A micro-fabricated colloid microthruster : high voltage electrostatic fields on a MEMS device

In recent years, there has been a resurgence of interest in colloid propulsion, sparked by a newly increased understanding of the underlying electrospray process. Electrospray produces sprays of charged droplets with an efficiency and specific charge that makes colloid propulsion very attractive. Traditionally colloid thrusters use hollow metal needles from which the spray is extracted using a nearby high-voltage electrode. However, this is a very problematic approach when mass is restricted, and lacks the dimensional consistency that is required across an array. The research reported here has produced a colloid microthruster, using micro-fabrication techniques, which aims to solve those problems and enable colloid propulsion as a microthrust technology with a novel approach to thrust throttling. Large arrays of electrospray nozzles were fabricated out of silicon. The amount of fuel lost by evaporation is drastically cut because the nozzles have very small exit diameters. A new packaging system delivers fuel to the nozzles at a prescribed pressure so the liquid remains at the exit, even at high vacuum. Extraction electrodes are situated on-chip; a silicon dioxide layer insulates them from the substrate. The electrodes are fundamental to the electrospray operation and this work is the first in which they are integrated with microfabricated nozzles. The approach benefits greatly from batch processing and the reduced dimensions mean significantly lower voltages are required. The design must create and withstand extremely high electrical fields produced by kV voltages. To ascertain the suitability of the design I tested the electrode systems and determined the major factors contributing to electrical breakdown. The breakdown strength and destructive mechanism have been discovered for differing geometries and oxide thickness, over a range of experimental conditions that simulate the predicted operational environment.