# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 3 Results
### Elemental Analysis of Phases in Cross-Section Samples  
High-resolution elemental mapping was performed on a TEM lamella extracted from cathode #1, using the multi-spectrometer EDS system of the FEI Talos F200X TEM. These results are presented in Figure 2, where the images and elemental maps show a cross-section of the cathode that was created during preparation of the TEM lamella. This specimen therefore reveals the structure and composition of the near-surface cathode region, including what is inside the W particles as well as what is on their surfaces and in the pores. The high-angle annular dark field (HAADF) image in Figure 2a shows the equiaxed shape of W particles, with flat edges corresponding to different surface facets of a given particle. Pt was deposited on the top cathode surface for protection during subsequent ion milling, as indicated by the white arrow in this image. Figure 2b displays a composite map of composition across the lamella, with different colors corresponding to the major elements of interest, and Figure 2c-h display elemental maps for W, Sc, Ga, Ba, Al and O, respectively. Comparing Figure 2d,h, specifically the area marked by the white arrow, it is seen that some compounds attached to the edges of tungsten particles show consistent Sc and O signals during EDS mapping, while certain other Sc-containing compounds located inside tungsten particles (e.g., the row of 4 particles inside the W grain, below the white arrow in Figure 2d,h) do not exhibit a noticeable corresponding O signal. In addition to the Sc-containing particles, it was observed in the cross-section maps of Figure 2f-h that other particles exhibit highly correlated signals of Ba, Al and O. These particles are interpreted to have a mixed Ba-Al oxide phase. The majority of these Ba-Al oxides exhibit only Ba, Al and O signals, e.g., as indicated by the black arrows in Figure 2a. However, several of the Ba-Al oxides also exhibit simultaneous Sc and O signals. In these cases, the Sc-O appears to be entrapped within the Ba-Al oxides, as indicated by red arrows in Figure 2b, as well as Figure 2d,f,g. Note that the Sc-O particles are distinct from the Ba-Al oxide particles. In addition, a number of Ba-Al oxide particles are observed to be immediately adjacent to other particles that show Sc and O signals, as marked by white arrows in Figure 2b, as well as Figure 2d,f,g. It is noted that a shell of W encases some of the particles seen in the cross-section lift-out specimen, for instance as indicated by the yellow arrow in Figure 2c, but this is attributed to redeposition of sputtered W from the ion-milling process during fabrication of the TEM lamella. This is also consistent with the Ga map in Figure 2e, since Ga is present in the redeposited layers. Figure 2: EDS elemental mapping of a cross-section TEM lamella from scandate cathode #1. (**a**) Low-magnification HAADF image of the electron-transparent sample region; (**b**) composite elemental map showing the distribution of W (red), Ba (blue) and Sc (green) in the TEM lamella; (**c–h**) elemental distribution maps for W, Sc, Ga, Ba, Al and O. The composition of a Ba-Al oxide particle near the cathode surface (visible as a dark particle at the upper left of Figure 2b), was investigated using EDS coupled with EELS. The results are presented in Figure 3. The HAADF image in Figure 3a shows the Ba-Al oxide particle located between three W particles and a Pt cap layer (from the FIB lift-out process). The EDS measurement of composition was taken from the region marked by the blue box in Figure 3a, and indicated the following composition: Ba (12.2 at.%), Al (20.7 at.%), O (63.4 at.%), W (0.5 at.%), and C (3.2 at.%). No Sc signal was detected here, i.e., no measurable peak was seen at 4.09 keV on the EDS spectrum acquired in the region outlined by the green box that encompasses the oxide particle. This measured composition provides a reasonable match to the compound BaAl\({}_{2}\)O\({}_{4}\), albeit with excess O.