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Re: inductive heaters
In a message dated 96-04-20 17:20:50 EDT, you write:
>g though (maybe this is a real dumb question, but I'm not
>seeing it): why not process the electrodes before heating up the unit in
>the oven? Sure, the emitter coating will burn off and the resulting
>particles will leave the emitter and then grab on to the inside surface
>of the tube somewhere. Will they not ultimately be released when the
>unit is heated in the oven, and everything is then fine? It would seem
>to me this would have to work, otherwise normal electrical bombarding
>would suffer from the coating particles adhering to the inside of
>the tube when the electrodes are heated up (because the rest of the
>glass is still at approx. 500F degrees, where it would be in the oven).
>
>
Emitter coating is a calcined, eutectic mixture of BaO2 with other rare earth
oxides, such as SrO2, Al2O3, members of the second periodic column, which are
instantly "delequatious" or water absorbing. They are shipped as the
carbonate form, in other words, Ba(CO3)2, which is what the BaO2 turns into
if exposed to air even for a few seconds. You must evacuate them and heat
them up, that "converts" then to the usable oxide form, releasing CO2 gas
into the vacuum, converting to the BaO2/Sr2O3 etc stuff (there are a lot of
formulations) which is the electron emissive substance. If you bring them
back up to air again, the whole thing has to be done again. This is easily
found if you look at the pressure rise from the electrodes on a vacuum gauge
when induction heating them. In fact, they even re-absorb the outgassing
products from the glass upon heating if the electrodes are not kept hot at
the same time, at least at the end of processing. Blasting the powder out of
the electrode is not the purpose of anything, that's a nasty thing to do and
will result in early tube failure. You need the powder right where it is for
the tube to work right. Any kind of heating will do the trick as long as it's
not so violent that it ejects the powder from the shell.
Sounds like you had a lot of misconceptions about what is happening during
bombarding.
Jeff Golin