by immersing the ceramic in an electric field, they can pre align domain junctions to cut time and temp WAY down. Sedit - 31-12-2013 at 23:23
This is along the lines of something I have dreamed of for a long time. I have worked with ceramics for well over a decade now and I envisioned a kiln
that would heat the ceramics to a dull red just to get them conductive then turning on a Magnetron to QUICKLY finish the job. This would take the
normal energy requirements and turn them down considerably not to mention the time it would save.3DTOPO - 15-2-2016 at 16:56
Wow - talk about an interesting coincidence. Just the other day I tried sintering some SiC in a microwave. All was well until the insulating alumina
blanket got hot enough to become a susceptor (I suspect) and the MW sounded overloaded so I shut it down (it made a WHHhhHHHOM noise).
But, despite the SiC attaining white heat it is still a green, unsintered part.
Morganism: is a magnetron sufficient to induce enough voltage in a susceptor to accomplish this? How many volts can one hope to induce with a consumer
magnetron?
[Edited on 16-2-2016 by 3DTOPO]
[Edited on 16-2-2016 by 3DTOPO]Twospoons - 15-2-2016 at 17:04
Morganism: is a magnetron sufficient to induce enough voltage in a susceptor to accomplish this? It mentions "100 volts per centimeter", which I am
not sure what that even means - centimeter of what? By itself a centimeters is not volume so I am a bit confused.
[Edited on 16-2-2016 by 3DTOPO]
100v/cm is a measure of electric field strength. If you had two electrodes 1cm apart and applied 100V to them, you would have a 100V/cm field in
between them (assuming flat plate electrodes). 100V/cm is not very much.
I suspect a DC field was used - the magnetron produces an AC field at 2.45GHz, so I would not expect it to have the same effect.3DTOPO - 15-2-2016 at 18:35
Excellent, thank you very much for the clarification!
So, just to make sure I understand correctly, if my part to be sintered dictated that the field plates be 2cm across, would I then need to apply a
field of 200v then to the the flat plate electrodes (regardless the dimension of the field plates). Or is the power required inverse square?
Will graphite plates be suffice for the electrode plates?
I'm definitely going to give this a try when I have a full understanding, so I will report back with my findings.IrC - 15-2-2016 at 20:01
Just the other day I tried sintering some SiC in a microwave. All was well until the insulating alumina blanket got hot enough to become a susceptor
(I suspect) and the MW sounded overloaded so I shut it down (it made a WHHhhHHHOM noise).
I do not think your diagnosis is correct. All a susceptor does is absorb radiation at microwave frequencies. The idea being it gets hot and radiates
this heat into the sample being heated. The better it absorbs RF the easier it is on the magnetron, think perfect dummy load for an RF transmitter.
Lowest possible SWR. So I believe the exact opposite condition was taking place, it was becoming more highly conductive, in effect reflecting
excessive amounts of energy back into the magnetron. This will cause it to overheat, draw excessive current, burn out. Outside of resistive and skin
effect losses in the cavity all energy not completely absorbed by the load will reflect back into the magnetrons internal cavity, a very bad thing,
destructive to it. The other possibility I can see is so much heat radiation getting to the magnetron itself causing it to begin self destructing. Or
some combination of both events.
"There are a wide variety of susceptor materials some of which reach very high temperatures -> 1800 degs F. These materials are generally a
patented or trade secret material combination – think graphite, magnetite, and/or various iron oxides."
[Edited on 2-16-2016 by IrC]3DTOPO - 15-2-2016 at 20:20
Your theory makes good sense now that I think about it. So I guess a glass of water or some other sacrificial susecptor was needed at that point on?
Thanks for the reading material too - checking out the Zirconia sintering paper now! IrC - 15-2-2016 at 20:33
At such high temperatures I don't think a glass of water is going to hang out long and you would have steam blowing out a vent carrying heat away. The
idea for a microwave kiln is to seal it, no vents. Measure the body of the magnetron for overheating without touching high voltage nor operating
unshielded. Modulation makes sense, I use it all the time on Salmon (at much lower temperatures carbon tastes like crap). Get a microwave that has
power settings that cycle the power. Or build something to do the job. Did you ever read this old thread?
Studying patents is useful when one is trying to discover what available materials would function best in the experiments.
[Edited on 2-16-2016 by IrC]3DTOPO - 15-2-2016 at 20:49
Actually, since I was trying to sinter a rather small SiC part, the microwave was no hotter than what I would expect if a turkey was being cooked in
it (I was keeping a good eye on the chamber temp). The small SiC susceptor could only embody so much energy relative to the chamber. So a even just a
couple inches away (and I had more room) I could have safely had a cup of water boiling away.
That said, I am currently planning on setting up a dedicated multi-magnetron furnace from scrap MWs; then I will be able to have everything properly
insulated (though actually thinking of using a water cooled copper furnace floor for the contraption). Then I could even have a water-cooled susceptor
in the faraday cage to act as the glass of water I suppose.
I realize it was a rather crude attempt described, but I am glad I mentioned it because I am learning a lot, thanks to all of you!
[Edited on 16-2-2016 by 3DTOPO]
[Edited on 16-2-2016 by 3DTOPO]wg48 - 16-2-2016 at 09:59
by immersing the ceramic in an electric field, they can pre align domain junctions to cut time and temp WAY down.
That's interesting and relatively easy compared to say 1000V/cm. I wonder if it helps sinter magnesium oxide.
I suspect it will still need a very fine powder (ball milling for days) and possible pressure.
I note that the magnesium insulation of metal clad heating elements is usually not sintered even after hundreds of hours of use at red heat and
subject to approximately 500Vrms/cm (UK voltages) in parts but then its at 50Hz.
3DTOPO - 16-2-2016 at 12:44
Depends exactly what you mean by red heat (dark red, cherry red, bright cherry, etc), but red isn't very hot in the grand scheme of things -
especially for MgO.wg48 - 16-2-2016 at 14:22
Depends exactly what you mean by red heat (dark red, cherry red, bright cherry, etc), but red isn't very hot in the grand scheme of things -
especially for MgO.
The red heat of an electric hob element I would call it cherry red about 900C. I just checked the melting point 2,852C I thought it was 1,800C. Now
thats what I call refractory.wg48 - 16-2-2016 at 15:37
With a sintering aid (0.2% iron oxide) the sintering temperature can be reduced to about 1400C. Don't know if it would still be compatible with
Nicrome or Kanthal heating element.
Probably still need serious ball milling to get the small particles and mixing.IrC - 26-2-2016 at 15:30
You might read through these (includes the one mentioned already).
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