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They are being studied for over 50 years, and it seems we are far from over. Preparation of transparent ceramics is tricky, since one has to achieve complete densification (removal of porosity) but try to maintain its grain size as small as possible. This is true especially for spinel and alumina ceramics, which are used for transparent armor and for which small grain size is critical either from the viewpoint of mechanical properties alone (spinel with its isotropic crystallites) or from the viewpoint of both mechanical and optical properties (alumina with its anisotropic crystallites). Sintering additives are readily utilized in some cases, for example LiF for spinel and MgO or SiO2 for yttrium aluminum garnet (YAG), one of the most frequently used host materials for solid state lasers. 

Currently, I am trying to comprehend the mechanisms behind fluoride sintering additives for the densification and the grain growth of various ceramics. It all started in 2016 with my attempts to prepare transparent spinel ceramics by spark plasma sintering (SPS) without any additives. Eventually I succeeded, but the optical quality was still quite bad compared to some of the results reported in the literature, especially when sintering additives such as LiF were employed. From the beginning, I tried very hard to avoid the use of LiF as it leads to exaggerated grain growth. But as they say: if you cannot beat them, join them. This sparked my interest in sintering additives as I began to uncover the complicated nature of their interaction with different ceramics. 

So far, things are suggesting that some sort of “unified theory” could be developed in order to predict the most suitable fluoride sintering additive for the purpose of densification and/or grain growth suppression. For example, I have observed very effective grain growth inhibition of spinel ceramics with NaF addition. According to my first draft of this theory, MgF2 should be suitable for densification of YAG ceramics … and indeed it was. Actually, the results even exceeded my expectations, as it turned out that MgF2 is really very efficient. 

Since current trends are towards more transparency in all fields, including open science, let me say, that I am particularly fond of new good ideas and open to discussions in the fields of transparent ceramics and advanced sintering techniques, which might finally grow into long-term collaboration. So, anyone interested in these topics, meet me online! 
 
Department of Glass and Ceramics 
University of Chemistry and Technology, Prague 
Prague 
Czech Republic 
necinav@vscht.cz