Fabrication of externally wetted emitter for ionic liquid electrospray thruster by low-speed wire cutting combined with electrochemical etching

Abstract

The micro-newton thrust generated by an ionic liquid electrospray thruster (ILET) can be used for drag-free control, attitude precise control, networking, and formation flight of micro-nano-satellites or spacecraft. Fabrication of high-quality emitters is one of the key technologies for the application of ILETs. A new method for fabricating externally wetted emitters is presented in this article. This method uses low-speed wire cutting (LSWC) combined with electrochemical etching, and the externally wetted emitter is fabricated in three steps. First, the tungsten cuboid is shaped by LSWC to get the geometry of the emitter. Second, the emitter is cleaned by superacid to remove the oxide layer that critically prevents the ionic liquid from wetting the emitter. Finally, microchannels are etched on the emitter surface by electrochemical etching. The tungsten emitter fabricated by the above-mentioned steps can obtain excellent wettability, and its microchannel can reduce the contact angle between the wall and the ionic liquid and enhance the capillary force that promotes the ionic liquid to climb along the emitter. The ionic liquid is spread into a uniform liquid film on the emitter surface to ensure a stable and continuous flow supply so that the thruster can work in pure ionic emission mode for a long time with low noise, high resolution, and a high thrust power ratio. The quantitative and qualitative analysis of all elements before and after the pickling emitter proves the feasibility of removing the oxide layer. The morphology features of the emitter are measured and observed using a metallographic microscope and profilometer, including the microchannel structure and its size and tip size and the thickness of the liquid film. Through repeated observation and evaluation, the etching parameters are recorded, including the amplitude and frequency of AC voltage, the etching time, the depth of emitter immersion in the solution, and the concentration of NaOH solution. In order to compare the performance of an externally wetted emitter and a porous emitter, the same process is used to fabricate two kinds of emitters. The conclusions on beam current, the resolution and noise of beam current, ignition reliability, and emission time are summarized during the emission testing. The emission tests indicate that the externally wetted emitter exhibits outstanding performance, which depends on its better wettability and the fact that the microchannels are not easily blocked by the products of electrochemical reaction.