(Digital Presentation) Carbon Nanotube Based Low-Reflective Coatings for Light Suppression
Authors: Wang Dan , Hall Timothy , Snyder Stephen , Inman Maria , Taylor E. Jennings 
Published: July 2022

Space-borne instruments, such as seeker telescopes, optical sensors, etc., need to minimize stray and reflected light for facilitating space observatory missions. Durable low-reflectivity surfaces/coatings are thus required for suppressing unwanted light within these optical systems. The low-reflectivity coatings need to withstand harsh space environments, including UV radiation, atomic oxygen, vacuum, etc. The excellent mechanical, thermal, optical, and electrical properties of carbon nanotubes (CNTs) make them as ideal coating materials for obtaining low reflectivity surfaces for space applications.

In this presentation, Faraday Technology Inc. will discuss an innovative electrophoretic deposition (EPD) manufacturing process, based on the use of pulsed electric fields, for controlled, reproducible, scalable application of low reflective CNT based coatings. The low-reflectivity CNT coatings have been successfully deposited on a variety of geometries: flat coupons, curved surfaces, sharp edges, and internal and external surface of square tubes using the FARADAYIC® EPD process with different anode-cathode configurations (Figure 1 A). The CNT coatings show the total hemispherical reflection of 0.5% - 1% across visible to near infrared (NIR) wavebands, which is much lower than the reflectance of the typically used Z306 black paint (Figure 1 B). Most importantly, the CNT coating shows minimal (~0.14-0.32%) reflectance increase after 5000 equivalent solar hours (ESH) UV/VUV illumination (Figure 1B). As a comparison, Z306 black paint shows ~2.40-4.05 % reflectance increases with same ESH UV/VUV illumination. Figure 1C indicates the CNT coatings has minimal 0-0.0015 (1/sr) BRDF increase in scatter. These CNT coatings also withstood simulated launch conditions vibrational tests, and demonstrated no weight loss and optical degradation. Furthermore, the CNT coatings were evaluated for atomic oxygen erosion resistance in a simulated low earth orbit (LEO) environment, and showed enhanced resilience when compared to Kapton and HOPG.

In summary, a scalable manufacturing process for fabricating CNT based coatings with low reflectivity across visible to near infrared wavebands has been demonstrated and shown great potential in facilitating space observatory missions by minimizing stray and reflected light on targeted telescope and baffle materials and construction. The CNT based coatings can also be utilized as broadband absorbers for solar thermal power generation and storage, solar-driven steam generation for sanitization and water purification, and so on.

Acknowledgements: The financial support of NASA SBIR program through contracts No. 80NNSC18P2062 & 80NSSC19C0177 is acknowledged. The authors acknowledge Alan Hopkins, Peter Fuqua, and Amber Hennessy from the Aerospace Corporation for simulated launch conditions vibration tests, optical characterization, and UV radiation effect tests. David Oakes and Daniel Hewett from Physical Sciences Inc. are acknowledged for the atomic oxygen erosion tests on the CNT based coatings.

Figure 1