Sciencemadness Discussion Board - Microwave Kiln "Microkiln"

Microwave Kiln "Microkiln"

kclo4 - 17-12-2009 at 18:03

This has me interested since it obviously has some other uses... http://www.youtube.com/watch?v=HAkSrMldzfU&feature=relat...

What would the white material be made out of, and what would the black material on the inside be?

I'd like to make one of these..

Any ideas? I assume the black inner material could be something like MnO2 or silicon carbide? The white could perhaps be Al2O3?

[Edited on 18-12-2009 by kclo4]

garage chemist - 17-12-2009 at 20:21

The white material is ceramic fibre. Just what I used to insulate my electric tube furnace.
The black material is granular silicon carbide, an excellent absorber of microwave energy.

Hm, this gives me the idea of simply mixing silicon carbide into a to-be-heated mix inside a ceramic or quartz retort, wrapping the retort with ceramic fibre blanket and placing it in the microwave. This should easily reach 800-1000°C, and SiC is very inert. One could easily make phosphorus or SO3 with such an arrangement.

bbartlog - 18-12-2009 at 07:44

There are two white materials: a thin layer of something right next to the (silicon carbide?) receiver, and the main body. The thin layer may well be ceramic fibre, but the main body looks like something else; maybe plaster.
The black stuff could also be some kind of ferrite or graphite compound. Though I think those would oxidize and/or melt more readily than silicon carbide.

bbartlog - 18-12-2009 at 07:47

As far as microwave phosphorus goes, there's a patent (US patent 6207024) on using finely divided carbon in phosphoric acid... the carbon both acts to absorb the microwaves and reduce the phosphoric acid.

not_important - 18-12-2009 at 08:23

http://www.microwavekiln.com/KILNINSTRUCTIONS.HTM

http://home.c2i.net/metaphor/mvpage.html

The white body is a castable ceramic. Plaster sets by hydration, heating it cause the water to be evolved which both weakens the solid and can cause it ti crack or even explode from internal pressure.

You need quite low thermal conductivity to make this work, else too much heat is lost through radiation. Ceramic insulating board or blanket is often used to line the oven, pottery and likn supply houses sell such and one vendor for example purposes is at http://www.agismfg.com/html/other_prod.html Note that these frequently use an organic binder that must first be burned out - smoke and smell alert, after which the ceramic is relatively brittle and fragile; bofore the burnout it can be cut to size.

Both SiC and graphic are used as susceptors, as is magnetite. As these are semiconductors and their resistance goes down as they heat up, and for some may be too high when cold, some installations use a bottom layer of the more conductive susceptor that does the early heating, then sidewalls containg a second susceptor take over as that warms enough to heat and the bottom effectively turns into a conductive sheet.

kclo4 - 19-12-2009 at 13:55

Can terracotta pots be heated without being destroyed?
If so, it seems like they might offer an easy solution to making one of these.. especially if you could perhaps use MnO2 for the heating- which the pot could be soaked in a solution of KMnO4, and then soaked in an alcohol or something to produce MnO2 in the terracotta pot...

MnO2 needs to be heated before the microwaves will affect it, so you'd still want some SiC to get it started or something else... right?

Or better yet - MnO2 is used to dye ceramics, right? So maybe you could just use a pot that was dyed with MnO2?

[Edited on 19-12-2009 by kclo4]

Sedit - 19-12-2009 at 14:09

I would think the Iron which I believe is present in terracotta would not be as transparent to U-waves as Alumina will be. You want to make sure it goes thru the first layer and heats the second well. I like the idea of carbon but won't you have the issue of the carbon vaporizing? I have SiC from old igniters for heaters but carbon would be a much easier solution to the problem if it would work.

argyrium - 21-12-2009 at 14:28

Very interesting thread.

Any actual suggestions at a starting "recipe" for a DIY material that would be sufficiently transparent to the microwave and have have good insulating (thermal) properties?

My wife is a jeweler and so have access to furnaces to burnout/fuse the cast the otherwise formed shell/assembly. I would assume the MnO2 and/or SiC could be applied to the fired inner surfaces with a Na silicate binder. I also have carbon microspheres and graphite that - guess would need to be incorporated along the lower or base areas (as suggested above) to initiate heat.

