Solution Combustion Synthesis of MnAl2O4 Brown Pigments Using Different Fuel Mixtures

this paper reports the synthesis of spinel manganese aluminates (MnAl2O4) brown pigments via microwave-assisted solution combustion using different urea-glycine fuel mixtures. The synthesized samples were analyzed through all the following methods: thermal analysis, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and colorimetry. The results showed that amorphous flakes are obtained via one-step solution combustion under microwave radiation. They are then converted to the MnAl2O4 and minor structural changes due to manganese valence variation after calcination at 1000 °C. The mean crystallite size in the case of the fuel mixture after calcination at 1000 °C was 23 nm. The fuel type slightly affected the combusted flake-like microstructure before calcination. However, the porous flakes converted to nonporous nanoparticles after calcination by increasing the glycine content. The sample obtained by the combustion of fuel mixture (75:25 weight ratio) after calcination at 1000 °C had the highest reflection intensity, especially in the red region due to the formation of the MnAl2O4 crystal structure. The hue and saturation of the sample were 62.7 and 27.69, respectively. The red, yellow, and brightness coordinates were 12.69 (a*), 24.61 (b*), and 45.76 (L*), respectively. As a result, the synthesized pigment can be a promising candidate for the production of ceramic inks compared with the commercial brown pigments [9.02 (a*), 13.62 (b*), and 33.61 (L*)]. The bandgap of the samples immediately after combustion was in the range of 4 eV, which was in good agreement with the results reported for the MnAl2O4 band gap (4.03 eV).