Analysis of a Microstructure for Highly Reflective Thermal Radiation Barrier Coatings

Abstract

A highly reflective multi-layer film is studied for use as a thermal radiation barrier coating (TRBC) to significantly reduce heat transfer to the structure from the high temperature environment. The multi-layer film consists of a three dimensional (3D) periodic microstructure with cylindrical nano-pores that are arranged periodically along the in-plane directions and with axes of pores parallel to the thickness direction. The pore diameter is varied periodically through the thickness to form a multi-layer film of alternating low and high porosities. This multi-layer microstructure acts as a photonic band gap (PBG) crystal, which gives rise to high reflectivity. 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 the multi-layer TRBC is computed from numerical simulations of the electromagnetic wave propagation through it. A homogenization method is employed to estimate the effective dielectric constant of each layer of the film. Consideration of homogenized layers reduces the 3D microstructure to an effective 1D structure, thereby reducing the computational expense. The results of numerical simulations reveal that the reflectivity found considering the 3D multi-layer microstructure explicitly is close to that computed using homogenized layers. Finally, using the homogenized layers, it is shown that the peak as well as the average reflectivities of a multi-layer TRBC can be increased if the film consists of sets of bilayers with different thicknesses.