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metalresearcher
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[*] posted on 23-10-2016 at 11:50
Making a really HOT electric resistance furnace


I have a kanthal furnace (http://kanthal.velp.info) which is very controllable and reaches 1100 C.
For obtaining higher temps I use a DC arc welder and carbon rods. Electric arc is VERY hot, but very uncontrollable.
I have done several EAF experiments (http://arcmelt,velp.info) with making CaC2 successfully, boiling metals like Al or Cu (2500 C), melting MgO (2800 C) like plastic, but the area getting that hot is very small.
I have no bricks which can withstand this heat concrete just melts. Blakite cement (rated 1650 C) melts as well.

I am still looking for how to make an electric resistance furnace for reaching, say, 1800 C ?
I have heard about I2R elements as a more affordable alternative to Kanthal Super.
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yobbo II
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[*] posted on 23-10-2016 at 18:27


Seeing this video on youtube I wonder is it possible to use these blades to heat a furnace to very hight temperature. The 'element' is Zirconium oxide. How long would they last?

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

https://en.wikipedia.org/wiki/Ceramic_knife

How do you manage with the S type theromcouple in your furnace. Do you not need it in an inert atmosphere?

There is a good read on the electrical needs of Mo disilicide elements here

http://www.wetcanvas.com/forums/showthread.php?s=55b7d6993a0...

It is spread out accross a few posts/threads but the dude did the controlls all himself.


You can only use graphite in an inert atmosphere
(unfortunately)


Long SiCarbide element here seems cheap. Perhaps you could cut it up into 3 or more elements to make a sensible size?
http://www.ebay.co.uk/itm/NEW-Keith-Silicon-Carbide-Heating-...

[Edited on 24-10-2016 by yobbo II]
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[*] posted on 24-10-2016 at 08:50


The Nernst lamp might be an interesting source as it can be used in open air.

Preheating up the Nernst filament by a Kanthal wire (or a burner) and then the Nernst gets conductive and gets a high temperature source for a furnace.

What material is the Nernst filament made of ?

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[*] posted on 24-10-2016 at 11:50




They are made from a Yittium / Sr oxide.

You can purchase silicon carbide elements on ebay but they are very small.
http://www.ebay.co.uk/itm/Gas-Dryer-Round-Ceramic-Ignitor-Ig...

The blades of ceramic knives I believe are made from Zr Oxide and may be usable as very high temperature elements. Not cheap though.
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[*] posted on 24-10-2016 at 12:33


In the largest online chemical textbook is stated that phase transition may crack the ZrO2 which makes it unsuitable to make a heating element from.it. And does it conduct electricity ?
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[*] posted on 24-10-2016 at 13:04


Quote from the youtube video above

This experimental lamp was made using a piece of Zirconium-oxide ceramics from a ceramic potato peeler :)

These ceramic knives are often sold as a set which includes a peeler knife like this set.

http://www.ebay.de/itm/Ceramic-Kitchen-Knife-Set-Black-Blade...
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[*] posted on 24-10-2016 at 14:19


Quote: Originally posted by metalresearcher  
In the largest online chemical textbook is stated that phase transition may crack the ZrO2 which makes it unsuitable to make a heating element from.it. And does it conduct electricity ?


It says that it can be stabilized against phase changes with yttria, and indeed most (nearly all?) applications of zirconium oxide are the stabilized form:

https://en.wikipedia.org/wiki/Yttria-stabilized_zirconia

And is alleged on that page that "YSZ" was used in Nernst lamps.




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[*] posted on 24-10-2016 at 15:14


From here

http://www.thermcraftinc.com/electrical-resistance-heating-e...



V. ZIRCONIUM OXIDE

This product was introduced in the early 1970’s and allowed chamber temperatures in excess of 2000°C. It was typically sourced from Sweden or China. The material was quite expensive, very sensitive to mechanical and thermal shock, limited to vertical mounting and had a maximum hot length of around six (6) inches. The elements had to be preheated to between 1000 to 1100°C before they became electrically conductive, which required a separate heating system. This feature, coupled with long duration controlled heat up/cool down rates, dictated a rather expensive power supply for successful operation. Since the only practical hot face insulation that can withstand these temperatures is a zirconium oxide based brick (which becomes electrically conductive at typical furnace operating temperatures), great care had to be taken in dealing with clearances between bricks, elements and a rather complex, expensive element support structure least element/system faults to ground occur with a corresponding negative impact on the internal refractories and element failure.

Almost all the applications were for a limited number of small, highly specialized laboratory furnaces used for nuclear or advanced aerospace programs and in the production of artificial gemstones, such as sapphires. As such, the material was deemed to be a nonviable product from an economic standpoint and, at this time, the writer is unaware of a readily available commercial source for zirconium oxide heating elements.







If you could set up something like what is here you could go to very high temperautres but you need inert gas to protect the graphite.

http://www.sciencemadness.org/talk/files.php?pid=368023&aid=...
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[*] posted on 26-10-2016 at 07:50


Interesting article.
It also states that metals like Mo, W or Ta are rather useless as they oxidize too quickly, which I already knew.
And about metals: not listed is Ir wire. Ir is a very noble metal, resistant to almost all chemicals and melts at 2450 C, so it can be used in air to 2000 C ?


