The Use of Boron Nitride for Improved Cold-Cathode Electron Field Emission Technology

Low-power Hall thrusters offer potentially important advantages for certain military applications but issues of lifetime and efficiency degradation at lower powers are issues hindering its utilization. A factor impacting efficiency is that thestate-of-the-art techniques for electron generation used for neutralization (such as hollow cathodes operating on the same propellant as the thruster) do not scale down in mass, power, and propellant consumption as readily as the miniaturized thrustersthemselves. This proposal outlines a possible solution utilizing Boron Nitride (BN), a chemically inert, tough, low work-function material, for improved cold-cathode electron field emission technology. The desirable characteristics of leading electronemission materials such as molybdenum tips and Carbon Nanotubes (CNT) are well known. However, the chemical reactivity of these materials, especially carbon, in oxidizing environ-ments presents significant limitations with respect to their application inHall thrusters and other propulsion technologies (both operational and handling factors). The propellantless nature of this approach eliminates the neutralizer as a degrading factor for efficiency and its superior material properties offer the possibilityof long lifetime operation. Producing a cold-cathode emitter using boron nitride would have application across the field of electric propulsion including thrusters and tethers. It could provide a long-life emitter that would not require a propellant andcould replace hollow cathodes and enable lower-power thrusters. Further developing boron nitride has applications beyond propulsion. Both phases of BN are chemically inert, highly insulating (if not doped), very good thermal conductors, and chemicallystable up to high temperatures. In addition, c-BN is very hard, and can be made semiconducting by doping with n- or p-type dopants.