Refining the Understanding of the Thermionic Emission Mechanism in Impregnated WBa and ScBa Cathodes Based on G.V. Samsonov’s Configurational Model I. Analysis of the Chemical Composition and Structure of the Emission–Adsorption Layer in WBa and ScBa Cathodes

Abstract

The first part presents an analysis of existing experimental studies on various types of impregnated WBa and ScBa cathodes (WBa-ICs and ScBa-ICs), underlying the development of polarization and semiconductor thermionic emission models intended to clarify the mechanisms whereby CaO and Sc₂O₃ oxides and platinum-group metals influence cathode emission. The analysis of existing polarization and semiconductor WBa-IC and ScBa-IC thermionic emission models shows that there is no universally accepted thermionic emission model. An interpretation of the chemical composition and structure of the emission–adsorption layer (EAL) in WBa-ICs is proposed. This interpretation serves as the basis for evaluating interatomic interactions within the EAL, relying on G.V. Samsonov’s configurational model of the electronic structure in solids. The EAL on the tungsten adsorbent in WBa-ICs consists of two structural–phase components: a two-dimensional Ba–O monolayer adsorbed on the tungsten surface and three-dimensional BaO–CaO oxides located within the pores and along their perimeters in the tungsten skeleton. The chemical composition and structure of the EAL in ScBa-ICs depend on the production technology. There is currently no consensus regarding the role of scandium and scandium-containing compounds in the characteristics of ScBa-ICs, which prevents the development of a unified thermionic emission model for ScBa-ICs. The polarization WBa-IC and ScBa-IC thermionic emission model proposed in the second part is for the first time analyzed from the standpoint of G.V. Samsonov’s configurational model. The new polarization model differs from existing ones in that it incorporates donor–acceptor interactions among valence orbitals of the adatoms within the EAL and between these adatoms and atoms of the adsorbent, initiated by changes in the energy stability of valence orbital configurations. In the proposed polarization WBa-IC and ScBa-IC thermionic emission model, the electron work function is determined by the potential barriers of polarized dipole complexes of two types. The first type is formed through donor–acceptor interactions between adatoms themselves and between adatoms and adsorbent atoms Ba (Ca, Sc)⁺–O^––A⁺. The second type involves adsorbed oxide molecules interacting with adsorbent atoms Ba⁺O^– (Ca⁺O^–)–A⁺, (\({\text{Ba}}^{+}{\text{O}}^{-}-{\text{Sc}}_{2}^{+}{\text{O}}_{3}^{-}-{\text{Al}}_{2}^{+}{\text{O}}_{3}^{-}\))–A⁺, and (\({\text{Ba}}^{+}{\text{O}}^{-}-{\text{Sc}}_{2}^{+}{\text{O}}_{3}^{-}-{\text{W}}^{+}{\text{O}}_{3}^{-}\))–A⁺. In all these complexes, the bond between the adsorbate and the adsorbent is mediated by oxygen, acting as an electron acceptor. The characteristics of the donor–acceptor interaction are defined by the energy stability of valence orbital configurations, d⁰, d⁵, and d¹⁰ for d-metals and s², sp³, and s²p⁶ for sp-elements, and their donor and acceptor capabilities. The new polarization WBa–IC and ScBa–IC thermionic emission model explains the effects of doping with CaO and Sc₂O₃ oxides and platinum-group d-metals on cathode emission. Although the presented results are qualitative, they are mutually consistent and correlate with observed emission characteristics. The simplicity of interpretation makes G.V. Samsonov’s configurational model suitable for examining charge transfer interactions in adsorbate–adsorbent systems.