Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: QUESTION: metal to glass seals and teflon
Date: Nov 14, 8:38 am
Poster: Richard Hull

On Nov 14, 8:38 am, Richard Hull wrote:

> Just a quick question, what is the metal alloy used in vacuum tubes for the pins, that bonds to the glass??. Is it similar to invar with a low expansion co-efficient???.


The metal is often and invar or kovar in expensive tubes with large leads. Some smaller tubes can us coated or treated stainless. Graded Seals of Uranium glass to the normal tube glass are often used as well.

Our own group's, Tim Raney, here in Richmond, who works with making tubes all the time, prefers nitrite treated .040 tungsten rod stock for thru glass connections. He preps a few ready for use normally with a bead of glass already on the leadin. From there it is a matter of just blowing a small hole in you tube and merging the two glasses together. He is pretty good at it, too.
A skill I wouldn't mind acquiring. I just have to pujt my foot in the path though. RH

>Finally how usefule is teflon for vacuum seals??? I realise it kind of flows under compression, but would it work if it was a fat, square cross section 'O' ring and located in grooves?.

Teflon is porous! I have used it at low vacuums, but it is uncertain at best. I used it as the main feed through seals for the HV lead on my fusor II and it worked well enough, but I wouldn't attempt its use on a neutron producing fusor where the conditions are a bit more severe. RH

>I have a book on aluminium coatings on glass which requires a vacuum of high quality and they are using stuff like waxes for the bell jar seal, also i have read of vacuum epoxies used in laser tubes with helium, would it be suitable???

Yes! There are a whole range of modern and older waxes, greases, and epoxies which are quite servicable in moderate vacuums. I have some apiezon grease which is the best they make and has a vapor pressure above 10e-6 torr. The stuff is $105.00 for a one oz. tube! All of these materials are expensive and should never wind up in or near hot, heated or beam bombarded components. They will outgass like crazy!

Used wisely, these sealing materials can be life savers that allow folks without welders, glass working skills or the money to purchase high end seals to do good work in medium to low vacuums.

Ricahrd Hull