Thermionic Emission Constants and Their Interpretation

Hensley, Eugene B.

doi:10.1063/1.1735994

Cited by 73 publications

(10 citation statements)

References 9 publications

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“…It is not the goal of the present work to develop a thermionic emission model that captures all of the physics behind our polycrystalline SrVO 3 emitters given its extreme complexity. Instead, we chose the “effective work function” approach described by Hensley [ 45 ] to identify Φ values directly from RLD Equation 1 with the theoretical A = 120 A (cm K) ‐2 as discussed in Section 2.3 and Section S5(Supporting Information). [ 62 ] We believe the simple RLD model will still capture the majority of the emission physics from our metallic SrVO 3 .…”

Section: Discussionmentioning

confidence: 99%

Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission

Lin

Jacobs

Chen

et al. 2022

Adv Funct Materials

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Engineering a material's work function is of central importance for many technologies and in particular electron emitters used in high‐power vacuum electronics and thermionic energy converters. A low work function surface is typically achieved through unstable surface functional species, especially in high power thermionic electron emitter applications. Discovering and engineering new materials with intrinsic, stable low work functions obtainable without volatile surface species would mark a definitive advancement in the design of electron emitters. This work reports evidence for the existence of a low work function surface on a bulk, monolithic, electrically conductive perovskite oxide: SrVO3. After considering the patch field effect on the heterogeneous emitting surface of the bulk polycrystalline samples, this study suggests the presence of low work function (≈2 eV) emissive grains on SrVO3 surface. Emission current densities of 10–100 mA cm–2 at ≈1000 °C, comparable to commercial LaB6 thermionic cathodes, indicative of an overall effective thermionic work function of 2.3–2.7 eV are obtained. This study demonstrates that perovskites like SrVO3 may have intrinsically low work functions comparable to commercialized W‐based dispenser cathodes and suggests that, with further engineering, perovskites may represent a new class of low work function electron emitters.

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Section: Discussionmentioning

confidence: 99%

Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission

Lin

Jacobs

Chen

et al. 2022

Adv Funct Materials

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“…[8] However, an experimental evaluation of various materials presents significant deviations from the universal value of 120 A / cm 2 K 2 . [9,10] Despite its wide band -gap characteristics diamond has long been studied for applications focusing on electron sources. With a set of promising material properties, i.e.…”

Section: Methodsmentioning

confidence: 99%

Thermionic electron emission from low work-function phosphorus doped diamond films

Koeck

Nemanich

Lazea

et al. 2009

Diamond and Related Materials

118

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Thermionic electron emitters are a key component in applications ranging from travelling wave tubes for communications, space propulsion and direct energy conversion. As the conventional approach based on metallic emitters requires high operating temperatures the negative electron affinity (NEA) characteristic of diamond surfaces in conjunction with suitable donors would allow an electronic structure corresponding to a low effective work function. We have thus prepared phosphorus-doped polycrystalline diamond films on metallic substrates by plasma assisted chemical vapor deposition where an NEA surface characteristics was induced by exposure of the film surface to a hydrogen plasma. Thermionic electron emission measurements in an UHV environment were conducted with respect to the Richardson-Dushman relation observing an emission current at temperatures < 375 ºC. Measurements were terminated at 765 ºC without significant reduction in the electron emission current indicating a stable hydrogen passivation of the diamond surface. A fit of the emission data to the Richardson equation allowed for the extraction of emission parameters where the value of the materials work function was

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“…As the effective work function was observed to follow ϕ e = ϕ + αT it follows that α = dϕ e /dϕ = k·ln(120/A R ). Thus, for materials that do not exhibit a temperature-dependent work function, the value for the Richardson constant will be 120 A cm −2 K −2 (Hensley, 1961). A further evaluation of expression (Eq.…”

Section: Single-crystal Nitrogen-doped Diamondmentioning

confidence: 99%

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

Koeck

Nemanich

2017

Front. Mech. Eng.

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Thermionic energy conversion, a process that allows direct transformation of thermal to electrical energy, presents a means of efficient electrical power generation as the hot and cold side of the corresponding heat engine are separated by a vacuum gap. Conversion efficiencies approaching those of the Carnot cycle are possible if material parameters of the active elements at the converter, i.e., electron emitter or cathode and collector or anode, are optimized for operation in the desired temperature range. These parameters can be defined through the law of Richardson-Dushman that quantifies the ability of a material to release an electron current at a certain temperature as a function of the emission barrier or work function and the emission or Richardson constant. Engineering materials to defined parameter values presents the key challenge in constructing practical thermionic converters. The elevated temperature regime of operation presents a constraint that eliminates most semiconductors and identifies diamond, a wide band-gap semiconductor, as a suitable thermionic material through its unique material properties. For its surface, a configuration can be established, the negative electron affinity, that shifts the vacuum level below the conduction band minimum eliminating the surface barrier for electron emission. In addition, its ability to accept impurities as donor states allows materials engineering to control the work function and the emission constant. Singlecrystal diamond electrodes with nitrogen levels at 1.7 eV and phosphorus levels at 0.6 eV were prepared by plasma-enhanced chemical vapor deposition where the work function was controlled from 2.88 to 0.67 eV, one of the lowest thermionic work functions reported. This work function range was achieved through control of the doping concentration where a relation to the amount of band bending emerged. Upward band bending that contributed to the work function was attributed to surface states where lower doped homoepitaxial films exhibited a surface state density of ∼3 × 10 11 cm −2 . With these optimized doped diamond electrodes, highly efficient thermionic converters are feasible with a Schottky barrier at the diamond collector contact mitigated through operation at elevated temperatures.

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Thermionic Emission Constants and Their Interpretation

Cited by 73 publications

References 9 publications

Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission

Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission

Thermionic electron emission from low work-function phosphorus doped diamond films

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

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Thermionic Emission Constants and Their Interpretation

Cited by 73 publications

References 9 publications

Demonstration of Low Work Function Perovskite SrVO3 Using Thermionic Electron Emission

Demonstration of Low Work Function Perovskite SrVO3 Using Thermionic Electron Emission

Thermionic electron emission from low work-function phosphorus doped diamond films

Advances in Thermionic Energy Conversion through Single-Crystal n-Type Diamond

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Product

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Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission

Demonstration of Low Work Function Perovskite SrVO₃ Using Thermionic Electron Emission