Processes for the Fabrication of SU-8 Structures and Sputtered Materials on Porous Glass for Electrospray Thruster Manufacturing

Abstract

Electrospray thrusters are electric propulsion devices that generate thrust through the use of an electric potential between the emitter, a concentrated point at which a propellant is flowed to, and downstream extractor electrodes that generates a high electric field at the emitter causing the propellant to be accelerated. Current electrospray thruster designs use sharp micron-scale cone-shaped emitters made from porous materials to generate ion emission through passive propellant feeding, but the current design has flaws that affect its lifetime, reliability, and performance. High specific impulse thruster firing occurs when operating in the purely ionic regime (PIR), in which an ionic liquid propellant (a room temperature molten salt or liquid metal) emits individual ions rather than larger droplets. These emitters must be built on the micron-scale to achieve PIR emission, resulting in their operation as large monolithic arrays with a single extractor to produce a usable amount of thrust, such that the failure of one emitter out of thousands could lead to full extractor and device failure. Futher, the broad parameter space (geometry, flow path, insulation, etc) is currently not selected according to the optimal requirements for operation in the PIR. Recent simulations show that PIR emission can be achieved in a relatively narrow domain that depends on the applied electric field, meniscus size, and hydraulic impedance for flat panel capillary emitters. These capillary emitters can be designed with individualized extractors that are connected through a series of fuses, isolating any shortage to a single emitter. Photolithography is a useful micromanufacturing tool that has not yet been utilized to build solid structures on top of porous structures. This is because a porous substrate would uptake any liquid photoresist applied during fabrication, making the susbtrate lose its porosity. To prevent this, and allow for the formation of solid structures on top of a porous substrate for electrospray thruster applications, this thesis develops a manufacturing plan in which the pores within the substrate are loaded with a volatile organic compound (VOC), allowing a structure to be fabricated on the substrate surface via photolithography, without the material entering the substrate’s pores. To regain the substrate’s porous structure, the VOC is removed post-manufacturing via sublimation and an acetone wash. Using the manufacturing techniques described in this thesis, a novel electrospray thruster design consisting of capillaries and fuses to optimize PIR performance and prevent shortage propagation is proposed to greatly increase the performance and reliability of electrospray thruster devices.