Thanks for any ideas.

kclo4 - 21-12-2009 at 16:20

Just put a small tarracotta pot in the microwave for five minutes.. it got hot enough to burn me, and hot enough to turn a napkin brown.... I feel this has potential... I am now coloring the inside of it with a pencil for the graphite - perhaps this could make a very small difference? I don't know...

Edit: colored the inside with pencil graphite, and tossed on a dish that would normally capture the water that drains from the pot - and put the pot on upside down so it sealed - close to the entire inside of the small pot was covered in a nice layer of graphite and only after a minute it seemed to hurt me upon touching it....

[Edited on 22-12-2009 by kclo4]

ChrisWhewell - 22-12-2009 at 07:24

Quote: Originally posted by bbartlog

As far as microwave phosphorus goes, there's a patent (US patent 6207024) on using finely divided carbon in phosphoric acid... the carbon both acts to absorb the microwaves and reduce the phosphoric acid.

Just be careful with that, phosphorous vapors are extremely toxic.

My favorite microwave experiment involves a lump of charcoal, and produces diamond powder.

not_important - 22-12-2009 at 08:13

Quote: Originally posted by argyrium

...
Any actual suggestions at a starting "recipe" for a DIY material that would be sufficiently transparent to the microwave and have have good insulating (thermal) properties?
...

The good high temperature insulators have low density, either from bonding together collections of fine fibers or particles, or forming a large amount of voids by burning out a high percentage of very finely powdered organic fillers.

High fire porcelains and mullite itself are commonly used. They have low RF losses; as well has high melting points, low thermal coefficient of expansion, and good strength and similar properties. Terra cotta does not have as good mechanical properties, and softens and melts hundreds of degrees lower than porcelain; it will work as a container but is liable to suffer frequent breakage.

Neither porcelain nor terra cotta in ordinary bulk form is a very good thermal insulator, you need a very high level of voids to get that.

12AX7 - 23-12-2009 at 01:31

Keep in mind that all ceramics become excellent succeptors around about half the lowest melting point (that includes glassy phases, particularly important in flux-sintered ceramics, porcelain and etc.), due to increased ion mobility. If you want to melt iron, I don't recommend silica and alumina; magnesia might be a better bet.

Actually, that might not be such a big problem. The ions that become mobile around red heat are sodium and potassium. If the ceramic is high purity (less than 1% Na, K), it may remain a fairly good insulator, even up near the melting point. Alumina and silica are pretty well covalently bonded; even in the liquid state, they probably don't conduct all that well.

Terra cotta will form an excellent black slag once you get it about yellow hot.

Tim

argyrium - 23-12-2009 at 01:39

The good high temperature insulators have low density, either from bonding together collections of fine fibers or particles, or forming a large amount of voids by burning out a high percentage of very finely powdered organic fillers.

High fire porcelains and mullite itself are commonly used. They have low RF losses; as well has high melting points, low thermal coefficient of expansion, and good strength and similar properties. Terra cotta does not have as good mechanical properties, and softens and melts hundreds of degrees lower than porcelain; it will work as a container but is liable to suffer frequent breakage.

Neither porcelain nor terra cotta in ordinary bulk form is a very good thermal insulator, you need a very high level of voids to get that.

[/rquote]
Thank you, not_important and Tim,

Wonder whether SiO2 (aerosil) mixed w/ "pearlite" or that expanded mica stuff (don't recall the name), a little CaSO4.H2O and a bunch more bentonite fired above the conversion T would work. Seems any type of "terra cotta" would have too much Fe and not withstand to many thermal cycles.

Argyrium

not_important - 23-12-2009 at 04:10

Quote:

When it reaches temperatures of 850–900 °C, perlite softens (since it is a glass). Water trapped in the structure of the material vapourises and escapes and this causes the expansion of the material to 7–16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles.
Unexpanded ("raw") perlite bulk density: around 1100 kg/m³ (1.1 g/cm³).
Typical expanded perlite bulk density: 30–150 kg/m³

so 900 C is the high nd temperature for using perlite. As for vermiculite, it softens around 1300 C.

