Burning (out) vacuum evap

Tales of woe that teach. We learn best through failure sometimes
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This is for tales of woe that teach. We often learn quickest through failure that shows us how our thinking was off, and of course, sometimes it's funny at least in hindsight. Share your "Doh!" moments here.

Burning (out) vacuum evap

Postby Doug Coulter » Mon Dec 26, 2011 9:46 pm

This worked eventually, but not this time. I made an evap fixture out of .020 tantalum wire and .032" titanium wire, the sizes I had, figuring the conductivity of the Ti is a lot less and it would maybe be ok.
Short answer, no. Liquid Ti dissolves Ta, collects into blobs...hot spots.
At least the bracket I made was a good idea. I put a little slot in the flattened Cu tubing so it could pump out and burnished it outside.
GndnHtr.jpg
First try heater and bracket.

Note adaptations for ship-in-bottle - I have difficulty seeing with one arm or working with both through a 6" hole - two feet away from the hole. There's a tapped hole in the big flange for that 90 deg to get into - and that bolt is nutted on so I can screw it in with the tubing - no screwdriver trying to start that screw. After its in place, I can bend it around to catch the ground end of the heater. The hot end is pre-captured in another nutted bolt to catch the end of the regular HV feedthrough. The idea is to hold the grid about where a fusor grid would be. Since this is going to make nasty metal fumes I don't want quite everywhere, I punched a 2" dia piece of mica to cover the end of the feedthrough, and taped (gasp) another 6x7" piece over the whole affair - I only want metal deposited in the sidearm.
HtrnMica.jpg
Installed almost

Mica.jpg
Gasp - tape in vacuum system! Not so bad, it's 600f rated stuff and won't get hot in this use anyway.


So - I'd tried to clip a bit of Pd foil on this after putting it in. Bad idea - failed several times, and then the one that stuck fell off during pumpdown...
And now, for the movie. Postmortem to follow.

http://www.youtube.com/watch?v=R9zefY3H320



So, I got a little enthusiastic on the variac and fried the Ta heater after the Ti melted. It bunched up into blobs, shorting out major parts of the Ta wire even though it's less conductive - at the same thickness.
So, the Ta burned up in one of the resulting hotspots. Here's a couple pix to show the effect. Evidently Ta is somewhat soluble in Ti when things are clean (they were, vacuum popped right down to a couple e-6 mbar in about 5 min) - which certainly didn't help - you can see the thinned Ta next to a blob of Ti in the shabby microscope photo.

BurntHtr.jpg
Heater pieces

MicroBurnout.jpg
Microscope pic


Well, knowing what the problem was - nothing beats a failure like another try - so I made a new heater, and this one worked fine, subject of another thread.
NewHrt.jpg
New one - 3 Ta wires, one Ti, and this time put on the Pd before putting it in the tank so I can make it stay.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
 
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Location: Floyd county, VA, USA

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