Surface chemical analysis and ab initio investigations of CsI coated C fiber cathodes for high power microwave sources

Abstract

CsI coated C fiber cathodes are promising electron emitters utilized in field emission applications. Ab initio calculations, in conjunction with experimental investigations on CsI-spray coated C fiber cathodes, were performed in order to better understand the origin of the low turn-on E-field obtained, as compared to uncoated C fibers. One possible mechanism for lowering the turn-on E-field is surface dipole layers reducing the work function. Ab initio modeling revealed that surface monolayers of Cs, CsI, Cs 2 O, and CsO are all capable of producing low work function C fiber cathodes ͑1 eVϽ⌽Ͻ1.5 eV͒, yielding a reduction in the turn-on E-field by as much as ten times, when compared to the bare fiber. Although a CsI-containing aqueous solution is spray deposited on the C fiber surface, energy dispersive x-ray spectroscopy and scanning auger microscopy measurements show coabsorption of Cs and I into the fiber interior and Cs and O on the fiber surface, with no surface I. It is therefore proposed that a cesium oxide ͑Cs x O y ͒ surface coating is responsible, at least in part, for the low turn E-field and superior emission characteristics of this type of fiber cathode. This Cs x O y layer could be formed during preconditioning heating. Cs x O y surface layers cannot only lower the fiber work function by the formation of surface dipoles ͑if they are thin enough͒ but may also enhance surface emission through their ability to emit secondary electrons due to a process of grazing electron impact. These multiple electron emission processes may explain the reported 10-100 fold reduction in the turn-on E-field of coated C fibers.

Improving field emission by constructing CsI–AlN hybrid nanostructures

“…The Cs signals could be attributed to Cs x O y , 34 and I to Cs x I y O z . 19,20 In addition, the O signal (see SI-6 in ESI †) could be divided into two peaks for O-Al (532.0 eV) and O-Cs (533.2 eV) 34 bonds, also suggesting the existence of Cs x O y . This result indicates CsI was decomposed during the FE, probably due to its exposure to X-ray irradiation arising from the electron emission, 18 in agreement with our (HR)TEM observation (Fig.…”

“…CsI is well known for its low work function 18 and CsI-coated nanostructures can effectively decrease the work function of the system. [19][20][21] In addition, CsI is a secondary electron emission material, 19,22,23 which may lead to the low turn-on field and high current density under a specific applied field. 19 Hence, decorating AlN with CsI is expected for the superior FE properties of the hybrid nanostructures.…”

“…[19][20][21] In addition, CsI is a secondary electron emission material, 19,22,23 which may lead to the low turn-on field and high current density under a specific applied field. 19 Hence, decorating AlN with CsI is expected for the superior FE properties of the hybrid nanostructures. In this work, we tried to improve the FE properties of AlN nanocone arrays by decorating CsI on their surfaces.…”

Work function and surface stability of tungsten-based thermionic electron emission cathodes

“…Examples include W, Mo, Ta, Re and Ni with adsorbed Cs, 30,31 (additional examples of work function lowering of other metals with adsorbed alkali metals are also summarized in Ref. 31), graphite with adsorbed Cs-I, 32,33 diamond with adsorbed H, Cs-O, and transition metals such as Cu, Ni, Ti and V, [34][35][36][37][38] GaAs and Ge with adsorbed Cs, 39,40 and W and Sc 2 O 3 with adsorbed Ba-O. 12,13,[41][42][43] The emitting properties of W have been studied previously with Density Functional Theory (DFT)-based approaches, including the stability and work function of Ba, Sc and O on the (001) surface, 41 the expected crystal structures and electronic properties of Os-doped W, 44 and the stability and emission characteristics of W coated with various oxide films and adsorbed alkali metals.…”

Modification of polymer velvet cathode via metallic Mo coating for enhancement of high-current electron emission performances

“…16,17 Surface coating with cesium iodide (CsI) on polymer velvet cathode is an alternative and effective strategy to reduce outgassing levels and decrease plasma expansion velocity, due to the low ionization potential and high mass of cesium. 9,18,19 However, the hygroscopicity and oxidizability of CsI in the atmosphere, 20,21 especially in the humid environment, restricted its wide application. Therefore, it is necessary to develop new surface modification technique for polymer velvet cathode to improve the high-current pulsed emission performances.…”