UC Irvine

Progress on miniaturization techniques in the last two decades has made it possible to scaledown spacecraft subsystems to smaller sizes and power levels, which resulted on a new category of satellites with mass below 600 Kg, called smallsats. The scale-down of satellites into smallsats, the decreasing service price of commercial rocket launchers, and standardization in the manufacturing of smallsats have boosted space technology activity. However, many smallsats in orbit lack a propulsion system due to the complexity of scaling it down to the volume, mass, and low power requirements. Electrospray propulsion is a natural fit for the micropropulsion required by smallsats because it is a soft ionization technique that does not involve the formation of a plasma. A single electrospray emitter, electrospraying a high conductive liquid such as an ionic liquid, efficiently converts electric power into beam kinetic power while operating at the available power levels. Successful integration of electrospray emitter arrays requires, however, both microfabrication expertise and detailed knowledge of the fundamentals of electrosprays. The first part of this work integrates an in depth study of a highly conductive liquid for its interest for electrospray propulsion with a scalable microfabrication method of silicon micro-emitters for a compact microfluidic electrospray propulsion systems. Its components, that include an emitter array electrode with fractal-like microchannels etched on the back side and perpendicular to the out-of-plane emitters, an extractor electrode, and a supporting micromachined glass substrate, are permanently bonded and precisely aligned using anodic bonding. The number of emitters, the hydraulic resistance of the microfluidic system, and the gap between the extractor electrode and the emitter electrode can be tailored during the initial design and fabrication steps to achieve the desired pressure and voltage operation range. The system presented demonstrates good performance, uniformity, synchrony of emission in each emitter part of the array, and a rapid response to the applied pressure in the propellant reservoir. The deposition of counter ions during operation is identified as key performance issue and addressed by adding an interface of platinum on the emitter array. The performance tests (86 hours and ongoing) demonstrate the largest operational lifetime of a microfabricated electrospray source with capillary-like emitters actively fed. The thrust measurements show thrust up to 174 μN with room for higher values. The design, fabrication, and performance of the micro-emitters shown in this work can lead to real primary propulsion solutions for smallsats. The second part of this work utilizes the electrospray expertise to study the fiber initiation on electrosprays of high molecular weight polymer solutions, also known as electrospinning. First, I focused on the electrostatic jet initiation of a SU-8 polymeric solution studied with two different geometries in the so-called Near-Field regime to quantify the initiation parameters and illustrate the optimization of the electric field. Then, I functionalized suspended glassy carbon fibers derived from the same polymer solution using chemical vapor deposition. The temperature required for the deposition of WO3-x is generated by imposing a constant electrical current through the wire that causes joule heating. The deposition starts in the midpoint of the wire, extends to its ends as the current increases, and can be monitored in real-time by measuring the voltage drop across the wire. The resulting thickness and length of the coating are functions of the imposed current. This work showcases uniform and polycrystalline WO3-x coatings with thickness from 71 nm to 1.4 μm in glassy carbon wires with diameters between 780 nm and 2.95 μm. The same process is scaled-up by using carbon nanofiber mats, fabricated with far-field electrospinning, where the uniform temperature increase is homogenous to a good approximation by the dominance of radiative heat transport leading to very uniform WO3-x coatings. The functionalization of carbon micro nano materials described in this work can lead to novel, inexpensive, and bioavailable sensing solutions.