# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 3 Results
### 3D Tomographic Reconstruction  
Standard characterization techniques such as SEM, TEM and AFM typically reveal planar, projected or surface information, and therefore may not be ideal for revealing internal microstructure. As such, 3D reconstruction can provide insights into the microstructure of a complex 3D object that is not attainable with 2D characterization [39]. In the current study, a 3D tomogram was constructed for the first time on a scandate cathode, by employing the dual-beam FIB-SEM microscope. This technique involves layer-by-layer milling/removal of thin slices of the sample, using the ion beam, with SEM images recorded after each slice. 10 nm thick slices were milled sequentially, culminating in 137 backscattered electron (BSE) images stitched together to form a 3D tomogram. A schematic of the serial sectioning technique is presented in Figure 9. In order to perform FIB milling, the sample stage is tilted to 52\({}^{\circ}\) and the region of interest is situated at the eucentric height, which is where the sample region of interest will not change its vertical position during tilting (it is also where the ion beam and the electron beam converge). Prior to slicing, the sample was coated with a layer of Pt to protect the underlying region of interest. The \(x\)-\(y\) plane is perpendicular to the ion beam, and the \(x\)-\(z\) plane (parallel to the ion beam) is imaged after each slice. The red lines in Figure 9 represent the serial sectioning that is performed at regular intervals along the \(y\)-axis of the sample. Figure 8: Crystal structure of the W matrix in scandate cathode #1, characterized using scanning nanobeam diffraction. (**a**) HAADF image showing the selected tungsten grain; (**b**–**d**) diffraction patterns corresponding to the sites of A, B and C as marked in (**a**). It is demonstrated that the W matrix has the BCC structure, which is inferred from diffraction patterns obtained at locations ranging from the middle of the W grain to its surface. The beam direction and zone axis for the grain orientation shown here are indexed as [\(-\)111]. The 3D reconstructed shape of a particular W particle was generated using Avizo software, and involved 137 BSE micrographs obtained after each step of automated serial sectioning. This W particle is shown in Figure 10a and exhibits surface facets. A noise reduction filter was applied to each micrograph prior to reconstruction. Each image was aligned with respect to a fiducial mark, resulting in the reconstructed tomogram displayed in Figure 10b. The SEM image of the selected W particle in Figure 10a and the 3D reconstruction of that particle in Figure 10b agree well with each other, and the surface facets are consistent and clear in both figures. The linear surface steps in Figure 10b result from the serial slicing process, specifically from slight misalignments between image positions after each slice, i.e., these surface steps are an artifact of reconstruction. Since BSE imaging was used to generate the tomogram, it was possible to optimize atomic number contrast for setting thresholds in the reconstruction. The metallic W regions exhibited consistent brightness levels, while the oxide particles were darker gray and clearly distinguishable from the main W particle. Leveraging this contrast, the (darker) oxide particles were differentiated, and Figure 10c reveals Sc/Sc\({}_{2}\)O\({}_{3}\) particles trapped inside the larger W particle. As shown in Movie Supplementary Movie S3, where the tomogram of Figure 10c is rotated and viewed from multiple directions, the smaller oxide particles can be seen to lie inside the W particle. These observations are consistent with the results described earlier, namely that Sc/Sc\({}_{2}\)O\({}_{3}\) particles are contained inside larger W grains. Figure 10: 3D reconstruction of a W particle from scandate cathode #2, generated by FIB serial sectioning and imaging in the SEM.