Going beyond SEM: bulk analysis of defects in transparent ceramics by optical microscopy

In this work we produced Yb:YAG using different procedures and process parameters. We observed, by SEM analysis, that when the transmittance is above 75 %, the main type of defect is porosity. SEM is the typical characterization for transparent ceramics, but it is not fully adequate when the number of defects is low. For transparent ceramics, even a small amount of defects heavily influences the final optical quality. For example, YAG with density of 99 %, or even 99.9 %, is not transparent. Stuer et al. were able to reconstruct the pore distribution in transparent alumina using a combination of SEM and FIB in three dimensions, but the analysis is destructive and the volume analysed was very small (30 µm³). It is therefore important to find alternative techniques that can allow to investigate a larger volume of material. We developed a fast and non-destructive method based on the use of focus stacking with a digital optical microscope. The resulting stacked image shows all the defects present in the portion of volume analysed (~0.022 mm3 or 22×106 µm3). Image analysis was performed to extract information about number of pores and their size. To present the potential of this novel analytical technique we analysed samples of Yb:YAG and it was possible to correlate the transmittance in the sample with the number of the pores and get information about the influence of the process on the average pore size.