Influence of mechanical activation on phase composition and structure of SHS spinel pigments

Ceramic spinel pigments were produced in the ZnO-CoO-Al ₂O ₃ and ZnO-MgO-Со0-А1 ₂0 ₃ systems by the self-propagating high-temperature synthesis method using mechanical activation of initial components. The powders of cobalt oxides (Со ₂О ₃, Со ₃О ₄), zinc, magnesium, aluminum, magnesium nitrate and SDA-4 aluminum powder were used as initial components. The SHS synthesis of pigments was conducted in the constant pressure installation heated in air at atmospheric pressure. The samples were ignited from the frontal surface of the sample by an igniting spiral. The temperature was controled using W-Re thermocouples placed in the center of the samples. Mechanical activation of initial reactionary mixtures was conducted in air in the M3 planetary ball mill with an acceleration of 45 g. The volume of milling drums was 10 dm , the mass of the green mixture was 50g, the ratio between the mass of the powder and the mass of balls was 1: 6. The initial and finished products of SH-synthesis were identified using X-ray diffraction analysis (DRON-UM1, filtered Со K _a-radiation), electron microprobe analysis (Camebax-Microbeam analyzer) and infrared spectroscopy (IR-Fourier spectrometer Nicolet 5700). Optical microscopy (Axiovert 200M) and scanning electron microscopy (Philips SEM 515) were used to study the microstructure of pigments obtained. Thermal analysis for the green pigment mixtures in the ZnO-MgO-CoO-Al ₂O ₃ system was carried out before and after mechanical activation using the SDT Q600 analyzer in the temperature range of 25-1200°C at a heating rate of 20°C/min in air. The dispersed pigment composition was determined by the dynamic light scattering method using the ZETASIZER NANO ZS device (Malvern «Mastersizer 2000). Oxidation of aluminum realized through direct oxidation of aluminum and a thermite reaction with cobalt oxide during the SH-synthesis of pigments was found to result in self-heating of the green mixture up to a temperature above 1000°C and the formation of spinels. Grinding of the green pigment mixtures in the M3 planetary mill with an acceleration of 45g for 60 seconds leads to a decrease in initial temperature of SH-synthesis (To) at 170°C, i. e. from 650°C to 480°C. The maximum temperature of synthesis (T _max) also slightly decreases after mechanical activation. The change АТ=Т ₀-T _maxfrom 1100°C to 1250°C indicates an increase in conversion efficiency of spinels. The difference in the synthesis temperature before and after mechanical activation is connected not only with grinding of green mixture components and the formation of various defects, but also with the occurrence of new nucleating phases. Thermal analysis of the green pigment mixture in the ZnO-MgO-CoO-Al ₂O ₃ system after mechanical activation in the M3 planetary mill has shown that there is decomposition of Co ₃O ₄ oxide to CoO during grinding, as evidenced by the absence of endoeffects at 912.8°C on a curve of differential scanning calorimetry. Metal inclusions of cobalt formed during a thermite reaction are not detected. The absence of the submelting and sintered surface in the finished pigment confirms the fact that mechanical activation of the green mixture leads to the solid-phase mechanism of spinel formation.

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