Time-dependent near-blackbody thermal emission from pulsed laser irradiated vertically aligned carbon nanotube arrays

Abstract

Vertically aligned carbon nanotube (VACNT) arrays have been reported to be the ``blackest'' material fabricated to date. They also have a thermal conductivity parallel to the VACNT axis that is much larger than their thermal conductivity perpendicular to the axis. Even so, because of their large length-to-radius ratio, they can be assumed to have the same temperature in any cross section that is perpendicular to the tube axis. Consequently, a pulsed-laser irradiated VACNT array is a candidate to produce fast thermal emissions from the face of a VACNT array grown on a high thermal conductivity substrate. This can be represented as a one-dimensional thermal conductivity problem, where the proximal end of the carbon nanotubes (CNTs) jumps in temperature when irradiated with a pulsed laser, and the tubes then cool by thermal conduction down their axis to their distal end on a substrate where the temperature is fixed. We have measured and calculated analytically the time-dependent infrared (IR) emission from a range of VACNT arrays with different tube lengths grown on aluminum-nitride (AlN) and silicon-dioxide--silicon (SiO${}_{2}$/Si) substrates. In a parallel effort to characterize their ``blackness,'' we measured the spectral reflectance and emissivity and single-wavelength bidirectional reflectance distribution functions (BRDFs) of these arrays and found these optical characteristics compare well to ideal blackbody behavior. Shorter CNTs exhibit faster cooling than longer nanotubes, and the effective axial conductivity has been determined by comparison between experimental IR signatures and theoretical modeling of expected temperature distributions. Our key finding is that VACNT arrays can act as very fast on-off blackbody sources, which can be useful in many applications requiring such a source.