Microwave Heating of Metals

Google "amorphous alloys" cooling 1780 hits

You think slowly and type quickly.

-- Uncle Al

(Toxic URL! Unsafe for children and most mammals)

Hi Al,

I'm afraid that after reading some Google hits, I was unable to come up with answers. I guess it wasn't so obvious to me. Are you saying that microwave heating is already being used for amorphous alloys formation? Are you saying that there is something intrinsic to amorphous alloy formation that is incompatible with microwave heating? Or are you saying that amorphous alloy formation would not usefully benefit from microwave heating?

I don't get it. Please let me know, thanks. :)

1) There is no reason to desire it in your version. 2) Microwaves don't penetrate metal (skind depth). 3) We already have several variants of induction heating. If cleverly executed, the crucible's contents stir themselves. 4) Your cooling argument is specious.

-- Uncle Al

(Toxic URL! Unsafe for children and most mammals)

quickly.

Read carefully, not the metal is heated directly, they create a microwave plasma around the part to heat it. Such a system has no benefits for rapid cooling, compaerd to an induction heater for example. It is just a very uniform heat source without the insulation needs of a conventiunal furnace.

if you want an amorphous alloy that works without crazy cooling rates, read here :

You will not be able to make it at home. Don't try to heat the stuff outside of a vacuum or inert gas furnace or you will get a nice highly toxic firework.

What I suspect that Al is suggesting is that even if you were stuck with a slow and hard to cut off heating technology, such as a furnace, you could just remove your melt from the furnace and transfer it to your quick-chilling device. Quick cooling simply doesn't require quick cut-off of heating.

Microwaves are generally reflected off of metals. I have found google links for microwave sintering of powdered metals. I don't truely understand how microwaves sinter powedered metals. I suspect that it would be a very dangerous thing to attempt without an inert atmosphere.

Now for non-metals, microwaves can facilitate ultra rapid volumetric heating. These guys have some interesting (if not sparse) info about using a gyrotron source for ceramic sintering

If you want rapid heating of metals you need an inductance heater. Check this out Inductance heaters can utilize a range of operating frequencys. Lower frequencys seem to heat from the inside out, higher frequencys seem to heat the outer surface.

Maybe you could couple a non-conductive coolant to the metal part as it is heated by a low frequency inductance heater.. Just a thought

It seems to me that what happens depends on the conductivity of the metal. A metal is a lattice of ions swimming in a sea of electrons, so when you impose a microwave (electric vector in rotary motion) the photon-electron interactivity coefficient (frequency dependent) the electron will be put into motion (ie, electric current). Given the resistivity (reciprocal of the conductivity) there will be an I-squared R loss. This energy term via the metals heat capacity would generate some temperature rise. I do not have the numeric databases available so can't estimate the temperature increase.

On the other hand, I once accidently put a ceramic dinner plate that had a decorative gold rim into the microwave. The sparking was impressive and the grub on the plate never warmed up at all. I expect the mobility of the gold rims electrons sucked up all the microwave energy. Someone else told me that was a good way to burn out a microwave.

Do not induction furnaces operate at much lower frequencies.

Read the material more carefully. It is the crucible that is heated, not the metal itself. The coatings on the crucibles are designed to absorb the microwave radiation, the metal itself does not. Pragmatist.

snipped-for-privacy@ties.k.mn.us (Jack Ferman) wrote in news: snipped-for-privacy@x134-84-252-91.dialup.umn.edu:

A loop of metal in an RF field tends to look like a short circuit. Try it with loosely crumpled aluminum foil. (Actually, don't!) Food seems to be innately higher impedance, and the mass is greater than a thin gold foil so heating is much slower, and less spectacular. It's not nuclear physics, just basic RF electronics.

I think so, but probably still in the RF range.

--Damon

The lack of penetratin of electromagnetic waves into metals has been classically well known for over a century.

There is a "skin depth" related to the conductivity of the metal and to the frequency of the electromagnetic wave. Both high frequency and high electrical conductivity limit the penetration of electromagnetic waves in metals to small depth.

That is why metals reflect electromagnetic waves.

At 10 Ghz, for example, the skin depths of good metallic conductors like Al, Au, Ag, Cu conductors are a little less than 1 micron.

It seems to me that what happens depends on the conductivity of the metal.

It seems that Uncle Al had a good observation.

Samnam does type faster than he thinks.

But, for nanotech, perhaps that isn't all bad, or else we could run out of hype for the otherwise valuable topic.

Jim

Depends on size/power requirements but generally 50/60 Hz for melting several tons, audio frequencies for for small melts (eg dental) going up to ~ 1MHz for welding/surface hardening.