Near-Surface Material Phases and Microstructure of Scandate Cathodes

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.

Desorption from Hot Scandate Cathodes: Effects on Vacuum Device Interior Surfaces after Long-Term Operation

“…As it is not, the immediate implication is that, unlike Ba, Ca, and Al, either Sc/Sc O desorbs so slowly that even after pre-activation, activation, and ~26,000 h of operation Sc is not detectable, or that Sc does not desorb from the cathode. A dissensus remains in the literature as to the behavior of Sc during operation: some report the presence of Sc at the surface of high-performing scandate cathodes after operation [ 20 ], while others hypothesize that Sc readily desorbs based on studies of model Ba-Sc-O on W surfaces [ 21 ].…”

A Review of Sc-containing "Scandate" Thermionic Cathodes

“…Liu et al [131], [132] conducted a thorough study of highperforming Sc-in-W cathodes fabricated with L-S powders. They reported a microstructure composed of distinct W grains "decorated" (as opposed to "coated") with impregnant oxide particles.…”

“…The relative stability of those surface configurations was reported to depend on the O chemical potential when the energies of Ba and Sc were referenced to BaO an Sc 2 O 3 . Based on observations of BaAl 2 O 4 , rather than BaO, in scandate cathodes [131], [132], combined with the possible presence of other Sc-containing compounds, Zhou et al [179] extended these results to also calculate energies of a number of surface configurations as a function of a full range of possible chemical potentials of Ba, Sc, and O. These extended calculations allowed comparison of surface energies of different surface configurations under any reasonable chemical conditions in which scandate cathodes might be found.…”

Электронная Структура Вариантов Составов Скандатных И Оксидно-Никелевых Катодов СВЧ Приборов

“…Начиная с 90-х годов прошлого века научные лаборатории университетов и промышленных предприятий во всех развитых странах проводят исследования, направленные на создание так называемых скандатных катодов, т. е. металлопористых катодов, содержащих в своем составе скандий. По результатам исследований установлено, что такой катод может обеспечить получение плотности тока термоэлектронной эмиссии до 100 А/cm 2 , а в перспективе и до 400 А/cm 2 [1][2][3][4][5][6]. Это открывает возможность создания принципиально новых типов электровакуумных СВЧ приборов.…”