You don't want calcium sufate in the mix. Starting at about 80 C the dihydrate starts losing water, by 150 C it will have lost 3/4 of its water to give the hemihydrate. This loses most of the remaining water above 170 C to give gamma anhydrite, above about 250 C it converts to the "dead burnt" form. At higher temperatures it will react with SiP2 and Al2O3, releasing sulfur oxides. Refractories just don't go well wit CaSO4.

Besides the castables, there are fiber board, sheet, and blanket refractories. These can be used to line a microwave oven to produce a small, well insulated inner space for the high temperature operation. These products generally are better insulators than the castables, with the boards being the best. Often they are coated with a castable refractory to give a more mechanically robust surface if they are likely to be exposed to physical trama.

One source, who also makes castables, is
http://www.hitempincusa.com/ceramic_fiber.asp

You might be able to home-brew something, depending on the target temperature range. It's more likely you can come up with container material to use, picked for the maximum temperature range it will be used in'

StevenRS - 12-1-2010 at 20:23

What about a small silicon carbide crucible, insulated with Kaowool (or any ceramic fiber like that)? The SiC acts as the susceptor and as the crucible, so the material to be melted is in direct contact with the heating element.

[Edited on 13-1-2010 by StevenRS]

Sedit - 12-1-2010 at 20:28

... Well that is kind of the point steven. They use fired Alumina but essentialy the same deal for the outer insulator.

StevenRS - 13-1-2010 at 19:56

I just placed a small lump of Silicon Carbide in my microwave, in the form of a broken knife sharpener, on a small pad of kaowool. The microwave heated it to red heat in under 60 seconds.

A small crucible made from this, maybe a "B4" from http://www.budgetcastingsupply.com/Crucibles.php could be a simple and powerful furnace. Most designs I have seen use a brick of SiC upon which the alumina (or whatever) crucible sits. I suggest making the crucible itself out of SiC.

Was my previous post unclear, or am I still restating something that has already been established?

Sedit - 13-1-2010 at 20:19

It sounds as though your considering basicly inverting the kilns design so that it can hold molten material. Even with a SiC crucible making the outside of Alumina would mean less heat loss and higher temperatures. Without that insulation I would expect the crucible to quickly melt thru your MW

Microkiln

dkjgator - 31-3-2010 at 11:50

I spent some time in the late 80's/early 90's doing research on the microwave densification of high temp superconductors. In those days, we made our own "microwave kilns". Actually it was out of necessity in order to control potential contaminants that could ruin the superconductor composition as well as the need for high temp continued use (930C for about an hour). The "kilns" were essentially refractory cylinder and board material from high purity alumina. Similar materials are available from Zircar http://www.zrci.com/ such as RS-99M. A high temp alumina mortar was mixed with SiC and applied to the interior.

The SiC particle size and volume% relative to the mortar along with the overall thickness of the layer were important factors as to whether or not any microwave energy made it to the samples in the kiln (as opposed to radiant energy from the microwave coupled SiC). If actual microwave- sample interaction is not required then it volume% and layer thickness are not as critical.

For those playing at home be extremenly careful. The post from 12AX7 is more accurate than hopefully most of you ever know. At elevated temps microwaves can couple with many materials that are normally excellent insulators leading to molten materials in and through your microwave.

For reference I've attached is a link to a book by one of my former professor on microwave processing of materials http://books.nap.edu/openbook.php?record_id=2266&page=1

franklyn - 1-4-2010 at 06:38

Quote: Originally posted by ChrisWhewell

Quote: Originally posted by bbartlog

Just be careful with that, phosphorous vapors are extremely toxic.

My favorite microwave experiment involves a lump of charcoal, and produces diamond powder.

From that cited patent 6207024
Quote
" reduction of phosphate with carbon in a microwave field has been found to occur
at unexpectedly low temperature , below 540 ºC. This compares to the approximately
1650 ºC of the electric furnace process."

