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Author: Subject: A high-temperature mini-furnace for temperatures above 2000 C.
Fulmen
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[*] posted on 4-12-2021 at 00:53


@Admagistr: My advice; start with a 1000-1200°C furnace. The challenge grows exponentially with temperature, working at 2000°C with no prior experience isn't realistic.



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[*] posted on 4-12-2021 at 09:01


+1 Fulmen

a kitchen oven is 250C.
a propane forge is about 1000C and an electric furnace capable of 1000C is not as easy as it seems.
Getting to even 1300C requires some engineering challenges (upgrading to fuel oil).
Even though the flame temperature of propane is higher than 1300C getting a working forge or furnace that heats that hot is not as intuitive as it may seem.
Most kilns do not go to cone 10 which is 1305C.
Kanthal A1 is usable up to 1400C with the right wire guage.
But you have to have thick walls (6" of insulation) and the right wattage.
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[*] posted on 5-12-2021 at 05:04


Quote: Originally posted by macckone  
There are only three normal types of heating that will get you to the temperature necessary to melt aluminum oxide.
1) carbon arc furnace (3000C)
2) hydrogen oxygen furnace (3080C)
3) microwave furnace (2200C for commercially available set ups)

All three use graphite crucibles to contain the melt.

Resistance elements top out at about 1850C with MoSi2

From a practical perspective the second two are better as no carbon particles get in the melt.
The microwave is just barely enough to reach the right temperature.
A hydrogen/oxygen type set up is used to make gem quality and/or clear alumina windows.


I have been looking for references about option 2, using a graphite crucible in a hydrogen-oxygen furnace, but there doesn't seem to be that common.

Does anybody have any info on this, i.e. how do you use a graphite crucible in a hydrogen-oxygen furnace?


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[*] posted on 5-12-2021 at 08:03


Quote: Originally posted by JohnnyBuckminster  


I have been looking for references about option 2, using a graphite crucible in a hydrogen-oxygen furnace, but there doesn't seem to be that common.

Does anybody have any info on this, i.e. how do you use a graphite crucible in a hydrogen-oxygen furnace?


Red hot carbon reacts with water vapor to produce CO and H2. Graphite probably does the same. So a red hot graphite crucible, particularly one at 3,000C, may not last very long when heated with a hydrogen oxygen flame.




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[*] posted on 5-12-2021 at 17:06


Quote: Originally posted by Fulmen  
@Admagistr: My advice; start with a 1000-1200°C furnace. The challenge grows exponentially with temperature, working at 2000°C with no prior experience isn't realistic.


I worked with a laboratory crucible furnace with a temperature up to 1000 C. Some time ago, I purchased a very precisely controlled modern laboratory furnace, with PID regulation, from a European manufacturer, up to a temperature of 1250 C. So some little experience I have...
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[*] posted on 5-12-2021 at 17:16


Quote: Originally posted by JohnnyBuckminster  
Quote: Originally posted by macckone  
There are only three normal types of heating that will get you to the temperature necessary to melt aluminum oxide.
1) carbon arc furnace (3000C)
2) hydrogen oxygen furnace (3080C)
3) microwave furnace (2200C for commercially available set ups)

All three use graphite crucibles to contain the melt.

Resistance elements top out at about 1850C with MoSi2

From a practical perspective the second two are better as no carbon particles get in the melt.
The microwave is just barely enough to reach the right temperature.
A hydrogen/oxygen type set up is used to make gem quality and/or clear alumina windows.


I have been looking for references about option 2, using a graphite crucible in a hydrogen-oxygen furnace, but there doesn't seem to be that common.

Does anybody have any info on this, i.e. how do you use a graphite crucible in a hydrogen-oxygen furnace?




Also, I haven't heard anything about oxy-hydrogen furnaces with graphite crucible in which Al2O3 melts...Carbon reacts with water vapor at high temperatures and creates dangerous gaseous products...In our country, it's called "Water Gas."
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[*] posted on 6-12-2021 at 09:57


You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.
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[*] posted on 6-12-2021 at 11:12


@Admagistr: That's good to know, it's hard to give advice without knowing your experience.



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[*] posted on 6-12-2021 at 15:51


Quote: Originally posted by macckone  
You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.


That might work well, thanks for the tip! I've thought about it, too, and I haven't seen a major problem. There's a company in our country that makes sapphire tubes and crucibles by modified Stepanov method. I spoke to their production experts and they told me they use large vessels of molybdenum and tungsten, working in a vacuum, or in an argon atmosphere. Those sapphires can then be surface contaminated with molybdenum. I suggested that they reheat them in the air at a high temperature to oxidize traces of Mo to MoO3, which they did, and when the product came to me it looked completely clean. I had to pay extra for that. Otherwise, their finished sapphire products have a chemical purity of 99.999%!
I'm also thinking and dreaming about the possibility of melting Al2O3 with an infrared CO2 laser. I don't know if any offered by Chinese retailers would be appropriate, I'd be more inclined to make my own CO2 laser, there's a few detailed instructions on how to do it on the internet, but it's not quite simple... The idea is very attractive for me, the laser heating would maintain perfect chemical purity...I even found a video on YouTube where Al2O3 melts in the focus of a giant solar furnace!


