Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Abstract

Diamond-based low-work function thermionic electron emitters are in high demand for applications ranging from electron guns and space thrusters to electrical energy converters. A key requirement of such diamond-based electron sources is hydrogen termination of the surfaces which can significantly reduce the emission barrier. However, at high temperatures (≤600°C), terminated hydrogen begins to desorb causing degradation in thermionic emission performance. The purpose of this study is to examine low-pressure hydrogen operating environments as a means to overcome this high-temperature performance limitation by enabling increased thermionic emission currents with improved stability at temperatures ≤600°C. A series of isothermal and isobaric experiments were performed in both nitrogen and hydrogen gas environments to determine the performance enhancement. Diamond electron emitters in both the as-grown and hydrogenated states were characterized at temperatures of 600, 625, and 650°C. An increase in thermionic emission current over vacuum operation was observed following the introduction of hydrogen. Upon evacuation of hydrogen to vacuum, the emission current decreased back to baseline levels. Further experiments in gas environments at a constant pressure (~5.5 × 10 −6 Torr) were conducted at temperatures ranging from 700 to 900°C. It was observed that the hydrogen environment promoted increased emission current while also enabling the diamond electron emitters to stably emit at increased temperatures compared with vacuum operation. Analogous experiments using nitrogen environments did not show any measurable performance enhancements, thus verifying that hydrogen is responsible for the observed effect. These results suggest diamond-based electron emitters can have improved thermionic emission performance at temperatures ≤600°C when operating in hydrogen gas environments.

A review of surface functionalisation of diamond for thermionic emission applications

“…There have been various methods reported to minimise space-charge. One approach is addition into the inter-electrode region of low-pressure caesium vapour [28] or gases such as H2 [34,35] or CH4 [36]. Cs readily ionises to Cs + ions, partially neutralising the negative cloud.…”

“…H2 and CH4 can absorb electrons to form negative ions, and then shuttle these electrons from the emitter to the collector, thereby preventing electrons from lingering near the emitter. Adding H2 gas to the inter-electrode region was reported to increase emission current while also enabling stable emission at increased temperatures compared with vacuum operation [35]. This improvement was probably due to the renewal of the surface hydrogenation, with new H atoms from the gas continually replacing those lost from the surface by thermal desorption.…”

A reactive molecular dynamics study of the hydrogenation of diamond surfaces

“…Experimentally, the most common temperatures at which the diamond hydrogenation is performed are in the range of 700-900°C, using hydrogen plasma [10,[35][36][37][38][39][40][41]. In order to gauge how the probability of hydrogenation changes with temperature, the hydrogenation simulation was done for the temperature range between 500-1200 C. In Figure 4(b) we present MD snapshots in perspective and top views of the resulting hydrogenated diamond surfaces at 900°C after the hydrogenation.…”