bereal511
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High Temperature Dark Body
Hello, I'm looking for a dark material (preferably black) capable of withstanding 1600+ degrees Celcius without decomposing or melting. I'm working
with a large Fresnel lens, and I'd like to see if I can make a solar furnace just for kicks and giggles. It's difficult to try to work with white
bodied materials because all the light simply reflects off of them. And it seems that most refractory oxides are white. Those that aren't just melt
and become reduced to their respective metals. And graphite burns like crazy in the open environment.
[Edited on 2-7-2006 by bereal511]
As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life -- so I became a
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alancj
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Try a clay-graphite crucible. They are pretty black. I doubt they would burn. There used to be a guy selling them on ebay for dirt cheap, but he sells
them no more! budgetcastingsupply.com would be the only other place to get them, unfortunately they charge an arm and a leg.
-Alan
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12AX7
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Graphite, but it burns. Clay-graphite burns off the surface. Silicon carbide may be suitable.
If you seal the furnace, the graphite won't burn away, the oxygen will be consumed down to carbon monoxide providing a reducing atmosphere.
Tim
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bereal511
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If I were to seal the furnace, how would I keep the glass cover (to allow in the light) sealed to the furnace body and still maintain its reusability?
Because I'd like to use graphite; it's readily available for me.
As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life -- so I became a
scientist. This is like becoming an archbishop so you can meet girls.
-- Matt Cartmill
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neutrino
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What about painting a clear flux over the graphite? As it warms up the flux should melt, forming a flat protective layer that lets light through (as
long as its coming in steep enough).
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unionised
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I know it sounds silly, but what about platinum. I don't mean a Pt crucible, I mean fireclay or some such, coated with Pt black.
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Mr. Wizard
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Why not just make a wash out of MnO2 from an old battery and paint the outside of the crucible. In my experience the stuff is so black you almost
can't get the stain off your hands. It also contains some carbon black which might burn off, but the MnO2 should stay black. You could also try
Magnetite the magnetic Iron oxide, available as black sand , mixed with a binder to adhere to the crucible with some clay wash. Both are also
available at pottery supply shops. Ilmenite is also pretty black.
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12AX7
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1600C will melt platinum (or nearly so), MnO2 and Fe3O4. Actually MnO2 would decompose, then melt. If nothing else, the platinum black would
recrystallize and undoubtedly lose its blackness.
Tim
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franklyn
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Try Obsidian, basalt, rhyolite any volcanic rock
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Mr. Wizard
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True, the MnO2 would decompose, but I think it would form Mn3O4 which is also black with a MP of 1560C, just a wild guess, as I haven't tried it. I'm
guessing the 'color' of the material won't matter much once it gets that hot, as it will be giving off more light. What 'color' is a ceramic pot in a
red hot kiln?
The Hematite and Iron oxides show melting points in the same 1590C range; in an oxidizing atmosphere what would happen to them? IMO bereal511's solar
furnace would be fighting an uphill battle by then, with his crucible radiating heat out of the opening almost as quickly as he was putting it in,
with the 'color' of the thing of minor importance.
Off subject, but has anyone ever seen the solar heated power plant near Barstow CA? They had acres of solar tracking mirrors aimed at a 'boiler'.
When they didn't want the boiler to get hot, they aimed the mirrors at an open air area above the oil filled heat absorbing 'boiler'. It made the air
GLOW with a dazzling brightness, visible for miles. They have since removed the trouble prone stainless steel unit, and I don't know what they are
doing with the facility.
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unionised
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It looks like most rocks would melt.
http://en.wikipedia.org/wiki/Lava
Finely divided tungsten does a fair job of blackening light bulbs.
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bereal511
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Yeah, I heard of about the solar mirrors. I think this is the article on about the solar array and its use.
http://www.energylan.sandia.gov/sunlab/snapshot/stfuture.htm
But back on subject, I think I'll try the tungsten powder or silicon carbide. Those two seem to have the most potential. Does molybdenum readily
oxidize at high temperatures?
