Emission mechanism of high current density scandia-doped dispenser cathodes

Abstract

High current, high current density field emitter array cathodes Emission uniformity enhancement between microfabricated tips in cold cathode arrays High current density field emission from arrays of carbon nanotubes and diamond-clad Si tipsThe emission mechanism of scandia-doped dispenser ͑SDD͒ cathodes is characterized based on systematic investigation of microstructure, surface behavior, evaporation, and emission. High current density SDD cathodes have been developed in recent years to meet the requirements of advanced vacuum electron devices. Space charge limited current densities of over 100 A / cm 2 have been reliably reached at a temperature of 950°C b . SDD cathodes are composed of a tungsten matrix with submicron structure and uniformly dispersed materials containing Sc, Ba, Ca, Al, and O as nanosized particulates in the matrix. The authors conclude that the copious electron emission from this cathode type results from a layer of Ba-Sc-O on W formed after activation on top of the matrix. This layer is about 100 nm thick and maintains a stable composition over its lifespan. Its emission can be explained in terms of a semiconductor layer with characteristics that differ from a metal-like emission model. The Ba-Sc-O layer is formed by codiffusion of Sc, Ba, and O ͑generated inside the W-matrix͒ to the surface of the W-matrix during activation. Furthermore, this layer guarantees a low evaporation rate of emission-relevant materials and thus enables a longer lifespan. A. Structure features of SDD cathodesUp to now, two types of SDD cathodes have been developed. They are the scandia-doped impregnated ͑SDI͒ cathodes and the scandia-doped pressed ͑SDP͒ cathodes. The SDI cathodes have a porous matrix fabricated by calcining, a͒ Electronic Wang et al.: Emission mechanism of high current density scandia-doped dispenser cathodes 04E106-2 04E106-2 Wang et al.: Emission mechanism of high current density scandia-doped dispenser cathodes 04E106-7 04E106-7 JVST B -Microelectronics and Nanometer Structures Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 84.88.136.149 On: Tue, 02 Dec 2014 08:38:57 Wang et al.: Emission mechanism of high current density scandia-doped dispenser cathodes 04E106-8 04E106-8

Tungstate formation in a model scandate thermionic cathode

“…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.…”

“…Wang et al 37 summarized their experience with a number of different types of scandate cathodes with several experimentally based conclusions. Wang et al 37 summarized their experience with a number of different types of scandate cathodes with several experimentally based conclusions.…”

DC emission characteristic of Sc- Doped Impregnated Dispenser cathodes

“…DC current densities at different operating temperatures were measured and evaluated by deviation points from Space Charge Limited (SCL) region as described in the previous paper [4]. The data were also compared with pulsed emission at same conditions.…”

“…These bring forward a great challenge for cathodes to provide high current density at high duty cycles or in DC model. Scandia Doped Impregnated Dispenser (SDI) cathodes with 411 Ba, Ca aluminates have been demonstrated to provide pulsed current densities of up to 100 A/cm 2 at operating temperatures of above 950 ºC b either in close-spaced diodes or Pierce electron guns at duty cycles up to 2.5% with pulse width of 250 µs [2][3][4]. In this work, DC emission characteristic of SDI cathodes have been investigated.…”

Near-Surface Material Phases and Microstructure of Scandate Cathodes

“…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.…”