Mikrostrukturierte Flüssigmetall-Ionenemitter-Chips: Untersuchung des Benetzungsverhaltens und der Betriebsstabilität von Flüssigmetall-Ionenquellen auf Basis von Glas-Substraten
Herstellung und Eigenschaften von Funktionsmaterialien
Zusammenfassung der Projektergebnisse
Electric propulsion is of growing interest for satellite manufacturers as it requires much less propellant compared to chemical engines as well enabling precision thrust control. Field-Emission- Electric-Propulsion (FEEP) uses liquid metal as propellant, which results in even smaller tank sizes due to the high propellant density with respect to traditional gas tanks. However, such FEEP thrusters only produce very small thrusts in the µN range, which limits its applicability. One solution is to cluster such emitters, which was done up to now using traditional machining. A new solution would be to achieve high cluster densities by using FEEP thrusters made out of glass, which can be batch manufactured. Few information is available up to now on the usability of glass emitters with metal propellants. Only one test has been reported so far which worked for low currents up to minutes before it stopped. In this project we tried to thoroughly investigate the bonding and wetting forces of different metal propellants on various glass substrates in order to check if such new glass-emitters are feasible or not. In order to investigate the suitability of glass substrates for Liquid-Metal Ion Sources, a series of bonding-strength tests with indium and gallium on fused silica and borosilicate was performed. The wetting properties of these combinations resulted in comparably weak with no self-sustaining wetting without mechanically forcing the liquid metal into the glass surfaces. Wetting attempts inside a glovebox with less than 0.1 ppm of oxygen proved an oxide layer on the metal to be necessary for the effective wetting of the glass substrates. In the subsequent project phase, a broad selection of emitters with various capillary sizes and geometries, different reservoir materials and surface coating were produced and tested. However, no stable and reproducible ion emission could be achieved. The operability of the testing facility and testing equipment was validated using comparable ion sources with metal capillaries, indicating that the reason for the unsuccessful ignition tests might be found in the design parameters of the emitters. The capillary forces appear to be too weak to permit a stable flow of propellant which is probably caused by the low wetting properties of the material combinations An approach to coat the inner capillary surface by gallium sputter deposition to improve the low wetting qualities was conducted and yielded a distinct and uniform gallium layer on the emitter surfaces. The inner capillary surfaces were coated as well, but the coating did not reach the capillary tip, such that the ignition tests continued to fail. Nevertheless, the coating improved the wetting properties, as expected, such as enabling the immediate and long-lasting wetting of the coated graphite reservoirs, which did not bond with gallium before at all. Our results provide valuable insights into the bonding behavior of gallium and indium on glass substrates, which may be further used in bonding applications. The application of glass emitters for liquid metal propellant seems not to be feasibly based on the results obtained so far. Future tests should use shorter capillaries that minimize the flow resistance and possibly enable to coat the inside of the capillaries by sputter deposition.
Projektbezogene Publikationen (Auswahl)
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"MEMS FEEP Thrusters – Miniaturized Liquid Metal Ion Source using Glass Capillaries", Proceedings of the Space Propulsion Conference, Paper SP2018_015, Sevilla, May 14-18 (2018)
Boy, C. and Tajmar, M.
