Spectral Emissivity and Thermal Conductivity Properties of Black Aluminum Films

Abstract

Black aluminum is a material characterized by high surface porosity due to columnar growth and exhibits unique optical properties that make it attractive for applications such as light trapping, infrared detection, and passive thermal radiation cooling. In this study, we correlate the structural and optical properties of black aluminum by comparing it with conventional reflective aluminum layers. These layers of varying thicknesses were deposited on fused silica substrates, and their optical properties were analyzed. COMSOL simulations, supported by experimental data, reveal that black aluminum's structure leads to a significant reduction in visible light reflectivity and an increase in emissivity in the near- and mid-infrared ranges. This enhanced emissivity is partly due to the presence of aluminum nitride (AlN) grain boundaries and an oxidized surface layer. Optically, black aluminum differs significantly from reflective aluminum by presenting a reflectivity below 5% in visible wavelength and an average emissivity of approximately 0.4-0.5 from 1.2 to 20 μm. Thermally, it possesses approximately ten times lower thermal conductivity and doubles the volumetric heat capacity. These differences are attributed to its porous structure, nanoscale crystallites, and the presence of aluminum nitrides and oxides within the material.

Figure 1

XRD diffraction patterns from R-Al and B-Al layers deposited on fused silica (FS) substrates as a function of their thicknesses. The solid lines indicate the Al peaks from a structure with the Fm3̅m space group and a lattice parameter a = 4.0490 Å.

Figure 2

SEM images of the surface of R-Al films with a thickness of 355 nm (a), 730 nm (b), and 1160 nm (c). B-Al films with a thickness of 240 nm (d), 660 nm (e), and 1010 nm (f).

Figure 3

Surface distribution of grain size morphologies extracted from the SEM images of the R-Al and B-Al films.

Figure 4

AFM images of the R-Al film surfaces in (a), (b), and (c) and B-Al film surfaces in (d), (e), and (f). The RMS surface roughness values R_q are indicated for each image.

Figure 5

Diffuse reflectivity of R-Al films and B-Al films for each studied thickness.

Figure 6

Simulated total reflectivity spectra for samples with various cell sizes for a 1-μm-thick sample. The dashed line indicates a 500-nm-thick sample.

Figure 7

Infrared reflectivities in (a) and emissivities in (b) of R-Al and B-Al films from a 1.5 to 20 μm wavelength.