Solvothermal synthesis and visible/infrared optical properties of Al/Fe3O4 core–shell magnetic composite pigments

• The Al/Fe3O4 magnetic composite pigment is prepared by the solvothermal method.
• The composite pigment is composed by coating Fe3O4 on the surface of flake aluminum powder.
• The phase structure and morphology mainly depend on the reaction solvent environment.
• The lightness and VIS reflectance is decreased by 45% and 0.56% after the surface coating.
• However, the infrared emissivity in the range of 8–14 μm waveband is only increases by 0.13.

A composite magnetic pigment with low infrared emissivity and low lightness was prepared by solvothermal method. This composite pigment was formed by coating Fe3O4 on the surface of flake aluminum powder. The phase structure, surface morphology, reflectance spectra and magnetic hysteresis loop were characterized by XRD, FE-SEM, UV/VIS/NIR spectroscopy, Fourier transform infrared spectrometer and VSM. The results show that the sample’s phase structure and morphology strongly depend on the reaction temperature and solvent environment, which lead a different synthesis pathway. The influence mechanism has also been studied. In order to form pure Fe3O4 phase, H2O must be completely excluded in the solvothermal process. The ethylenediamine solvent and high reaction temperature can promote evolution and crystallization of Fe3O4, and helps to form dense Fe3O4 granular film on the surface of the flake aluminum powder. Then, the black magnetic Al/Fe3O4 composite pigment with low lightness and low infrared emissivity can be obtained after solvothermal treatment in an ethylenediamine solvent at 250 °C for 24 h. The lightness L* and visual light reflectivity can be decreased by 45% and 0.56% compared with uncoated flake Al pigment, while the infrared emissivity, in the range of 8–14 μm waveband, is only increased by 0.13. Also, the saturation magnetization value is 25.9 emu/g. Therefore, the Al/Fe3O4 magnetic composite pigments can be used as a novel low infrared emissivity pigment to improve the multispectral compatible property of the low infrared emissivity coating.

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