So it is an option to buy a ceramic knife and (ab)use it as a heating element ?

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[*] posted on 26-10-2016 at 13:06


Check this out

http://www.ebay.com/itm/1pcs-Zirconium-Oxide-Ceramic-Rod-Dia...

They say it's good to 1900 degree C. Would it last? Don't know myself.

The seller has other sizes of rod as well. What sort of a power supply would you need and you still need to heat the oven/element before you can get the stuff to conduct but that would be dooable.
The knives would be a great experiment of you came accross one that was broken but thats a long shot.
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[*] posted on 27-10-2016 at 17:37


Some pdf's on Pt elements. It would appear Rhodium Pt alloy is best. They will cost a fortune IMO.

If you keep in eye on ebay sometimes cheap Kanthal super (Mo Disilicide) elements appear.

High temperature refractory bricks cost a fortune if you can find them. I believe this is because the stuff that goes into them must be very pure + there is not much of them manufactured.
You would be better off purchasing high temperature refractory cement for the inside layer which seems to be cheaper than ready made bricks and you can shape it to perhaps a shape that suits yourself.



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[*] posted on 28-10-2016 at 16:10


Very high temperature bricks here
http://www.ebay.com/itm/3200-Degree-INSULATING-Firebrick-64-...

You do not see these too often. They are very dense and therefor not great insulators. You would need a layer in insulating bricks on the outside but they will take the blunt of the very high temperature.
Perhaps if it is in your neck of the woods.........

More here
http://www.ebay.com/itm/Korundal-Refractory-Fire-Brick-Arch-...


[Edited on 29-10-2016 by yobbo II]
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[*] posted on 28-10-2016 at 22:34


If you want to push really high temperatures your options for materials dwindle (as well as tools for measuring the temperature).

One way to increase your options is too accept the idea of using an inert atmosphere. After all, welders use argon gas (and other mixtures) in the open air to shield their hot zone from oxidation.

Making a fairly tight outer structure is not that challenging - air does not travel through cemented brick, or metal walls. Maintaining a modest gas flow can keep out any air intrusion.

Graphite is cheap, easy to work, and extremely refractory (though not an electrical or thermal insulator) so being able to use it could be very handy.

Alumina crucibles good to 1800 C are not terrifically expensive up to the 1000 mL size, and so could be used as a furnace chamber:

http://www.ebay.com/itm/1pcs-Alumina-Aluminum-Oxide-Ceramic-...

Graphite-clay crucibles rated to 1510 C are even cheaper:
http://www.budgetcastingsupply.com/Foundry-Crucibles-s/1830....

Since the expected use of graphite-clay crucibles is for them to be lifted up with tongs while full of molten metal, I suspect their operating temperature could be pushed even higher in a no-load situation.

One though I had was to use the graphite crucibles with lids available cheap on eBay as a disposable chamber wherein the material to be processed is placed, and then an electric arc struck on the top and the bottom (or even just one of these). Graphite has very good thermal conductance so the heat should get conveyed to the interior quite efficiently. Exact temperature control would be hard to arrange (an optical pyrometer hole, and an arc on-off switch?) but if you just need to hit a threshold temperature (e.g. a decent melting point to boiling point gap) then this might not be a serious problem.




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[*] posted on 29-10-2016 at 04:20


Another thought - I think you might not be giving electric arc heating its due. The extremely high temperature is indeed in a small area and difficult to control, but that can be beside the point.

A high temperature heating element is anything that dissipates a lot of power in a small volume, and can withstand the temperatures produced.

An electric arc is able to dissipate very large amounts of power in a small volume, same as any resistance heater. If you just ignore the actual arc zone, its just a heating element same as any resistance heater, though the geometry needs to be adapted to, for example placing your material in an annular chamber around the arc, which is then externally heavily insulated to retain the heat produced. The high initial temperature, and energy density gives some inherent lee way in design.

Immersed arcs, in which the arc is literally buried in the material being processed is a common industrial process. Even without actually burying the arc in you stuff, you have it surrounding the arc, as close as you like it.




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[*] posted on 1-11-2016 at 00:35


AFAIK, tungsten is the metal with the highest melting point that we know of. Carbon has the highest melting point of all the elements. Thorium oxide has the highest melting point of any oxide, although the lanthanides are pretty close, and not radioactive.

TIG welding rods often have 2-4% lanthanide oxides or thorium oxide in them, to protect them from oxygen when they get hot. The oxides will form a powdery coating on the surface that is by no means permanent, but is pretty effective at prolonging the life of the rod.
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[*] posted on 1-11-2016 at 06:27


Employing the bakery synthesized carbon foam idea from the concurrent thread, how about this for a high temperature furnace.

Take one graphite crucible, and a graphite tube, with enough diameter that you can insert your graphite electrode without causing any side arcs.

