Study On The Mechanism Of Composite Coating With Low Infrared Emittance And Low Radar Reflectance

It is vital for the armaments to efficiently control their infrared signatures in order to survive in the war field. Compared with the experimental research, the theoretical study was undeveloped. The interaction between the infrared radiation and the pigment particles, and the radiative transfer equation within the coating are studied in this work.To begin with, we discuss the single scattering property of the pigment particle, and compute the scattering parameters of homogeneous sphere, coated sphere, spheroid, and other nonspherical particle. We concentrate on the scattering by homogeneous sphere, discuss the phase function, forward scattering ratio and other parameters. When the particle clearance is larger than 0.3 wavelength, the dependent scattering effect due to the interaction of particles can be ignored, and the independent scattering approximation is valid.Secondly, the optical properties of metal, semiconductor, and dielectric are studied. Metals own low emittance, therefore metals can be selected as pigment particles to construct low emittance coating. However, the high reflectivity due to the free electrons scattering make the metallic continuous film highly reflective for visible light and radar. The plasma frequency of doped semiconductor can be adjusted by changing the dope concentration. Therefore we can realize low reflectivity in the visible region, high reflectivity (low emittance) in the infrared region. The reststrahlen mode of polarizable material can be made use to construct the low emittance film, and the polarizable material doesn't cause high reflectivity for radar and visible light due to the absence of free charge carriers.Thirdly, we study the radiative transfer within the infrared coating. Considering the semi-isotropic property of thermal radiation, we can take advantage of the Kubelka-Munk theory under the standard application conditions to solve the radiative transfer equation within the coating. By use of the approach of Maheu et al., we discuss the relation of the effective scattering coefficient and effective absorption coefficient to the scattering property of the pigment particle.Fourthly, we extend the two kinds of classical effective medium theory to the case of large particle size parameter. By use of the Newton iteration, we obtain the numerical solutions of the two kinds of extended effective theory. The effective optical constants of the coating can be used to determine the boundary conditions of the radiative transfer equation within the coating.At last, taking the spherical aluminium particles as the pigments, in the case of sparse concentration which ensures the validity of the independent scattering approximation, we obtain the dependence of the emittance of the infrared coating with respect to the particle radius and the thickness of the coating. Firstly, the numerical results indicate that there exists an optimal radius range of the aluminum particle. The emittance of the infrared coating consisting of aluminium particles and resin in the optimal radius range decreases from 0.9 to lower than 0.5 even for a thin infrared coating. In the 3～5μm window, the optimal particle radius range is 0.3～0.8μm, and in the 8～14μm window, the optimal radius range is 0.7～1.6μm. Secondly, in the optimal radius range, when the thickness of the infrared coating exceeds a critical value called saturation thickness, the emittance of the coating would not decrease. For the 3～5μm window the saturation thickness is about 100μm, and for the 8～14μm window it is nearly 300μm. For the coating which is constructed by the pigment particles with radius exceeding the optimal radius range, the emittance still decreases.The composite coating is constructed by attaching the low infrared emittance coating to the radar absorbing materials (RAM) coating. When the thickness of the infrared coating is about the saturation thickness, the radar reflectivity of the composite structure decreases a little in the low frequency region, whereas increases in the high frequency region. The absorbing peak shifts to the low frequency region. The influence of the infrared coating on the performance of the RAM coating is negligible, and this kind of composite coating can realize a low infrared emittance and a low radar reflecivity.