Emission life and surface analysis of barium-impregnated thermionic cathodes

Abstract

Emission life of small Ba-impregnated thermionic cathodes suitable for use in TV tubes is investigated systematically. Cathodes are covered with an Os layer to enhance the emissivity. They have an unusual emission life in that the emission current decreases gradually with time and then drops abruptly. The early gradual decrease in emission at high cathode temperatures is caused by the phase change of the Os layer to OsW2. A linear relationship is found to exist between the logarithm of the lifetime and the reciprocal of the cathode temperature. This life end is believed to happen after the end of the reduction reaction between BaO and the porous W body and after ceasing replenishment of the low work function Ba+-O− monolayer on the metal surface. A formula for the emission lifetime is derived from the assumption that the rate determining step is the Ba diffusion inside the pellet. Emission lifetime τ is shown to be given by τ=Ad2 exp(E/kT), where A is a constant, d is the pellet thickness, and E is the activation energy of Ba diffusion. This relation is confirmed experimentally.

Hollow cathode modeling: I. A coupled plasma thermal two-dimensional model

“…Some common emissive inserts such as BaO-W are constituted from a metallic matrix impregnated with a low work function emissive element. This results in an emissive surface which displays a porous aspect [32] and may lead to an effective emissive surface much larger than the geometrical surface of the emitter (under the assumption that the plasma penetrates in the pores of the emitter). This mechanism is important in obtaining a good agreement with experimental measurements of the plasma density along the axis of the cathode [8].…”

A Review of Sc-containing "Scandate" Thermionic Cathodes

“…[165]- [167]. In B-and M-type cathodes discrete (and mobile) atomic or molecular species containing Ba cations adsorbed on the emitting surface form the outward facing dipoles [12], [37], [51], [91]- [94], and are representative of the broader (and widely studied) phenomenon of WF reduction due to the adsorption of alkali and/or alkali-earth metals [28], [86]. Sc cations have been proposed to adsorb with Ba cations to give a more optimal density of adsorbed cations and therefore maximize work function reduction.…”

“…Although the work functions of the refractory metal coatings are higher than that of W itself, the overall effective work function of the emitting surface-understood to consist of an atomically thin layer of BaO atop the refractory metals coating the W grains-is lower than that of B-type cathodes. M-type cathodes exhibit effective work functions of 1.1-1.8 Oxide Ni [25]- [27] -Ba-Sr-O [26], [28] 1.0 [26], [28] 700-800 [26], [28] 1.5 [26], [28] B-type W Ba-Ca-Al-O* -5.0 [29], [30] 1150 [31] 2.0-2.1 [28], [31] M-type W Ba-Ca-Al-O* Os-Ru [32], Os [33], 1-100 [29], [33]- [37] 950-1050 [33], [35], [37] 1.16-1.72 [33], [38] Ru [39], Ir [39] Scandate W, W+Sc Sc/Ba-Ca-Al-O* ** 1-400 [15], [40] 750-950 [41] ∼1.15-1.5 [40], [41] * This is an oxide mixture containing Ba, Ca, and Al (and Sc in scandate cathodes). ** Top-layer scandate cathodes have a W+Sc 2 O 3 coating [16], [40].…”

Study on the Emission Properties of the Impregnated Cathode With Nanoparticle Films

“…For another example, smaller evaporation of the cathode is required to improve the reliability, life, and other properties of the microwave devices [13]. To meet these requirements, the impregnated cathode has attracted attention, and various impregnated cathode types have been developed [14]- [16]. The emission characteristics of an impregnated cathode are substantially improved by coating it with a top layer of Os, Re, Ir, and/or Os/Ru or by increasing the pore density on the surface of the cathode.…”