Boulder, CO--Researchers from the National Institute of Standards and Technology (NIST), Kansas State University (Manhattan, KS), and the Colorado School of Mines (Golden, CO) have demonstrated a spray-on mixture of carbon nanotubes and ceramic that has a very high laser-damage threshold.1
Such a coating, which absorbs laser energy but withstands it, could be used for high-power laser-beam detectors. The new material improves on NIST's earlier version of a spray-on nanotube coating for optical power detectors2 and has already attracted industry interest.
"It really is remarkable material," says NIST co-author John Lehman. "It's a way to make super-nanotubes. It has the optical, thermal, and electrical properties of nanotubes with the robustness of the high-temperature ceramic."
CNTs and ceramic
The new composite consists of multiwall carbon nanotubes (CNTs) and a ceramic made of silicon, boron, carbon, and nitrogen. Boron boosts the temperature at which the material breaks down. The nanotubes were dispersed in toluene, to which a clear liquid polymer containing boron was added drop by drop, and the mixture was heated to 1100 °C. The resulting composite was then crushed into a fine powder, dispersed in toluene, and sprayed in a thin coat on copper surfaces. Researchers baked the test specimens and then exposed them to a far-IR laser beam of the type used to cut hard materials.
The composite was developed by Kansas State. NIST researchers suggested using toluene to uniformly coat individual nanotubes with a ceramic shell. They also performed damage studies showing how well the composite tolerates exposure to laser light.
NIST has developed and maintained optical power standards for decades. In recent years, NIST researchers have coated optical detectors with nanotubes because of their unusual combination of desirable properties, including intense black color for maximum light absorption.
Analysis revealed that the coating absorbed 97.5% of the light and tolerated 15 kW/cm2 of laser power for 10 seconds. This is about 50% higher damage tolerance than other research groups have reported for similar coatings—such as nanotubes alone and carbon paint—tested with the same wavelength of light, according to the paper. The nanotubes and graphene-like carbon absorb light uniformly and transmit heat well, while the oxidation-resistant ceramic boosts damage resistance. The spray-on material also adheres well to the copper surface. As an added bonus, the composite can be produced easily in large quantities.
1. R. Bhandavat et al., ACS Applied Materials & Interfaces (2013), ASAP Publication Date March 19. DOI: 10.1021/am302755x.
2. 2009 NIST Tech Beat article, "New Nanotube Coating Enables Novel Laser Power Meter," at www.nist.gov/pml/div686/laser_050509.cfm.