Work Function Measurement of Lanthanum - Boron Compounds

Jacobson, D. L.; Storms, Edmund

doi:10.1109/tps.1978.4317109

Cited by 29 publications

(18 citation statements)

References 3 publications

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“…(1) from the present work and literatures. 6,[13][14][15] The work functions, the Richardson constants, and the temperature-modified coefficient used to plot Figure 6(a) are listed in Table I. The tendency of the current density depicted using the present work shows good agreement with other published data.…”

Section: Emission Propertiessupporting

confidence: 77%

Emission properties and back-bombardment for CeB6 compared to LaB6

Bakr

Kawai

Kii

et al. 2015

Journal of Applied Physics

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The emission properties of CeB6 compared to LaB6 thermionic cathodes have been measured using an electrostatic DC gun. Obtaining knowledge of the emission properties is the first step in understanding the back-bombardment effect that limits wide usage of thermionic radio-frequency electron guns. The effect of back-bombardment electrons on CeB6 compared to LaB6 was studied using a numerical simulation model. The results show that for 6 μs pulse duration with input radio-frequency power of 8 MW, CeB6 should experience 14% lower temperature increase and 21% lower current density rise compared to LaB6. We conclude that CeB6 has the potential to become the future replacement for LaB6 thermionic cathodes in radio-frequency electron guns.

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Section: Emission Propertiessupporting

confidence: 77%

Emission properties and back-bombardment for CeB6 compared to LaB6

Bakr

Kawai

Kii

et al. 2015

Journal of Applied Physics

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“…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%

“…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%

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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“…Since the bulk material is emitting, there is no chemistry involved in establishing the low work function surface and LaB 6 cathodes are insensitive to impurities and air exposures that would normally destroy a BaO dispenser cathode. In addition, the cathode life is determined primarily by the evaporation rate of the bulk LaB 6 material at typical operating temperatures [19][20][21] . The higher operating temperature of LaB 6 and the need to make contact with LaB 6 with compatible materials has perhaps unjustly limited their use in the US space program.…”

Section: Introductionmentioning

confidence: 99%

High-Current Lanthanum Hexaboride Hollow Cathode for High-Power Hall Thrusters

Goebel

Chu

2014

Journal of Propulsion and Power

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Space missions continue to demand higher power Hall and ion thrusters capable of providing high thrust and long life. For high power Hall thrusters in the range of 20 to 100-kW being developed for future cargo and manned-missions, the hollow cathodes will be required to produce discharge currents of 50 to 400 A with lifetimes in excess of 10 khrs. A lanthanum hexaboride (LaB6) hollow cathode has been previously developed for space applications that features graphite tubes or sleeves to provide a diffusion boundary to protect the LaB6 insert from chemical reactions with the refractory metals, and includes a long life heater capable of igniting the higher temperature emitter. Several versions of this cathode design with different LaB6 insert diameters have been built and tested at up to 250 A of discharge current to demonstrate both the high current capability and ease of handling and gas purity requirements obtained with LaB6 cathodes. While the LaB6 cathode insert operates at a higher temperature than the conventional BaO dispenser cathode, LaB6 offers the capability of very high discharge currents, long life and orders of magnitude less sensitivity to propellant impurities and air exposure than conventional dispenser cathodes.

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Work Function Measurement of Lanthanum - Boron Compounds

Cited by 29 publications

References 3 publications

Emission properties and back-bombardment for CeB6 compared to LaB6

Emission properties and back-bombardment for CeB6 compared to LaB6

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

High-Current Lanthanum Hexaboride Hollow Cathode for High-Power Hall Thrusters

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