Synthesis of Inorganic Cobalt-Containing Spinel-Type Pigments by Self-Propagating Synthesis

Cobalt-containing ultramarine spinel-type pigments are fabricated in the ZnO–MgO–CoO–Al(OH)3–Al system by self-propagating high-temperature synthesis (SHS). the initial components are oxides of cobalt (Co3O4) and zinc (ZnO), aluminum hydroxide (Al(OH)3), and magnesium nitrate hexahydrate (Mg(NO3)2 · 6H2O). An aluminum powder of the ASD-4 brand is used as the reducer metal. The synthesis is performed using samples 40 mm in diameter. The combustion wave propagation velocity is 1–2 mm/s, and the maximal synthesis temperature is 1180°C. Parallel processes of oxidation of aluminum and aluminothermic reactions are leading reactions that provide the synthesis of spinel-based ceramic pigments in the layer-by-layer combustion. These processes result in charge self-heating to the synthesis temperature of spinels, which are also formed with heat liberation. The rapid destruction of Al(OH)3 upon heating leads to the formation of active submicron γ-Al2O3 participating in the subsequent synthesis of finely dispersed spinel structure. Endotherms associated with the decomposition of Al(OH)3 lead to cooling of combusting sample, which complicates the SHS implementation are requires an additional heat supply. Gases liberated during thermal decomposition loosen the charge in the heating zone and decrease the maximal combustion temperature, which makes it possible to perform the synthesis in the solid phase without product fusion and acquire it in the finely dispersed state. Microstructural investigations into the samples by means of scanning electron microscopy confirmed the finely dispersed structure of pigments. IR spectroscopic and X-ray phase analyses revealed a spinel structure. Particle-size-distribution histograms for the initial Al(OH)3 and after its heating, as well as for synthesized spinels, are presented in the work. It is shown that the content of particles 903 nm in diameter is maximal in pigment. Thus, the fabrication of spinel-type pigments in the finely dispersed state by solid-phase synthesis immediately in the combustion wave considerably simplifies the process flowsheet of their production due to the absence of a grinding state.