Scandium oxide coated polycrystalline tungsten studied using emission microscopy and photoelectron spectroscopy

Wan, Congshang; Vaughn, Joel; Sadowski, Jerzy; Kordesch, Martin E.

doi:10.1016/j.ultramic.2011.10.001

Cited by 16 publications

(6 citation statements)

References 19 publications

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“…Our observation of dewetting for the tungstates on W does not support the continuous 100 nm thick layer of tungstate proposed by Wang et al 37 In our earlier work, [4][5][6] it was necessary to postulate electrical conduction through a thick BaO-Sc 2 O 3 bilayer. We also show that the Sc and Ba tungstates that we observe and identify are not good electron emission materials.…”

Section: Discussioncontrasting

confidence: 92%

Tungstate formation in a model scandate thermionic cathode

Wan

Kordesch

2012

Journal of Vacuum Science &Amp; Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phen

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Crystalline compounds found at the surface of model Ba-Sc-O-W thermionic cathodes (“scandate”) are uniquely identified using Raman spectroscopy. Thin films of sputtered BaO and Sc2O3 on W have been observed in thermionic emission microscopy, field emission scanning electron microscopy, optical microscopy, and Raman Spectroscopy. While the best thermionic electron emission is observed from areas that at the end of the cathode life are completely devoid of thin film BaO, Sc2O3 or observable bulk oxide or tungstate material, the poor emission areas are characterized by BaWO4, Ba2WO5 and long chain linear tungstates (νas = 860 cm−1) that are related to Scx-WOy components. There is no evidence from Raman spectroscopy that tetrahedral Sc2(WO4)3 is present or forms on the surface of the model cathode, or for the presence of Ba3WO6.

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Section: Discussioncontrasting

confidence: 92%

Tungstate formation in a model scandate thermionic cathode

Wan

Kordesch

2012

Journal of Vacuum Science &Amp; Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phen

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“…It has been reported that substrate metals with larger numbers of unoccupied d-orbitals bind strongly with Ba-O complexes, enhancing work function reduction for W compared to Pt substrates [86]. Similar effects are expected to be at play for the effects of adsorbates on different crystallographic planes of the same substrate [150], [175], [176]. Brodie [172] suggested that disparities in work function for different facets is related to direction of the effective mass of an electron with the Fermi energy inside the crystal.…”

Section: Semiconductor Modelmentioning

confidence: 86%

“…Like Lesny and Forman [12], these authors studied emission properties of various model surfaces synthesized to reproduce cathode systems-BaO on W, Sc 2 O 3 on W, Sc and Ba co-sputtered on W, Sc 2 O 3 on BaO on W, and BaO on Sc 2 O 3 on W. Thermionic electron emission microscopy (ThEEM) and photoelectron emission microscopy (PEEM), described in Ref. [150], were used to image the electron emitting regions and correlate brightly emitting regions with areas of W films coated with the dis-similar materials. While the emission behavior and deposited layers of many samples were observed to evolve with time at high temperature, Vaughn and Kordesch reported that the BaO/Sc 2 O 3 /W system was both stable and the only multilayer system that showed thermionic emission comparable to that reported for scandate cathodes.…”

Section: B Ba Adsorbed On Sc-containing Layer Modelmentioning

confidence: 99%

A Review of Sc-containing "Scandate" Thermionic Cathodes

Seif¹,

Zhou²,

Liu³

et al. 2022

Preprint

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Although thermionic emission has been studied for more than 100 years, recent interest in novel electron devices for military and civilian use has led to a surge in demand for cathodes with enhanced emission properties (e.g. higher current density, more uniform emission, lower operating temperatures, or extended in-service longevity). Sc-containing "scandate" cathodes have been widely reported to exhibit superior emission properties compared to previous-generation thermionic cathodes, including oxide, B-, and M-type cathodes. Despite extensive study spanning several decades, the mechanism by which the addition of Sc enhances cathode emission remains ambiguous, and certain limitations -non-uniform emission, low reproducibility, inconsistent longevity -continue to prevent widespread commercial integration of scandate cathodes into electron devices. This review attempts to survey the literature to-date addressing the fabrication, structure, and properties of scandate cathodes, with particular attention to studies addressing the role of Sc in enhancing emission.

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“…This result that the emitted current density varies across different grains due to the difference in their work function values is consistent with experimental thermionic electron emission microscopy (ThEEM) images obtained in the TL region. [16,[18][19][20][21][22][23][24][25][26][27] As the schematic figures show, in the TL (Fig. 6a) and the transition region (Fig.…”

Section: Two-dimensional Emission Mapmentioning

confidence: 99%

“…[3,5,6,[14][15][16][17] The non-uniform nature of thermionic electron emission from polycrystalline W has been observed experimentally by using thermionic electron emission microscopy (ThEEM). [16,[18][19][20][21][22][23][24][25][26][27] In a representative ThEEM image, at a particular temperature, certain grains of the W surface are bright while others remain dark, indicating that some grains are more emissive than others, due to factors such as lower work function, surface topography, etc.…”

Section: Introductionmentioning

confidence: 99%

Physics-based Model for Nonuniform Thermionic Electron Emission from Polycrystalline Cathodes

Chen¹,

Jacobs²,

Petillo³

et al. 2021

Preprint

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A new physics-based model is developed that predicts the emitted current from thermionic emission that accurately spans from the temperature-limited (TL) to the full-space-charge-limited (FSCL) regions. Experimental observations of thermionic electron emission demonstrate a smooth transition between TL and FSCL regions of the emitted-current-density-versus-temperature (J − T ) (Miram) curve and the emitted-current-density-versus-voltage (J − V ) curve. Knowledge of the temperature and shape of the TL-FSCL transition is important in evaluating the thermionic electron emission performance of cathodes, including predicting the lifetime. However, there have been no firstprinciples physics-based models that can predict the smooth TL-FSCL transition region for real thermionic cathodes without applying physically difficult to justify a priori assumptions or empirical phenomenological equations. Previous work detailing the nonuniform thermionic emission found that the effects of 3-D space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work function values), and Schottky barrier lowering can lead to a smooth TL-FSCL transition region from a model thermionic cathode surface with a checkerboard spatial distribution of work function values. In this work, we construct a physics-based nonuniform emission model for commercial dispenser cathodes for the first time. This emission model is obtained by incorporating the cathode surface grain orientation via electron backscatter diffraction (EBSD) and the facet-orientation-specific work function values from density functional theory (DFT) calculations. The model enables construction of two-dimensional emitted current density maps of the cathode surface and corresponding J − T and J − V curves. The predicted emission curves show excellent agreement with experiment, not only in TL and FSCL regions but, crucially, also in the TL-FSCL transition region. This model provides a method to predict the thermionic emission from the microstructure of a commercial cathode, and improves the understanding on the relationship between thermionic emission and cathode microstructure, which is beneficial to the design of vacuum electronic devices.

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Scandium oxide coated polycrystalline tungsten studied using emission microscopy and photoelectron spectroscopy

Cited by 16 publications

References 19 publications

Tungstate formation in a model scandate thermionic cathode

Tungstate formation in a model scandate thermionic cathode

A Review of Sc-containing "Scandate" Thermionic Cathodes

Physics-based Model for Nonuniform Thermionic Electron Emission from Polycrystalline Cathodes

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