Abstract
A finite-difference time-domain (FDTD) method is used to model thermal radiative properties of vertical arrays of multi-walled carbon nanotubes (MWCNT). Individual CNTs are treated as solid circular cylinders with an effective dielectric tensor. Consistent with experiments, the results confirm that CNT arrays are highly absorptive. Compared with the commonly used Maxwell-Garnett theory, the FDTD calculations generally predict larger reflectance and absorbance, and smaller transmittance, which are attributed to the diffraction and scattering within the cylinder array structure. The effects of volume fraction, tube length, tube distance, and incident angle on radiative properties are investigated systematically. Low volume fraction and long tubes are more favorable to achieve low reflectance and high absorbance. For a fixed volume fraction and finite tube length, larger periodicity results in larger reflectance and absorbance. The angular dependence studies reveal an optimum incident angle at which the reflectance can be minimized. The results also suggest that an even darker material could be achieved by using CNTs with good alignment on the top surface.
Highlights
Ever since large-scale aligned CNT arrays have been synthesized [1], they have exhibited promise for various applications
The structure of a nanotube array is characterized by many parameters, which can be consolidated to tube diameter, intertube distance, and tube length for two-dimensional square lattice
The typical diameters of multi-walled carbon nanotubes (MWCNT) are from 40 to 240 nm, the intertube distances vary from 200 to 600 nm, and the tube lengths are from 1 μm to 3 μm
Summary
Ever since large-scale aligned CNT arrays have been synthesized [1], they have exhibited promise for various applications. Optical properties of CNT arrays depend first on the atomic structure of each individual CNT and on their collective arrangement. For an individual CNT, the optical properties of singlewalled (SW) CNTs and MWCNTs should be considered separately. The optical properties of SWCNTs, especially those with diameters less than 1 nm, exhibit very strong dependence on the detailed atomic structure (chirality) [12,13,14]. SWCNT arrays usually contain many different chiralities with a random distribution, which complicates the engineering of optical properties and functionalities. It is reasonable to treat each MWCNT as a homogeneous medium [4] and design the structure for different applications
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