Synthesis of inorganic cobalt-containing spinel pigments by SHS method

Cobalt-containing spinel-type ultramarine pigments were obtained by self-propagating high-temperature synthesis (SHS) in the ZnO-MgO-CoO-Al(OH)₃-Al system. Starting components were oxides of cobalt (Co3O4) and zinc (ZnO), aluminum hydroxide (Al(OH)₃), and 6-water magnesium nitrate (Mg(NO₃)₂•6H₂O). ASD-4 grade aluminum powder was used as a reducing metal. The samples with a diameter of 40 mm were synthesized. The combustion wave velocity was 1-2 mm/s, and the maximum synthesis temperature was 1180 °С. Parallel aluminum oxidation and aluminothermic reactions were the leading reactions providing the synthesis of spinel-based ceramic pigments in the layer-by-layer combustion mode. They result in charge self-heating up to the synthesis temperatures of spinels that are also formed with the release of heat. The fast destruction of Al(OH)₃ upon heating leads to the formation of active submicron y-A1₂O₃, which is involved in the further synthesis of finely dispersed spinel. Endothermic effects associated with Al(OH)₃ decomposition lead to burning sample cooling. This complicates the SHS implementation and requires additional heat supply. Gases emitted during thermal decomposition loosen the charge in the heating zone and reduce the maximum combustion temperature that allows solid-phase synthesis without any melting of the product to obtain it in a finely dispersed state. The microstructural analysis of samples by scanning electron microscopy confirmed the finely dispersed structure of pigments. IR spectroscopy and X-ray diffraction analysis revealed spinel structures. The paper presents the particle size distribution histograms for starting Al(OH)₃, Al(OH)₃ after heating, and synthesized spinels. It was shown that the pigment contains the maximum number of 903 nm particles. Therefore, obtaining finely dispersed spinel-type pigments by solid-phase synthesis directly in the combustion wave greatly simplifies their production process due to the absence of a grinding stage.

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