Optical absorber coatings have been developed from carbon-based paints, metal blacks, or glassy carbon. However, such materials are not truly black and have poor absorption characteristics at longer wavelengths. The blackness of such coatings is important to increase the accuracy of calibration targets used in radiometric imaging spectrometers since blackbody cavities are prohibitively large in size. Such coatings are also useful potentially for thermal detectors, where a broadband absorber is desired. Au-black has been a commonly used broadband optical absorber, but it is very fragile and can easily be damaged by heat and mechanical vibration. An optically efficient, thermally rugged absorber could also be beneficial for thermal solar cell applications for energy harnessing, particularly in the 350–2,500 nm spectral window.
It has been demonstrated that arrays of vertically oriented carbon nanotubes (CNTs), specifically multi-walled-carbon- nanotubes (MWCNTs), are an exceptional optical absorber over a broad range of wavelengths well into the infrared (IR). The reflectance of such arrays is 100× lower compared to conventional black materials, such as Au black in the spectral window of 350–2,500 nm. Total hemispherical measurements revealed a reflectance of ≈1.7 % at λ ≈1 μm, and at longer wavelengths into the infrared (IR), the specular reflectance was ≈2.4 % at λ ≈7 μm.
The previously synthesized CNTs for optical absorber applications were formed using water-assisted thermal chemical vapor deposition (CVD), which yields CNT lengths in excess of 100’s of microns. Vertical alignment, deemed to be a critical feature in enabling the high optical absorption from CNT arrays, occurs primarily via the crowding effect with thermal CVD synthesized CNTs, which is generally not effective in aligning CNTs with lengths < 10 μm. Here it has been shown that the electric field inherent in a plasma yields vertically aligned CNTs at small length scales (
This work was done by Anupama B. Kaul and James B. Coles of Caltech for NASA’s Jet Propulsion Laboratory.
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NPO-47876
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Thermally Resilient, Broadband Optical Absorber From UV to IR Derived From Carbon Nanostructures
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Overview
The document presents a technical disclosure from NASA's Jet Propulsion Laboratory (JPL) regarding a novel thermally resilient, broadband optical absorber derived from carbon nanostructures. This technology addresses the limitations of traditional optical absorbers, which have been primarily based on carbon-based paints, metal blacks, and glassy carbon. These conventional materials often exhibit poor absorption characteristics at longer wavelengths and are not truly black.
The new optical absorber utilizes vertically aligned carbon nanotubes (CNTs) synthesized through a process called plasma-enhanced chemical vapor deposition (PECVD). This method allows for the creation of CNT arrays with exceptional optical properties, demonstrating ultra-low reflectance across a broad spectrum from ultraviolet (UV) to infrared (IR) wavelengths (approximately 350 nm to 2500 nm). The CNT absorbers exhibit reflectance values that are significantly lower than those of traditional materials, such as Au-black and other standard optical blacks, making them highly effective for applications requiring efficient light absorption.
Key advantages of the CNT absorbers include their thermal resilience, as they maintain optical performance at elevated temperatures (up to 400ºC) without significant degradation. This is in stark contrast to other materials, like porous Au-black, which show performance decline at much lower temperatures (around 200ºC). The document highlights that the vertical alignment of the CNTs enhances their ability to trap incoming light, attributed to the unique structure of the CNT arrays, which feature long pores that improve light absorption efficiency.
The research also emphasizes the potential for lower synthesis temperatures using PECVD, which could facilitate the integration of these absorbers with thermal detector materials that are sensitive to high temperatures. The findings suggest that the CNT absorbers could revolutionize the design of optical coatings in various applications, including thermal solar cells and other aerospace technologies.
Overall, this document outlines a significant advancement in optical absorber technology, showcasing the potential of carbon nanostructures to create materials that are not only effective in light absorption but also resilient to thermal stress, paving the way for innovative applications in science and engineering.
