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Admagistr
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A high-temperature mini-furnace for temperatures above 2000 C.
I'm thinking about the possibility of creating a mini-furnace capable of reaching a temperature of just over 2,000 C. The heat source would come from
Nernst's lamp. Has anyone looked into this and gotten any results? The furnace would be usable, for example, for melting alumina to create rubies and
sapphires, and for similar processes requiring very high temperatures...
https://www.youtube.com/watch?v=aYvWC-mjfdI
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metalresearcher
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Seems interesting. The Nernst lamp appeared to be a commercial success in the late 1890s, but the tungsten filament lamp superseded it.
The disadvantage is that the 'conductor' made of a Y2O3 / ZrO2 mixture bar only conducts above a certain temperature, hence the video shows the user
heating it with a burner until it conducts. Now is the question: how could one get (or make) these bars ?
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zerodan
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Quote: Originally posted by Admagistr  | I'm thinking about the possibility of creating a mini-furnace capable of reaching a temperature of just over 2,000 C. The heat source would come from
Nernst's lamp. Has anyone looked into this and gotten any results? The furnace would be usable, for example, for melting alumina to create rubies and
sapphires, and for similar processes requiring very high temperatures...
https://www.youtube.com/watch?v=aYvWC-mjfdI
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At these temperatures the problem isn't providing the heat, rather it's preventing the furnace from melting down.
How do you plan to do that?
Most refractory bricks are made from silca+alumina and some other things, if you want to melt alumina it's going to be tough.
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Jenks
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The bottom of the wikipedia article you link mentions that bars of silicon carbide "globars" have replaced Nernst bars for IR spectroscopy. So maybe
one of those would do.
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metalresearcher
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| Quote: Originally posted by Jenks |
The bottom of the wikipedia article you link mentions that bars of silicon carbide "globars" have replaced Nernst bars for IR spectroscopy. So maybe
one of those would do. |
Indeed Globar are SiC bars which can be heated till 1600 C (not that hot as Nernst bar), but does not heat preheating as it conducts at room
temperature.
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Admagistr
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The "Nernst stick" has the great advantage of being able to heat up to over 2,000 C in air and not be oxidized!! And is extremely chemically
resistant. Tungsten wire doesn't stand a chance in this. The "Nernst stick" can withstand higher temperatures than SiC! It's just a
problem of how to make it or where to get it, it's ZrO2 stabilized by Y2O3.
I'd make a furnace lining out of ZrO2 with a little "water glass" mixed in. "Water glass" is approximately 35% Na2SiO3 solution in water. ZrO2 as a
furnace lining could be a great choice, it's one of the heat-resistant materials with the highest heat resistance ever. It can hopefully be found in
ceramics supplies, in larger quantities.
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phlogiston
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What material is used to make the electrical connections to the actual Y2O3/ZrO2 ceramic?
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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yobbo II
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Some ceramic blades (on expensive kitchen knives) are made from this type of ceramic I belived
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Admagistr
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True, but which particular one to choose? Occasionally the dealer labels by zirconia ceramics, another kind of similar but totally useless ceramics,
with a mixture of Al2O3+ZrO2...Personally, I don't know what's used as a conductor to the Nernst stick, I'd probably choose tantalum? Speaking of SiC,
it oxidizes in the air, only superficially, but still. This oxidation protects it from another attack of oxygen by turning part of the silicon into
oxide, which prevents carbon from being destroyed. In the case of ZrO2-Y2O3 there is nothing to oxidize
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macckone
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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.
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metalresearcher
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MoSi2 is the material of which Kanthal Super is made.
It is really horror stuff for amateurs. It is VERY fragile, particularly under 1000 C and requires a strict heating up scheme, so it is only suitable
for commercial purposes.
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Jenks
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| 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. |
Back to light bulbs, the original Edison bulb used carbon (mp 3,550C) as the filament, the novelty being evacuating the bulb. A graphite crucible
would also have to be protected from oxygen at high temperature. What I don't understand about the above furnaces is that it would seem simpler to
heat the graphite crucible by simply passing an electrical current through it. Why not that?
