Sciencemadness Discussion Board - Making a really HOT electric resistance furnace

Making a really HOT electric resistance furnace

metalresearcher - 23-10-2016 at 11:50

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.

yobbo II - 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]

metalresearcher - 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 ?

yobbo II - 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.

metalresearcher - 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 ?

yobbo II - 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...

careysub - 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.

yobbo II - 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&...

metalresearcher - 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 ?

yobbo II - 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.

yobbo II - 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.

Attachment: pmr-v2-i2-038-044.pdf (1.3MB)
This file has been downloaded 729 times

Attachment: pmr-v12-i3-086-088.pdf (242kB)
This file has been downloaded 709 times

yobbo II - 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]

careysub - 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.

careysub - 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.

Melgar - 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.

careysub - 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.

yobbo II - 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]

mikeehlert - 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.
Mike

careysub - 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.

metalresearcher - 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.

mayko - 1-11-2016 at 11:52

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

https://archive.org/details/TemperaturesVeryLowVeryHigh

yobbo II - 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.

yobbo II - 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.

3DTOPO - 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.

3DTOPO - 3-12-2016 at 20:44

Quote: Originally posted by yobbo II

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

3DTOPO - 3-12-2016 at 20:48

Quote: Originally posted by careysub

With enough carbon foam insulation you should be able to use a reasonably affordable refractory container

That would only work with an inert atmosphere or vacuum. Somewhere over 600C carbon sublimes, and the hotter it is, the faster it goes.

3DTOPO - 3-12-2016 at 23:52

Quote: Originally posted by metalresearcher

I have no bricks which can withstand this heat concrete just melts. Blakite cement (rated 1650 C) melts as well.

BTW: I use this castable refractory for a steel melting furnace - its really nice stuff. Its rated to 1760C. I use that for the hot face. Not quite 1800C - but darn close - I imagine it would last quite a while.

Backed with insulating refractory for insulation. Its rated for 1648C.

The only higher rated stuff (for working in earth's atmosphere) that I aware of is MgO and ZrO refractory, but its expensive and difficult to source. Perhaps a thin layer of ZrO for your hot face, then the 1760C followed by the 1648C insulating refractory.

careysub - 4-12-2016 at 11:52

Quote: Originally posted by 3DTOPO

Quote: Originally posted by careysub

With enough carbon foam insulation you should be able to use a reasonably affordable refractory container

That would only work with an inert atmosphere or vacuum. Somewhere over 600C carbon sublimes, and the hotter it is, the faster it goes.

Yes, I discuss the necessity and use of an argon inert atmosphere several times on this thread - not a real problem IMHO, a really high temperature furnace is going to be enclosed anyway due to the intolerable loss of heat otherwise and argon is cheaply and readily available to provide inert atmospheres for welding.

An inert atmosphere is likely required for most processes of interest at extremely high temperature - most refractory metals for example oxidize readily far below their melting points.

An additional benefit: argon has a lower thermal conductivity that air (or nitrogen) and thus improves the insulation factor.

Reference indicating that carbon sublimes at atmospheric pressure at anything like 600 C? This is the first I've heard of it. Its widespread use in high temperature crucibles and the like seems to refute this. Most sources claim it has one of the highest sublimation temperatures.

3DTOPO - 4-12-2016 at 14:35

Quote: Originally posted by careysub

Quote: Originally posted by 3DTOPO

Quote: Originally posted by careysub

With enough carbon foam insulation you should be able to use a reasonably affordable refractory container

That would only work with an inert atmosphere or vacuum. Somewhere over 600C carbon sublimes, and the hotter it is, the faster it goes.

Reference indicating that carbon sublimes at atmospheric pressure at anything like 600 C? This is the first I've heard of it. Its widespread use in high temperature crucibles and the like seems to refute this. Most sources claim it has one of the highest sublimation temperatures.

I may have used the wrong term, but graphite crucibles are definitely consumable, and the higher temperature I use them at, the shorter they last. I will be lucky to use a brand new thick-walled high purity graphite crucible 10 times when melting copper before it is used up. Clay graphite crucibles last longer than the high purity crucibles because of their clay content (usually alumina based clays).

