Very large cathode current and long term stability of vacuum sealed tubes with engrafted-carbon-nanotube emitters

For the future commercial applications of carbon nanotubes (CNTs) in high power vacuum microwave amplifiers or compact X-ray tubes, we have attempted to fabricate engrafted CNT field emitters on a metallic substrate using both screen printing and chemical vapor deposition. Cobalt nano-grains are doped in the printed CNT paste and act as the catalyst for the engrafted growth of CNTs by the cold wall chemical vapor deposition. Stable cathode current (~ 30 mA) from a small area (~ 1.5 mm²) of engrafted CNT emitters was measured in a vacuum-sealed diode tube. High current density (> 1.6A/cm²) has also gotten in the vacuum sealed tube in which the emitters spread about 0.78 mm² after an aging process that lasts more than 12 h in DC mode with the water cooling of the anode.