Effect of pH on the chemical composition, morphology, and densification of magnesium aluminate spinel nanopowder synthesized by the coprecipitation method

Magnesium aluminate (MgAl₂O₄) spinel nanopowder was prepared by coprecipitation using ammonium bicarbonate solutions with pH values varying from 7.7 to 10.2. At lower pH (7.7 and 8.0), the precipitates obtained were equiaxed ammonium dawsonite and hydrotalcite with low crystallinity. As the pH was increased above 9.0, the precipitates transformed into rod-like ammonium dawsonite and hydrotalcite with higher crystallinity. The precipitates were calcined at 1100 °C for 2 h, resulting in spinel nanopowders with Al₂O₃/MgO molar ratios ranging from 2.74 to 0.96, depending on the pH of the precipitant solution. The morphology of the spinel nanopowders evolved from equiaxed primary particles to rod-like structures with increasing pH. The sinterability of the spinel nanopowders was enhanced at higher precipitant solution pH, with the nanopowder synthesized at pH 10.2 (Al₂O₃/MgO molar ratio of 0.96) achieving 99% theoretical density after sintering at 1550 °C for 4 h. This study demonstrates the significant influence of precipitant solution pH on the composition, morphology, and sintering characteristics of spinel nanopowders.