I guess it is oxidization that is the issue and not sublimation?

http://jes.ecsdl.org/content/110/6/476

yobbo II - 4-12-2016 at 14:54

How tolerent are moly elements to water vapour, lots of water vapour. I was led to believe they are fine in an oxidizing atmosphere.
How delicate are moly d when it comes to dust etc from use in a more diry furnace? I thought dust etc would not bother them.

I have purchased two moly d elements cheap on ebay, about 160 dollars). The heating part is around 200mm long and 6 mm dia. They run at around 160 amps (200 amps is there max current.) They are rated at 1800C.
I am hopeing to run them off a large variac and a buzz box welder transformer. I will perhaps heat the furnace to 600C or so simply by placing some electric heater elements (domestic, 'pencil' elements) into the furnace which can then be removed as the buzz box is brought into action. The difficult part with moly d is when they are cold. Very low resistence.

There is details of running mold d's at the following link

http://www.wetcanvas.com/forums/showthread.php?t=490969

Would the following elements , quartz halogen heater elements, work as heaters in a furnace.
http://www.ebay.co.uk/itm/3-Pack-400W-Elements-Halogen-Heate...

http://www.ebay.co.uk/itm/RTC-LA-306-LA-304-IR-FURNACE-INFRA...

The second link are actually furnace elements. More expensive than the domestic heater spares.
The spares are cheap and would be great for agressive atmosphers (I presume). I have purchased 6 and we will see.
Perhaps I will need to attach Nickel wire to each end?

Yob

3DTOPO - 4-12-2016 at 16:09

Quote: Originally posted by yobbo II

How tolerent are moly elements to water vapour, lots of water vapour. I was led to believe they are fine in an oxidizing atmosphere.

Apparently not good according to this paper.

Quote: Originally posted by yobbo II

How delicate are moly d when it comes to dust etc from use in a more diry furnace? I thought dust etc would not bother them.

I don't know exactly, but when I was researching them, I read that they are susceptible to dust and best used in a clean room. I guess the cleaner they operate, the longer they will last.

Quote: Originally posted by yobbo II

The second link are actually furnace elements. More expensive than the domestic heater spares.

It states on that page they are rated up to 1000C.

yobbo II - 4-12-2016 at 16:31

Quote: Originally posted by 3DTOPO

Apparently not good according to this paper.

Balls. There goes the Moly D idea out the window!

careysub - 4-12-2016 at 22:54

Researching refractory metals I came across this report about greatly reducing the oxidation of refractory metals by phosphiding them.

Phospiding is done by heating the metal in an oven with a phosphorus atmosphere.

Not too attractive a process.

Yet, thinking about, methods of producing phosphorus using aluminum and any of many phosphates at not too high a temperature have been detail on this site. The main problem is containing the phosphorus while also getting it out of the reaction vessel safely.

Perhaps a sealed vessel phosphiding process could be devised, in which the phosporous is produced in the sealed vessel via the aluminothermic reaction, and then destroyed through oxidation to P2O5 before it is opened.

From the report:
ABSTRACT
Samples of niobium, tantalum, molybdenum and tungsten metals
have been given phosphided cases by heating in a phosphorous
atmosphere. The samples were subsequently exposed to air at an
elevated temperature. In tests at 800°C, the phosphided samples
resisted air oxidation markedly compared to the pure untreated
samples. The phosphided cases were only a few microns thick yet
oxidation resistance of all these samples extended for several hours.
Phosphided samples with 90° edges did not resist oxidation at the
edges as well as rounded edge samples.

And

Phosphide cases have been prepared on a number of metals by the
direct reaction of elemental phosphorus vapor on the massive metal.
Investigators at the Nuclear Corporation of America (4) have prepared
phosphide cases on sheets of four refractory metals, yttrium, zirconium,
hafnium and molybdenum by this means at temperatures ranging between
600° and 1000°C. Although the phosphiding treatment was for periods
of up to 20 hours, no values of total phosphorus pick up or phosphide
case thickness were reported.

