Near-Field Radiative Heat Transfer around the Percolation Threshold in Al Oxide Layers

We present a theoretical calculation of the near-field radiative heat transfer (NFRHT) around the percolation threshold of oxide layers that forms on aluminum surfaces. As this layer grows the volume fraction of Al decreases and that of Al2O3 increases. During this process, the Al plasmon response degrades, and the surface phonon modes of Al2O3 become active, thus increasing the spectral radiative heat flux. This occurs when the content of aluminum oxide on the surface approaches 67% in volume, which corresponds to the percolation threshold according to the Bruggeman effective medium theory. This oxidation fraction defines two regions: for volume fractions below the percolation threshold, the total heat flux is lower than that of unoxidized Al; otherwise, it is larger. Thus, oxidation considerably affects both the spectral and total radiative heat transfer.