Electrothermal characterization of tungsten-coated carbon microcoils for micropropulsion systems

Abstract

Laser-induced chemical vapor deposition is used to deposit tungsten-coated carbon microcoils from the gas phase. Because the microcoils are used as heating elements in cold/hot gas microthrusters in nanosatellites, it is important to characterize their electrothermal behavior so that the performance of the thruster may be predicted. The coils are deposited using an argon-ion laser (wavelength 514.5nm) at 360mW and 930mbar of C2H4. Bent arms are then deposited at the ends of the 1mm long coils using 600mW of laser power and 700mbar of C2H4. The arms serve as electrical contacts and as mechanical supports to hold the coils in the thruster by a locking mechanism. A layer of tungsten is then applied to the carbon coils by the hydrogen-reduction of WF6 using a 20:1 (H2:WF6) pressure ratio at a total pressure of 105mbar and 400mW of laser power. High-resolution scanning electron microscopy analysis showed the tungsten coating to be 1.5–3.5μm thick on the body of the coil and less than 2μm on the contact arms. The tungsten coating reduced the resistance of the carbon coils by an order of magnitude and reached higher temperatures at lower voltages. In vacuum, the tungsten coating is the dominant current carrier below 1300–1700°C; above this range carbon dominates. Peak temperatures for the tungsten-coated coils are 2050°C in vacuum and 1940°C in N2 – several hundred degrees higher than the non-coated coils.