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kclo4
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Microwave Kiln "Microkiln"
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]
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garage chemist
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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.
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bbartlog
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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.
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bbartlog
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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.
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not_important
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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.
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kclo4
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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]
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Sedit
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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.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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argyrium
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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.
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kclo4
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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]
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ChrisWhewell
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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.
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not_important
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| 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.
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12AX7
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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
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argyrium
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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
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not_important
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| 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'
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StevenRS
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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]
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Sedit
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... Well that is kind of the point steven. They use fired Alumina but essentialy the same deal for the outer insulator.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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StevenRS
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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?
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Sedit
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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
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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dkjgator
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Microkiln
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
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franklyn
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| Quote: Originally posted by ChrisWhewell | | 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. |
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
.
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JohnWW
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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]
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JohnWW
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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.
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dkjgator
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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.
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Margarette
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Do you know or could you try if Calcium Silicate absorbs microwaves?
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12AX7
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Not at low temperatures. Calcium silicate is a primary ingredient in glass, an insulator.
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