# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 5 Conclusions  
Multiple scandate cathodes were prepared using the liquid-solid fabrication method and were subjected to CSD emission testing. Subsequently, they were characterized using multiple advanced microscopy techniques, including (S)TEM imaging, high spatial resolution EDS mapping, EELS analysis, scanning nanobeam diffraction, and 3D reconstruction/tomography. The experimental observations and discussion above are summarized with these concluding remarks:  
* Most of the larger (\(\sim\)100 nm diameter) particles that are attached to the surfaces and edges of W grains are BaAl\({}_{2}\)O\({}_{4}\), as revealed during characterization of scandate cathode cross-section samples. In some cases, BaAl\({}_{2}\)O\({}_{4}\) particles are contiguous with Sc\({}_{2}\)O\({}_{3}\) particles and/or envelop Sc\({}_{2}\)O\({}_{3}\) particles.
* No evidence for chemical interaction or mixing between BaAl\({}_{2}\)O\({}_{4}\) and Sc\({}_{2}\)O\({}_{3}\) was observed, i.e., these oxide particles appear to remain distinct.
* Scanning nanobeam electron diffraction in the TEM showed that the crystal structure of W grains remains body-centered cubic in emission-tested scandate cathodes. The crystal structure and lattice constant of W particles have not changed as a result of the complex chemical reactions that occur during cathode fabrication, nor during activation or emission testing.  
* 3D tomographic reconstruction revealed the interior microstructure of W grains in a scandate cathode. Interestingly, nanoscale Sc/Sc\({}_{2}\)O\({}_{3}\) particles tend to cluster or align in certain regions of a given W grain, rather than being distributed uniformly throughout the W grain.  
The following are available online at [[http://www.mdpi.com/1996-1944/12/4/636/s1](http://www.mdpi.com/1996-1944/12/4/636/s1)]([http://www.mdpi.com/1996-1944/12/4/636/s1](http://www.mdpi.com/1996-1944/12/4/636/s1)), Video S1: 3D tomogram in Figure 5, Video S2: Scanning nanobeam diffraction in Figure 8, Video S3: 3D reconstruction in Figure 10.  
Conceptualization, X.L. and T.J.B.; Methodology, X.L. and T.J.B.; Software, X.L.; Validation, X.L., T.J.B. and M.J.B.; Formal Analysis, X.L., T.J.B., M.J.B. and B.K.V.; Investigation, X.L., T.J.B. and M.J.B.; Resources, T.J.B., B.K.V. and M.J.B.; Data Curation, X.L.; Writing-Original Draft Preparation, X.L.; Writing-Review & Editing, T.J.B. and X.L.; Visualization, X.L.; Supervision, T.J.B.; Project Administration, T.J.B. and M.J.B.; Funding Acquisition, T.J.B. and M.J.B.  
This research was funded by the Defense Advanced Research Projects Agency (DARPA) Innovative Vacuum Electronics Science and Technology (INVEST) program (N66001-16-1-4041). The views, opinions, and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.  
The authors thank [PERSON] at FEI, Inc. (Thermo Fisher Scientific) for some of the TEM characterization. The authors also thank JEOL USA Inc. for some of the TEM observations that were included in this paper. Additionally, the authors thank [PERSON] for assistance with 3D reconstruction, using the FIB-SEM at the University of Kentucky Electron Microscopy Center.  
The authors declare no conflict of interest.