Microwave absorbtion material >600°C

[ElectrostaticHairdo]

As part of a mineralogy study, I am attempting to heat small samples (5-15g) of material (rock) to 450°C in a microwave oven. This has cause the oven(s) to blow fuses and/or self destruct.

I understand that microwave ovens should not be operated empty or nearly empty, and so I need to add some microwave absorbing material along with what I'm trying to heat. Normally this could be water, but I need to heat my sample material to 450°C, so I'm looking for a material that can absorb microwaves and withstand temperatures of >600°C.

Any suggestion on what material would be suitable? And roughly how much material would be required to prevent damage to the microwave? This is for a household 1200W microwave with inverter.

Thanks!

[Marco]

You can buy microwave kilns.

Or DIY one, I remember a youtube video making one out of fire cement with perlite mixed in with graphite on the inside.

PS. oh the video is members only now, Robert Murray-Smith's "How to make a microwave kiln (Furnace) from scratch for £5".

[T3sl4co1l]

Magnetite and SiC are typical microwave susceptor materials.  (Obviously, magnetite runs out at mid-high temps, where it melts, but SiC can go quite far.)  As mentioned, kiln kits are available, or you can make your own.

Tim

[twospoons]

Since you wont be heating the entire interior of your MW to 450C you could still just put a glass of water in the corner.
Wrt possible susceptors, I believe graphite also works ( and melts at 3000C )

[T3sl4co1l]

Yup -- though graphite burns in air well below 3000C; but at 600C it's pretty slow, and negligible at 450.

Tim

[Berni]

Graphite could work (is conductive but not very conductive) and crucibles made out of it are pretty easy to find online.

Tho note that if you use a crucible out of microwave absorbent material, then there is not much difference from simply placing your crucible on top of a gas burner, so the microwave doesn't have that much of a point anymore. Especially since 600°C is easy to reach using a flame.

There are some youtubers that used microwaves to produce glass. The tricky part there is getting it going, but once glass starts to go molten it becomes conductive and starts soaking up microwaves. This did require wrapping it up in a LOT of insulation to keep the heat in. Maybe you just need to try that.

And as people said if your microwave can't handle running at low load for so long just put some water in a corner.

[Marco]

Yup -- though graphite burns in air well below 3000C; but at 600C it's pretty slow, and negligible at 450.

From memory, the video applied it with a water glass slurry.

[T3sl4co1l]

Worth noting that bulk graphite is probably too conductive (it's fairly metallic as things go); but a thin layer of it, or a mixture, can have higher resistance and thus good absorption.

Graphite is an excellent susceptor at low frequencies, but this is also done with inductive coupling, where the near fields are quite low impedance; impedance in the cavity will be say 100s of ohms, where a higher resistivity material is desirable.

Ideal conductivity is more or less to have a 1/4 wave thick layer and skin depth such that it absorbs in the bulk; or a sheet/foil/film layer that, while lower resistivity, has comparable sheet resistance (i.e. low 100s Ω/sq, I suppose).

Conversely, dielectric absorption approaches from the other way, where impedances might be kohms at lower frequency (various industrial processes that aren't amenable to induction), but converges towards cavity impedance here.  I'm not sure what offhand would have reliable dielectric loss though, in the same temperature range, or if it matters much at all how we express the lossiness at this point.  (We'd pretty reasonably assume a material like magnetite has a combination of hysteresis and conduction loss, but beyond something like water..?)

Yup -- though graphite burns in air well below 3000C; but at 600C it's pretty slow, and negligible at 450.

From memory, the video applied it with a water glass slurry.

Hmm, interesting catch is, once the glass itself melts (~red hot), ionic conduction takes over.  Which might end up making that a bit goopy, I guess?  Upshot, it should slow the combustion of graphite, though on the other hand it's also not very sticky, the graphite will tend to float out of it.  450C is probably still okay, but I would be concerned about possible hot spots, I guess?

