Promising variants of single crystal tungsten electrodes for high-performance thermionic converters

Kobyakov, V. P.

doi:10.1134/1.1514813

Cited by 7 publications

(4 citation statements)

References 1 publication

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“…The relationships among the growing conditions, orientation, and morphology of single-crystal tungsten grown by vapor-phase epitaxial deposition have been extensively studied [5,6]. However, the growth rate anisotropy and growth mechanism of tungsten single-crystal layers at different growth temperatures have been rarely researched.…”

Section: Introductionmentioning

confidence: 99%

“…Their investigations showed that the nucleation energy of crystal planes is affected by reactant supersaturation, which causes changes in the crystal plane morphology. Recently, Russian researchers focused on how to achieve maximum coverage of tungsten single-crystal {1 1 0} planes using chemical (chloride) vapor-phase epitaxial deposition [6].…”

Section: Introductionmentioning

confidence: 99%

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Deposition temperature effects on tungsten single-crystal layer by chemical vapor transport

Tan

et al. 2011

Journal of Crystal Growth

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Deposition temperature effects on tungsten single-crystal layer by chemical vapor transport

Tan

et al. 2011

Journal of Crystal Growth

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“…It works at a high temperature of approximately 1773 K throughout the service period, which requires the high temperature, mechanical strength, and microstructural stability of the emitter material. Tungsten coatings rely on their high electron work function, low diffusion penetrability, and superior hightemperature creep strength, which are vital to the cathode material [2][3][4]. Emitters with high work functions enable electronic fuel components to operate at high voltages and increase their efficiency and load capacity [5].…”

Section: Introductionmentioning

confidence: 99%

Microstructures and Kinetics of Tungsten Coating Deposited by Chemical Vapor Transport

Wang

Hao

Xiao

et al. 2019

KEM

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Chemical vapor transport deposition (CVTD) is an effective method for preparing large tungsten coatings for space thermionic reactors. In this study, a high-density, high-work-function polycrystalline tungsten coating was prepared using a WCl6 transport agent in a concentric tube-type closed transport system. The relationship between the kinetics and the microstructures of the CVTD polycrystalline tungsten coating at the substrate temperature of 1593 K-1793 K and system pressure of 15.93 Pa-106.8 Pa was studied, which provided a basis for the preparation of high-quality tungsten coatings. At a low temperature or a low pressure, the activation energy was approximately 2 kJ/mol, the deposition rate was almost independent of the temperature changes, and the control mechanism was mass transport limited. The tungsten coating had nodules on the surface with pores in the grain boundaries and grew preferentially along <111>. At a high temperature and a high pressure, the apparent activation energy was approximately 90 kJ/mol, the value of order was approximately 1, and the control mechanism in this process range was surface limited. The tungsten coating exhibited a hexagonal pyramidal structure, and the growth direction was preferred along <110>. The average work function of the tungsten coating prepared at a temperature of 1673 K and a system pressure of 106.80 Pa was as high as 5.20 eV.

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“…Nevertheless, the cathode of the thermionic energy converter, which is the "heart" of the reactor, must operate at a high ambient temperature of about 1500°C throughout the service period. The tungsten single crystal has a combination of many excellent properties including a high melting point, high anti-creeping strength, high elastic modulus, high resistance to radiation, and a good thermionic emission work function [3,4]. These make W single crystals an ideal material for the functional layers of the cathode.…”

Section: Introductionmentioning

confidence: 99%

On the tungsten single crystal coatings achieved by chemical vapor transportation deposition

Shi

Shen

Yao

et al. 2016

Materials Characterization

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The tungsten single crystal has many excellent properties, namely a high melting point, high anti-creeping strength. Chemical vapor transportation deposition (CVTD) is a possible approach to achieve large-sized W single crystals for high-temperature application such as the cathode of a thermionic energy converter. In this work, CVTD W coatings were deposited on the monocrystalline molybdenum substrate (a tube with <111> axial crystalline orientation) using WCl6 as a transport medium. The microstructures of the coatings were investigated by a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The as-deposited coatings are hexagonal prisms--rough surfaces perpendicular to <110> with alternating hill-like bulges and pits at the side edges of the prisms, and flat surfaces perpendicular to <112> with arc-shaped terraces at the side faces. This can be explained by the two-dimensional nucleation -mediated lateral growth model. Some parts of the coatings contain hillocks of an exotic morphology (noted as "abnormal growth"). The authors hypothesize that the abnormal growth is likely caused by the defects of the Mo substrate, which facilitate W nucleation sites, cause orientation difference, and may even form boundaries in the coatings. A dislocation density of 10 6 to 10 7 (counts/cm 2 ) was revealed by an etch-pit method and the synchrotron X-ray diffraction. As the depositing temperature rises, the dislocation density decreases, and no sub-boundaries are found on samples deposited over 1300°C, as a result of the atom diffusion and the dislocation climbing.

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Promising variants of single crystal tungsten electrodes for high-performance thermionic converters

Cited by 7 publications

References 1 publication

Deposition temperature effects on tungsten single-crystal layer by chemical vapor transport

Deposition temperature effects on tungsten single-crystal layer by chemical vapor transport

Microstructures and Kinetics of Tungsten Coating Deposited by Chemical Vapor Transport

On the tungsten single crystal coatings achieved by chemical vapor transportation deposition

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