# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 4 Discussion  
Since the primary materials difference between scandate cathodes and conventional impregnated tungsten cathodes is the addition of scandia to the W matrix, it is reasonable to assume that scandia (or scandium) is responsible for the significant improvement in thermionic emission and the low work function of scandate cathodes. However, the exact role of Sc/Sc\({}_{2}\)O\({}_{3}\) in cathode processing and/or performance remains unclear. [PERSON] et al. [PERSON] et al. (2018) suggested it is impossible for Sc\({}_{2}\)O\({}_{3}\) to migrate from interior regions of the porous W matrix to the cathode surface during activation, due to the low diffusion coefficient (1.6 \(\times\) 10\({}^{-18}\) cm\({}^{2}\)/s at 1370 K). Therefore, in order for Sc to diffuse or migrate to the cathode surface within a reasonable timeframe, Sc\({}_{2}\)O\({}_{3}\) would need to be transformed into metallic Sc, as proposed by [PERSON] (2018) and [PERSON] (2018). However, the reaction(s) required to generate free Sc and the mechanism of Sc migration have not been experimentally verified, despite these research efforts. In this section, chemical reactions that may enable the creation of metallic Sc are discussed, as are Sc-containing compounds that may be relevant to the role of Sc in scandate cathodes. Sc has a stronger affinity for O than does Ba, which means that Ba cannot reduce Sc\({}_{2}\)O\({}_{3}\) to Sc directly [PERSON] et al. (2018). In a cathode study by [PERSON] et al. [PERSON] et al. (2017), it was proposed that metallic Sc can be produced via a chemical reaction between Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\) and Ba, as described in Equation (1), which was based on their work on (W-Sc\({}_{2}\)O\({}_{3}\))-coated [PERSON] et al. (2018) and (W-Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\))-coated impregnated cathodes [PERSON] et al. (2017). \[\text{Sc}_{2}\text{W}_{3}\text{O}_{12}+3\text{Ba}=3\text{BaWO}_{4}+2\text{Sc} \tag{1}\]  
In that reaction scenario, metallic Ba was created by impregnate materials reacting with W at pore walls, similar to the case for regular impregnated tungsten cathodes. Multi-step chemical reactions that generate Ba were described by [PERSON] (2018) as:  
\[5\text{BaO}+2\text{Al}_{2}\text{O}_{3}=3/2\text{ Ba}_{3}\text{Al}_{2}\text{O}_{6}+1/2\text{ Ba}\text{Al}_{2}\text{O}_{4} \tag{2}\]  
Figure 11: SEM micrographs of (**a**) tungsten powder and (**b**) scandia-doped tungsten powder. Both images were recorded in secondary electron imaging mode. \[2\text{Ba}_{3}\text{Al}_{2}\text{O}_{6}\ (\text{s})+\text{W}\ (\text{s})=\text{BaWO}_{4}\ (\text{s})+2\text{BaAl}_{2}\text{O}_{4}\ (\text{s})+3\text{Ba}\ (\text{g}) \tag{3}\]  
According to these reactions, the production of free metallic Ba is accompanied by generation of the compound BaAl\({}_{2}\)O\({}_{4}\). As demonstrated by [PERSON] [43], this compound can be regarded as an inert material, i.e. it is thermally stable and does not participate in thermionic electron emission from the cathode surface. Since Equations (2) and (3) take place at W pore walls, i.e., the surfaces of W grains, it is reasonable to observe BaAl\({}_{2}\)O\({}_{4}\) particles at W surfaces and junctions in cathode cross-section samples, as was shown for example in Figure 3.  
As described by [PERSON] [35], the compound Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\), which can result from oxidation during heat treatment of W-Sc\({}_{2}\)O\({}_{3}\), is the most likely source of metallic Sc and would be produced according to Equation (1) above. However, Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\) is not observed in the cathode samples described in the current paper, and this may be due to complete reaction of the compound Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\). By virtue of their nanoscale size and uniform distribution across W surfaces, the Sc\({}_{2}\)O\({}_{3}\) nanoparticles fabricated during L-S processing in the current study may react more readily with W, and thereby enhance the formation of Sc\({}_{2}\)W\({}_{3}\)O\({}_{12}\).