Electrothermal characterization of tungsten-coated carbon microcoils for micropropulsion systems Original Research Article

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 514.5 nm) at 360 mW and 930 mbar of C2H4. Bent arms are then deposited at the ends of the 1 mm long coils using 600 mW of laser power and 700 mbar 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 105 mbar and 400 mW of laser power. High-resolution scanning electron microscopy analysis showed the tungsten coating to be 1.5–3.5 μm thick on the body of the coil and less than 2 μm 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.