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The average weight loss for the three samples was 21 x 10-6 grams;
thus the per cent weight loss for the samples during the 100-hr test at 1200*F
is 0.007. Under these conditions, it would take 160 yr for the 0 001-in.-thick
foil to completely sublime. The experimental test and the theory are in good
accord.
3. Composite Low-Emittance Coating System
The low-emittance coating (E < 0.05) will operate in a space environment
for 1 yr in the temperature range of 850 to 1250*F. At these temperatures,
gold and rhodium exhibit the lowest emittance of the oxidation resistant metals.
When the gold or rhodium is in intimate contact with a metallic substrate such
as SS, operation of this composite system at temperatures greater than 850*F
results in rapid diffusion of the stainless constituents through the gold or
rhodium layer.
Previous discussion has shown that gold or gold-molybdenum foils on SS
offer a satisfactory radiation control surface. This approach, while suitable,
creates problems in component assembly and handling; thus, a major effort con-
tinues in developing an integral, composite low-emittance coating system. Vit-
reous ceramic diffusion barriers interposed between SS and a Liquid Brite gold
film appear effective. This composite low-emittance coating system has demon-
strated high-temperature stability. However, the application temperature re-
quirements of the vitreous ceramic to the SS (1700-2000*F) make this approach
unattractive for any component or piping process procedure.
In general, metals have much lower emittances than non-metallic com-
pounds such as oxides. At high operating temperatures, the emittance of metal-
lic coatings increases markedly if they have a tendency to oxidize. Since the
SNAP system components undergo checkout at design temperatures in soft
vacuums, this fact effectively limits the choice of a low-emittance control sur-
face to gold, rhodium, palladium, or platinum.
In the temperature range of 800 to 1250*F, gold has the lowest emittance
(0.04 to 0.05) of these oxidation resistant metals. At 1000*F, the emittance of
palladium is 0.08 and the emittance of platinum is 0.10. Rhodium (E = 0.06 at
1000*F) is a secondary choice to gold.
Most data on diffusion of solids take into consideration only the diffusion
of ions in ionic crystals or of metals in metals. Comparatively little data is
NAA-SR-9908
45
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Crosby, J. R. The Development and Qualification of Thermal Control Coatings for Snap Systems, report, September 30, 1965; Canoga Park, California. (https://digital.library.unt.edu/ark:/67531/metadc1028869/m1/49/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.