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[*] posted on 6-12-2021 at 23:49


A laser is a rather inefficient way to put heat into bulk material. CO2 lasers are about 10% wall-plug to light output.



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[*] posted on 7-12-2021 at 00:22


Quote: Originally posted by Admagistr  
Quote: Originally posted by macckone  
You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.


That might work well, thanks for the tip! I've thought about it, too, and I haven't seen a major problem. There's a company in our country that makes sapphire tubes and crucibles by modified Stepanov method. I spoke to their production experts and they told me they use large vessels of molybdenum and tungsten, working in a vacuum, or in an argon atmosphere. Those sapphires can then be surface contaminated with molybdenum. I suggested that they reheat them in the air at a high temperature to oxidize traces of Mo to MoO3, which they did, and when the product came to me it looked completely clean. I had to pay extra for that. Otherwise, their finished sapphire products have a chemical purity of 99.999%!
I'm also thinking and dreaming about the possibility of melting Al2O3 with an infrared CO2 laser. I don't know if any offered by Chinese retailers would be appropriate, I'd be more inclined to make my own CO2 laser, there's a few detailed instructions on how to do it on the internet, but it's not quite simple... The idea is very attractive for me, the laser heating would maintain perfect chemical purity...I even found a video on YouTube where Al2O3 melts in the focus of a giant solar furnace!





There are some reports about holding Al3+ in graphite crucibles, it is problematic because of the formation of alumina carbides, see for example Hoseinpur & Safarian,

"Results showed that Al in Si–Al melt infiltrates into graphite leading to the formation of aluminum carbides, which accompanies with volume expansion and therefore the crucible destruction."

That might be why it is necessary to use molybdenum / tungsten crucibles.





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[*] posted on 7-12-2021 at 16:08


Quote: Originally posted by JohnnyBuckminster  
Quote: Originally posted by Admagistr  
Quote: Originally posted by macckone  
You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.


That might work well, thanks for the tip! I've thought about it, too, and I haven't seen a major problem. There's a company in our country that makes sapphire tubes and crucibles by modified Stepanov method. I spoke to their production experts and they told me they use large vessels of molybdenum and tungsten, working in a vacuum, or in an argon atmosphere. Those sapphires can then be surface contaminated with molybdenum. I suggested that they reheat them in the air at a high temperature to oxidize traces of Mo to MoO3, which they did, and when the product came to me it looked completely clean. I had to pay extra for that. Otherwise, their finished sapphire products have a chemical purity of 99.999%!
I'm also thinking and dreaming about the possibility of melting Al2O3 with an infrared CO2 laser. I don't know if any offered by Chinese retailers would be appropriate, I'd be more inclined to make my own CO2 laser, there's a few detailed instructions on how to do it on the internet, but it's not quite simple... The idea is very attractive for me, the laser heating would maintain perfect chemical purity...I even found a video on YouTube where Al2O3 melts in the focus of a giant solar furnace!





There are some reports about holding Al3+ in graphite crucibles, it is problematic because of the formation of alumina carbides, see for example Hoseinpur & Safarian,

"Results showed that Al in Si–Al melt infiltrates into graphite leading to the formation of aluminum carbides, which accompanies with volume expansion and therefore the crucible destruction."

That might be why it is necessary to use molybdenum / tungsten crucibles.






On the other hand, Henry Moissan, winner of the Nobel Prize in Chemistry, successfully used his electric arc furnace and graphite crucible to synthesize rubies! I wrote a new topic here recently in a forum. So you can see that even graphite can be used, albeit in a limited way...;)
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[*] posted on 7-12-2021 at 23:50


Quote: Originally posted by Admagistr  
Quote: Originally posted by JohnnyBuckminster  
Quote: Originally posted by Admagistr  
Quote: Originally posted by macckone  
You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.


That might work well, thanks for the tip! I've thought about it, too, and I haven't seen a major problem. There's a company in our country that makes sapphire tubes and crucibles by modified Stepanov method. I spoke to their production experts and they told me they use large vessels of molybdenum and tungsten, working in a vacuum, or in an argon atmosphere. Those sapphires can then be surface contaminated with molybdenum. I suggested that they reheat them in the air at a high temperature to oxidize traces of Mo to MoO3, which they did, and when the product came to me it looked completely clean. I had to pay extra for that. Otherwise, their finished sapphire products have a chemical purity of 99.999%!
I'm also thinking and dreaming about the possibility of melting Al2O3 with an infrared CO2 laser. I don't know if any offered by Chinese retailers would be appropriate, I'd be more inclined to make my own CO2 laser, there's a few detailed instructions on how to do it on the internet, but it's not quite simple... The idea is very attractive for me, the laser heating would maintain perfect chemical purity...I even found a video on YouTube where Al2O3 melts in the focus of a giant solar furnace!