Attachment: Oxidation studies of some refractory metals.pdf (645kB)
This file has been downloaded 823 times

yobbo II - 5-12-2016 at 08:04

The Moly D is now back in the window........

My understanding of Moly D was that it formed a coating of SiO2 on the outside which protected it. This coating has to form the first time the elements are fired up. If the elements are held at a 'low' temperature the first time they are fired up (with or with or without extra water vapour) in air they will oxidize.
Once they are properly fired up and 'cured' on their first run they should be OK from then on (minus dust).
Fused quartz is OK in a water vapour oven and thats what is on the outside of the MD elements.

@Careysub
I have to get my oven going before I can make P!

3DTOPO - 5-12-2016 at 19:51

Quote: Originally posted by yobbo II

The Moly D is now back in the window........

My understanding of Moly D was that it formed a coating of SiO2 on the outside which protected it.

True, but the synopsis of the paper I referenced states:

Quote:

It is shown that the oxidation rate increases drastically in the presence of water vapour, and the growth Of MoO3 crystals on the oxide surface increases considerably. The different re.-ions in the oxide cross-section are Mo-depleted compared with the corresponding regions in the bulk when oxidised in oxygen saturated with 10% water vapour. However., the samples oxidised in dry oxygen only shows Mo-depletion in some outer parts of the oxide. Accelerated growth of the MoSi2-oxide layer during exposure in O-2+10%H2O compared to that in O-2 can be related to the fact that more volatile Mo-species form in the presence of water vapour, resulting in a substantial loss of MoO3 from the inner part of the oxide. The voids left behind are not healed by the silica at this low temperature, which leaves the oxide with an open structure. As a result, the oxidation rate increases.

That doesn't sound good.

Radient Energy Furnace

Chemetix - 17-12-2016 at 14:37

Has anyone tried this?

I'd think you could get some serious W.m^-2 at the focal point. It's a matter of just how much you want to melt dictates how big the overall furnace is. But a smallish (microwave oven) size could have a target of 20-30g???

wg48 - 18-12-2016 at 05:38

Unfortunately even if the reflector is 100% reflective, perfectly focused on to all sides of the target and the target is suspended in a vacuum such a scheme can only heat the target to the same temperature as that of the elements.

D4RR3N - 7-1-2017 at 18:00

MAPP gas burns a 2000C in air

yobbo II - 7-2-2017 at 16:38

Super Kanthal heater 1800C for sale
It is very small. Would make a 'one brick hollowed out' furnace.
They don't come up very often on ebay at this price.
I am not the seller!

http://www.ebay.co.uk/itm/Kanthal-Super-1800-heating-element...

Radiant energy furnace

Chemetix - 13-5-2017 at 16:29

A few days ago I had the opportunity to talk furnace design with a furnace engineer. I made a small deviation in our discussion to cover the workability of the radiant energy furnace design. His software couldn't model the focusing effect of the reflective dome, but an approximation to assume high levels of gain like any solar concentrator has, meant the target saw some extraordinary temperatures. His concern became one of there not being any material that would cope with the temperatures. We modeled graphite as a crucible surrounded by zirconia surrounded by mica in fire brick.

The floor of a 0.5m^2 oven with kanthal wire at an optimum spacing for steady state operation at 1500C surrounded by kiln designed to be losing 300W.m^-2 gave a power rating of around 3.5kW for the elements. The elements gave off most of their energy as radiation once the furnace was at 1500C.

Some of the details were lost to me, I wasn't looking at his screen and didn't get to see how the software interpreted the models,or even what was supplied as inputs, so I'll call this a sort of back of an envelope calculation until someone gets to either do empirical work or better modeling.

But it boiled down to the elements giving off about 2kW.m^-2 in radiation from the array on the floor and focused from the reflector, with a collection area of 0.5m^2, to a target of 2x10^-3 m^2. (25mm radius) gives a magnification factor of about 250.

If the incoming radiation is at 1500 W.m^-2( because the whole floor area is not radiating at the energy level of 2kW.m^2) multiplied by 250 then the target should be receiving 125kW.

