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°C, 625°C, 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 x 10-6 Torr) were conducted at temperatures ranging from 700-900°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 to 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.
Highlights
Previous research has demonstrated diamond to be an exceptional low-temperature electron emitter for numerous applications, including thermionic energy conversion (TEC)
Given that the diamond films are grown in a hydrogen-rich, methane-limited environment, a substantial amount of hydrogen is likely incorporated within the bulk of the diamond cathodes
As the emission current is monitored under increased heating, these hydrogen atoms diffuse from the interior of the diamond to the surface, bonding temporarily with surface carbon atoms
Summary
Previous research has demonstrated diamond to be an exceptional low-temperature electron emitter for numerous applications, including thermionic energy conversion (TEC). In addition to increasing the bulk conductivity of diamond films, hydrogen has been shown to Diamond Thermionic Emission in Hydrogen interact with the diamond surface to form polarized C–H bonds, reducing the electron affinity and in turn, reducing the work function (Maier et al, 2001). This effect was illustrated in previous work by Paxton et al (2012a), which observed that exposure of diamond cathodes to a low-energy hydrogen plasma prior to testing resulted in drastically enhanced thermionic emission current relative to as-grown diamond films by four orders of magnitude. In order to capitalize on diamond’s favorable properties for thermionic emission applications, methods that allow such cathodes to overcome the aforementioned performance limiting factors associated with the desorption of hydrogen need to be identified
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