There are some reports about holding Al3+ in graphite crucibles, it is problematic because of the formation of alumina carbides, see for example Hoseinpur & Safarian,

"Results showed that Al in Si–Al melt infiltrates into graphite leading to the formation of aluminum carbides, which accompanies with volume expansion and therefore the crucible destruction."

That might be why it is necessary to use molybdenum / tungsten crucibles.






On the other hand, Henry Moissan, winner of the Nobel Prize in Chemistry, successfully used his electric arc furnace and graphite crucible to synthesize rubies! I wrote a new topic here recently in a forum. So you can see that even graphite can be used, albeit in a limited way...;)



Moissan, didn't he just throw everything into water, so the crucible was "consumed" in the experiment?

Forth and back, I have been thinking about a small scale furnace, ~50 ml volume, for high-temperature experiments, > 2 000 C. I ruled out the electric arc furnace because it seems difficult to control. A hydrogen-oxygen furnace is appealing, highly controllable, can reach temperatures up to 3 000 C, and you don't need to worry about blowing the fuses. But, unwanted chemistry can be induced, because of the atmosphere.

A Nernst lamp is a very interesting option as a heat source, it should be highly controllable, and can operate in an ordinary atmosphere.

The company in your country, that makes sapphire tubes in molybdenum and tungsten crucibles, how do they heat the crucible?
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[*] posted on 8-12-2021 at 03:32


Maybe this Chinese supplier of induction furnace might give you an idea ?

https://www.cdocast.com/category/precious-metal-melting-furn...

Or (cheaper and easier) the Youtube channel of Nighthawkinglight, he has some nice ideas of obtaining high temperatures.
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[*] posted on 8-12-2021 at 07:37


Moissan doesn't say if the crucible went down or not, it just says what voltage and current he used and how long the meltdown lasted, it was only 10 to 15 minutes! Our manufacturer uses induction heating, it's standard procedure for the Czochralski method and its like. The manufacturer is CRYTUR and is based in Turnov, Czech Republic. It has a great tradition and experience, now it is focused on the American market. I thought of powering up that Moissan furnace with four 12V batteries, designed for automobiles, and using an induction, a silencer, some coil to guard the batteries for overload and to keep them from exceeding the prescribed current. An electrician told me that he had once built a similar battery power suply and it worked as a welder for him. In Prague, at Charles University, they use an optical furnace to synthesize rubies and sapphires and other gems, when the necessary temperature is generated in a pipe of quartz glass, in the focus of ellipsoidal mirrors! They use for it powerful halogen lamps. I spoke to their expert and he invited me to tour their labs. I haven't been there yet, because of Covid. Thanks so much for the link and the tips, Greetings German colleagues and friends!


[Edited on 8-12-2021 by Admagistr]
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[*] posted on 8-12-2021 at 20:22


Quote: Originally posted by Admagistr  
Quote: Originally posted by macckone  
You have to use a reverb type set up.
The aluminum oxide is heated from the top, not the bottom.
Hydrogen oxygen flame will ignite graphite.
The graphite crucible is often heated from the bottom with traditional electrical heating to 1400C.
Since graphite is a pretty good insulator, the aluminum oxide in the crucible stays fluid with the flame heat input and the flame doesn't impinge on the graphite.
The aluminum oxide is preheated in a chute as it is dropped into the crucible.

This how they do it for large panes of aluminum oxide.


That might work well, thanks for the tip! I've thought about it, too, and I haven't seen a major problem. There's a company in our country that makes sapphire tubes and crucibles by modified Stepanov method. I spoke to their production experts and they told me they use large vessels of molybdenum and tungsten, working in a vacuum, or in an argon atmosphere. Those sapphires can then be surface contaminated with molybdenum. I suggested that they reheat them in the air at a high temperature to oxidize traces of Mo to MoO3, which they did, and when the product came to me it looked completely clean. I had to pay extra for that. Otherwise, their finished sapphire products have a chemical purity of 99.999%!
I'm also thinking and dreaming about the possibility of melting Al2O3 with an infrared CO2 laser. I don't know if any offered by Chinese retailers would be appropriate, I'd be more inclined to make my own CO2 laser, there's a few detailed instructions on how to do it on the internet, but it's not quite simple... The idea is very attractive for me, the laser heating would maintain perfect chemical purity...I even found a video on YouTube where Al2O3 melts in the focus of a giant solar furnace!




