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.

Keywords: 3D reconstruction; 3D tomography; electron diffraction; elemental mapping; scandate cathodes.

Figure 1

Surface morphology of two scandate cathodes. (a,b) are low- and high-magnification SEM images of the surface of cathode #1. (c,d) are low- and high- magnification SEM micrographs of cathode #2. All micrographs were obtained in secondary electron imaging mode.

Figure 2

EDS elemental mapping of a cross-section TEM lamella from scandate cathode #1. (a) Low-magnification HAADF image of the electron-transparent sample region; (b) composite elemental map showing the distribution of W (red), Ba (blue) and Sc (green) in the TEM lamella; (c–h) elemental distribution maps for W, Sc, Ga, Ba, Al and O.

Figure 3

Structural and elemental analysis of a Ba–Al–O particle at the cathode surface. (a) HAADF image, with green square showing the area for EELS mapping, as well as blue and yellow boxes showing sites for EDS analysis of the Ba–Al–O particle and neighboring W grain, respectively; (b) selected area diffraction pattern acquired from the Ba–Al–O particle; (c–f) EELS elemental maps of O, Al, Ba and W; these four maps share the same scale bar.

Figure 4

EDS elemental analysis of a Ba-Al oxide particle that also exhibits an encased Sc signal. (a) Low-magnification HAADF image of near-surface region of the cathode sample; (b) high- magnification HAADF image showing the area scanned for EDS mapping, which corresponds to the red box in image (a); (c) composite elemental map showing distributions of W, Ba and Sc; (d–h) individual elemental maps of W, Ba, Al, O and Sc, where the red box in (h) outlines the region of EDS quantitative measurement.

Figure 5

(a) HAADF image of near-surface cathode region where structure and elemental distribution were investigated by tomography. (b–e) HAADF image and EDS elemental maps, shown as 2D slices from the 3D tomogram, of a Ba–Al oxide particle that encases a smaller Sc-containing particle and sits on a W grain. Sc is distinct from both the W particle and the Ba–Al oxide.

Figure 6

High spatial resolution EDS elemental mapping of Ba–Al oxide and adjacent Sc-containing particle. (a) Low-magnification HAADF image; (b) high-magnification HAADF image of area mapped by EDS, corresponding to red box in image (a), blue and yellow boxes denote locations of EDS point analysis; (c–g) elemental maps of Sc, Ca, Ba, Al and O; (b–g) share the same scale bar.

Figure 7

High spatial resolution characterization of a Sc-containing particle located in the interior region of a W grain. (a) Low-magnification HAADF image showing the near-surface region of the cathode; (b) zoomed HAADF micrograph corresponding to the red box in (a). Inset in (b) are a HAADF image and W elemental distribution map, revealing nanoscale W particles within the 100 nm Sc-containing particle.

Figure 8

Crystal structure of the W matrix in scandate cathode #1, characterized using scanning nanobeam diffraction. (a) HAADF image showing the selected tungsten grain; (b–d) diffraction patterns corresponding to the sites of A, B and C as marked in (a). It is demonstrated that the W matrix has the BCC structure, which is inferred from diffraction patterns obtained at locations ranging from the middle of the W grain to its surface. The beam direction and zone axis for the grain orientation shown here are indexed as [−111].

Figure 9

Schematic illustration of the sample, ion beam and electron beam for serial sectioning in the FIB-SEM, in order to perform 3D tomographic reconstruction.

Figure 10

3D reconstruction of a W particle from scandate cathode #2, generated by FIB serial sectioning and imaging in the SEM. (a) Secondary electron SEM micrograph of the selected W particle, (b) reconstructed tomogram, and (c) spatial distribution of Sc/Sc₂O₃ inside the W particle. Note that the viewing direction of image (c) is rotated with respect to that of images (a,b).

Figure 11

SEM micrographs of (a) tungsten powder and (b) scandia-doped tungsten powder. Both images were recorded in secondary electron imaging mode.