Chemical vapor deposition of thoriated tungsten protective cups Page: 17 of 45
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3. PROCESS DEVELOPMENT AND DISCUSSION
Tungsten may be chemically vapor deposited from any one of 3 gaseous systems,
namely, the hexafluoride, the pentachloride, or the carbonyl. All of these systems require
the use of hydrogen as the reducing gas, depositing the tungsten on a hot mandrel.
For this program, tungsten hexafluoride was selected as the metal reactant primarily
because it is available in a very pure form and has an excellent deposition rate of about
0.0005 inch/minute of tungsten at a relatively low temperature of 6000C. In contrast,
deposition of tungsten from the chloride is usually conducted at 10000C, and the carbonyl
results in the formation of a high carbide tungsten product.
Tungsten metal is deposited from the hexafluoride onto a substrate by the following
(1) WF6 + 3H2 6000 W + 6HF
There are few compounds of thorium which have the necessary combination of vapor
pressure and chemical stability to facilitate chemical vapor deposition. Of the compounds,
thoria from thorium acetylacetonate and powdered thoria (1/2 micron diameter) appeared to
be most compatible for co-depositing with the tungsten hexafluoride system.
The powdered thoria was entrained into the deposition gas stream using an argon
carrier gas prior to entering the reactant chamber. This carburetor-type entrainment method
proved most difficult to control, and it was not possible to obtain a constant thoria flow rate.
This non-uniform dispersion of thoria in the tungsten was of a magnitude that a non-uniform
recrystallized grain structure resulted as shown in Figure 2.
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Blay, J.S. Chemical vapor deposition of thoriated tungsten protective cups, report, January 1, 1968; Pittsburgh, Pennsylvania. (https://digital.library.unt.edu/ark:/67531/metadc1026564/m1/17/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.