than 30 molecules that greatly intrigue scientists and technologists. Nanotubes are stronger and far lighter than steel, and they have superior electrical properties—about one-third are metals and the rest are semiconductors.

Researchers have used both types to make electronic components like molecular wiring and molecular transistors that are 100 times smaller than those found in today’s most advanced microchips. Nanotubes also are being considered for use in the manufacture of extremely strong yet lightweight materials and many other applications.

Until now, though, there has been a significant bottleneck to nanotube commercialization —the supply of these intriguing molecules has been limited. Not only are the processes used to make nanotubes expensive—today’s going rate for a gram of nanotubes is $500—they also are so complex that, until a couple of years ago, all the single-walled nanotubes ever created worldwide totaled less than one pound. All that may change thanks to chemists in Rice University’s Carbon Nanotechnology Laboratory (CNL) who have created the first process that can continuously produce single-walled carbon nanotubes in bulk. Known as HiPco (high-pressure carbon monoxide process), the method is a watershed achievement in nanoscience.

The process used to create the first buckyballs at Rice in 1985 was carried out in a laser oven. A rod of carbon graphite was vaporized by a laser, creating a cloud of gaseous carbon atoms that reformed into buckyballs. In the early 1990s, researchers adding trace metals to the graphite discovered carbon nanotubes—the metal atoms react with the carbon atoms, causing them to grow into long tubular structures containing thousands or even millions of atoms.

Prior to the advent of HiPco, virtually all single-walled carbon nanotubes have been produced at research laboratories, either in laser ovens or in carbon arcs. Both processes are labor-intensive and time-consuming, and moreover, they yield just a few grams of nanotubes per day and cannot be scaled up to produce larger quantities needed for commercial applications.

One of these laboratories was in Rice’s Center for Nanoscale Science and Technology, where a research group headed by buckyball co-discoverer Richard Smalley perfected a laser oven process for making single-walled nanotubes. The group began providing nanotubes to research groups at Rice, NASA, and other institutions in the late 1990s under a program called Tubes@Rice, and Rice helped NASA’s Johnson Space Center set up its own laser facility for nanotube production in 1997.

In 1998, NASA and Rice entered into a five-year program to collaborate on nanotube research. One thrust of that program was the development of a continuous flow process suitable for large-scale production of nanotubes and led to the development of HiPco. In the HiPco process, the gaseous carbon atoms don’t come from vaporized graphite rods. Instead, they come from carbon monoxide gas, which is continuously pumped into a high-pressure reaction chamber and mixed with an industrial gas containing the necessary catalysts to sustain the chemical reactions that create nanotubes. The temperature and pressure conditions required in the HiPco process are common in industrial plants, making HiPco both less expensive and faster for producing nanotubes than the laser-oven method.

In 2000, Smalley, who also is a University Professor at Rice and director of CNL, and colleagues formed Houston-based Carbon Nanotechnologies Inc., a start-up company that holds exclusive worldwide license to the HiPco process and other Rice intellectual properties. In exchange for its support, NASA materials scientists have now received more than a pound of nanotubes, some 500 grams. “NASA was one of the first organizations to understand the tremendous potential of single-walled carbon nanotubes, and it was also one of the first to invest in that potential,” says Smalley. “It’s fitting that they are the first to benefit from HiPco.”

Ultimately, NASA hopes to develop nanotube applications for space exploration. Because of their superior strength-to-weight ratio, single-walled nanotube composites may one day reduce the weight of spacecraft by 50 percent or more compared to conventional materials. Other space-exploration applications include energy storage, life support systems, thermal materials, nanoelectronics, nanosensors, electrostatic discharge materials, and biomedical applications.

—Jade Boyd