Optical and magnetic properties of Al/Fe3O4 core-shell low infrared emissivity pigments

A core-shell Al/Fe3O4 magnetic composite pigment has been prepared by solvothermal method in order to obtain low infrared emissivity pigment with low lightness and visible reflectance. The influence of the shell layer thickness, surface roughness and radiation wavelength on spectral reflectivity of Al/Fe3O4 composite pigment is systematically researched by geometric optics theory and rough surface theory. The Al/Fe3O4 composite pigment with low visible reflectance and high infrared reflectance can be designed by thin thickness and high surface roughness of the shell layer. Then, Al/Fe3O4 composite pigments with different covering content of Fe3O4 are prepared by adjusting the molar ratio of Fe3+: Al. The phase structure, surface morphology, reflectance spectra and magnetic hysteresis loop of samples are characterized by XRD, FE-SEM, UV/VIS/NIR spectroscopy, Fourier transform infrared spectrometer and VSM. The result shows that the surface covering content rises with the increase of Fe3+: Al, which significantly reduces the VIS reflectance and lightness of flake Al pigment but only slightly increases the infrared emissivity at low molar ratio of Fe3+: Al. There are only a few of the surface coated with a thin Fe3O4 layer when the molar ratio of Fe3+: Al is 0.075: 1, but the surface roughness of flake Al pigment significantly increases. The lightness L* and visual light reflectivity can be decreased by 12.8 and 0.34 as compared to the uncoated flake Al pigment, but the infrared emissivity is only increased by 0.04. The saturation magnetization value of the composite pigment is 11.9 emu/g, which could help improve the radar stealthy performance. Therefore, these Al/Fe3O4 magnetic composite pigments can be used as a novel low infrared emissivity pigment to improve the stealthy performance of the coating in the visual, IR and Radar wavebands. (C) 2013 Elsevier B. V. All rights reserved.