finishing Ti surfaces for UHV 2

[meekg]

Anyone familiar with best practices for finishing Ti surfaces for a UHV enclosure?
Mechanical polishing to what level?
any chemical or electro-chemical processes?

 

[vacuumsage]

It really depends on the level of vacuum you want to achieve. You specify UHV so I am assuming 10E-9 torr or better. Cleanliness is essential, no oil or grease is permissible of course, so a thorough hot degrease would be good.

Polishing is essential on sealing surfaces. O ring seals work fine on 32 microinch surfaces, but you might want to improve on that for any metal wire seals you might be using.

Theoretically, the smoother the surface finish, the better it is for vacuum, since less surface area means less gas to desorb, just make sure you don't force polishing compounds into the surface. I have always found machined surfaces to work just fine.

I have never made a vacuum chamber from Titanium, stainless steel has always been an appropriate material for my systems, but I don't see any reason Ti wouldn't work.

 

[meekg]

my omission.
The target base pressure is 1E-7 Torr.

The concern is about the total quantity absorbed in the walls (as opposed to the outgassing rate), since after initial pump-down to 1E-3 torr, only a getter is available to pump it down to and maintain to 1E-7 torr.

I guess the question was more about the process itself - what is achievable using common mechanical and electro-chemical processes.

 

[vacuumsage]

I have been giving this some thought, and I am concerned about the ability of a getter alone to reduce the pressure from the 1E-3 range to 1E-7. I have never tried that myself, though I have used getters to maintain that level of vacuum once pumped down by other means. However I don't have any hard and fast data to say it can't be done, and anyway, I don't like saying "can't be done". (Too defeatist).

For cleaning, I would certainly proceed with a hot degrease (in isopropyl alcohol or similar), followed by an acid clean. This type of pre clean is used in the vacuum tube industry, (travelling wave tubes, magnetrons and so on)where construction of devices from copper, nickel and stainless steel is common. You could try an electropolish, but it is the removal of grease, dirt and oil that is most critical. I would very much like to know how you get on with the getter, if you are able to divulge any info. If I manage tyo dig up any info on similar things, I will forward it. Good luck!

 

[AndyO1]

A paper 'Vacuum characteristics of titanium' from J.Vac.Sci.Technol.A13(3) May/June 1995 agrees with the cleaning process mentioned before and adds a bakeout of 250 C, a common practice in stn. stl. UHV systems...As fas a getters go my exp. is ion pumps and Ti Sublimation pumps for UHV. Conclusion from paper is that Ti has excellent characteristics.