Ba Sc O  on W (0 0 1), (1 1 0), and (1 1 2) in scandate cathodes: Connecting to experiment via <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si13.gif" overflow="scroll"><mml:mrow><mml:msub><mml:mrow><mml:mi>μ</mml:mi></mml:mrow><mml:mrow><mml:mi>O</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> and equilibrium crystal shape

Scandate cathodes have exhibited superior emission properties compared to current state-of-the-art “M-type” thermionic cathodes. However, their integration into vacuum devices is limited in part by a lack of knowledge regarding their functional lifespan and behavior during operation. Here, we consider thermal desorption from scandate cathodes by examining the distribution of material deposited on interior surfaces of a sealed vacuum device after ~26,000 h of cathode operation. XPS, EDS, and TEM analyses indicate that on the order of 1 wt.% of the initial impregnate is desorbed during a cathode’s lifetime, Ca does not desorb uniformly with time, and little to no Sc desorbs from the cathode surfaces (or does so at an undetectable rate). Findings from this first-ever study of a scandate cathode after extremely long-time operation yield insight into the utility of scandate cathodes as components in vacuum devices and suggest possible effects on device performance due to deposition of desorption products on interior device surfaces.

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“…The presence of metallic Ba in the sealed glass envelope indicates that after the Ba-Al flash, the residual amount of O

in the system is extremely low, specifically lower than the threshold O

partial pressure (that is, chemical potential) required to oxidize Ba. This is consistent with previous results [ 19 ] suggesting that O

partial pressure may significantly affect scandate cathode performance.…”

Section: Resultssupporting

confidence: 94%

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

2020

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“…Other studies in the scientific literature ascribe the enhanced emission capability and low work function of scandate cathodes to a Ba–Sc–O monolayer [ 29 , 45 ] or a relatively thick (~100 nm) Ba–Sc–O layer [ 7 , 8 , 32 ] that covers W grains at the cathode surface. However, as was described in a recent paper from the current authors’ group [ 37 , 46 ], no Ba–Sc–O layer with thickness on the order of 10–100 nm was observed, despite using several pertinent and complementary techniques in the characterization of multiple scandate cathodes that had yielded good emission test results. Finally, as suggested by Hasker [ 29 ] and Zhou [ 46 ], one possible function of Sc is to regulate the chemical potential of oxygen on the emitting surface of a scandate cathode, since the affinity of O for Sc is greater than it is for Ba.…”

Section: Discussionmentioning

confidence: 53%

Near-Surface Material Phases and Microstructure of Scandate Cathodes

et al. 2019

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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.

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“…Our calculation results indicate that the local electric fields around the nanoparticle could be enhanced [46]. Recently, Zhou et al [47] claimed that Sc 2 O 3 and BaAl 2 O 4 crystallites with the particle size of 100 nm were separated, distinct, interspersed and Ba or BaO dots with diameters of ~ 10 nm were distributed on the surface of the W grain. Meanwhile, a serial of works [48][49][50][51][52] are done to explore the surface structure and stability of Ba x Sc y O z monolayer cathodes by the method of ab initio modeling.…”

Section: Mechanism Study Of the Sdi Cathodementioning

confidence: 56%

A review on scandia doped tungsten matrix scandate cathode

et al. 2019

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As the cathode with the highest electron emission capability in the thermionic cathode, the scandate cathode has attracted more and more attention in recent years, especially the scandia doped tungsten matrix scandate cathode. Experimental studies show that scandia doped tungsten matrix scandate cathodes with submicron microstructures exhibit excellent emission capacity, and the pulse emission current density in the space-charge region can be over 35 A cm −2 at 850 °C b. In the direct current (DC) condition with temperature compensation, the emission current density could reach 25 A cm −2 at 850 °C b. The device lifetime is over 3700 h after operating at 950 °C b with the DC loading of 40 A cm −2. The emission mechanism of the scandate cathode including the effect of the surface structure and composition on the work function of the cathode are systematically reviewed.

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Ba Sc O on W (0 0 1), (1 1 0), and (1 1 2) in scandate cathodes: Connecting to experiment via μO and equilibrium crystal shape

Cited by 36 publications

References 50 publications

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

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

Near-Surface Material Phases and Microstructure of Scandate Cathodes

A review on scandia doped tungsten matrix scandate cathode

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