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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Be0 is apparently only toxic at or below a certain particle size (about 10µ or less if memory serves) On one occasion I broke an argon laser tube,(which contains loads of BeO) and I`m pretty much alive. BeO tends to splinter into large fragments (like china). Obviously I would advise against taking an angle grinder to such ceramic, since this would liberate plenty of dust. I think the main fear of Berylliosis is, that when you ARE exposed to a dangerous quantity of dust, and it starts to do harm, there is nothing that can be done, except wait to die. This applies to other nice dusts like talc, and asbestos.
Common magnetrons do not contain Be0 ceramic, as there really is no need, and its extraordinarily expensive. Be0 is used in very high temp environments (2500+ degrees, like in argon lasers), whereas even in an uncooled magnetron, the temperature would be quite modest, maybe 400 degrees or so.
I`m pretty sure I read somewhere that someone had built low current ( few mA) 5kV supply and connected it to a household magnetron (instead of the 800mA supply) , in order to experiment with 2.450GHz at a few milliwatts. Its something I have considered doing myself, as I have no shortage of magnetrons!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've also shattered RF power transistors that contain BeO while changing them out (and picked up the pieces with my bare hands!) and I'm still around to tell the tale. I know they did because the equipment was covered in BeO warning stickers.
These transistors don't run at 2500 degrees. BeO is used because it's part of the cooling path, and it has the highest thermal conductivity of all the ceramics. However I believe the ceramic in magnetrons is alumina, which is just aluminium oxide.
It's still not a good idea to breathe in the dust of anything you're attacking with a grinder. Just cut the metal part of the envelope with a hacksaw.
Also, I think magnetrons have a limited operating range. They don't oscillate at all below a certain power, and go into chaotic mode-hopping above another power. I believe microwave ovens use that doubler circuit to feed the magnetron with pulsating DC for that reason: it's easier to design one that runs with 3kW pulses than with 800W continuous. I guess it saves money on capacitors and diodes too, though.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Aye thats right enough, BeO has excellent thermal conductivity, better even than some metals!
I read that too somewhere as well, that there are thresholds for current and voltage in magnetrons, so I was a little skeptical myself, if I find the link I will post it.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ... I read that too somewhere as well, that there are thresholds for current and voltage in magnetrons, so I was a little skeptical myself, if I find the link I will post it.
Magnetron operating parameters are determined by both the Buneman-Hartree threshold voltage and the Hull cutoff condition.
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