# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 3 Results
### Elemental Analysis of Phases in Cross-Section Samples
The Ba-Al oxide particle marked by the leftmost white arrow of Figure 2 was characterized at high-magnification, since it was adjacent to a Sc-oxide particle and offered an opportune location for more detailed study of these distinct but adjacent oxides. Figure 6 presents high spatial resolution images and elemental maps of these particles; as seen by comparison of Figures 2a and 6a, the TEM lamella has been imaged from the opposite orientation in Figure 6, i.e., the features are reversed left-to-right. The HAADF image of Figure 6b, corresponding to the red box in Figure 6a, shows a wide darker gray region that indicates lower average atomic number and therefore suggests the presence of oxide particle(s). The elemental maps of Figure 6c-g agree with other observations of distinct BaAl2O4 and Sc2O3 particles and, moreover, they show a sharp boundary between Sc and Al. These high spatial resolution EDS maps also revealed a Ca signal, albeit a weak one. Ca may be expected to exist in the tested cathode, as it is intentionally added as part of the impregnate mix during cathode fabrication;  
Figure 4: EDS elemental analysis of a Ba-Al oxide particle that also exhibits an encased Sc signal. (**a**) Low-magnification HAADF image of near-surface region of the cathode sample; (**b**) high-magnification HAADF image showing the area scanned for EDS mapping, which corresponds to the red box in image (**a**); (**c**) composite elemental map showing distributions of W, Ba and Sc; (**d**–**h**) individual elemental maps of W, Ba, Al, O and Sc, where the red box in (**h**) outlines the region of EDS quantitative measurement. however, Ca is not typically detected at significant levels during EDS analysis. However, when it is detected in the near-surface region of an activated and tested scandate cathode, the Ca signal is usually coupled with a Sc signal, as is the case in Figure 6. This may be due to the presence of a mixed Sc-Ca-oxide at certain locations, or remnant CaO (from the impregnate material) situated above/below the Sc\({}_{2}\)O\({}_{3}\); however, the low number of such observed particles precludes a definitive statement at this point. Quantitative EDS analysis was performed on the Ba-containing and Sc-containing oxide particles in Figure 6, in the regions marked by blue and yellow boxes in Figure 6b, respectively. These results are presented in Table 1. The region marked by the blue box had a composition (at.%) of Ba (9.3), Al (18.4), O (67.3), W (0.7), C (4.3), which is similar to the result for the oxide particle shown in Figure 3. This region is therefore interpreted to be BaAl2O4, which agrees with the EDS elemental maps in Figure 6. The region marked by the yellow box had a composition (at.%) of Sc (29.8), Ca (5.2), Ba (3.0),  
Figure 5: (**a**) HAADF image of near-surface cathode region where structure and elemental distribution were investigated by tomography. (**b–e**) HAADF image and EDS elemental maps, shown as 2D slices from the 3D tomogram, of a Ba–Al oxide particle that encases a smaller Sc-containing particle and sits on a W grain. Sc is distinct from both the W particle and the Ba–Al oxide. Figure 3.0 High spatial resolution EDS elemental mapping of Ba–Al oxide and adjacent Sc-containing particle. (**a**) Low-magnification HAADF image; (**b**) high-magnification HAADF image of area mapped by EDS, corresponding to red box in image (**a**), blue and yellow boxes denote locations of EDS point analysis; (**c–g**) elemental maps of Sc, Ca, Ba, Al and O; (**b–g**) share the same scale bar. Al (0.2), O (60.7), W (1.1). Again, this agrees with observations described above, except for the presence of Ca. Given the additional presence of a low but not insignificant amount of Ba in the yellow box region, it is proposed that these signals may arise from remnant amounts of CaO and BaO that were added in the impregnate mix. Therefore, the primary oxide particle in the yellow box region would be Sc\({}_{2}\)O\({}_{3}\), along with low amounts of CaO and BaO.