Materials characterization of impregnated W and W–Ir cathodes after oxygen poisoning

Abstract

A c c e p t e d M a n u s c r i p t Highlights: Hollow cathodes with impregnated tungsten and tungsten-iridium emitters were operated with 100 ppm of oxygen in the xenon gas flow for 480 and 80 hours, respectively.  Chemical and morphological changes were studied using scanning electron microscopy, energy dispersive spectroscopy, and laser profilometry.  High concentrations of oxygen accelerate the formation of tungstate layers in both types of emitters.  Deposits of pure tungsten were observed on the W-Ir emitter, indicating that tungsten is preferentially removed from the and transported in the insert plasma.  The W-Ir emitter exhibited less erosion and redeposition at the upstream end than the pure W emitter surface.Page 2 of 8 A c c e p t e d M a n u s c r i p t AbstractElectric thrusters use hollow cathodes as the electron source for generating the plasma discharge and for beam neutralization. These cathodes contain porous tungsten emitters impregnated with BaO material to achieve a lower surface work function and are operated with xenon propellant. Oxygen contaminants in the xenon plasma can poison the emitter surface, resulting in a higher work function and increased operating temperature. This could lead directly to cathode failure by preventing discharge ignition or could accelerate evaporation of the BaO material. Exposures over hundreds of hours to very high levels of oxygen can result in increased temperatures, oxidation of the tungsten substrate, and the formation of surface layers of barium tungstates. In this work, we present results of a cathode test in which impregnated tungsten and tungsten-iridium emitters were operated with 100 ppm of oxygen in the xenon plasma for several hundred hours. The chemical and morphological changes were studied using scanning electron microscopy, energy dispersive spectroscopy, and laser profilometry. The results provide strong evidence that high concentrations of oxygen accelerate the formation of tungstate layers in both types of emitters, a phenomenon not inherent to normal cathode operation. Deposits of pure tungsten were observed on the W-Ir emitter, indicating that tungsten is preferentially removed from the surface and transported in the insert plasma. A W-Ir cathode surface will therefore evolve to a pure W composition, eliminating the work function benefit of W-Ir. However, the W-Ir emitter exhibited less erosion and redeposition at the upstream end than the pure W emitter.

Effect of Anodizing Time on Morphology and Wettability of TiO₂ Nanotubes Prepared by Carbon Cathode

“…These tube sizes were similar even though the anodizing time was doubly increased. This observation was also found by Guohua et al [7]. The comparable tube diameters gained from the different anodizing times pointed out that anodizing time has no significant effect on the nanotube diameter under a constant voltage.…”

“…The average lengths of the nanotubes prepared for 5 h and 10 h were 284.6 nm and 376.5 nm, respectively. The increase in nanotube length with time was also pointed out by the previous studies of Guohua et al [7] and Regonini et al [17]. Obviously, anodizing time played an important role defining the length of nanotube arrays.…”

“…There are several factors such as electrolyte concentration, electrolyte pH, anodizing voltage, temperature, time and counter electrode that can be varied to control the morphology of TiO2 nanotubes. The effect of electrolyte concentration, pH and anodizing voltage on the morphology of TiO2 has been studied by many researchers [6][7][8]. The majority of the previous experiments have been used the platinum as the counter electrode for titania anodization.…”