Thermionic electron emission microscopy of metal-oxide multilayers on tungsten

Introduction Thermionic cathodes are used in many industrial vacuum electronic applications. Vacuum tubes for high-frequency or high-power applications are one example (e.g., see [1]). The emission properties of thermionic electron sources in vacuum devices are often Bimproved[ by the addition of oxide coatings on the metal cathode surface [2, 3]. The explanation of the improvement is usually based on the creation of a surface dipole. BaO is often used. The oxygen atom (ion) is embedded in the tungsten surface, and the Ba atom (ion) is outermost at the surface-vacuum interface [4–6]. Practical cathodes, however, are made with bulk powder loading of a porous tungsten plug [7]. Some are made by spray coating with a layer on the order of 65 m [8]. The monolayer surface dipole structure is thought to develop as the cathode is heated and Bactivated[ [9]. Recent experimental [10, 11] and theoretical [12] studies have suggested that multilayer oxides on the order of 10 nm can also reduce the work function of a metal. The work function is reduced by the alteration of the buried metal-oxide interface rather than at the oxide-vacuum surface. In this paper, we demonstrate that 200to 400-nm-thick oxide layers can increase the electron yield from a heated tungsten surface. The thermionic electron emission microscopy (ThEEM) images of the oxide layers on tungsten show a unique Bwindow[-like view of the tungsten grain structure through the oxide. No structures resulting from the oxide are resolved in the image. The window effect is proposed to result from a reduced interfacial energy barrier at the W-Sc2O3 interface and the electrical conductivity of Sc2O3 at high temperatures (91,100 K).