As an adolescent I aspired to lasting fame, I craved factual certainty, and I thirsted for a meaningful vision of human life -- so I became a
scientist. This is like becoming an archbishop so you can meet girls.
-- Matt Cartmill
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12AX7
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Molybdenum and tungsten are best used in a reducing atmosphere.
Moly silicide might be worth a look?
Tim
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franklyn
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He's right
Reading over the specifications I take back what I said. Assuming that
the wattage of your collector would be sufficient to melt any sizable
chunk of substrate. The problem with refractory materials used in kilns
is that they are all , refractory (white). Your alternative would be
abrasive ceramics that are grey to black in color.
Carborundum ( Silicon Carbide ) melts at 2700° C
See -> http://en.wikipedia.org/wiki/Carborundum
Tunsten Carbide melts at 2870° C
See -> http://en.wikipedia.org/wiki/Tungsten_Carbide
Another thought would be to use carbon aerogel such as used in
spaceshuttle heat sheilds. See -> http://en.wikipedia.org/wiki/Aerogel
Also known as pyrolytic carbon
See -> http://en.wikipedia.org/wiki/Pyrolytic_carbon
But before you spend big bucks try the simple clay tile approach.
.
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neutrino
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Any form of carbon will burn, as we said before.
The carbides may be worth a try. But I still get the feeling those would be oxidized to carbon dioxide and a metal oxide.
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unionised
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Come to think of it, how hot will this thing get anyway.
You have so many square metres of sunlight . That gives you a fixed input power. How hot will the crucible need to get before it is losing as much by
radiation as it is gaining from the sun. I don't fancy doing the maths but I'd start by googling "black body radiation".
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Twospoons
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Silicon carbide is what you want. Its emissivity is high all the way into the deep IR, and stays high at elevated temperatures. Has been used (in
sponge form) in massive solar power powerstations as the target, as it lasts longer than anything else.
Failing that - there is a kiln lining material called ITC-100, which is painted on the inside of kilns to improve the radiation efficiency of the kiln
walls. (I suspect ITC-100 is mostly SiC anyway!)
Helicopter: "helico" -> spiral, "pter" -> with wings
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JohnWW
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Carbides can be used as refractories only if they are kept away from the presence of air at high temperatures. Metal or Si or B carbides would be
largely oxidized to metal oxides and silicates and B2O3, with the C going off as CO or CO2; and may also partly react with N2 to form cyanogen, (CN)2,
and often brittle metal nitrides.
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ethan_c
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| Quote: | Originally posted by JohnWW
Carbides can be used as refractories only if they are kept away from the presence of air at high temperatures. Metal or Si or B carbides would be
largely oxidized to metal oxides and silicates and B2O3, with the C going off as CO or CO2; and may also partly react with N2 to form cyanogen, (CN)2,
and often brittle metal nitrides. |
Mmmm…cyanogen gas…
I think tungsten carbide would work better than the 'metalloid' carbides, and its pretty bloody cheap too (ebay, tungstenheavypowder.com, etc). I
certainly have never been able to melt or even visibly change it at all, where I can easily oxidize the aforementioned Si and B carbides, burn through
firebrick, melt high-cone ceramic, all with a gas torch, even using a reducing flame.
Totally OT, I have two rather beautiful and quite substantial pieces of B<sub>4</sub>C and SiC- if one is interested in gems, crystals,
even just less common compounds that are quite pretty, check some of that stuff out!
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12AX7
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Well, the thing about SiC and silicides is SiO2. Keep it below 1800C or so and you won't fuse the silica away.
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MI...
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=CA... (etc.)
And perhaps others.
ITC-100 is a reflective, not an absorbent. Kilns heat inside from the walls, not the walls themself; thus, heat is better reflected away from the
walls. It's based on zirconia, AFAIK.
Is your SiC iridescent? 
Tim
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