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Twospoons
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| Quote: Originally posted by Jenks | | What I don't understand about the above furnaces is that it would seem simpler to heat the graphite crucible by simply passing an electrical current
through it. Why not that? |
Because you have to get the current into the crucible with lower resistance connections than the crucible itself, otherwise the heating goes into the
contacts. And of course the contact material also has to withstand the furnace temperatures.
So while in theory you could use fat graphite lead-ins for the current, they also would be very good thermal conductors, which presents a heat loss
problem.
Personally I would go the microwave route - you can completely insulate the crucible for better heating efficiency, and cheap microwave ovens are
readily available.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Admagistr
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| Quote: Originally posted by Twospoons | | Quote: Originally posted by Jenks | | What I don't understand about the above furnaces is that it would seem simpler to heat the graphite crucible by simply passing an electrical current
through it. Why not that? |
Because you have to get the current into the crucible with lower resistance connections than the crucible itself, otherwise the heating goes into the
contacts. And of course the contact material also has to withstand the furnace temperatures.
So while in theory you could use fat graphite lead-ins for the current, they also would be very good thermal conductors, which presents a heat loss
problem.
Personally I would go the microwave route - you can completely insulate the crucible for better heating efficiency, and cheap microwave ovens are
readily available. |
Thanks you for the advice! You mean a regular microwave for kitchen use? When I tried to melt a powdered Al2O3 in it with a little amount of Cr2O3, by
placing a closed ring of 1 mm thick Al wire on this mixture surface, I had to terminate the experiment prematurely after about 20 seconds, because the
microwave started to stink and something was probably burning. How do you make not burn the magnetron or any other part of the microwave? Would help
the perfect thermal insulation of the crucible? Otherwise, the experiment went well, I created tiny rubies and one formed on the end of a overburnt
aluminum wire. Thank you!
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Twospoons
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Ideally you want to modify the oven by inserting a circulator or isolator between the magnetron and waveguide. This can be used to dump reflected
power into a dummy load, protecting the magnetron. But this is into the realm of RF engineering - not trivial.
It would probably help to have a bigger susceptor in the oven for heating, probably SiC or graphite rather than Al metal. Al is too conductive to use
as a heater - and it melts too low.
The other thing worth doing is to use a variac to control the magnetron power, but to do this you will need a separate transformer for the filament
supply - which you can pull from a second MW oven ( same model preferably).
So a bit of engineering there, but probably easier than trying to make a zirconia element.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Jenks
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| Quote: Originally posted by Twospoons | | Quote: Originally posted by Jenks | | What I don't understand about the above furnaces is that it would seem simpler to heat the graphite crucible by simply passing an electrical current
through it. Why not that? |
Because you have to get the current into the crucible with lower resistance connections than the crucible itself, otherwise the heating goes into the
contacts. And of course the contact material also has to withstand the furnace temperatures.
So while in theory you could use fat graphite lead-ins for the current, they also would be very good thermal conductors, which presents a heat loss
problem.
Personally I would go the microwave route - you can completely insulate the crucible for better heating efficiency, and cheap microwave ovens are
readily available. |
The heat loss isn't ideal, but otherwise your description sounds very simple - take a carbon rod and narrow it in the middle, drill a hole in the top
of the narrowed part to serve as a crucible, surround the thing with a steel box or can using a ceramic to insulate the carbon from the can, purge
with nitrogen and pass enough current through to heat the narrowed portion to the desired temperature. Maybe spray the can with water if it gets too
hot. I like that this doesn't require costly or complicated technology. Is there some reason this wouldn't work?
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Twospoons
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Of course that would work - but that wasn't your original question was it? Making a graphite heating element with a narrow mid section is not the
same as passing current through a thick graphite crucible.
Looking around the net it would seem induction heating of graphite crucibles is a common method. Safer than microwave engineering, but you'd need an
induction heater supply.
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Fulmen
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@Admagistr: What is your current level of proficiency? You can't just jump in at 2000°C, that's wizard level alchemy...
We're not banging rocks together here. We know how to put a man back together.
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yobbo II
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Thread here of use.
http://www.sciencemadness.org/talk/viewthread.php?tid=70334
Super Kanthal can be run from a large welder transformer and a variac. I have done it. The Super K is hard to purchase cheap and you need a large
variac (and welder).
Yob
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wg48temp9
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Induction heating of a graphite crucible with water cooled copper tube coil would be the way to go. However at 2,000C and higher the graphite will
be burning in air and the heat loss will be very high.
