Fusor Forums

Mark -

A Steven notes below, tri -alkali cathodes start from tri-carbonate cathode coatings... which are usually strontinum, barium and calcium carbonates. After reaching a good vacuum in the -6's, heat is applied to burn off the binder first, and dry the carbonates, then temperature is raised in two stages, if memory serves correctly.... to convert the carbonate to the oxides. Temps are around 1500 K or above for this part.

The oxide blend results in a low work function surface... around 1-1.1 electron volts. Compared to the work function of Tungsten at 4.25 eV and Thoriated Tungsten at about 2.5 - 2.6 eV, the tri oxide cathode surface has a higher current density at lower temperature.

Dave Cooper

In tests here of electron beam generating devices for ionizers and such, we tried the tri-alkali stuff, old tube cathodes and homebrew mix, doing our own activations and so on. It's hard to get right, but not impossible. I have a couple older books on vacuum tube tech that give the process.

The thing is, they won't take post activation exposure to air and live. They form hydroxides that you can't reactivate at all, so you get maybe two shots total out of that type.

Based on some work I read on mass spectrometer ion source design, we tried yttria (Y2O3) and while it doesn't work quite as well in lowering the work function, it does handle air exposure after that, and doesn't fry if you get a little too hot on the filament...so that's what we stuck with from then on. Instead of the dull red you can run the alkali stuff at, this gets closer to orange, but that's still a great deal over yellow hot thoriated tungsten (which also is ruined by air exposure to a large degree, and further by ions hitting it, reducing too much thoria which then evaporates) or the white hot you need for pure tungsten.

A compromise, but a decent one if you make things that have to be air exposed once in awhile.
We got for example, about 10 ma emission off a 6v, 10w filament running on 3.5v coated with it, which was maybe space charge limited (we didn't have a very strong field to pull electrons off).
At any rate, it solved that for us.

Hello:

Aremco materials makes a bunch of ceramic "glue type" of high temp ceramic compounds. This stuff is like toothpaste when wet, and it becomes a hard ceramic upon heating. They have many versions, based on MgO, Al2O3, and even Zircoa.

Another common use is for cementing in Xe and halogen quartz lamps, into their reflector assemblies, which have to take a lot of heat. Actually you may be on to something here, this stuff could come in very handy for fusor, and high temp vacuum applications. Enjoy folks, maybe bookmark this one : )

http://www.aremco.com/a4.html

I looked at the Aremco stuff a few years back when I was thinking of making a spark-plug feedthrough without any organics like epoxy, etc. I never followed through, though.

Removing the filament feedthru assembly from a magnetron was fairly easy with a lathe. Unfortunately this time was more of a practice run as the filament was bad. Once out, I noticed a coating on the heater plate. It can be seen in th attached photo. Is this an oxide I should be concerned about?

Mark Rowley

Thanks Lutz
This will be very useful
Steve

Mark,

I have removed the filament from dozens of magnetrons, and I can assure you that you get better with time. I broke a few in the beginning.

Later I actually found a source in China where I could buy them open.

Be careful if cutting or grinding the ceramic plug, many of them are made from beryllium oxide, best to cut or grind under water.

viewtopic.php?f=12&t=5013#p32321

Steven

I didnt know that. I took some classes on scientific glassblowing and they had told me the white ceramic stuff was torr-seal. Good to know.