Abstract
Scandate cathodes that were fabricated using the liquid-solid process and that exhibited excellent emission performance were characterized using complementary state-of-the-art electron microscopy techniques. Sub-micron BaAl2O4 particles were observed on the surfaces and edges of tungsten particles, as seen in cross-section samples extracted from the scandate cathode surface regions. Although several BaAl2O4 particles were observed to surround smaller Sc2O3 nanoparticles, no chemical mixing of the two oxides was detected, and in fact the distinct oxide phases were separately verified by chemical analysis and also by 3D elemental tomography. Nanobeam electron diffraction confirmed that the crystal structure throughout W grains is body-centered cubic, indicating that they are metallic W and did not experience noticeable changes, even near the grain surfaces, as a result of the numerous complex chemical reactions that occur during cathode impregnation and activation. 3D reconstruction further revealed that internal Sc/Sc2O3 particles tend to exhibit a degree of correlated arrangement within a given W particle, rather than being distributed uniformly throughout. Moreover, the formation of Sc/Sc2O3 particles within W grains may arise from W surface roughening that occurs during the liquid-solid synthesis process.
Highlights
Fabrication included the following steps: for each cathode, the W-scandia powder mixture was pressed into a pellet ~2 mm diameter and ~1 mm tall; the pellet was sintered and subsequently impregnated with barium calcium aluminate (6BaO–1CaO–2Al2 O3 ); afterwards, the sample was washed with deionized water to remove residual impregnate material; the cathodes were activated by heating at 1150 ◦ Cb for 1 h
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
Since the primary materials difference between scandate cathodes and conventional tungsten cathodes is the addition of scandia to the W matrix, it is reasonable to assume that scandia impregnated tungsten cathodes is the addition of scandia to the W matrix, it is reasonable to assume is responsible for the significant improvement in thermionic emission and the low work that scandia is responsible for the significant improvement in thermionic emission and function of scandate cathodes
Summary
Scandia-doped tungsten thermionic cathodes (referred to as scandate cathodes) are promising candidates for application in microwave tubes [1,2], traveling wave tubes (TWTs) [3,4], satellite communication [5] and vacuum electron devices (VEDs) [6,7,8,9,10] owing to their reported enhancement of electron emission, delivering higher current densities at lower temperatures [1,3,5,6,8,11,12] than conventional dispenser cathodes, i.e., oxide cathodes [13,14,15,16], B-type cathodes [17,18,19,20], and M-type cathodes [21,22,23]. Various scandate cathodes have been developed, including impregnated [24,25,26], pressed [27,28], and top-layered types [29,30,31]. Scandate cathodes fabricated from starting powders of micron-scale tungsten (W) and nanoscale scandia (Sc2 O3 ) are reported to exhibit the most promising emission characteristics and have been widely investigated. This scandate cathode variant is a powder metallurgy (P/M) porous tungsten plug fabricated with nanosized scandia-doped tungsten powder and impregnated with barium calcium aluminate (in a specific molar ratio) prior to activation at the proper temperature. In order to enable commercialization and application of scandate cathodes, several issues must first be addressed, including emission uniformity, poor reproducibility, and an incomplete understanding of the mechanisms that govern emission [7,16,19,27,32,33,34,35]
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