noxx
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Low thermal conductivity coating ?
Hello guys,
I recently built an electric furnace and I would like to make it even more efficient. I taught I could make my own coating like
ITC-100
I taught about Sodium Silicate + Fine Perlite but I'm no sure of the result... I want to get high IR reflectivity.
Thanks in advance.
[Edited on 27-12-2007 by noxx]
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chemrox
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How about sheets of mica crystal? One can obtain muscovite sheets over a foot in diameter; biotite too but you wanted high IR reflectivity. For that
you would also paint the inner surface of the muscovite crystal with ZnO. I'e. the painted surface to the ceramic side and the uncoated mica surface
to the inside of the oven .
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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noxx
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Good idea. But how can I paint ZnO ? It's insoluble in water.
Thanks a lot.
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not_important
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You don't want to use ZnO - zinc oxide - for two reasons. The first is that it turns a yellow color as it gets hot, reducing its reflectivity. The
second is that any reducing agents, including dust in the air, that come into contact with hot ZnO are going to reduce some of to free zinc, which
will vapourise off and then reoxidise in the air.
The kiln and forge finishes are generally based on Al2O3, MgO, ZrO2, amd/or zircon - zircronium silicate. Sometimes a certain amount of clay is
added, most commonly this is a small amount of bentonite used to help suspend the solid materials in water so that they may be brushed or sprayed onto
the target surface.
Perlite softens around 1100 C, that may be too low for your application.
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Rosco Bodine
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What about TiO2 as an IR reflector , maybe mixed
with porcelain slip and painted onto the walls .
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noxx
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Thanks for the answers guys.
I forgot to mention that my electric furnace is made to melt gold. So it must go around 1200°C max.
not_important, if perlite is mixed with another higher melting point material, will the resulting mix still softens at 1100°C ?
Rosco, the thermal conductivity of TiO2 is quite high... about 10W/K*m
Thanks in advance.
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Fleaker
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ITC-100 is a proprietary blend of zirconia, silicates/deflocculents, and other ingredients. It does work well, and I have used it before. Excellent
for stabilizing ceramic fiber wools and protecting against mechanical and flux damage. Easy to use too: make a thin slurry and paint on to your
furnace walls.
It is also expensive at 26 USD/ 16 fluid ounces and that's the cheapest source I know of, granted I last looked the guy up about 2 years ago. I have
posted info about him before, but I can look it up for you.
Keep in mind, ITC is most useful for electric furnaces, where it drastically decreases heat up times and has an almost unlimited service life. When
used in a combustion furnace, one must be very careful to ensure that the atmosphere is always oxidizing--any free carbon soot will negate the
reflective effects of the zirconia.
Neither flask nor beaker.
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--The Dark Lord Sauron
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noxx
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That's why I want to make my own 
If only I could know the composition... lol
Any idea who is making ITC-100 HT ?
Thanks
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noxx
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I just found this while searching on this forum.
Posted by Cyrus:
IIRC ITC-100 is made of zircon/zirconia, (colloidal?)silica, alumina, sodium silicate, magnesium silicate, and water, but I have no exact proportion
data, just general ranges, (ie 5-15%) and have done a test on this, it's not perfect yet, but moderately ok so far.
He does not seem to be active anymore on this forum. Anyone has his email ?
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not_important
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| Quote: | Originally posted by noxx
...if perlite is mixed with another higher melting point material, will the resulting mix still softens at 1100°C ?
... |
Consider some chocolate chip cookies. If you warm one up and it has only a few chips in it all that happens is you get a few soft bits. But if the
cookie is mostly chips, you end up with a blob of melted chocolate with some crumbs in it. Now substitute perlite for chocolate chips. There's also
a chance that the molten perlite will dissolve or react with the higher melting material.
MgO, ZrO2, and zirc silicate in part work because they reflect much of the heat back, as Fleaker mentioned in combustion heater furnaces carbon
deposits can greatly impair the effectiveness of such coatings.
The ITC mixes are improvements on other older ones. Zirconium oxide/silicate gives high temperature resistance and good reflectivity. Sodium silicate
functions both as a pre-firing glue and as a fluxing agent, not much is used. Alumina + silica can give mullite, important in porcelain and a high
temperature refractory (1600 to 1800 C) Al, Mg, and Si give more complex silicates, and along with the sodium silicate make a glass that helps bind
the structure together. The trick is to match thermal expansions well enough that the refractory doesn't break apart.
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noxx
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Ok thanks.
And does the Infrared reflectance is in direct relation with the Thermal Conductivity ?
For example, Zirconium Silicate has a thermal conductivity of 4.22W/m*K but it's infrared reflectance is supposed to be 60% - 75%
So ZnO (1.2W/m*K) would have an even better infrared reflectance ?
Thanks a lot guys !
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not_important
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Note that I mentioned that hot ZnO turns yellow, meaning it becomes less reflective. Then there's the reduction issue. And beyond that ZnO is a
strong fluxing agent, and may react with whatever ceramic materials it is in contact with at temperature.
Reflectance has little to do with thermal conductivity. For non-metallics refractive index and particle size and shape are likely the strongest
influences.
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noxx
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Ok Thank You
I'll try to find the best mix of all those materials...
But I'll have to buy them all first 
[Edited on 29-12-2007 by noxx]
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chemrox
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@not_important
what if he painted one sheet with a metal oxide and then sandwiched the coating with another sheet of mica? The mica sheets are thin enough for that
kind of approach.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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noxx
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Oh and what is considered to be a low thermal conductivity value ?
Below 1 W/m.K ?
Thanks
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12AX7
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Yeah, around there.
Can't remember what kaowool typically runs...
Tim
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not_important
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| Quote: | Originally posted by chemrox
@not_important
what if he painted one sheet with a metal oxide and then sandwiched the coating with another sheet of mica? The mica sheets are thin enough for that
kind of approach. |
Why bother with the complexity? Alumina, magnesia, and Zr oxide and silicate don't have the problems and are commonly used in such applications, ZnO
isn't. Even mica isn't that commonly used.
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Fleaker
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If I were noxx, I would probably head to the pottery supply, look for finely powdered zircon, alumina, and silica. Make a paste, and just apply that.
It won't cost much, and it won't be as durable, but it will work well.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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Texium
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