The influence of mechanical activation and sintering process on the formation of the spinel phase of MgAl2O4

Magnesium aluminate has the spinel structure and, due to good mechanical, chemical, and thermal properties, has a wide range of applications including refractory ceramics, optically transparent ceramic windows, and armors. Calcined MgO and Al₂O₃ powders were mixed in a one-to-one molar ratio to produce phase-pure spinel. The MgO powder was calcined at 1000 ^oC for 1 h prior to mechanical activation to avoid presence of hydroxide and carbonate at the powder surface. The powders were mechanically activated for 15, 30, and 60 min in a high-energy planetary ball mill in air atmosphere. The mechanically activated powders were pressed into pellets and heated to 1300 ^oC at 10 ^oC/min and held for 1 h for the reaction. Afterwards pellets were ground and sieved. Synthesized powders were sintered at 1450 ^oC at 10 ^oC/min for 2 h and examined for phase composition, crystal structure, and morphology. The results showed that mechanical activation and sintering led to formation of pure spinel phase. Mechanical activation times of 30 min and 60 min led to spinel that was more highly crystalline and phase-pure. Raman spectroscopy showed the presence of all five Raman active modes (A_1g + E_g +3T_2g), and their positions were in a good agreement with previous investigations. Powder morphology analysis showed that particles were comminuted, but that agglomerates formed for longer activation times. The maximum in the particle size distribution curves decreased from 14.7 μm for non-activated powder to 9.1 μm after 15 min of mechanical activation and 8.1 μm after 30 min of mechanical activation, but increased to10.5 μm after 60 min of activation.