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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 MgAl2O4 spinel 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 MgAl2O4 spinels 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 m, 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 m whereas alumina-rich spinels with 90% alumina had an average grain size of 20 m. 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.
Ceramic Materials Division, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, India 2:
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology (IICT), Hyderabad 500007, India