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Physics Forum Q&A (an excerpt)

QUESTION: “I recently had some discussions about the use of aluminum in a ultra-high vacuum (UHV) apparatus. There seem to be those who believe Al is good in UHV and those who believe it is bad. I was wondering if anyone has some authoritative source for information on this…”

ANSWER: “I’ve researched this topic pretty extensively for some UHV projects I’d like to work on. I want to use Al it’s much cheaper and easier to machine than stainless steel, I have access to tons of it, and I don’t have tons of money to throw at buying stainless steel UHV components.

Aluminum is actually considered to be one of the best materials available for UHV applications if it is used properly. Reasons being that it’s extremely easy to machine (cost), it has a much higher thermal conductivity than stainless steel (reducing bakeout time/temperature), low nuclear activation (no contamination issues in nuclear/particle applications), and can have lower outgassing rates if properly prepared. The troubles are that you need to do surface preparation for low outgassing, aluminum is not easy to weld to UHV standards, and it’s not hard enough to compress standard Conflat UHV flanges.

There are ways around all of these troubles. Aluminum’s typical high outgassing rate is due to a porous Al2O3 layer which forms on exposure to air during the casting or extrusion process. This can be etched away chemically, allowing a very dense oxide layer to form which doesn’t create trapped volumes and decreases outgassing by orders of magnitude. I know that accelerators at Brookhaven National Lab used an etching protocol based on strong acids. However, I’ve also seen papers where people have used pretty common, cheap detergents to do the oxide layer removal. Can’t remember the chemicals, but papers are out there if you look hard enough.

Welding is also doable. However, the only way I’ve heard of to get a UHV tight weld is to use special protocols based on TIG welding. Anyone who has ever welded aluminum can tell you it’s not easy. Getting a UHV weld is even tougher. But there are companies out there that routinely do this, so I have to imagine that you could eventually get it right with the proper amount of practice.

Final problem is creating a UHV flange from aluminum components. Typical UHV flange uses knife edges made out of stainless steel to bite into and compress a copper gasket. Aluminum is not hard enough to bite into copper, so it can’t be used with Conflat flanges as is. Again, there are ways around it. Some people coat the aluminum knife edge with a hard material such as TiN (by vapor deposition) or nickel (by electroless plating). This hardens the knife edge enough to make it possible to bite into special aluminum gaskets which are unfortunately much more expensive than copper gaskets. The price difference is something like 10-20 times as expensive for aluminum gasket versus copper gasket. There are also compression fittings based on ISO flanges that can achieve a UHV seal with aluminum O-rings. Instead of the ISO wingnut fitting, they use chain clamps to provide lots of force evenly across a flange connection. Since they’re not routinely used (except by Japanese semiconductor equipment manufacturers), the gaskets and chain clamps are not cheap. However, they allow UHV gaskets to be re-used, so they might be nice. Also, they can be used with ISO flanges which are much cheaper (or easier to make if you’re doing it that way) than Conflat flanges. The final solution is to use an aluminum to stainless steel transition on the flange, so that the bulk of the flange is aluminum, but the knife edge made out of stainless steel is capable of forming a standard Conflat UHV seal. These flanges are commercially available. If you were looking into making things for yourself, you might be able to find plates of the transition metal available. It’s probably not cheap, as the joints are made by so-called explosive bonding methods.”

Submitted by D.K. and reposted here

The author goes on to suggest checking out, among a few others, Atlas Technologies for chambers and flanges. He gives the home page and also the Atlas Technologies bibliography page:

By the way, Atlas is home to an amazing welder who has mastered the Art of Aluminum.

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