Self-propagating high-temperature synthesis of spinel-containing pigments in the ZnO-МgO-CoO-Al₂O₃ system

The ceramic spinel-type pigments were obtained by the self-propagating high-temperature synthesis in the ZnO-CoO-Al2O3 and ZnO-MgO-CoO-Al2O3 systems. A stable mode of layered combustion is found to be observed for the small samples (10-20 mm in diameter) due to the uniform heating in a short time. The insufficient heating of the green mixture leads to the decrease in the stability of the stationary mode that passes into a spin mode for the small and large samples (60-90 mm in diameter). The spin effect is formed by the temperature redistribution in the cross-section of the sample. Dark color of the pigment indicates the higher temperatures and is connected with the partial destruction of the pigment. In the layered combustion (Tmax = 1600°C) a spinel crystallizes as octahedrons and flat triangular plates during the solid-phase reaction. The temperature increase (~ 1700-1800°C) during the SHS process leads to the formation of aluminum suboxides and the epitaxial growth of spinel layers starting with the appearance of individual embryos on the substrate and developing according to the PC mechanism. The edge skeleton crystals are formed under conditions of the accelerated growth. The pigment synthesis temperature is determined by the ratio mAhO3/mAl. The spinel is damaged to form corundum, cobalt oxide CoO and a partially inverse spinel at the temperatures >2000°C (mAhO3/mAl<5.8). The mechanical activation of the green mixture leads to the fact that the synthesis products from the gas phase can not be found due to the predominance of the termite SHS-process reactions over the direct oxidation of aluminum, which are the leading SHS-process reactions and the appearance of new phase embryos that lead to the decrease in the spinel synthesis temperature. Intimate mixing of the green mixture increases the completeness of the spinel synthesis. The amount of metal cobalt is significantly reduced in the synthesis products, but the a-AhO3phase is maintained. During NPA combustion, aluminum nitride (AlN) is formed in the synthesis products. If aluminate spinels are obtained using the powders of different grades (ASD-1, ASD-4, NPA) then aluminum nitride is not formed, perhaps, due to the formation of MgAhO4 from AlN. The absence of fixed nitrogen in the pigments is confirmed by the Kjeldahl distillation method. The composition of the products is confirmed by the X-ray and IR spectroscopic analysis.

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