lahthffire
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Tantalum - undiscoloration?
Suppose you had some tantalum that was in contact with some molybdenum and you put it in a vacuum furnace and heated it up real hot. Now suppose the
high vacuum part of your vacuum system had some issue that caused it to not work as well as usual so the ultimate vacuum was somewhere between normal
roughing pump vacuum and high vacuum. Now you have some slight discoloration on the tantalum where it was touching the molybdenum. It looks kind of
bluish - kind of like some oxide of molybdenum, or maybe lightwave interference caused by a thin Ta2O5 layer.
1. What is the most likely cause of the discoloration?
2. What do you think would be a suitable chemical (non abrasive/physical) method of removing it?
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12AX7
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Depending on vapor pressure of MoOx's and distance, I would guess it's either vaporized or diffused onto the Ta. Problem with diffusion is that would
require a very tight fit, and I don't know of your furnace even got hot enough to diffuse anything.
Hydrofluoric acid ought to etch the Ta and clean it, but I don't know if you'd like to mess with that...
Heh, on the subject of vacuum furnaces, at work they have several nice sized vacuum furnaces for heat treating. A good bunch of kilowatts (think 460V
three phase rated a few hundred amps), and the diffusion pump is about two feet in diameter. We've (maintainance) been having problems with coolant
flow, probably a blockage somewhere in the house system...
Tim
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not_important
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If it is just near the Mo, the it is moly oxides, hard to tell which or if it could be a mix. Molybdenum oxidises fairly readly when hot.
Polishing is the simplest way to get it off, given that it would be tough to say exactly which oxide(s) it is.
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The_deadly_dustbin
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Heating under a flow of hydrogen should reduce the MoxOy fairly rapidly to the element and water.
Done routinely in the lighting industry. Heating to approx. 1200° to 1400° C for a couple of seconds should suffice.
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lahthffire
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I've looked at one of the parts under a microscope, and I'm thinking it is some kind of Mo-oxide compound. The colored material is composed of tiny
blue granules sitting on top of the silver colored tantalum. So does anybody know how to dissolve MoO2? (I believe that's the blue molybdenum oxide,
although there are more complicated Mo structures that are blue as well).
Unfortunately I can't readily achieve 1200° to 1400° C at the moment, and besides, tantalum has hydrogen embrittlement problems. HF doesn't sound
like much fun, either, and I don't have any on hand.
2 ft. diameter diffusion pump = huge! I'm more used to ~8 in.
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lahthffire
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These guys list a tantalum cleaning procedure as:
Tantalum parts must not be cleaned by hydrogen firing. Cleaning and degreasing present no special problems and conventional methods and materials may
be used. However, note that hot caustics must be avoided. Electronic tube parts that must be chemically cleaned require more careful treatment.
The first step for tantalum parts that have been blasted with steel grit is to immerse the parts in hot hydrochloric acid to remove particles of iron.
The hydrochloric acid may be used as hot and as strong a solution as desired; it will not attack the tantalum. The parts should then be thoroughly
rinsed with distilled water. Tap water often contains calcium salts that may be converted to insoluble sulfates in the subsequent cleaning process. If
the tantalum parts have not been grit-blasted, the hydrochloric acid cleaning may begin with the second step below.
The second step is a chemical cleaning process. A hot chromic acid cleaning solution, commonly used for cleaning glass, may be applied. A saturated
solution of potassium dichromate in hot concentrated sulfuric acid may be used for this purpose. However, chromium trioxide is preferred to potassium
dichromate because its use eliminates the possibility of potassium residues in crevices or elsewhere on the tantalum parts. The cleaning solution
should be applied at approximately 110'C and should maintain its red color at all times. When the liquid becomes muddy or turns green, it should be
discarded.
After the chromic acid wash, a third step is applied to rinse the parts. The preferred rinse is hot distilled water. If running distilled water is not
available, three lip washes will suffice, but it is important that all cleaning solution be removed. The parts should be dried in clean, warm air,
free from dust. The parts should not be wiped with paper or cloth and they should not be handled with fingers.
Is the hot chromic acid solution likely to remove the Mo oxide?
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not_important
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Try a solution of "ammonium carbonate" with a little H2O2.
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