Abstract
Tomographic analysis using focused ion beam-scanning electron microscopy (FIB-SEM) provides three-dimensional information about solid materials with a resolution of a few nanometres and thus bridges the gap between X-ray and transmission electron microscopic tomography techniques. This contribution serves as an introduction and overview of FIB-SEM tomography applied to porous materials. Using two different porous Earth materials, a diatomite specimen, and an experimentally produced amorphous silica layer on olivine, we discuss the experimental setup of FIB-SEM tomography. We then focus on image processing procedures, including image alignment, correction, and segmentation to finally result in a three-dimensional, quantified pore network representation of the two example materials. To each image processing step we consider potential issues, such as imaging the back of pore walls, and the generation of image artefacts through the application of processing algorithms. We conclude that there is no single image processing recipe; processing steps need to be decided on a case-by-case study.
Highlights
The focused ion beam (FIB) instrument was first developed for machining devices at a micrometre or sub-micrometre scale and as a method for preparing samples of various shapes [1,2,3,4,5,6]
Of biological samples, usually between 50 and 250 nm thick, for transmission electron microscopy (TEM) observations in two dimensions (2D) and in three dimensions (3D) through tomographic acquisition [7,8]. This tradition of sample preparation in FIB has been extended into materials sciences [9,10,11], for simple planar foils and for more complex shapes such as conical or cylinder forms required for optimizing scanning (S)TEM tomography and atom probe tomography (APT) [12,13]
The serial sectioning procedure itself occurs in three steps: (1) the ion beam mills away a thin layer at a current similar to the one used for polishing; (2) the focus of the electron beam is automatically adjusted based on the working distance (WD) variation that is related to the thickness of the slice erased during the milling; and (3) a scanning electron microscope (SEM) image is recorded from the newly uncovered material on the face of the cube
Summary
The focused ion beam (FIB) instrument was first developed for machining devices at a micrometre or sub-micrometre scale and as a method for preparing samples of various shapes [1,2,3,4,5,6]. This can be achieved using the dual-beam systems, where imaging and slicing are directly correlated creating a FIB-SEM cross section series [15,38] Performing such 3D acquisitions on porous materials can resolve mesopores and macropores [39], and characterize the interactions between them. The most challenging aspect of using FIB-SEM for such 3D tomography is the treatment of the data, i.e., slice image stack, to produce representative reconstructions This is a important challenge for porous materials, where the porosity and size distribution of the pores are unknown prior to analysis, which makes the accuracy of the segmentation procedure of the internal microstructures difficult to evaluate [45,46,47,48,49].
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