Using carbon nanotubes as field emitting sources
Vacuum microwave devices for telecommunication systems in satellites and spacecrafts that will operate at higher frequency bands than currently available require high electron current densities ranging from 5 to 20A/cm2. Conventional thermionic cathodes technology can provide these high current densities at the expense of reduced lifetimes and increased life-cycle costs by operating the cathode at high temperatures. Field emitting cathodes fabricated from nano-structurally rich carbon materials are an attractive alternative for low-cost replacement of the thermionic cathodes. In particular, carbon nanotubes (CNs) grown in the form of small, sharp spikes exhibit extraordinary field emission properties due to their electrical conductivity and remarkable thermal stability. Researchers at the THALES ELECTRON DEVICES S.A. were successful in constructing arrays of uniform individual carbon nanotubes vertically aligned and spaced at a distance corresponding to approximately twice their height. Each carbon nanotube of 5μm height and 50nm diameter was grown by plasma enhanced chemical vapour deposition. After growth, rapid thermal annealing was used to improve both the crystallinity of the carbon nanotubes and their electrical contact to the substrate (E. Minoux et al., 2005). This allowed the fabrication of CNs each capable to emit very high currents of up to 100μA at relatively low electric fields. The device was operated at a frequency equal to 1,5GHz in a continuous mode without degradation or a decrease in current output which reached a current density of 12A/cm2 (K. Teo et al., 2005). This miniaturised electron source because of its unique properties holds promise for a new generation of efficient, lightweight and compact microwave devices. The most promising applications of this new type of cold cathodes are power amplifier systems operating at high frequencies (30-100GHz) and in particular for travelling wave tubes used in satellites. Additionally, their ability to generate and directly modulate electron beams makes them ideal candidates for low voltage field emitters used in parallel electron beam lithography and X-rays generation.
