[NewCandle] Vacuum, outgassing, etc.

[John Winterflood]

Kyle wrote:
> Anyone here with experience working with vacuum?
Not much actual experience, but I've picked up plenty of talk from the 
guys that do.
> 1. Anything good to seal (semi-permanently, should hold 0.1 torr for 
> at least a few months) metal to glass? I am no glassblower, so I'd 
> prefer not to go down that route.
If you really mean 0.1 torr, then that is hardly a vacuum at all.  A 
standard backing pump alone will do a couple of orders of magnitude 
better than that.  I am sure that a simple little nylon ball valve (as 
used in water lines for instance) would be able to seal to that level 
for a reasonable amount of time.
> 2. What metals outgas the LEAST in this moderate vacuum environment?
Metal outgassing should no way be detectable in such a high partial 
atmospheric pressure!

Even for vacuums as good as 10^-6 torr, you can get away with rubber 
o-ring seals.  It is only when you want to get to the 10^-8 to 10^-10 
sort of level that you have to worry about what sort of metal is used.
> I am capable of pulling a decent vacuum for making homebrew Crookes 
> tubes, and have built quite a few. But I'd like to be able to seal it 
> off from the pump, and have it standalone. I did this a short time ago 
> (read: 1 hour) and the tube still works. Seal off was made by heating 
> a glass tube *epoxied*
If you are want to get a good vacuum, you should be rather careful of 
things like plastics and epoxies.  Basically anything that you can smell 
(and I can smell araldite) has got to be pretty bad in a vacuum!  
Plastics are far worse than any sort of metal.  But at the 10^-1 torr 
level, I doubt if anything much matters!
> to the side of the bottle where a hole was drilled. The tube softened 
> and collapsed, at which point I used the flame end to narrow the 
> dimple further, so I could snap it off. The tube, so far, holds vacuum 
> and makes a nice electron beam.
>
> Are there epoxies or other substances (what did they used back in 
> Crookes' time??) to seal a 0.1 torr vacuum in?
>
> The cathode is brass, the anode copper. I seem to remember reading 
> that they were bad choices. Oops. What's good to use?
I believe brass is not so good because it contains zinc - which has a 
"high" vapor pressure.  But "high" being a very relative term - have you 
ever smelt the galvanizing on a hot piece of corrugated iron - I doubt 
it!  In high vacuum applications (high vacuum meaning low pressure) the 
equipment needs to be heated up as hot as possible - often enough to 
blacken the surface of the metal - in order to get as much outgassing as 
possible to happen in a hurry (hours or days).  Under these conditions 
the zinc will actually sublimate to a gas to some extent and this can 
produce problems in some cases.

We have managed to make high vacuum seal-offs which have never (to the 
best of my knowledge) leaked, by finally pumping through a fairly small 
(say 1/4") copper tube and then cutting the tube off with a pair of 
modified bolt cutters.  The bolt cutters are modified by grinding away 
the cutting edges of the jaws and welding or brazing a pair of short 
round rods made of hardened steel in their place.  These rounded jaws 
then squash the the copper tube completely to form a perfect seal, 
eventually cutting right through it so that the end being sucked on 
separates from the piece remaining on the apparatus.  The same tube can 
be cut off a bit shorter to re-open it and the process repeated several 
times until it is too short.  Then a new long piece of copper tube can 
be brazed into its place to start the process again.  Joining the copper 
tube on with a metal olive type of compression fitting also works just 
fine and saves the brazing business.

Nick wrote:
> If you are not up for experimenting with graded glass to metal seals, you could do a couple of things - you could get some ready made lectrode seals at a neon sign shop, or you could use Torr-Seal which is a vacuum epoxy.  You can get that from Kurt J. Lesker Co.  KJL is the Radio Shack of vacuum - a bit pricier than some places but always there in a pinch.  Been using them for 25 years.
>   
Yes Torr-Seal is good stuff, I have seen it used a lot.
> Tungsten, stainless steel, brass, aluminum, zirconium, molybdenum - good.  Zinc, cadmium, tin, lead - bad.  Copper is meh.
>   
Sounds about right for high vacuum, but hardly anything matters at 0.1 torr!

John