The National Institute of Standards and Technology (NIST) has developed an ultra-black, multiwalled carbon nanotube coating that can be sprayed on to a substrate such as copper. The rugged coating can withstand high temperatures without melting or oxidizing. One of the first applications for the coating was a NIST-designed power meter for calibrating high-power lasers used by the U.S. military to defuse unexploded mines. The new laser power meter will be used to measure the light emitted by 10-kW laser systems. Light focused from a 10 kW laser is more than a million times more intense than sunlight reaching the Earth.

Until now, NIST-built power meters were barely portable and operated slowly. NIST claims that the new power meter is much smaller—about the size of a crock pot rather than a refrigerator. It also features a new design that enables it to make continuous power measurements. According to a NIST description of the meter, light is absorbed by the power meter in a cone-shaped copper cavity, where a spinning mirror directs the light over a large area and distributes the heat uniformly. The cavity is lined with multiwalled carbon nanotubes held together by a potassium silicate (water glass) binder, and surrounded by a water jacket. The coating absorbs light and converts it to heat. The resulting rise in water temperature generates a current, which is measured to determine the power of the laser.

The coating is a critical component of the device, says NIST researcher, John Lehman. "There aren't many things that are intrinsically black," he explains, pointing out that the blacker the coating the higher the optical absorption, which leads to more accurate laser power measurements. The problem with really black coatings, including carbon nanotubes, is that they oxidize or age in other ways. "The best commercially available black coatings have binders that melt at high temperatures," notes Lehman. "And then the carbon oxidizes, making these coatings a poor choice for high-power applications."

In recent years, NIST researchers have experimented with a variety of coatings made of nanotubes because they offer an unusual combination of desirable properties, including intense black color for maximum light absorption. Designing a detector to collect and measure all of the energy from a high-power laser is a significant challenge. The new power meter uses the latest version of NIST’s nanotube coating, which absorbs light efficiently, is more stable than some conventional coatings such as carbon black, and resists laser damage as effectively as commercial ceramic coatings.

Carbon nanotubes (black coating in photo, right) form the inner lining of NIST’s new laser power meter, enabling the copper instrument to withstand the intensity of military lasers while precisely measuring their power. Laser light is distributed evenly inside the water-cooled cavity by a mirror (diagonal component at center of left graphic). Credit: C. Cromer/NIST

From the beginning, NIST recognized the value of carbon nanotubes for this application, but until now carbon nanotube coatings weren't rugged enough. "So we needed a binder that would retain all the advantages of the carbon nanotubes while making them robust," Lehman explains. The secret is in the binder, which can withstand high temperature and ensures even dispersal of the carbon nanotubes throughout the binder. The coating is simply sprayed onto a substrate—copper, in the case of the power meter.

Although the power meter was developed for the military, Lehman sees commercial applications for the meter and the carbon nanotube coating. Industry currently uses high-power lasers for cutting, welding, and marking. IPG Photonics, for instance, makes a 50 kw fiber laser for welding and precision-cutting applications. The ability to accurately and quickly measure power output is key to maintaining a predictable manufacturing process. The coating itself would be of interest to those looking for a coating with high optical absorption (really black), high thermal conductivity, and resistance to damage and aging.

Among other test results, NIST has found that multiwalled carbon nanotubes perform better than single-walled nanotubes, according to NIST. Researchers are continuing to seek nanotube formulas that are durable and easy to apply, such as enamel paint, but have even higher damage thresholds than today’s coatings.

NIST’s nanotube coating technology already has been transferred to industry for use in commercial products. According to Lehman, the process for manufacturing the coating is straight forward, as is the spray application.

Development of the new power meter was funded by the Air Force.