Characterization of the microstructure of YAG ceramics via stereology-based image analysis

The microstructure of transparent YAG ceramics is investigated by stereology-based microscopic image analysis using SEM and FE-SEM micrographs. Interface densities, mean curvature integral densities and the related grain size measures (mean chord length and Jeffries size) have been determined with relative errors of 9-12 % for interface densities and mean curvature integral densities and 6-9 % for the corresponding grain size measures. A comparison of the two grain size measures confirmed an excellent linear correlation between the Jeffries size and the mean chord length, with a mean-chordlength-to-Jeffries-size ratio of 0.928 ± 0.086. The overall range of average grain sizes is approx. 11-34 ?m. It has been found that the sintering time has a significant influence on the grain size, especially for YAG ceramics without Yb doping. When the sintering time is increased by a factor 8 (from 2 h to 16 h) the grain size increases by more than 200 % in undoped YAG ceramics, whereas the grain growth is much weaker in Yb-doped YAG ceramics (grain growth only 50-70 % for YAG ceramics with 5-10 at.% Yb). Thus it can be concluded that the Yb dopant acts as a grain growth inhibitor in YAG ceramics, at least for sufficiently long sintering times (8 h and more). The influence of the sintering additive (tetraethyl orthosilicate TEOS) content on the grain size is negligible in the concentration range tested (0.3-0.5 wt.%).