This may have use for the preparation of carbides notably CaC2. As we see the electrical
conductivity of carbon translates the microwave radiance into heat. A well mixed amount
of finely powdered lime and activated charcoal half filling a coffee mug which is then topped
with about a centimeter of lime powder lightly wetted on top to seal off the contents and
placed on top of a refractory tile ( to protect the microwave's turntable ) may just work.
The reaction produces considerable carbon monoxide so it needs ventilation to the outdoors.

CaO + 3 C => CO + CaC2

A related thread.
http://www.sciencemadness.org/talk/viewthread.php?tid=2492

.

JohnWW - 1-4-2010 at 14:03

Here are two useful patents:

An apparatus for microwave heating processing of materials, US Patent 6222170:
http://www.pat2pdf.org/patents/pat6222170.pdf

Plasma-assisted microwave processing of materials, US Patent 5847355:
http://www.freepatentsonline.com/5847355.html or
http://www.pat2pdf.org/patents/5847355.pdf

Here are some important unrestricted articles on the subject:

Microwave Processing Of Ceramics:
http://www.mri.psu.edu/centers/mpec/media/025.pdf 427 Kb

Microwave Processing Of Variety Of Materials (2006):
http://www.mri.psu.edu/centers/mpec/media/133.pdf 773 Kb

Microwave Refractory Processing Research At Virginia Tech: http://www.microwaves.mse.vt.edu/article.pdf 950 Kb

A Novel Approach To Understanding Microwave Heating Of ZrO2:
http://www.ceralink.com/publications/A_NOVEL_APPROACH.pdf 135 Kb

Enhanced Computer Modeling Of High Temperature Microwave Processing Of Ceramic Materials:
http://www.crg.cs.nott.ac.uk/~mpc/mrs96tlm.pdf 104 Kb

[Edited on 1-4-10 by JohnWW]

JohnWW - 1-4-2010 at 14:30

Quote: Originally posted by dkjgator

(cut) For reference I've attached is a link to a book by one of my former professor on microwave processing of materials http://books.nap.edu/openbook.php?record_id=2266&page=1

That book is Microwave Processing Of Materials, published both in print and as a PDF by the National Academy Press (1994). However, except for excerpts, it cannot be directly downloaded as a PDF from their site.

I found a downloading link for it as a PDF on
http://www.vo2ov.com/Microwave-Processing-of-Materials-Publi... and
http://download.f60s.com/forums/t/209599.aspx . However, one of two links provided, on Rapidshare, has been recently deleted, thanks to the Copyright Gestapo, so I intend reuploading it to my Rapidshare Premium account and posting the link in the References section.

dkjgator - 5-4-2010 at 18:11

Thanks JohnWW for the links/references you posted.
If you are able to get a pdf of the book can you forward me a copy(I lost my hard copy during a move).

Interesting footnote, the Virginia Tech reference you posted is about Dr. Clark, author of that book and my former professor at UF.

Margarette - 25-5-2010 at 10:44

Do you know or could you try if Calcium Silicate absorbs microwaves?

12AX7 - 25-5-2010 at 17:29

Not at low temperatures. Calcium silicate is a primary ingredient in glass, an insulator.

un0me2 - 29-8-2010 at 19:43

This is probably the best review of the idea I've yet seen. It describes the use of MW Irradiation for the preparation of glasses, the sintering of glasses & the film formation on glasses. It has some very useful information.

NB For those who are interested in mounting a reflux condenser through the top of a domestic MW, the schematic of how it HAS been done is in this paper. :cool:

(they enclose the bottom half of the condenser in a metal Farraday trap).

Attachment: Kharissova.Kharisov.RuizValdez.Review.the.Use.of.MW.Irradiation.in.the.Processing.of.Glasses.and.Their.Composites.pdf (1.8MB)
This file has been downloaded 1879 times

Sedit - 30-8-2010 at 17:52

Quote: Originally posted by 12AX7

Not at low temperatures. Calcium silicate is a primary ingredient in glass, an insulator.

Insulators are a very loose term when it comes to Electromagnetic frequencys in the range of radiowaves and microwaves. A HF tesla coil makes glass(as with almost every other substances iv tried) act like a conductor. This was the reason tesla was so gung ho with his project in the first place since it would eliminate the need for wires allowing the entire landscape and infact the air itself to be the conductor for the electric.

