Theoretical models of dispenser cathode surfaces

Abstract

The work function of impregnated tungsten dispenser cathodes (B type) can be modified by coating the surface with metal or alloy films. Using simple thermochemical ideas this paper attempts to explain and unify the available data. Data are presented which enable the effects of as yet untried metals to be estimated.

Composition and work function relationship in Os–Ru–W ternary alloys

“…The observation that minimum values of cathode work function from emission tests correspond closely with maximum values of alloy film work function from CPD measurements supports the idea that a high work function coating will result in a cathode with a low work function, at least in certain cases. This obviously does not hold true for all coatings, as it has been shown 3,18 that high work function coatings such as gold and platinum do not produce low work function cathodes.…”

“…1 They suggested that the differences between heats of formation for O 2 and Ba on the coatings are a better predictor of emission change than the work function of the coating alone. 3 Their model also accounts for Au and Pt, making it more robust than Zalm's. Looking strictly at the cathode work function (i.e., not the work function of the coating, but the work function determined during emission testing), Thomas and Gibson asserted that there are both compositional and structural reasons for their observed work function minima.…”

“…They show that the charge transfer between an adsorbed species and the metal oxide is most efficient when the work function of the substrate (metal oxide) is greater than the ionization energy of the adsorbed species. The current understanding of Ba at the cathode surface is that it exists in metallic form 3,5,20 as an adsorbed monolayer above O, which in turn sits atop the alloy substrate. 2 The alloy surface forms very early and may even form during the initial heating stages that activate the cathode.…”

A review on recent progress of thermionic cathode

“…They stated that the metal film adsorbed on the cathode surface would increase the electronegativity of metal matrix, which reduces energy barrier of surface and provides favorable conditions to reduce work function of W. However, this model is inapplicable to highly electropositive Pt and Au films, because these films did not make any contribution to emission performance. Skinner et al [51] stated ΔH O 2 -ΔH Ba model ( ΔH O 2 -ΔH Ba is the difference between the heat of sorption of oxygen and barium on the surface) to further explain the specificity of Pt. According to their theory, the low emission properties of Pt and Au are originated from the formation of intermetallic compounds between noble metals and fresh formed Ba reduced from the weaken-bonded BaO.…”

Recent Development of Cathodes used for Cathode Ray Tubes.

“…The lower work function, however, could possibly be explained by a higher final coverage due to reduced dipole-dipole interaction and/or Ba binding energy (Mueller, 1989)22). Coating materials used for improving emission have also been discussed based on the difference in the Gibbs free energy of bonding between barium/ coating materials and oxygen/ coating materials (Skinner et al, 1982)…”