# Effect of Structural Materials on Monopropellant Thruster Propulsion Performance in Micro Scale
## 2 Materials for Microthruster Fabrication  
Micro-propulsion systems, which produce thrust in the range of sub-micronewton to sub-millinewton, generally have components in the micro scale, such as the microinjector, microchamber, and micronozzle. For example, 50 um was the smallest dimension for a microinjector design in [20], and 30 um was the smallest dimension for a micronozzle throat in a previous study [9]. Those dimensions are out of the manufacturing capability of conventional machining, and the MEMS fabrication technology is essential to downsize space propulsion systems to this scale. Various photolithography techniques for the MEMS fabrication processes have been employed to experiment with different substrates for microthruster manufacturing. Silicon has been one of the most commonly used materials in the MEMS process. [PERSON] et al. [9] manufactured a silicon microthruster using a deep reactive ion etching (DRIE) process. This was a monoppellant thruster fueled by a hydrogen peroxide propellant to produce 500 uN of thrust, with a microchamber filled with diamond-shaped pillar structures to increase the reaction surface of the catalyst, and it was coated with silver, a catalyst active material conducive to propellant decomposition. Silicon was also used by [PERSON] et al. [21] who reported a microthruster manufactured in the DRIE process using silicon with a platinum thin film, which functioned as an integrated resistance temperature detector and a heater and enabled propellant decomposition. The silicon structure for the microthruster was found to be sufficiently robust when the temperature sensor and the thin film heater were in operation. [PERSON] et al. [22] utilized porous silicon processed with a bulk etching in the fabrication of a microthruster. The microthruster chamber was coated with platinum, an active catalyst material for propellant decomposition. The robustness of the porous silicon was proved in experimental tests, despite a part of the catalyst having been lost due to insufficient adhesiveness between the catalyst and the silicon structure. [PERSON] et al. [23] manufactured a silicon microthruster using inductively coupled plasma etching. The silicon surface of the thruster was coated with titanium to protect the material from the corrosive propellant, hydrazine, which was supplied through a multichannel distributor before it reached the catalyst bed. Iridium, an active catalyst material, was coated on circular pillars that filled the thruster catalyst bed to increase the reactive surface of the catalyst. [PERSON] et al. [10] also utilized silicon for a microthruster, with a microheater to increase the decomposition efficiency of a propellant while it was decomposed by the catalyst, manganese dioxide in the form of nanowires, in the condition of excessive heat loss due to the large surface-to-volume ratio in the micro scale. These studies demonstrate the feasibility of the material in the construction of a microthruster using a MEMS fabrication process, as well as their robustness and compatibility with propellants that involve chemical reactions. However, as reported in the silicon-based studies, there was evidence of propellant decomposition efficiencies occasionally necessitating the preheating of the catalyst chamber to compensate for the large heat loss in the micro scale. Accordingly, it has not yet been demonstrated that the material is the best for conserving thermal energy in the micro scale. Other materials employed in different MEMS fabrication processes were also examined, such as low-temperature co-fired ceramics (LTCCs), high-temperature co-fired ceramics (HTCCs), and glass, which have much lower thermal conductivities than silicon and thus are more desirable for thermal energy loss suppression. [PERSON] and [PERSON] [24] employed low-temperature co-fired ceramic (LTCC) tape technology in the fabrication of a planar monopropellant microthruster.