Diego Roldan

• The knowledge of structural properties in microscopic materials contributes to a deeper understanding of macroscopic properties. For the study of such materials, several imaging techniques reaching scales in the order of nanometers have been developed. One of the most powerful and sophisticated imaging methods is focused-ion-beam scanning electron microscopy (FIB-SEM), which combines serial sectioning by an ion beam and imaging by a scanning electron microscope. FIB-SEM imaging reaches extraordinary scales below 5 nm with large representative volumes. However, the complexity of the imaging process results in the addition of artificial distortion and artifacts generating poor-quality images. We introduce a method for the quality evaluation of images by analyzing general characteristics of the images as well as artifacts exclusively for FIB-SEM, namely curtaining and charging. For the evaluation, we propose quality indexes, which are tested on several data sets of porous and non-porous materials with different characteristics and distortions. The quality indexes report objective evaluations in accordance with visual judgment. Moreover, the acquisition of large volumes at high resolution can be time-consuming. An approach to speed up the imaging is by decreasing the resolution and by considering cuboidal voxel configurations. However, non-isotropic resolutions may lead to errors in the reconstructions. Even if the reconstruction is correct, effects are visible in the analysis. We study the effects of different voxel settings on the prediction of material and flow properties of reconstructed structures. Results show good agreement between highly resolved cases and ground truths as is expected. Structural anisotropy is reported as resolution decreases, especially in anisotropic grids. Nevertheless, gray image interpolation remedies the induced anisotropy. These benefits are visible at flow properties as well. For highly porous structures, the structural reconstruction is even more difficult as a consequence of deeper parts of the material visible through the pores. We show as an application example, the reconstruction of two highly porous structures of optical layers, where a typical workflow from image acquisition, preprocessing, reconstruction until a spatial analysis is performed. The study case shows the advantages of 3D imaging for optical porous layers. The analysis reveals geometrical structural properties related to the manufacturing processes.