Formation Process of Magnesium Aluminate due to Solid-State Reaction of Highly-Dispersed and Nanometer-Sized Particles

摘要

The formation process of magnesium aluminate (MgAl_2O_4) due to the solid-state reaction of highly-dispersed and nanometer-sized aluminum and magnesium compounds has been examined by high-temperature X-ray diffractometry (HT-XRD), synchrotron radiation diffractometry (SRD) and X-ray photoelectron spectroscopy (XPS). The starting compounds were alpha- and gamma -aluminum oxide (alpha and gamma-Al_2O_3; primary particle sizes, 105 and 31.6 nm, respectively) as aluminum sources, and magnesium oxide (MgO; 41.3 nm) and magnesium hydroxide (Mg(OH)_2; 61.1 nm) as magnesium sources. Through the combination of these compounds, four powder mixtures were prepared, namely, (i) alpha-Al_2O_3 and MgO, (ii) gamma-Al_2O_3 and MgO, (iii) alpha-Al_2O_3 and Mg(OH)_2, and (iv) gamma-Al_2O_3 and Mg(OH)_2. Phase change investigation during the heating of these mixtures indicated that the formation of MgAl_2O_4 due to the reaction of gamma-Al_2O_3 with Mg (OH)_2 was faster when compared to the other combinations; almost single phase of MgAl_2O_4 could be obtained when this mixture was heated at 1200 deg C for 1 h. More detailed investigation on the formation process of MgAl_2O_4 was conducted using the precursor mixture of gamma-Al_2O_3 and Mg(OH)_2 heat-treated at 800 deg C for 1 h. The data obtained from SRD and XPS suggested that small amounts of MgAl_2O_4 and alpha-Al_2O_3, together with gamma-Al_2O_3 and MgO, were present in this precursor. The formation of MgAl_2O_4 due to the reaction of gamma-Al_2O_3 with Mg(OH)_2 was found to occur readily due to active mass transfer as a result of the very small primary particle and agglomerate sizes.