Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes

Gaertner, Georg; Barratt, David S.

doi:10.1016/j.apsusc.2005.03.213

Cited by 31 publications

(4 citation statements)

References 12 publications

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“…Apart from the two models discussed above, Raju and Maloney proposed a semiconductor model [118] in 1994, which has recently been reconsidered by Shih [119], Wang [120] and Gaertner [121] based on their experimental results.…”

Section: Theoretical Aspects Of Cathode Operationmentioning

confidence: 99%

Fundamental physics of vacuum electron sources

Yamamoto

2005

Rep. Prog. Phys.

131

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The history of electron emission is reviewed from a standpoint of the work function that determines the electron emission capability and of applications in the fields of scientific instruments and displays. For years, in thermionic emission, a great deal of effort has been devoted to the search for low work function materials with high melting temperature, while reduction of the local change in time of the work function rather than the work function itself has been the main issue of field emission investigations. High brightness and long life are the central targets of emission material investigations for scientific instrument applications, while high current density and low power consumption are the guiding principles for display applications.In most of the present day industries, thermionic emission materials are exclusively used in such fields requiring high current and high reliability as cathode ray tubes, transmission and receiving tubes, x-ray sources and various electron beam machines. Field electron emission sources, however, since applied to high resolution electron microscopes in the 1970s have recently become dominant in research and development in the fields of scientific instruments as well as in the fields of various electron tubes and beam machines.The main issue in this report is to analyse the work function on the atomic scale and thereby to understand the fundamental physics behind the work function, the change in time of the local work function leading to field emission current fluctuation and the relationship between microscopic (on atomic scale) and macroscopic work functions.Our attempt is presented here, where the work function on the atomic scale is measured by utilizing a scanning tunnelling microscopy technique, and it is made clear how far the local work function extends its influence over neighbouring sites. As a result, a simple relationship is established between microscopic and macroscopic work functions.

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Section: Theoretical Aspects Of Cathode Operationmentioning

confidence: 99%

Fundamental physics of vacuum electron sources

Yamamoto

2005

Rep. Prog. Phys.

131

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“…Scandia-doped tungsten thermionic cathodes (referred to as scandate cathodes) are promising candidates for application in microwave tubes [ 1 , 2 ], traveling wave tubes (TWTs) [ 3 , 4 ], satellite communication [ 5 ] and vacuum electron devices (VEDs) [ 6 , 7 , 8 , 9 , 10 ] owing to their reported enhancement of electron emission, delivering higher current densities at lower temperatures [ 1 , 3 , 5 , 6 , 8 , 11 , 12 ] than conventional dispenser cathodes, i.e., oxide cathodes [ 13 , 14 , 15 , 16 ], B-type cathodes [ 17 , 18 , 19 , 20 ], and M-type cathodes [ 21 , 22 , 23 ]. Consequently, scandate cathode technology has received significant attention over recent decades.…”

Section: Introductionmentioning

confidence: 99%

“…This scandate cathode variant is a powder metallurgy (P/M) porous tungsten plug fabricated with nanosized scandia-doped tungsten powder and impregnated with barium calcium aluminate (in a specific molar ratio) prior to activation at the proper temperature. In order to enable commercialization and application of scandate cathodes, however, several issues must first be addressed, including emission uniformity, poor reproducibility, and an incomplete understanding of the mechanisms that govern emission [ 7 , 16 , 19 , 27 , 32 , 33 , 34 , 35 ].…”

Section: Introductionmentioning

confidence: 99%

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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“…Vacuum electronic devices have a wide application in civil and military applications, for example communication, radar, and industrial heating [1]. Cathodes are the important components in the devices, because they provide the electron beams required.…”

Section: Introductionmentioning

confidence: 99%

Preparation and characterization of scandia-doped tungsten powders prepared by a spray-drying method

2011

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Scandium oxide-doped tungsten powders were prepared by a new method of spray drying combined with two-step hydrogen reduction. The particle size of doped tungsten, powder morphology, and distribution of doped scandium oxide were characterized by scanning electron microscopy, X-ray diffraction, and laser diffraction particle size analysis. Experimental results indicated that the predecessor powders prepared by spray drying were spherical in shape. Two compounds, WO 3 and Sc 2 W 3 O 12 , in the raw powder calcined at 600°C were transformed into metallic tungsten and scandia after reduction at 850°C by hydrogen for 1 h. The scandia-doped tungsten powders obtained had an average size of 950 nm and scandium oxide was distributed evenly throughout the tungsten powder. The mechanism of reduction of the doped tungsten oxide is discussed in this paper.

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Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes

Cited by 31 publications

References 12 publications

Fundamental physics of vacuum electron sources

Fundamental physics of vacuum electron sources

Near-Surface Material Phases and Microstructure of Scandate Cathodes

Preparation and characterization of scandia-doped tungsten powders prepared by a spray-drying method

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