Near-surface material phases and microstructure of scandate cathodes

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 BaAl ₂ O ₄ 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 BaAl ₂ O ₄ particles were observed to surround smaller Sc ₂ O ₃ 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 throughoutWgrains 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/Sc ₂ O ₃ particles tend to exhibit a degree of correlated arrangement within a givenWparticle, rather than being distributed uniformly throughout. Moreover, the formation of Sc/Sc ₂ O ₃ particles withinWgrains may arise fromWsurface roughening that occurs during the liquid-solid synthesis process.