This work characterizes the bulk emission properties of carbon nanotube (CNT) forest cathodes fabricated with various geometries. Geometries explored include dense nanotube forests of varying height grown on 5 mm x 5 mm silicon (Si) substrates and discrete, patterned CNT pillars fabricated using UV photolithography. Dense forest heights ranged from 683 µm to 1.25 mm. Patterned sample micropillar heights ranged from 47 µm to 393 µm, with pillar widths on tested samples ranging from 250 µm to 270 µm. Properties explored include emission current, turn-on field, emission current performance over time, and emission site mapping. A parallel plate electron beam diode with a 100 µm A-K gap and an automated test apparatus were developed to provide a configurable experiment that gives accurate and repeatable measurements for DC, DC sweep, and timed performance testing. Testing has shown evidence of a hysteresis effect on the emission current tied to the applied field history as well as shifting of the turn-on field magnitude throughout the testing period, suggesting a conditioning effect during use. Three separate emission regions in the I-V curves during sweep testing have also been observed. In the geometric study, dense forest and patterned samples were sweep tested up to a peak applied voltage of -250 V, with the taller samples generally producing higher emission currents. Currents produced in the geometric study from the dense forest emitters ranged from 36.8 µA to 572.34 µA, with current densities ranging from 0.15 mA/cm² to 2.29 mA/cm². Currents produced from the patterned micropillar emitters ranged from 39.4 µA to 317.51 µA, with current densities ranging from 0.67 mA/cm² to 3 mA/cm². DC time testing showed a relatively stable output current over a 4.5-hour testing period. Another diode structure with a gridded anode was designed for emission site mapping using a mid-wave infrared (IR) camera. Localized emission sites with elevated temperatures were observed across several samples of both geometries during DC sweep testing. These elevated temperatures at the emitter surface were correlated with an increase in emission current measured at the anode grid. The correlation between emission current and the rise in temperature at the emitter surface led to the conclusion that the CNT emitters tested in this work do not operate solely in the field emission region. Instead, they operate according to a thermal-field (TF) relation, with the contribution from thermal fields resulting from self-heating of the CNTs. The findings of this work aim to explore CNT emitters as a viable alternative to thermionic cathodes used in large RF systems and to characterize connection between CNT emitter geometry and the resulting emission performance. The insights gained from this work will be used for informed design and optimization of future emitters.
