Synthesis of Magnesium Aluminates
As with many ceramics, MgAl2O4 spinels can be made by solid-state sintering of the component oxides MgO and Al2O3 . Pure stoichiometric spinel (MgAl2O4) is made by solid-state reaction of high purity end-members at high temperatures. Starting materials are either oxides (Al2O3 and MgO) or carbonates (MgCO3). The synthesis relies on solid-state reactions between the grains of starting material and so depends on the fineness of the powders used. An additional problem is the potential for Mg(II) to volatilize at high temperatures from the Mg-starting powder, which can lead to nonstoichiometric phases. In some instances this is desired, since more Al2O3- rich spinels are more stable under reducing atmospheres.
For some applications, better control on porosity is required and alternatives to solid-state synthesis methods have been sought requiring synthesis temperatures much lower than those used for the sintering route (1,600-1,800°C).
Chemical synthesis of MgAl2O4 spinels has been attempted using gibbsite (Al(OH)3) and MgO precursors. Spinel precursors are formed by coprecipitation and the resultant material is then calcined to produce spinel. The starting material gibbsite, which is a by-product of the Bayer process, is dissolved in a solution of HCl and HNO3. MgO is added in a molar ratio 2:1 Al/Mg (i. e., stochiometric spinel). A precipitate is formed by adding NH4OH to maintain a pH of 8.5-9.0. The precipitate is filtered and rinsed before calcining at temperatures of up to 1,400°C. Finally, nanophase spinel aggregates are formed with a reduced (83%) density.
Even greater control of the microstructure of spinels is achieved by joint crystallization of mixtures of magnesium and aluminum salts [74, 75]. The magnesium salt, magnesium nitrate hexahydrate (Mg(NO3)26H2O) can be used to form highly reactive spinel precursors by mixing in solution with different aluminum compounds. Vasilyeva and coworkers [74, 75] for example report synthesis of nano-phase spinels with porosity of up to 50% through use of aluminum nitrate monohydrate (Al(NO3)39H2O), aluminum isopropoxide (Al( (CH3)2CHO)3), and aluminum hydroxide (AioOh, Boehmite). The stoichiometric mixtures of salts are dissolved in water and the pH is adjusted by the addition of nitric acid (HNO3). The solutions are evaporated and then calcined at 250-900°C. The porosity is variable and depends on the aluminum compound used, and a combustible synthesis aid such as carbon can be added to further increase the porosity.
Sol-gel techniques have also been developed to make MgAl2O4 spinel . In some applications [15, 77], such as filtration membranes for the food industry, spinels, which have greater chemical stability, are prepared on the surfaces of y-Al2O3 nanoparticles. In this technique, boehmite, produced by sol-gel process, is used as a starting sol. In situ modification of the sol surface is achieved by adding Mg(NO3)2 and ethylene-dinitro-tetra-acetic acid (EDTA) to the aged boehmite sol, and polyvinyl acetate (PVA) solution and polyethylene glycol (PEG) is added to prevent defect formation. During calcining, at 550-850°C, magnesium oxide diffuses to the core and reacts with the alumina to form a spinel coat on the y-Al2O3 particles.
A modified sol-gel method can also be used to make spinel directly . Magnesium oxide is dispersed into an isopropanol solution of aluminum sec-butoxide. Water is added to the solution to promote alkoxide gelation and the slurry is evaporated to remove excess water and alcohol. The precursors are then dried and calcined at 300-800°C. In this case the formation of spinel is through reaction of nanophase MgO and Al2O3 in the spinel precursor during the calcining process.