Emission Capabilities of Nafion-Based Emitting Geometries

Abstract

The propulsion field is home to numerous technological advancements, ranging from the mechanisms of their operation to the propellants used. All of these have enabled numerous space missions, either for technological demonstrations, or for other commercial, and even government use. Fortunately, with the dawning age of miniaturized electronics, these engines can be downsized to lower the system mass requirements, while ensuring the execution of mission specifications. Electrospray propulsion, therefore, is one of the propulsion areas in which this is possible.

Numerous technological advancements have been realized in the field of electrospray propulsion, ranging from the use of exotic plasma propellants, otherwise referred to as ionic liquids (IL, ILs), to the exploitation of various materials to fabricate surfaces for ion emission. All of these have reported attractive results ranging from competitive emission currents that range between nano- and micro-Amperes for single and array emitters, respectively, to characteristic velocity outputs on the order of kilometers per second.

Although the thrust levels are small, usually of the order of micro-newtons, the specific impulse values go as high as 3000 seconds. Unfortunately, with the available materials used in fabricating emitter geometry, with either porous to non-porous bulk properties, the emission characteristics are normally compromised either by off-axis emissions or high hydraulic impedance due to low to no IL transportation to emission sites. As such, it warrants the necessity to opt for materials that ensure smooth geometries for predictable and axially symmetric emissions, thereby fostering increased thrust densities.

Fortunately, ionomers, a group of polymers capable of transmitting electricity, pose as attractive options for this situation. Nafion, a fluorocarbon main-chain and sulfonic end groups composition, is the material used in the research presented in this work. Due to its extensive use in fuel cells, Nafion presents as a viable choice since numerous research has been performed to understand the material's morphological and physical behavior in situ. This resource was beneficial, especially in the research presented in this composition.

From the work done in the past using this material, it was clearly proven that ion emission, initiated via an electrified meniscus, was possible. Unfortunately, due to some unreliable manufacturing results, including bubble-filled tip structure, it was theorized that the emission results were, to some capacity, compromised. The presence of these air pockets inside the tip bulk, including broken apexes, interfered with emission characteristics, either by limiting effecting liquid transport through the bulk or by necessitating high start-off voltages for ion emissions.

Therefore, the main contribution of the research presented herein is to develop a manufacturing process that eliminates bubble structures in the tip bulk, and testing said geometries under high enough electrostatic forces for ion emissions to occur. Thus, this research presents the new manufacturing process and communicated some results obtained for fabricated single and array geometries.

Fortunately, it was proven that for tip geometries impregnated with IL, the absorbed solution is available for transportation to the emission site. It was also shown that the Nafion geometries always mimicked the structural properties of the parent counterparts.

For emission results, it was shown that for single emitters, the emission characteristics were competitive, with emission currents that were higher than those obtained from other single emitters reported elsewhere.