Did you get any information about why they work in vacuum or in an argon atmosphere, is it to protect the crucible or the melt?
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[*] posted on 9-12-2021 at 11:03


They use an inert environment to protect the crucible.
The melt is a very stable oxide.
Graphite crucibles used in industry have a very limited life span but are considerably cheaper than molybdenum or tungsten in the same size.
Once the large chunk is made, they cut off the edges that are 'imperfect' then slice it to make windows.
Diamond saw blades etc, the waste dust is used as a polishing abrasive.
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[*] posted on 9-12-2021 at 18:41


https://www.youtube.com/watch?v=VTzKIs19eZE

i have no idea the temperature that could be reached with this arc furnace, but it is small, uses "common" materials and it is pretty simple to operate (even though i wouldn't call it 100% safe foor newbies).

the electrodes get burned pretty quickly, and the insulating brick he used in the video got pretty beat up by the heat, so i suppose finding a refractory/insulating brick that survives 2.000°C is a challange in itself

i know this post was about a small furnace that uses nerst elements, i have 0 experience with nerst lamps (i only know of them because i had an IR spectroscopy course)





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[*] posted on 9-12-2021 at 19:38


Quote: Originally posted by Ubya  
https://www.youtube.com/watch?v=VTzKIs19eZE

i have no idea the temperature that could be reached with this arc furnace, but it is small, uses "common" materials and it is pretty simple to operate (even though i wouldn't call it 100% safe foor newbies).

the electrodes get burned pretty quickly, and the insulating brick he used in the video got pretty beat up by the heat, so i suppose finding a refractory/insulating brick that survives 2.000°C is a challange in itself

i know this post was about a small furnace that uses nerst elements, i have 0 experience with nerst lamps (i only know of them because i had an IR spectroscopy course)


Great video, thanks! I'll be making a brick of zirconia (ZrO2) reinforced with sodium silicate solution! Zirconia has a melting temperature of 2700C!

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[*] posted on 10-12-2021 at 16:43




Once you 'reinforce' the ZrO2 you will decrease its melting temp.
PURE ZrO2 has a melting temp. of 2700C.

Pure bricks are very expensive. I think it's because they are pure or perhaps they are a low volume produced product or both?
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[*] posted on 10-12-2021 at 17:38


Quote: Originally posted by yobbo II  


Once you 'reinforce' the ZrO2 you will decrease its melting temp.
PURE ZrO2 has a melting temp. of 2700C.

Pure bricks are very expensive. I think it's because they are pure or perhaps they are a low volume produced product or both?


That's a good point, and unfortunately true...Thanks for that. In which case, it would be better to use sodium zirconiate, or a zirconium dioxide water gel... Some of the "zirconia" bricks are actually ZrSiO4, not ZrO2. Also, ZrO2 is mixed with Al2O3 for those purposes. Why these bricks are so expensive I don't know, but maybe because the sintering process that they're probably making takes a long time and high temperatures. Plus ZrO2 is much more expensive than, say, Al2O3...
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[*] posted on 10-12-2021 at 18:42



There is a link here showing Zirconia blocks. They range from 99% to 99.999%

https://www.americanelements.com/zirconium-blocks-7440-67-7

Funny thing is they give the same melting temperautres?

Bricks here at about 12 bucks each!
https://www.ebay.co.uk/itm/264205633418?_trkparms=amclksrc%3...

Tubes of ZiO2+Yit Oxide here but they are probably a silly price.

https://www.preciseceramic.com/products/zirconia-ysz-tube/

Yob

[Edited on 11-12-2021 by yobbo II]

[Edited on 11-12-2021 by yobbo II]
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[*] posted on 10-12-2021 at 23:36


You can also use magnesia (mp 2800 C). I tried to find a supplier of MgO bricks, but could not find one.
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[*] posted on 11-12-2021 at 15:50


Quote: Originally posted by metalresearcher  
You can also use magnesia (mp 2800 C). I tried to find a supplier of MgO bricks, but could not find one.


Thanks a lot! That would be a cheaper solution, I believe:). Moissan used calcium oxide in his furnace as thermal insulation, which surprises me, because of its, to water rather reactive and then corrosive nature...Before it turns into CaCO3.
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[*] posted on 12-12-2021 at 14:02


Magnesia is easily synthesized.
Start with sorel cement, heat until all the HCl is given off.
To make it impermeable you can treat it with milk of magnesia (use the low sodium variety).
Then treat with linseed oil, which will crosslink and carbonize under heat.
At the working temperature you are hoping to achieve, the carbon with graphitize.
This yields a magnesia/graphite composite that is very durable and capable of withstanding all but the highest temperatures (> 2500C) and is more resistant to oxidizing conditions.
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