Date

2023-1-27

Author

Keskin, Ahmet

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In recent years, infrared low-observable technology has come to the fore in the aerospace and defense industry to conceal military aircraft against rapidly developing infrared search and tracking systems. Pigment materials developed for coatings applied to aircraft surfaces based on materials science and engineering in line with electromagnetic principles play a crucial role in this regard. Although low-observable structures were obtained in the 8-14 µm infrared wavelength range in particle studies, radar and visible region invisibility, which are among the stealth requirements, posed the major challenge. Core/shell particles developed to provide low observability in the multiband range resulted in succesful demonstrations on this subject. Although efficient results were obtained for the radar and visible region in the literature, sufficiently low infrared emissivity could not be obtained. In addition, there are only limited number of studies investigating the efficiency of the particles embedded in the coating in the literature. In this study, the focus is on the development of infrared low emissivity core/shell particles, as well as radar and visible region reflectance. In the first step, candidate materials were screened for the development of core/shell particles. Aluminum, which is the most efficient material in this field, was determined as the core, and magnetite (Fe3O4), silica (SiO2), titania (TiO2) and zinc oxide (ZnO) were determined as the shell materials. Depending on the shell thickness of the determined materials, 8-14 µm infrared reflectance performances in the form of metal/metal oxide core/shell particles in the simulation environment under the same conditions were analyzed by FDTD method. Al/ZnO core/shell particles were decided to be synthesized, showing the highest performance in all shell thicknesses. Solvothermal synthesis method was used for the synthesis of Al/ZnO core/shell particles and the effect of temperature and Zn molar ratio were investigated in detail. A systematic material characterization study was performed on the Al/ZnO particles using X-Ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-Ray Analysis (EDX) methods. Following characterisation, powders were combined with aerospace-standard polyurethane by 1/1 weight ratio and the composites were coated onto glass and aluminum substrates. Firstly, 8-14 µm FTIR spectrometer transmittance and reflectance performances of powder and composite coatings were investigated, and then the visible region reflectance of the composite coatings were investigated with UV-Vis spectrophotometer. As the closest value to Al absorbance, Al/ZnO core/shell powder with flake-shaped shell structure synthesized at a reaction temperature of 150 °C demonstrated the closest absorbance to Al in LWIR band. The absorbance and reflectance characteristics of the PU-matrixed composite coatings with pure Al and core/shell particles were evaluated. The average FTIR reflectance of the core/shell composite coating closest to composite coating with Al particle was measured as 45.45%. The results obtained from coatings were consistent with the powder samples. Thermal images of composite materials were obtained at 60, 80 and 100 °C, and the closest value to Al/PU composite coating (emissivity of 0.888) was obtained from Al/ZnO composite coating (emissivity of 0.931). The coatings of Al/PU and Al/ZnO/PU resulted with low-emissivity in previous test were examined depending on the filler fraction from 0.5/1 to 2/1 by weight. The lowest emissivity values of 0.724 and 0.805 were measured in Al/PU and Al/ZnO/PU composite coatings at a particle/binder ratio of 2/1 by weight, respectively. In addition, it has been proven that lower emissivity values were obtained from Al/ZnO-filled composites than pure Al/PU composites when the core/shell filler fraction is increased. The X-band radar absorbance performances of the composite materials with the lowest emissivity value were examined. As average, X-band radar reflectance was decreased by 10% and visible reflectance was reduced by 4.1% in composite materials with core/shell particles when compared to Al/PU composite coatings. In this study, the visible band absorbance and radar absorbance performances were improved compared to pure Al with development core/shell particles. Consequently, considering all the requirements of stealth characteristics, besides providing LWIR low radiation, it has been proven that the core/shell structures are promising materials to meet radar and visible range low observability.

Subject Keywords

Infrared stealth technology, Solvothermal synthesis, Low-emissivity, Core/shell, FDTD method, Multiband low observability

URI

https://hdl.handle.net/11511/102107

Collections

Graduate School of Natural and Applied Sciences, Thesis