Lifetime and Failure Mechanisms of an Arrayed Carbon Nanotube Field Emission Cathode

There is interest in the use of carbon nanotubes (CNTs) to create a field emission (FE) cathode for the neutralization of exhaust plumes of low-power (<; 500 W) electric propulsion devices since FE cathodes do not require a gas flow to operate. To incorporate CNT emitters into propulsion systems, the current emission output over the lifetime of the cathode must be understood. Multiple FE cathodes that consist of multiwalled CNT arrays have been fabricated. Seven cathodes are characterized at pressures below 10 5 T at constant voltage between the CNTs, and the gate until failure occurs. The maximum current density observed is 9.08 mA/cm
²
with average current densities up to 2.52 mA/cm
²
, and the maximum life span is 368 h. The behavior of the cathode current emission is highly unstable and consists of oscillations and sudden shifts. Resistive heating is believed to be the primary cause for failure in two thermally assisted modes: 1) oxidative ablation at the root of the nanotube and 2) field evaporation at the tip.