Modeling Emissivity Of Infrared Low Emissivity Coating Containing Horizontally Oriented Metallic Flake Particles

With the development of advanced infrared sensors, there are big challenges to the infrared stealth performance of weapon platforms, especially aircrafts. Nowadays, the low infrared emissivity coating is an important way to cut the infrared radiation characteristics of the target. For application background of the aircrafts, the requirement is that the coating emissivity is as low as possible in 3~5 μm and 8~14 μm infrared window. Typically, the low infrared emissivity coating is a composite that consists of metallic particles and organic matrix. Several experiments have found that the low infrared coating which contains horizontally oriented metallic flake particles perform better infrared stealth property than the coating that contains spherical particles. We also find that several factors, such as the particle volume fraction, particle size, particle thickness, the coating thickness and binder absorption, affect the coating emissivity. However, there is no effective model to explain how those factors work. So it is meaningful to build a model to accelerate the research process of low infrared emissivity coating.This thesis is divided into following two parts:(1) The scattering model of a single metallic disc when illuminated from its normal. For a metallic disc which is in the “resonant zone”, we build a so-called generalized geometric optic model which has two versions to describe the scattering property of a single metallic disc. The total scattering cross section is divided into three parts: reflection term of back-hemispherical space, diffraction term of forward-hemisphere space and the edge term. The core of our generalized geometric optic model is the introduction of edge term which is particle size related. In the first version, diffraction term is described by Fraunhofer formulae of a circular hole while reflection term and edge term are made only to the contribution to backward 180°. The second version is supposed that reflection term has the same angle distribution in back-hemisphere space as diffraction term in forward-hemisphere space. And edge term has the same scattering energy in both forward- and backward-hemisphere and the same angle distribution as diffraction term.(2) Emissivity formulae of the low infrared emissivity coating containing horizontally oriented metallic discs is developed. Two simplified versions of the radiative transfer equation—Kubelka-Munk theory(combined with the first version of generalized geometric optics model) and six-flux theory(combined with the second version of generalized geometric optics model) are used to derive emissivity formulae of the coating and then we discuss in detail how Al disc content, size, thickness, coating thickness and matrix absorption affect emissivity of the coating in 8~14 μm infrared window. By comparison between simulation result and experiment result, the validity of the model is proved. Notably, the trend of how particle size affect coating emissivity successfully predicted for the first time by combination of the six-flux theory and the second version generalized geometric optic model.