Drill a shallow socket for said tube in the exact center of the inside bottom of the crucible, and embed the crucible is a big chunk of carbon foam. Place this arrangement inside a suitably refractory container that can be kept flooded with argon.

Voila, a high temperature arc heated furnace that might be able to reach 3000 C.

With enough carbon foam insulation you should be able to use a reasonably affordable refractory container, since the heat flow to a much larger surface area, through a good layer of insulation, should be tolerable. You could either use high temperature firebrick, or perhaps just thin steel with external air cooling keeping the temperature down. After all, insulating the outer shell is not really going to help the central furnace at all.




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[*] posted on 1-11-2016 at 10:21


Do all carbon arc furnaces have an inert gas cover. They must have IFAIK if the rods are going to last as the carbon oxidizes badly as the temp goes over 1000 degrees C or so.

The Lanthanum oxide etc additions do not protect the tungsten rod much from oxidation it is the inert shield of gas (argon) that you use when welding that does the bulk of the shielding.

There is a quick and easy carbon arc furnace made with a brick here.
https://www.youtube.com/watch?v=VTzKIs19eZE

Things get difficult when you want to heat a larger area to a high temperature and hold the heat for a period of time (say an hour or so). The carbon arc is the way to go but you need the inert gas which is somewhat expensive if you have to purchase it just for some experimentation. The bottles can be the expensive bit if you must rent or purchase outright.

Is there any easy way/device to remove O2 from the air and use that.

Another possible heater material which is not too expensive is Mo wire so long as you have inert gas.

[Edited on 1-11-2016 by yobbo II]
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[*] posted on 1-11-2016 at 10:34


Quote: Originally posted by yobbo II  


Is there any easy way/device to remove O2 from the air and use that.

[Edited on 1-11-2016 by yobbo II]

Oil Tankers use their own exhaust gas (CO2, CO, N, C, & hydrocarbons) to inert the cargo tanks. Works well enough to prevent most explosions.
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[*] posted on 1-11-2016 at 11:05


Quote: Originally posted by yobbo II  
Do all carbon arc furnaces have an inert gas cover. They must have IFAIK if the rods are going to last as the carbon oxidizes badly as the temp goes over 1000 degrees C or so.

[Edited on 1-11-2016 by yobbo II]


Well, submerged arcs don't have a problem. And copper coated carbon welding rods may not have much problem (but I am not sure). Arc lights ran in open air, but it may be that the oxidation did not add much to the rod consumption that happened in any case.

But if you want a really high temperature furnace, >2000 C, to melt metals and such you probably need an inert atmosphere anyway since nearly everything oxidizes at those temperatures.

If you can afford welding equipment, you can probably afford a bottle of argon.




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[*] posted on 1-11-2016 at 11:17


Great tips !
I already have experimented with electric arcs. Making CaC2 from CaO + charcoal, boiling metals like Al or Cu.

Argon may help a lot, particularly for attempts to make Na or K by heating their carbonates with charcoal to 1200 C. That does work, but the metal vapor immediately burns in air, so flushing the retort with Ar might allow to distill off the alkali metal.
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[*] posted on 1-11-2016 at 11:52


this book might be useful here and is generally quite interesting:

https://archive.org/details/TemperaturesVeryLowVeryHigh




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[*] posted on 7-11-2016 at 15:40


There is a Pt / Rh type s 1600 centrigade thermocouple here

http://www.ebay.com/itm/WRP-100-225mm-probe-S-type-platinum-...

It seems a bargain.
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[*] posted on 9-11-2016 at 18:00



There is Molybdenium Disilicide elements available here which are much cheaper than usually found on ebay. Problem is they sell ten at a time.

Seller
jinyuelectricheatingmaterial

eg
http://www.ebay.com/itm/10pcs-DB-Dumbbell-Sic-Jinyu-Electric...

They are lots of different shapes/sizes.
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[*] posted on 3-12-2016 at 20:42


Quote: Originally posted by yobbo II  


They are made from a Yittium / Sr oxide.

You can purchase silicon carbide elements on ebay but they are very small.
http://www.ebay.co.uk/itm/Gas-Dryer-Round-Ceramic-Ignitor-Ig...

The blades of ceramic knives I believe are made from Zr Oxide and may be usable as very high temperature elements. Not cheap though.


I bought some large SiC elements through Alibaba - but SiC is only good to about 1500C (1600C max with short life).

MolyD is good to I think about 1800C. MolyD elements bend towards each other when they are heated and are very unforgiving about dust/etc. They also need a low voltage, high current power supply - you can't just plug them into a socket like you can with properly sized SiC.
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[*] posted on 3-12-2016 at 20:44


Quote: Originally posted by yobbo II  
Do all carbon arc furnaces have an inert gas cover. They must have IFAIK if the rods are going to last as the carbon oxidizes badly as the temp goes over 1000 degrees C or so.
[Edited on 1-11-2016 by yobbo II]


Yes - or vacuum. Otherwise the carbon foam insulation would sublime in a hurry!
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