A black body absorber from vertically aligned single-walled carbon nanotubes

Read from local Zotero full-text PDF (group 5183627 / Joule Heist).

Key findings

• A vertically aligned single-walled carbon nanotube (SWNT) “forest” has a measured emissivity of 0.98-0.99 over the 5-12 um range (standard deviation only 0.003 — nearly wavelength-independent), the highest ever reported and well above conventional black materials.
• Reflectance is extremely low at 0.01-0.02 and constant across an exceptionally wide spectral range from UV (0.2 um) to far IR (200 um); specular reflectance in the mid-to-far IR was <0.002.
• This beats conventional black surfaces: NiP alloy, black coatings (Astro Black), and chemically treated surfaces peak near 0.96 at 5-9 um but drop notably beyond 9 um; carbon black/graphite emissivity is limited to 0.8-0.85 by air-dielectric interface reflection.
• The black-body behavior is structural, not intrinsic to nanotubes: roll-pressed sheet and buckypaper from the same SWNTs had emissivity only 0.62-0.76, and spray-coated film 0.92, all far below the 0.98 forest.
• The forest is an aerogel-like structure occupying only ~3% of total volume (97% air); on average a 15x15 nm area holds one nanotube. Low material density lowers electron density, permittivity, and hence refractive index, suppressing reflection (Fresnel’s law); most solids cannot get below ~0.028 reflectance because indices exceed ~1.4.
• Emissivity rises with area mass density (height x density), reaching 0.987 at 2.33 mg/cm2 (~3.1 x 10^4 graphene-sheet-equivalent layers per unit area); even a 2-um-tall forest reaches >0.97, and >50 um reaches >0.98.
• Nanotubes are tilted ~20 degrees from vertical; light enters with low reflection at the flat top surface, then is repeatedly absorbed by the imperfectly aligned tubes. Specular reflectance stayed below 2% even at incident angles up to 70 degrees.
• Forests synthesized by water-assisted CVD (“SuperGrowth”) on silicon at 750 degrees C using ethylene as carbon source; applications cited include solar energy collectors and infrared thermal detectors/heaters, especially in vacuum where convective cooling is negligible.