Spinel powders for the production of transparent polycrystalline ceramic windows have been produced using a number of traditional ceramic and sol-gel methods. We have demonstrated that magnesium aluminate spinel powders produced from the reaction of organo-magnesium compounds with surface modified boehmite precursors can be used to produce high quality transparent spinel parts. The new powder production method allows fine control over the starting particle size, size distribution, purity and stoichiometry. The new process involves formation of a boehmite sol-gel from the hydrolysis of aluminum alkoxides followed by surface modification of the boehmite nanoparticles using carboxylic acids. The resulting surface modified boehmite nanoparticles can then be metal exchanged at room temperature with magnesium acetylacetonate to make a precursor powder that is readily transformed into pure phase spinel.
Densification and microstructural evolution of transparent magnesium aluminate spinel (MgAl2O4) were examined through systematic experiments that focus on the role of impurities in the starting powders. Up to 1 wt. % LiF was added to very fine spinel powder synthesized by a metal-exchange method. The material was densified by pressing at elevated temperatures. Very high transmissivity was achieved under appropriate conditions. A layered spinel/LiF/spinel specimen was also fabricated with the intent of better understanding the role of LiF on microstructure development. Its presence can lead to accelerated grain growth, or growth suppression, depending on a complex interplay of chemistry, concentration and processing schedule.