There's also graphite paper, fiber, foam, etc.; though probably fairly specialty products?  (Note, those pyrolytic sheets for thermal conduction and shielding are too conductive, they're in the area of ~ohms I think.)

Tim

[TimFox]

Thin layers of carbon can be deposited on glass, etc. using a colloidal suspension of graphite in water, commercially known as Aquadag.
https://www.agarscientific.com/catalog/product/view/_ignore_category/1/id/57427/s/bonderite-l-fg-adag-graphite-lubricant-500g-eco/

[twospoons]

How about a carbon foam? aka burnt toast?

[ElectrostaticHairdo]

Thanks for the replies. Those microwave kilns are fascinating but not quite applicable for what I'm trying to do.

My intention is to have the minerals in a crucible and heat it directly with microwaves. The minerals in question aren't particularly receptive to microwaves so I also need a have a dummy load / heat sink, and water will likely boil before the minerals reach the temperature required.

I did pick up some silicon carbide. Wow, does that stuff ever effectively convert microwaves to heat. Too effectively for me to use it pure as a heat sink. But I should be able to mix it with something less reactive to find the right balance of absorption without too much heat.

Thanks again for the help.

[Marco]

Without transporting the heat outside the microwave, you will not find a better sink than water. Eventually anything else is just going to melt a hole in your microwave or cause fumes as it boils any way. The water boiling is what allows it to take up more energy than any reasonable alternative. Is high humidity a problem for the mineral? What's the problem with the water boiling?

If you really don't want boiling water, open up the microwave a bit so you can get access to the outside of the cavity metal. Paint a susceptor on a square on the inside of the cavity, bolt a CPU heatsink on the outside (with a thermal pad) in the same area. If the heat energy has to stay inside, use water, otherwise bring it outside. Or find/make a 10 Watt microwave.

[Zero999]

Why not use a low power microwave? My Panasonic inverter microwave can be set as low as 100W. I expect it'll be able to handle a lighter load, at lower power output because the magntron won't get as hot.

[Marco]

It will take even longer for the sample to heat up ... essentially what he's looking for is a high density of microwaves, which is hard to achieve with a kitchen microwave without a load dump.

In a purpose build machine you wouldn't even have a cavity for microwaves to jump around, you'd just have the magnetron pointed at a load dump (with a heatsink and fan) and put the sample in between.

[Zero999]

It will take even longer for the sample to heat up ... essentially what he's looking for is a high density of microwaves, which is hard to achieve with a kitchen microwave without a load dump.

In a purpose build machine you wouldn't even have a cavity for microwaves to jump around, you'd just have the magnetron pointed at a load dump (with a heatsink and fan) and put the sample in between.

I don't see how it's possible to have such a tightly focused beam of 122mm radiation because the wavelength is too long.

[Marco]

You can contain it in a waveguide towards the load dump. Which the microwave cavity is too, except a bit haphazard and more prone to reflecting power back to the magnetron.

[Zero999]

You can contain it in a waveguide towards the load dump. Which the microwave cavity is too, except a bit haphazard and more prone to reflecting power back to the magnetron.

You still can't get away from the fact the object is much smaller than the wavelength of the radiation.

[T3sl4co1l]

Well, you can use a ridged or pinched waveguide, and get extreme electric field at a point; self-igniting plasma torches can be made this way.  Putting the crucible right in front of this gives the best chance of success, then, I suppose an absorbent surround to deal with the rest.

I still don't get why the crucible can't simply be heated with a susceptor though.  This sounds like added worry and effort for no benefit.

If it can't be heated too quickly, then adding thermal mass, and running at a low duty cycle, and perhaps also a dummy load in the chamber (which at the lower duty cycle, can be something like water, or SiC painted tiles, without getting disastrously hot), will do fine.  You don't want to run a microwave continuously for extended periods of time anyway, they're really not made for that.

Tim

[twospoons]

Theres also the question of 'why microwaves?'. Why not use a simple electrically heated tube furnace for such a  small sample? 450C should be easily obtainable without using exotic materials - quartz tube and nichrome wire should do it.