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

Journal Article · Thu Dec 15 00:00:00 EST 2016 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2016.10.016· OSTI ID:22689665

Shi, J. Q.; Shen, Y. B.; Yao, S. Y.; Zhang, P. J.; Zhou, Q.; Guo, Y. Z. ^[1]; Yu, X. D. ^[1]; Nie, Z. H. ^[1]; Ma, H. L. ^[2]; Cai, H. N. ^[1]

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1. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081 (China)
2. China Astronaut Research and Training Center, Beijing 100094 (China)

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 WCl{sub 6} 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 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{sup 6} to 10{sup 7} (counts/cm{sup 2}) was revealed by an etch-pit method and 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 atom diffusion and dislocation climbing. - Highlights: •The varied growth rate causes the different morphologies of different planes. •The W coating is a single crystal when only single hillocks appear. •The (110) plane tends to have the lowest dislocation density. •The dislocation density tends to decrease as the temperature increases.

• OSTI ID: 22689665
• Journal Information: Materials Characterization, Vol. 122; Other Information: Copyright (c) 2016 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 1044-5803
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
BACKSCATTERING
CHEMICAL VAPOR DEPOSITION
COATINGS
DEPOSITION
DISLOCATIONS
ELECTRON DIFFRACTION
MELTING POINTS
MICROSTRUCTURE
MOLYBDENUM
MONOCRYSTALS
MORPHOLOGY
NUCLEATION
SCANNING ELECTRON MICROSCOPY
SPECTROSCOPY
SUBSTRATES
SYNCHROTRON RADIATION
TEMPERATURE RANGE 0400-1000 K
TUNGSTEN
TUNGSTEN CHLORIDES
X-RAY DIFFRACTION