Mr. Wizard - 31-8-2010 at 05:58

A very easy OTC source of SiC would be 'wet or dry' sandpaper.

Sedit - 31-8-2010 at 16:57

Or one could just buy SiC grinding wheels which would more then likely pose less of a hassle in extraction and be cheeper. A few minute in the microwave would remove most of the materials such as binders and glues leaving you with SiC and possible some alumina oxides or whatever other rarefactory material they may contain.

watson.fawkes - 31-8-2010 at 17:34

Quote: Originally posted by Mr. Wizard

A very easy OTC source of SiC would be 'wet or dry' sandpaper.

Quote: Originally posted by Sedit

Or one could just buy SiC grinding wheels [...]

What are you all on about? You can buy SiC grit, in bags. Search for "silicon carbide grit". Or visit a lapidary shop, real or virtual.

Eclectic - 31-8-2010 at 19:01

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=28049...

Mr. Wizard - 1-9-2010 at 09:14

Yes, SiC is available in many places and ways. I just mentioned it as a source, not THE source. The fact it is already spread out on a flat waterproof carbon based sheet might help attach it in certain situations.

watson.fawkes - 1-9-2010 at 11:43

Quote: Originally posted by Mr. Wizard

The fact it is already spread out on a flat waterproof carbon based sheet might help attach it in certain situations.

Granted, although it would take quite a burnout process, and then you still need a binder for it. I would have to imagine that using sodium silicate (or other refractory binder) is easier. Disclaimer: I've never tried either myself.

Eclectic - 1-9-2010 at 18:12

There is some indication that B4C, B, B2O3 are effective binders for SiC using microwave radiation to heat to sinter temperatures. Even BN will wet SiC if heated strongly enough.

ümit - 8-10-2010 at 21:20

As a DIY sick I've been planning to make one of these kilns and found your site.I really thanks you all who wrote your opinions,I learnt a lot.

Before I found here I was trying to cover inside a red casserole with white clay (white one is stronger and is for protection of red casserole),but after reading what you wrote about terracotas I have some doubts,will that white clay protect casserole?

I found some kind of low cement concretes which contain SIC between %30-80 in İstanbul where I live (could'nt found pure SIC concrete,they contains some AL2O3 as well)http://www.remsan.com/en/06.cimento.asp

By the way this guy made his own microkiln out of firebricks and SIC,is that logical and what do you think he used as mortar to attach bricks to each other?It seems as if there is no mortar

This is the link
http://www.periodictable.com/PopSci/2003/09/1/index.html

[Edited on 9-10-2010 by ümit]

Halcyon - 9-10-2010 at 20:01

As a simple microwave-heated crucible, how well would the following work?

A small glazed porcelain crucible inside a larger one, with the gap (~10mm for instance?) filled with slightly compacted 1000 mesh SiC powder.

I've tried to find the temperature up to which porcelain will hold, but my search-fu is failing me...

12AX7 - 9-10-2010 at 21:40

Probably work well. Mind that porcelain will crack from thermal stress....

There are lots of kinds of porcelain; most are probably good up to cone 6 (~2100F / 1150C).

Tim

Halcyon - 10-10-2010 at 04:44

Cheers Tim.

When you refer to thermal stress, do you mean heating to certain temps, heating/cooling in general, or just sudden/uneven change in temperature?

bbartlog - 10-10-2010 at 12:52

I cracked a porcelain mortar while heating CuO powder in it in a microwave, and I doubt it (the powder) got beyond 800C just based on the glow temp. Porcelain can take the heat, but not the thermal stress - also the stresses end up being enough that it shatters fairly violently (though it did not break the glass door of the microwave). If you are determined to try it I would say that you could try to introduce some sort of insulation between the outer porcelain and the SiC susceptor (perlite? rock wool?) but I think you are better off just trying to find something less liable to crack.

(edit): Another alternative is to heat slowly. Thinking about it some more, I was running at full power (1 kilowatt) with just a few grams of powder in my mortar, which resulted in very rapid heating, maybe 15C/sec or so. If you are more sensible about the power versus susceptor mass you might well have better results than I did.

[Edited on 10-10-2010 by bbartlog]