The burning can be easily solved with an inert atmosphere or a vacuum. A vacuum would have the advantage of eliminating convective cooling but that
still leaves radiative cooling which at 2,000C and above will be very high even in a vacuum. That heat loss can be reduced with a one or more
reflective shields and if sufficiently far from the crucible can be made from stainless steel or even copper. The shields may have to be water cooled
unless they are made of a refractory metal.
With a small 20 ml crucible in a vacuum with reflectors as above, perhaps only one or two kW of heating power would be required.
A 2.5kW (probably more accurately half of that) high frequency induction heating supply is relativly cheap from China £90.5 on ebay see https://www.ebay.co.uk/itm/402913450608?hash=item5dcf835a70:...
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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Twospoons
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I'd put the graphite crucible inside a zirconia crucible for a first layer of insulation. Vacuum would be more of a PITA than inert gas, probably
not worth the effort since radiative heat transfer will far outweigh convective heat transfer (from memory radiative goes up as the fourth power of
delta T).
I hadn't realised those chinese induction heaters were so cheap - might have to put one on my wish list to play with.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Admagistr
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| Quote: Originally posted by Fulmen | | @Admagistr: What is your current level of proficiency? You can't just jump in at 2000°C, that's wizard level alchemy... |
@Fulmen I'm sorry, I don't know if I understand your question correctly? English is not my mother's language. What exactly do you find unrealistic?
I'm not an electrician or a radio amateur,..I have some theoretical knowledge, but theory is not the same as practice...
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wg48temp9
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| Quote: Originally posted by Twospoons | I'd put the graphite crucible inside a zirconia crucible for a first layer of insulation. Vacuum would be more of a PITA than inert gas, probably
not worth the effort since radiative heat transfer will far outweigh convective heat transfer (from memory radiative goes up as the fourth power of
delta T).
I hadn't realised those chinese induction heaters were so cheap - might have to put one on my wish list to play with. |
I wounder how insulating zirconia crucibles with a lid would be. Perhaps with an outer high alumina insulation it could be heated in a domestic
microwave oven and reach 2,000C. With a lid the oxidation of the graphite would be significantly reduced.
You need a multi-kilowatt 48V power supply to power the induction heater. I noticed in the "12V PSU for electrolysis" thread referred to a cheap
server 70A@12V power supply. I checked on ebay and yes they are cheap for used and even tested ones they cost about £10 to £15. Three or four of
them would easily drive the induction heater. From memory they are also suitable for powering carbon gouging rods (copper removed) used as heating
elements.
PS: There are youtube videos showing how these server power supplies can be modified to change the output voltage or even make it variable.
They also show what the output connections are and how to enable them.
The following link has the connections and mod info for cheap and commonly available server power supplies: https://www.xsimulator.net/community/threads/the-ultimate-po...
[Edited on 12/4/2021 by wg48temp9]
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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metalresearcher
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These cheap induction furnaces from ebay are very picky on overpowering. They are not protected to overpowering which is dependant on the contents of
the crucible. The MOSFETS decease easily when overpowered.
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wg48temp9
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| Quote: Originally posted by metalresearcher | | These cheap induction furnaces from ebay are very picky on overpowering. They are not protected to overpowering which is dependant on the contents of
the crucible. The MOSFETS decease easily when overpowered. |
Can you give more detail as to what you mean by "overpowered MOSFETS"?
My understanding is as follows:
1) The minimum component count circuits don't have protection against both MOSFETS being on at the same time which can happen during switch "on" the
resulting high dissipation in the MOSFETS destroys them.
2) There is usually little to no limits on what the generators can drive other than the maximum power supply voltage and current drawn. The more
expensive generators do include a coil which I would assume the generator is capable of driving at least without anything loading the coil.
I am also aware that maximum voltage, current and in particular power handling specs of cheap modules from China tend to be optimistic.
I am wg48 but not on my usual pc hence the temp handle.
Thank goodness for Fleming and the fungi.
Old codger' lives matters, wear a mask and help save them.
Be aware of demagoguery, keep your frontal lobes fully engaged.
I don't know who invented mRNA vaccines but they should get a fancy medal and I hope they made a shed load of money from it.
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