Formation and Densification Behavior of Magnesium Aluminate Spinel: The Influence of CaO and Moisture in the Precursors

Abstract

Different grades of stoichiometric and non‐stoichiometric dense magnesium aluminate spinel (MgAl2O4) grains were prepared by a conventional double‐stage firing process using two types of alumina and four types of magnesia raw materials. The MgAl2O4spinel formation was found to be highly influenced by CaO and moisture present in the precursor oxides as confirmed by thermogravimetry (TG), differential thermal analysis (DTA), and X‐ray diffraction (XRD) techniques. The Fourier transform‐infrared spectroscopy (FTIR) study of the precursor oxides revealed the presence of moisture. Influence of alumina and magnesia composition on the densification behavior of MgAl2O4spinels was assessed by characterizing bulk density (BD), apparent porosity (AP), water absorption (WA) capacity, and the microstructures of the stoichiometric, the magnesia‐rich, and the alumina‐rich spinels sintered at 1650°C for 1 h. Sintering studies indicate that to obtain dense stoichiometric spinel grains with >3.35 g/mL BD, <2.0% AP, and <0.5% WA, the spinel powder should possess a median particle size of <2 μm, CaO content of >0.9%, compact (green) density of >1.95 g/mL, and spinel content of >90%. Among various spinels synthesized, the magnesia‐rich spinels exhibited superior properties in terms of high BD, low percentage of AP, and low WA capacity, whereas alumina‐rich spinels showed inferior properties. Stoichiometric spinels exhibited an average grain size of 10 μm whereas alumina‐rich spinels with 90% alumina had an average grain size of 20 μm. The increase in holding time at higher temperatures enhanced the sintering properties of the spinels, particularly the magnesia‐rich spinels. Further, raw mixtures having >0.9% CaO exhibited better sintered properties as compared with others.