Anything that is getting that sort of energy directed at it is going to be seriously hot.

yobbo II - 15-5-2017 at 12:02

What would you make the reflector from?

You would need to start with a lower temperature (1000C?) in order to have a refector that could cope. A metal (shiny) reflector would need to be in an inert atmosphere to stop it oxidizing and becoming non reflecting.

All above IMO, I really don't have clue.

I ordered a zirconium oxide rod from ebay (about 20 dollars) and hope to make a 'one brick' oven that will go to approx 1800C.
No idea what to do when it is made. Measuring the temperature and controlling the temperature would be a problem.

Chemetix - 15-5-2017 at 14:55

My original idea was Ti coated with TiN, an excellent reflector of IR. Also chemically inert to temperature induced oxidation. CVD Si is an option.

This is getting way past backyard science I have to admit.

Dan Vizine - 25-5-2017 at 10:47

Well, trying to return to more of a backyard approach....

Crucibles are always a problem, and about the best you're going to find commercially made are yttria. It's easy to find small-volume ones, larger ones seem to be more difficult to find.

Dollar for dollar, you simply aren't going to find a more cost-effective solution than an optical pyrometer for temperature measurement. This is also good if you need to cheap-out and not provide an inert atmosphere. In fact, using one with an inert atmosphere is the trick.

Not providing an inert atmosphere makes no sense. As was mentioned a number of posts back, it's not hard to achieve and it isn't expensive. I would think that unless the furnace is just a passing toy, this would be the only way to go. And really, what do you even want to heat in air?

Inert gas opens up a way to get away from the many, many restrictions you get with molybdenum disulfide, silicon carbide, and other specialty conductive ceramics.

1) the control of flow of electricity through the carbon resistance heaters is straightforward. Simple even.

2) the elements are not subject to cracking, they require no special conditioning, they tolerate dust. They are much cheaper than the ceramic options.

I'm a firm believer in taking cues from established procedures when you're trying to replicate industrial or laboratory grade results. Carbon heaters have been used for a long time to achieve very high temperatures.

The drawback is that the elements are not as clean as the ceramics.

There are a myriad of choices. The only way you can really choose one rationally is if you look at the intended use first.

[Edited on 5/25/2017 by Dan Vizine]

Sulaiman - 25-5-2017 at 11:39

I think that the comment by wg48 should be carefully considered,

Quote: Originally posted by wg48

Unfortunately even if the reflector is 100% reflective, perfectly focused on to all sides of the target and the target is suspended in a vacuum such a scheme can only heat the target to the same temperature as that of the elements.

... the target will heat up to the same temperature as the source as it will then be in equilibrium,
(e.g. real solar furnaces only reach about 4,000 K from a 6,000 K source)
This is a fundamental constraint. (I believe)

Magpie - 25-5-2017 at 12:15

Quote: Originally posted by yobbo II

I ordered a zirconium oxide rod from ebay (about 20 dollars) and hope to make a 'one brick' oven that will go to approx 1800C.
No idea what to do when it is made...

Wiki says "white heat" is ≥1315°C. With that heat you can make phosphorus from dried H3PO4/C. I'm not sure what you would use for the retort, possibly steel or a ceramic. (ref: The Chemical Process Industries by Shreeve)

Chemetix - 25-5-2017 at 14:06

Before I give up on this one, consider the old light bulb, the filament gets to nearly 3kK (3000K) but the bulb is almost touchable. I don't think that the oven could ever reach anything close to the same temperatures as the target.
And by extension from the bulb logic, if you were to try to heat a furnace of 0.5 m^3 with a 5cm^3 source at 3kK, you'd be lucky to get the walls to 1kK. The oven would be leaking energy at somewhere around 300-700 W.m^-2 at this temperature so they could never attain any where near the source temperature.

Ponder again the prospect of pouring 125kW into something reasonably small....hmmmm

floridajohnny - 31-5-2017 at 13:34

Quote: Originally posted by mikeehlert

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.
Mike

Is this true 100% of the time???