Analysis of a nano-porous multi-layer film for thermal radiation barrier coatings

Abstract

A highly reflective thermal radiation barrier coating (TRBC) made of a multi-layer film is studied for high temperature applications. The multi-layer film with a periodic microstructure consisting of cylindrical nanopores acts a photonic band gap (PBG) crystal. The coating is constructed such that pores are arranged periodically along the in-plane directions and axes of the pores are parallel to the thickness direction. The pore diameter is varied periodically through the thickness to form a multilayer film of alternating low and high porosities. The primary motivation behind considering this microstructure is that it can be fabricated from aluminum dioxide, which remains stable at high temperatures. The reflectivity of a single layer with uniform porosity is computed by numerically solving Maxwell's equations, by considering both the microstructure explicitly and a homogenized layer to gain insights into the effect of pore size on the reflectivity of the nano-porous layer. Based on the study of the single layer, two microstructures with different arrangements of pores are designed to exploit the effect of microstructure to widen the band gap of the PBG crystal and to increase the reflectivity of the TRBC. Results of numerical simulations reveal that a wider band gap and higher reflectivity can be achieved by making the interpore distance of alternate layers of the multi-layer film comparable to the wavelength of the incident thermal radiation. A TRBC, which is made of a microstructure with a wider band gap and has increased reflectivity, will reflect a greater amount of incident heat energy over a wider range of frequencies.