Structures, Characteristics Of Infrared Bands,Compatibility, And Spectral Selectivity Of Composite Coatings

Infrared band is divided into the near-infrared(1~3 μm), mid-infrared(3~5 μm, 5~8 μm), and far-infrared(8~14 μm), the different bands with different application backgrounds. The near-infrared band(1~3 μm) mainly for the 1.06 μm and 1.54 μm laser stealth, requiring strong absorption and low reflection. The mid-infrared band(3~5 μm) mainly for the high-temperature infrared stealth, requiring low absorption and high reflection(low emissivity). The mid-infrared band(5~8 μm) requires high infrared radiation(high emissivity), in order to achieve the objectives with heat radiation. The far-infrared band(8~14 μm) mainly for the room-temperature infrared stealth, requiring low absorption and high reflection(low emissivity). This paper expands the research based on the characteristics of each band: In the far-infrared atmospheric window of 8~14 μm, according to the structural characteristics of PU/flaky metal composite coatings, the theories of one-dimensional photonic structure(1DPS) were used to systematically investigate the related theoretical issues of the above low infrared emissivity composite coatings. In the near-infrared band of 1~3 μm, PU/Sm2O3 composite coatings were prepared by using PU and Sm2O3 particles as adhesives and pigments, respectively. The near-infrared absorption and mechanical properties of as-prepared coatings were systematically characterized. In the near-infrared band of 1~3 μm and far-infrared band of 8~14 μm, PU/(bronze & Sm2O3) and PU/(Al & Sm2O3) composite coatings were prepared by using PU and bronze powders, Al powders, Sm2O3 particles as adhesives and pigments, respectively. The infrared emissivity, near-infrared absorption, and mechanical properties of as-prepared coatings were systematically characterized. In the mid- and far-infrared bands of 3~14 μm, three kinds of Ge/Zn S 1DPSs with low infrared emissivities at the wavelengths of 3~5 μm, 8~14 μm, both 3~5 μm and 8~14 μm were designed and prepared by using Ge and Zn S as high and low refractive index dielectric materials, respectively. The microstructure and spectral emissivity of as-prepared 1DPSs were systematically characterized.The following results are obtained:(1) In the far-infrared atmospheric window of 8~14 μm, a simulation method of optimum pigment content and a prediction model of infrared emissivity for resin/flaky metal composite coatings with low infrared emissivity were established. Then the above method and model were experimental verified by using PU/Al, PU/bronze, and PU/Ag composite coatings as research objects. The verification results indicate that the calculated values of the three kinds of coatings are basically metal powders to Sm2O3 increasing, metal powers content is increased, Sm2O3 content is decreased, leading to the conductivity of the coatings increased, then the emissivity is reduced, while the near-infrared absorption capability is decreased, leading to the reflectivities at 1.06 μm and 1.54 μm increased. In addition, when the total pigment content of the coatings is fixed, the mechanical properties of PU/(bronze & Sm2O3) composite coatings are not sensitive to the mass ratio of bronze powders to Sm2O3, the coatings have good mechanical properties in different mass radios. But the mass radio of Al powders to Sm2O3 has important influence on the flexibility of PU/(Al & Sm2O3) composite coatings, and the value can be tuned from 2 mm to 5 mm.(4) In the mid- and far-infrared bands of 3~14 μm, the effects of refractive index difference of film-based materials and the number of periods on the infrared emissivity of 1DPS were systematically investigated by theoretical analysis. Three kinds of Ge/Zn S 1DPSs with low infrared emissivities at the wavelengths of 3~5 μm, 8~14 μm, both 3~5 μm and 8~14 μm were designed and prepared by using the optical coating technology. The study found that the larger the refractive index difference of film-based materials is, the smaller the emissivity, with the number of periods increasing, the emissivity decreased obviously, and always kept stable after 5 periods. The average emissivities of as-prepared Ge/Zn S 1DPSs(3~5 μm and 8~14 μm) can be as low as 0.052 and 0.195, respectively, illustrating that not only the high conductive precious metal films have ultra-low infrared emissivity(lower than 0.1), the semiconductor materials such as Ge and Zn S through rational design of 1DPS can also get ultra-low infrared emissivity. The as-prepared Ge/Zn S one-dimensional heterostructure photonic structure(1DHPS) has obviously infrared spectrally selective low emissivity characteristic, the average emissivities in the atmospheric windows of 3~5 μm and 8~14 μm can be as low as 0.046 and 0.190, respectively, but the average emissivity in the non-atmospheric window of 5~8 μm can be as high as 0.579, the difference between the high and low emissivity can reach more than 12 times. consistent with the experimental values. “U” type variation of emissivity changed with increasing pigment content of resin/flaky metal composite coatings is derived from the blue-shift of reflection peak center wavelength with increasing pigment content of 1DPS in the coatings. The reflectivity from the 1DPS in the coatings is fist increased then decreased due to the blue-shift of reflection peak with increasing pigment content, resulting in “U” type variation of emissivity changed as fist increased then decreased. The orientation of flaky metallic pigment in the resin/flaky metal composite coatings has significantly effect on the emissivity, the emissivity can be gradually increased with increasing the orientation angle of the flaky metallic pigment, and it increases to the highest value when the orientation angle is 45o. Using flaky Al powders with different thicknesses, or flaky Al powders and flaky bronze powders as composite metallic pigments can significantly reduce the infrared emissivity of the coatings. The reason for this phenomenon is that the composite metallic pigments can lead the reflection spectra of 1DPS in the composite coatings has multiple reflection peaks, leading to the overall reflectivity of the coatings for the infrared radiation is increased, thus the emissivity of the coatings is obviously reduced.(2) In the near-infrared band of 1~3 μm, PU/Sm2O3 composite coatings were prepared by using Sm2O3 and PU as pigments and adhesives, respectively. The near-infrared absorption and mechanical properties of as-prepared coatings were systematically characterized. The existence of Sm2O3 can give the acute absorption properties for near-infrared lights of 1.06 μm and 1.54 μm. The existence of PU can give the good mechanical properties of such coatings. The absorption capabilities and reflectivities at 1.06 μm and 1.54 μm enhanced and decreased obviously with the Sm2O3 content and thickness of the coatings increasing, respectively. When the Sm2O3 content is 40 wt% and thickness is 90~100 μm, the reflectivities at 1.06 μm and 1.54 μm can be as low as 58.7% and 34.7%, respectively, then kept stable. Meanwhile, the coatings have good mechanical properties, the hardness can reach 5 H, the adhesion strength can reach 1 grade, the flexibility can reach 2 mm, and the impact strength can reach 40 kg·cm.(3) In the near-infrared band of 1~3 μm and far-infrared band of 8~14 μm, PU/(bronze & Sm2O3) and PU/(Al & Sm2O3) composite coatings were prepared by using bronze powders, Al powders, Sm2O3, and PU as pigments and adhesives, respectively. The infrared emissivity, near-infrared absorption, and mechanical properties of as-prepared coatings were systematically characterized. The existence of metal powders can give the low infrared emissivity of such coatings. The existence of Sm2O3 can give the low reflective properties for near-infrared lights of 1.06 μm and 1.54 μm. The existence of PU can give the good mechanical properties of such coatings. With the mass ratio of...