>There was a comment a while back about emission from a tungsten-thoria
>filament being the result of carburization. I had been under the impression
>that the thorium was used to enhance emission. And that as it evaporated,
>the emission would decrease. Well, here's the answer from Eimac's Web site:
>
>"By proper processing during vacuum pumping of the tube envelope, the
>metallic thorium is brought to the surface of the filament wire, and emission
>increases approximately 1000 times. The thoriated tungsten filament is also
carburized.
>The small amount of tungsten carbide formed in the carburizing process
>reduces the evaporation rate of the thorium and thus increases the life of the
>filament."
>
As it was explained to me, when Econco Broadcast Services (916-662-4495)
rebuilds a thoriated-tungsten filament tube, No thorium is used to
restore emission -- no matter how many times the tube is rebuilt.
Acetylene gas (C^2 H^2) is the sole agent. When tungsten/1.5%-thorium
alloy is heated to c.1800 deg K in the presence of acetylene, carbon
atoms from the acetylene molecule bond with tungsten atoms to form a
tungsten-carbide layer at the surface, leaving hydrogen gas byproduct.
During recarburization, a continual input of acetylene brings in a new
supply of carbon atoms and forces out hydrogen.
- - How does one determine how much tungsten carbide has been formed?:
. Tungsten is a better electrical conductor than tungsten-carbide, so
one can tell how thick the tungsten-carbide layer is by measuring the
decrease in filament current as the process takes place. When the
layer's thickness is determined to be sufficient, acetylene flow is
stopped, the tube is pumped to a high vacuum and resealed. With careful
control of fil. potential, emissive life is typically c.20k hours between
recarburizations.
>FYI, the URL is: http://www.eimac.com/cf.htm
>
>73's
>Chris, KF0FX
>KF0FX@aol.com
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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