Aluminum wire evaporation problem.

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Umit demir
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Aluminum wire evaporation problem.

Post by Umit demir »

Hello everyone, I made a thermal evaporator. However, I have one question. The machine melts the aluminum wire. Everything is normal until this time. As the aluminum wire evaporates, the vacuum value rises rapidly. About 4 seconds. And after it reaches a value, the vacuum rise stops. And the vacuum value drops again at the same speed. Therefore, the coating quality becomes dark. I need advice for this.
JoeBallantyne
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Re: Aluminum wire evaporation problem.

Post by JoeBallantyne »

It's not clear from your post what material you are trying to evaporate. Do you want the aluminum wire to evaporate, or are you trying to evaporate some other material using the aluminum wire as a heater. I'm pretty sure most evaporators use tungsten or some other material with extremely high melting point/vaporization point for their boats that are used to heat the material being evaporated.
Umit demir
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Re: Aluminum wire evaporation problem.

Post by Umit demir »

Of course I use tungsten wire to melt the aluminum.
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Richard Hull
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Re: Aluminum wire evaporation problem.

Post by Richard Hull »

Remember, Tungsten has a water cycle issue, If the tungsten is not first brought to near incandesce and then 9999 or better aluminum is melted onto the tungsten you might create an oxide of tungsten. You do not state the depth of your vacuum but I would think a minimum of 10e-4 torr would be needed. A single pump will not do. However, most successful coaters always use a cold trap of some sort to condense out water and oil vapor.

Detailed info regarding your system and operational technique would be nice. Note: While some of us have evaporatively coated. This is a fusion focused site and the vacuum forum focuses on fusion requirements. Have you tried contacting the Bell Jar web site on this topic. Steve Hanson is the expert in such issues. The bell jar deal solely with vacuum only focused projects.

Richard Hull
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Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Umit demir
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Re: Aluminum wire evaporation problem.

Post by Umit demir »

The system I produced is large in size. The issue I noticed is this: During the evaporation of the aluminum wire, intense gas is produced in the environment. The diffusion pump, on the other hand, cannot keep up with the intense gas output. Could this problem occur because the nozzle gaps of the christmas tree inside the diffusion pump are wide? Can I have your comments on this?
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Richard Hull
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Re: Aluminum wire evaporation problem.

Post by Richard Hull »

The diff pump is what it is. You say your system is large. What is the pumping rate of the diff pump? The evaporation rate must be controlled based on the possible outgassing rate and the diff pump's ability to keep the pressure at a suitable evaporation level.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Rich Feldman
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Re: Aluminum wire evaporation problem.

Post by Rich Feldman »

I remember aluminum evaporation from college lab, in a bell jar a little bigger than 12 inches diameter,
for depositing conductive metal film on semiconductor wafers. Two stage pump, with manually operated valve opened after rough evacuation of chamber.

Not my expertise, but could the gas load in Umit's experiment be molecules originally dissolved in the solid aluminum?
Or molecules originally stuck to chamber walls, dislodged when bombarded by "hot" aluminum atoms?
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Jerry Biehler
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Re: Aluminum wire evaporation problem.

Post by Jerry Biehler »

Sounds like you either have a poor vacuum or not using pure aluminum.

What pressure are you at?
John Futter
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Re: Aluminum wire evaporation problem.

Post by John Futter »

aluminium evaporation usually needs a cold finger cooled with nitrogen to remove oxygen from the chamber.
the pressure rise problem can be overcome by using a shutter between the aluminium boat and the piece being coated. Get the boat to temperature and wait until the pressure drops back and open shutter
aluminium is a premium getter of oxygen while it is a vapour... also are you using pure aluminium not bits cut from sheet that contains silicon that will come out dark
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Richard Hull
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Re: Aluminum wire evaporation problem.

Post by Richard Hull »

I obtained 99996, (4n6), Aluminum bar stock in the late 90's for free! I tried to machine it and it was like machining play-dough or bubble gum. I sold what I had to a friend in the early 2000's at one of the HEAS events here who used it in his evaporation efforts . I have been told by those in the pure metals biz that the Japanese can take Aluminum to 999999, (6n0 as spoken in the metals biz), provided you have a few kilo bucks per kilogram. Sold in vacuumed out and argon refilled HDPE containers. I figure once carelessly opened it is back to (5n?) and if not baby handled after that the 5n rating will drop off all over the surface to some molecular depth.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
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Re: Aluminum wire evaporation problem.

Post by Frank Sanns »

It is definitely an air problem (oxygen). Just like other wires that were smoking and people thought might have been oil, it is oxides coming off.

The diffusion pump may be on but my guess is that the vacuum is no better than 10E-2 torr.
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Dennis P Brown
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Re: Aluminum wire evaporation problem.

Post by Dennis P Brown »

I evaporate aluminum on a regular bases. Yes, I get a pressure rise but I start at mid range of 10^-6 torr in a fairly large bell jar. One should not evaporate much above 10^-4 torr for Al and lower is far better. My films are of high quality. I use simple off-the-store-self aluminum foil, no less. The key is starting with and roughly maintaining the high vacuum. Dark films mean too much oxygen is getting in. I simply wrap the Al foil on a tungsten wire and heat via current.

I assume you have at least a baffle for the diffusion pump. I use a turbo so my system is clean (and only used for metal coatings.)
Umit demir
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Re: Aluminum wire evaporation problem.

Post by Umit demir »

I found the source of the problem. The intervals of the copper cooler on the diffusion pump are more frequent than they should be. Therefore, the gas released during evaporation is suffocated.
Gustavo Cervantes
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Re: Aluminum wire evaporation problem.

Post by Gustavo Cervantes »

So it was a pumping problem after all?
In my experience aluminum purity is not a big deal unless you have a very demanding application, i have used comercial aluminum foil with good enough results, since its over 99% for health reasons it can be used to do coatings for telescope mirrors, i have changed to high purity aluminum after that.

Outgassing on the first seconds of the filament being turned on is really noticeable, if the pump has a low pumping speed you might have issues, but it should not be that big of a deal if you have a shutter to block aluminum coating during this time.
Umit demir
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Re: Aluminum wire evaporation problem.

Post by Umit demir »

What kind of blind are you talking about? Can you give details?
Aleksa Djokic
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Re: Aluminum wire evaporation problem.

Post by Aleksa Djokic »

Who said Blind? You mean the shutter? Shutter is a piece of flat sheet metal that sits just above the boat (resistive boat)and restricts the flow of metal vapor until the outgassing/cleaning has settled down. I still didn't understand what the problem was in the first place, what kind of copper cooler are you mentioning?
Gustavo Cervantes
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Re: Aluminum wire evaporation problem.

Post by Gustavo Cervantes »

A photo of your setup would be useful if you require more help
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