Comparison of the effect of different alkali halides on the preparation of transparent MgAl2O4 spinel ceramics via spark plasma sintering (SPS)

The effect of different alkali halides additives (LiF, LiCl, NaF, NaCl, KF, KCl) on the densification of spinel ceramics prepared via spark plasma sintering (SPS) is systematically studied. Significant differences are found between different additives considering the onset temperature of densification, grain growth and the possibility of densification up to transparency. It is confirmed that LiF is the best sintering additive as far as densification is concerned. It is found, however, that NaF has a quite similar effect on densification, while it leads at the same time to significant grain growth inhibition (contrary to LiF which generally promotes grain growth). Moreover, it is found that LiCl promotes grain growth to a very similar degree as LiF, thus indicating that the Li+ ion is responsible for exaggerated grain growth. Finally, the mechanism of LiF in the densification of spinel ceramics is revisited and discussed.