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Re: How to measure the temperature of tungsten wire?

Posted: Sat Jun 04, 2016 5:26 am
by Jeroen Vriesman
Thank you Roberto! (and the rest).

One question, benzene or toluene is used as the hydrocarbon for carburization, both are a bit toxic.

Is there any reason why something like butane would not work? The temperature seems high enough to break down any hydrocarbon.
Or is does the reaction require double bindings to form tungsten carbide? Would ethylene work?

And just to completely understand the process:

a) Is the carbon also reducing the ThO to Th, so the layer of tungsten carbide is not the only reason for carburization, it also provides a bigger reservoir of Th?
b) Is a short reduction cycle with H2 after the carburization useful? From some texts it seems that on top of the WC+Th layer a thin layer of W + Th is formed by reduction with H2, but Roberto's procedure doesn't include that step.

Re: How to measure the temperature of tungsten wire?

Posted: Sat Jun 04, 2016 6:55 am
by Roberto Ferrari
Jeroen

The carbide layer will keep the Th in the filament, reducing its evaporation, no higher reservoir.

I never learned of using gaseous hydrocarbons but they may work. Is just a little more cumbersome to handle them. Try xylene, easy and low toxicity.

I never decarburized and don't see the need of. The chemistry is complex, several alternative carbides can be deposited. Horsting talk about them, related to C6H6 concentration.

See two more papers.

Re: How to measure the temperature of tungsten wire?

Posted: Mon Jun 06, 2016 10:07 pm
by Bruce Meagher
There’s lots of great info in this thread. One of the things I learned in George’s vacuum tube workshop is making temperature measurements of fine filaments using a micro optical pyrometer (like Rich and Frank mentioned). These routinely come up on eBay such as this link, and can often be had for a reasonable price. You need to correct for the absorption of the glass, and for the emissivity as described in figure 10 of the Harbaugh paper Roberto posted, to get an approximate absolute temperature.

I’ve been activating Th/W filaments in some diy triodes, but have not been able to get to a vacuum level where the Th level emission lasts but for a few moments. Also see viewtopic.php?f=10&t=9810&p=66058&hilit ... ion#p66058

Below are a couple pics of my setup looking at a halogen light filament (excuse the poor picture quality through the eye piece).

I look forward to hearing about your progress.

Bruce

Re: How to measure the temperature of tungsten wire?

Posted: Tue Jun 07, 2016 2:02 pm
by Roberto Ferrari
Hi Bruce

Let us know how is your vacuum system.
Do you use some getter?

Re: How to measure the temperature of tungsten wire?

Posted: Wed Jun 08, 2016 2:55 pm
by Bruce Meagher
Roberto,

My first triode (the pickle triode) was continuously pumped with a mechanical pump (as a proof of concept). To my surprise it worked, and I even observed increase emission after activating the filament (even for just a brief time). I’m currently using a cryo trapped, turbo pump based, vacuum setup where the tube bases are epoxy to the envelope, baked, getter flashed, and then tipped off under vacuum. Th/W activation is more of a curiosity right now, and just learning practical tube building techniques is more my focus. There is a new forum http://diyvacuumtubes.com dedicated to home built vacuum tubes. Discussion about the vacuum setup might be more appropriate for that forum.

Bruce

Re: How to measure the temperature of tungsten wire?

Posted: Wed Jun 08, 2016 4:43 pm
by Roberto Ferrari
Right!
"See" you there...