CHARACTERIZATION OF THE MICROSTRUCTURE OF YAG CERAMICS VIA STEREOLOGY-BASED IMAGE ANALYSIS

Abstract

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 confi rmed 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 μm. It has been found that the sintering time has a signifi cant 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 suffi ciently long sintering times (8 h and more). The infl uence of the sintering additive (tetraethyl orthosilicate TEOS) content on the grain size is negligible in the concentration range tested (0.3-0.5 wt.%).

Keywords

• YAG ceramics, Yb-doping, solid state lasers, microstructure, image analysis, stereology, stereological relations, grain size (mean chord length, Jeffries size), interface density (grain boundary density), mean curvature integral density, edge length density (triple junction line length density)