Carbon Nano-Materials for Vacuum Electronics

A number of vacuum electronic devices, including MW generators and amplifiers, CRT, X-ray tubes etc., utilize electron beams produced by cathode. An essential improvement of the devices may be obtained using field emission cathodes providing intense monoenergetic electron beam and operating at room temperature. Material of these cold cathodes must sustain very strong electric field and residual gas ion bombardment in the vacuum tubes. This requirement limits list of appropriate materials by a few hard metals (like Mo, W), semiconductors (Si) and carbons with diamond and graphite atomic configurations. The carbon materials have strongest interactomic sigma-sigma bonding but diamond has no free electrons to emit them into vacuum and graphite itself has layered structure with very weak interlayer bonding. Carbon nanotubes seems to be ideal field emitters having strongest graphite-type interactomic bonding cylindrical configuration of atomic layers, preventing their destruction under strong electric field and highest aspect ratios allowing significant decrease of used voltages. This report presents our comparative study of field emission from various carbon nano-materials. Carbon nanotubes synthesized by arc-discharge methods, nano-diamond powders and films, and nano-graphitic CVD films were tested for field emission and other properties. The conditions of used CVD process were optimized to obtain most efficient carbon cold cathode material. We propose mechanism of electron emission from carbon nano- materials based on assumption of heterogeneous sp2-sp3 structure formation. Device applicability of the cold carbon cathodes is demonstrated in prototypes of cathodoluminescent light sources.