Influence of hydrogen on the thermionic electron emission from nitrogen-incorporated polycrystalline diamond films

Paxton, William F.; Howell, M.; Kang, W.P.; Davidson, J.L.

doi:10.1116/1.3684982

Cited by 29 publications

(31 citation statements)

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“…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. Thermionic emission can be described by the Richardson equation seen in Eq.…”

mentioning

confidence: 67%

“…The temperature control loop was used to increase the temperature in roughly 5° increments up to ~900°C. Each temperature was maintained for 30 s and the emission current was constantly monitored using the same technique as previously reported (Paxton et al, 2012a). These isobaric tests were intended to determine if the presence of hydrogen would increase the emission current of diamond thermionic emitters and/or raise their operational temperature ceiling.…”

Section: Methodsmentioning

confidence: 99%

“…Samples were deposited at 1.5 kW microwave power in a 50 Torr hydrogen, methane, and nitrogen environment. SEM micrographs of samples deposited under these conditions can be found elsewhere (Paxton et al, 2012a). The samples were examined in the testing configuration previously reported (Paxton et al, 2012b).…”

Section: Methodsmentioning

confidence: 99%

“…Polycrystalline nitrogen-incorporated diamond films were deposited on molybdenum substrates using a microwave plasma-enhanced chemical vapor deposition (MPCVD) reactor in the manner previously described (Paxton et al, 2012a(Paxton et al, ,b, 2014. Samples were deposited at 1.5 kW microwave power in a 50 Torr hydrogen, methane, and nitrogen environment.…”

Section: Methodsmentioning

confidence: 99%

“…However, previous studies have reported that the emission current begins to deviate from Eq. 1 at temperatures between 650 and 800°C in that it exhibits a decreasing trend with increasing temperature (Suzuki et al, 2009;Kataoka et al, 2010;Paxton et al, 2012a). This trend has been attributed to the desorption of hydrogen and results in a "temperature ceiling" (defined here as the temperature at which the maximum emission current is achieved) that prevents higher emission currents from being obtained.…”

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confidence: 98%

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Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Paxton

Ravipati

Brooks³

et al. 2017

Front. Mech. Eng.

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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.

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mentioning

confidence: 67%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 98%

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Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Paxton

Ravipati

Brooks³

et al. 2017

Front. Mech. Eng.

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Progress Toward High Power Output in Thermionic Energy Converters

Campbell

Celenza

Schmitt³

et al. 2021

Advanced Science

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Thermionic energy converters are solid‐state heat engines that have the potential to produce electricity with efficiencies of over 30% and area‐specific power densities of 100 Wcm−2. Despite this prospect, no prototypes reported in the literature have achieved true efficiencies close to this target, and many of the most recent investigations report power densities on the order of mWcm−2 or less. These discrepancies stem in part from the low‐temperature (<1300 K) test conditions used to evaluate these devices, the large vacuum gap distances (25–100 µm) employed by these devices, and material challenges related to these devices' electrodes. This review will argue that, for feasible electrode work functions available today, efficient performance requires generating output power densities of >1 Wcm−2 and employing emitter temperatures of 1300 K or higher. With this result in mind, this review provides an overview of historical and current design architectures and comments on their capacity to realize the efficiency and power potential of thermionic energy converters. Also emphasized is the importance of using standardized efficiency metrics to report thermionic energy converter performance data.

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Experimental Studies of Electron Affinity and Work Function from Aluminium on Oxidized Diamond (100) and (111) Surfaces

James

Cattelan

Fox

et al. 2021

Physica Status Solidi (b)

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Three different procedures are used to deposit aluminium onto O‐terminated (100) and (111) boron‐doped diamond, with the aim of producing a thermally stable surface with low work function and negative electron affinity. The methods are 1) deposition of a > 20 nm film of Al by high‐vacuum evaporation followed by HCl acid wash to remove excess metallic Al, 2) deposition of <3 Å of Al by atomic layer deposition, and 3) thin‐film deposition of Al by electron beam evaporation. The surface structure, work function, and electron affinity are investigated after annealing at temperatures of 300, 600, and 800 °C. Except for loss of excess O upon first heating, the Al + O surfaces remain stable up to 800 °C. The electron affinity values are generally between 0.0 and −1.0 eV, and the work function is generally 4.5 ± 0.5 eV, depending upon the deposition method, coverage, and annealing temperature. The values are in broad agreement with those predicted by computer simulations of Al + O (sub)monolayers on a diamond surface.

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Influence of hydrogen on the thermionic electron emission from nitrogen-incorporated polycrystalline diamond films

Abstract: Articles you may be interested inSubstrate-diamond interface considerations for enhanced thermionic electron emission from nitrogen doped diamond films

Cited by 29 publications

References 13 publications

Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Thermionic Emission from Diamond Films in Molecular Hydrogen Environments

Progress Toward High Power Output in Thermionic Energy Converters

Experimental Studies of Electron Affinity and Work Function from Aluminium on Oxidized Diamond (100) and (111) Surfaces

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