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 Obtaining transparent ceramics, which can be an alternative to single crystals for laser applications, requires precise control of the sintering trajectory to achieve a polycrystalline structure of almost ideal density, practically free of pores (the permissible fraction of residual pores for laser applications is <10-3 vol.%). In the vast majority of cases, ceramic technologies are based on controlling the sintering trajectory by modifying the surface energy and, as a result, the diffusion mobility of crystal grains and the boundaries between them using additives to the initial powder composition (usually called sintering aids). In my work, I mainly study the features of the consolidation of ceramics based on yttrium-aluminum garnet (YAG), which is controlled using additives based on silica and magnesium. Current trends in laser engineering request YAG-based laser mediums, doped by Nd3+, as well as with Sm3+. This requires ceramic technology to have significant flexibility in terms of the chemical composition of sintered ceramics. The ultimate goal is to be able to obtain materials with varying dopant sorts and concertation within single sintering (in particular, for layered laser media as gradient-doped YAG:Nd3+ or YAG:Nd3+/YAG:Sm3+).

         Our recent research was focused on sintering trajectory control of YAG ceramics, consolidated by reactive sintering, by bi-component sintering additive of Si4++Mg2+.  Both of these ions cause a strong influence on sintering dynamics due to the heterovalent substitution of Al3+ sites in YAG crystalline matrix. Herewith, Al3+ → Si4+ substitution with exceed positive charge provoke the formation of cationic vacancies, which accelerate both densification and recrystallization in ceramics. While Al3+ → Mg2+ substitution creates opposite, anionic vacancies, which are found to inhibit grain growth, the latter is vital to keep an effective diffusion path for vacancies for residual pores to eliminate during sintering. I with colleagues studied the influence of both silica and magnesium concentrations in bi-component additive on the YAG ceramics sintering, final microstructure, and optical properties, details of which can be found in our JECS publication [1].

We found that besides a particular amount of each component of aid, the Si4+/Mg2+ ratio is crucial to achieving the transparent state of ceramics due to the mutually suppressive interaction between these ions. Results of this work were then used to optimize the obtaining condition of doped ceramics of two compositions, which are in high demand for contemporary laser development – 1 at.% Nd3+:YAG and 3 at.% Sm3+:YAG. Using sintering trajectory control by bi-component additive, we extend the temperature range of obtaining transparent ceramics for both Nd3+:YAG and Sm3+:YAG compositions to the point of overlapping (initially, Sm3+:YAG required lower sintering temperature due to lower energy threshold of the abnormal grain growth). This result opens the opportunities to drastically reduce technical complexity and cost for fabrication of composite laser media, such as layered Nd3+:YAG/ Sm3+:YAG. Usual for such composites multi-stage fabrication process involving diffusion bonding can be substituted by single sintering of ceramics from the composite green body.

Dr. Ihor Vorona (vorona.ihor@gmail.com)

Institute for single crystals NAS of Ukraine, Kharkiv

Links:

[1] https://doi.org/10.1016/j.jeurceramsoc.2022.05.017