Work function of sintered lanthanum hexaboride

Pelletier, J.; Pomot, C.

doi:10.1063/1.90769

Cited by 45 publications

(24 citation statements)

References 11 publications

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“…The lanthanum-boron system can consist of combinations of stable LaB 4 , LaB 6 , and LaB 9 compounds, with the surface color determined by the dominant compound 13 . The evolution of LaB 4 to LaB 9 compounds is caused either by preferential sputtering of the boron or lanthanum atoms at the near surface by energetic ion bombardment, or by preferential chemical reactions with surface atoms [14][15][16] . Lanthanum-boride compounds, heated to in excess of 1000 ˚C in vacuum, congruently evaporate their components at a rate that produces a stable LaB 6.0 surface.…”

Section: Lab 6 Characteristicsmentioning

confidence: 99%

“…Polycrystalline LaB 6 cathodes have a work function of about 2.67 eV depending on the surface stoichiometry 13 , and will emit over 20 A/cm 2 at a temperature of about 1700 C. Since the bulk material is emitting, there is no chemistry involved in maintaining the surface work function and LaB 6 cathodes are insensitive to impurities and air exposures that would destroy other cathodes. The LaB 6 cathode life is determined primarily by the evaporation rate of the bulk LaB 6 material at typical operating temperatures [14][15][16] . The American Institute of Aeronautics and Astronautics 3 higher operating temperature and the need to make contact with LaB 6 with compatible materials has likely limited its use in the U.S. space program.…”

Section: Lab 6 Characteristicsmentioning

confidence: 99%

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High Current Lanthanum Hexaboride Hollow Cathode for 20-to-100 kW Class Hall Thrusters

Goebel¹,

Chu²

2012

48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference &Amp;amp; Exhibit

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NASA is continuing to develop high power Hall thrusters in the range of 20 to 100 kW for future cargo and manned-missions. The cathodes for these thrusters will be required to produce discharge currents in the 50 to likely over 300 A range with lifetimes in excess of 10 khrs. A prototype high current hollow cathode with a 2-cm-dia lanthanum hexaboride (LaB 6 ) insert was previously developed for these applications. The original design featured a graphite cathode tube to interface with the LaB 6 insert and an Al 2 O 3 insulated sheath heater capable of heating the higher temperature emitter to ignition temperatures. A new version of this cathode has been designed and built that uses a refractory metal cathode tube and features a robust design similar to the small LaB 6 cathode used for the H6 Hall thruster. The cathode has been successfully tested at steady-state discharge currents from 25 A to 300 A. This cathode is intended to be used in the X3/80 80-kW Nested Hall thruster being built at the University of Michigan at discharge currents over 250 A. Nomenclature d = variable disk diameter in the gas flow model D o = temperature modified coefficient in the Richardson-Dushman thermionic emission equation e = electron charge k = Boltzman's constant l = disk length in the gas flow model n = neutral density T= temperature T r = temperature normalized to 289.7 K P 1,2 = pressure Q = xenon gas flow rate ζ = gas viscosity φ wf = work function

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Section: Lab 6 Characteristicsmentioning

confidence: 99%

Section: Lab 6 Characteristicsmentioning

confidence: 99%

High Current Lanthanum Hexaboride Hollow Cathode for 20-to-100 kW Class Hall Thrusters

Goebel¹,

Chu²

2012

48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference &Amp;amp; Exhibit

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“…However, in practical applications, a number of alloys and compounds, such as thoriated tungsten 10,11,12,13 and lanthanum hexaboride 14,15 , are used because they offer relatively low work functions (≈ 2.5 eV) in combination with much better chemical and thermal stabilities. It has also been known since the 1930s 16,17 that it is possible to create surfaces with work functions lower than those of any elemental, bulk materials by using coatings with thicknesses on the order of a monolayer.…”

Section: Introductionmentioning

confidence: 99%

An orbital-overlap model for minimal work functions of cesiated metal surfaces

Chou

Voss

Bargatin

et al. 2012

J. Phys.: Condens. Matter

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We introduce a model for the effect of cesium adsorbates on the work function of transition metal surfaces. The model builds on the classical point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates and its effect on the strength and orientation of electric dipoles along the adsorbate-substrate interface. The new model improves upon earlier models in terms of agreement with the work function-coverage curves obtained via first-principles calculations based on density functional theory (DFT). All the cesiated metal surfaces have optimal coverages between 0.6 to 0.8 monolayers (ML), in accordance with experimental data. All the cesiated tungsten surfaces we have considered are shown to have minimum work functions (MWF) lower than most cesiated metals, also in accordance with experiments.

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“…A similar value of 2 kV was also used by Pelletier et al [35] to determine the work function of LaB 6 sintered pellets. Figure 10 shows the dependence of emitted electron current I on LaB 6 ionizer temperature, expressed as log(I/T 2 ) versus e/kT (so-called Richardson plot [74]), for the TaThO576-MK4 unit operated in 2016 at ISOLDE, together with the electron emission currents measured with various MK4 units previously operated at ISOLDE-SC. It can be seen that the historical MK4 ion sources were operated with a LaB 6 whose work function was close to 2.9 eV, somewhat larger than the 2.66 eV found for a nominal LaB 6 material.…”

Section: Isotopementioning

confidence: 99%

Production of negatively charged radioactive ion beams

2017

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Beams of short-lived radioactive nuclei are needed for frontier experimental research in nuclear structure, reactions, and astrophysics. Negatively charged radioactive ion beams have unique advantages and allow for the use of a tandem accelerator for post-acceleration, which can provide the highest beam quality and continuously variable energies. Negative ion beams can be obtained with high intensity and some unique beam purification techniques based on differences in electronegativity and chemical reactivity can be used to provide beams with high purity. This article describes the production of negative radioactive ion beams at the former holifield radioactive ion beam facility at Oak Ridge National Laboratory and at the CERN ISOLDE facility with emphasis on the development of the negative ion sources employed at these two facilities.

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Work function of sintered lanthanum hexaboride

Cited by 45 publications

References 11 publications

High Current Lanthanum Hexaboride Hollow Cathode for 20-to-100 kW Class Hall Thrusters

High Current Lanthanum Hexaboride Hollow Cathode for 20-to-100 kW Class Hall Thrusters

An orbital-overlap model for minimal work functions of cesiated metal surfaces

Production of negatively charged radioactive ion beams

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