Hollow Cathode With Low Work Function Electride Insert

Rand, Lauren P.; Waggoner, Ryne M.; Williams, John D.

doi:10.1115/imece2011-65785

Cited by 4 publications

(3 citation statements)

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“…Operation of the thermionic electrode under current flow at temperatures exceeding 825°C indicates the stability of the hydrogen passivation layer that induces the NEA surface which eliminates the surface emission barrier and contributes to the low work function. This is one of the lowest work function values reported; comparable to the measured work function of 0.82 eV for 12CaO-7Al 2 O 3 , or C12A7:electride and 0.9 eV for polycrystalline phosphorus-doped diamond (Koeck et al, 2009;Rand et al, 2011).…”

Section: Single-crystal Phosphorus-doped Diamondsupporting

confidence: 71%

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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Section: Single-crystal Phosphorus-doped Diamondsupporting

confidence: 71%

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

Koeck

Nemanich

2017

Front. Mech. Eng.

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“…It is believed that the electron deficient layer on the emitter surface may cause trouble to the thermionic emission of C12A7 electride. Without eliminating the electron deficient layer, relying solely on external heating to ignite the C12A7 hollow cathode is likely to fail [18,19].…”

Section: Performance Of Iodine Hollow Cathodementioning

confidence: 99%

Early experimental investigation of the C12A7 hollow cathode fed on iodine

Hua

Wang

Ning

et al. 2022

Plasma Sci. Technol.

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To fully realize the superiority of the iodine electric propulsion system in streamlining the size and reducing the operating cost, iodine hollow cathode technology must be developed. Considering the corrosiveness of iodine and the possible impurity of working propellant, the C12A7 hollow cathode with promising chemical ability was developed and tested. The C12A7 hollow cathode with a nominal current of 1-4 A was successfully ignited with iodine from the reservoir outside the vacuum chamber. It was operated at 1 A of anode current with a 1.2 mg/s iodine mass flow rate. Despite involuntary extinguishment, the C12A7 hollow cathode could be restarted repeatedly with a single operation time of up to twelve minutes and a total duration of half an hour. The unexpected fluctuation of iodine flow may be the reason for the short operation time. Experimental results and microscopical observation of the electride emitter show the compatibility of the iodine and electride emitter. For the development and demonstration of future single iodine electric propulsion of Hall thrusters, the iodine storage and supply system with precise control and regulation may be the critical technology.

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“…This led to the rise of lanthanum hexaboride, a refractory compound characterized by its low work function, high melting point, and robust chemical stability, making it an ideal choice for cathode applications, particularly in satellite missions [21][22][23]. Calcium aluminate electride (C12A7) is a promising contender for future electric propulsion endeavors [24,25]. While the exact work function of C12A7 remains a topic of debate, there is a consensus in the academic community that C12A7 boasts a notably lower work function compared to traditional emitter materials [16].…”

Section: Introductionmentioning

confidence: 99%

Analysis of anode surface roughness influence on heaterless hollow cathode discharge

Hsieh,

Huang,

Huang

et al. 2024

Phys. Scr.

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This research delves into the influence of cylindrical and planar anode surface roughness on heaterless hollow cathode discharge characteristics. Three surface roughness levels, Ra 1.6 µm, Ra 3.2 µm, and Ra 6.4 µm, along the cylindrical anode's azimuthal direction and the planar anode's radial direction, have been selected and modified by the machining process. A central finding is the correlation between the discharge voltage and anode surface roughness. As the roughness increases, cylindrical and planar anodes require less discharge voltage to sustain the primary discharge. This reduction is likely due to the enhanced surface area from the roughness, which aids in electron current collection. The discharge voltage oscillations appear inversely related to the surface roughness of planar anodes, which may be associated with changes in the neutral gas density gradient between the planar anode and the cathode. In contrast, cylindrical anodes show fewer effects from their surface roughness, likely due to the distinct neutral gas flow dynamics. These findings offer insights into standardizing hollow cathode testing and allow future research to explore these interactions more deeply.

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Hollow Cathode With Low Work Function Electride Insert

Cited by 4 publications

References 0 publications

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

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

Early experimental investigation of the C12A7 hollow cathode fed on iodine

Analysis of anode surface roughness influence on heaterless hollow cathode discharge

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