The effect of MnO2 additive on the microstructure and mechanical properties of magnesium aluminate spinel

In this study, varying amounts of MnO 2 up to 5 wt.% were added to magnesium aluminate spinel (MA) bodies using a solid-state sintering method at 1200–1600°C. The effect of MnO 2 addition on the phase composition, microstructure, distribution of elements, and ionic valence of MA was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy, respectively. The results showed that Mg 2+ ions in MA crystals were replaced by Mn 2+ ions, resulting in the formation of the (Mg 1- x Mn x )Al 2 O 4 solid solution. The distorted crystal structures promoted the sintering reactions, and the mechanical characteristics of MA were greatly improved by the solid solution strengthening process. When the additive amount of MnO 2 was 5 wt.% and the sintered temperature reached at 1600°C, excess manganese ions hardly dissolved into the lattice of MA. And these ions were only distributed at the grain boundaries of MgAl 2 O 4 , forming a “barrier” that hindered the migration and diffusion of particles, thereby suppressing the sintering process and weakening the mechanical strength of MA.