1500K Oven Advice 2

[Hellomyfriend]

Hello, I’d like to thank you for bearing with me before hand. I wanted to see if anyone had tips for a project I’m working on. I intend on using resistive heating wire to bring a custom oven up to about 1500K inside an ultrahigh vacuum.

I’m not familiar with any resistance alloys which will reach this temperature so I was thinking pure metal wire, Tantalum or Tungsten wire. It’s my understanding that Tungsten wire is not easily bent and it will be very brittle at 1500K. I’m not familiar with how easily Tantalum wire is bent or shaped, but I believe it’s less brittle (although more pricy). I was wondering if anyone knew how workable Tantalum wire is or if anyone had any knowledge of other heating elements (at these temperatures) that would be practical.

Secondly, I was wondering if anyone could offer any advice about contact with the oven. The body is going to be a small Molybdenum cylinder (maybe 10cm tall, 4cm radius). I was planning on having grooves along the body, winding around it, to wrap the wire in. Then I would just wrap a sheet of Molybdenum over the grooves. Are there any general concerns I should have with this setup? Is there some thermal conductor (a soft metal?) that I could instead fill in the grooves to improve heat transfer?

Thanks for any tips you can give. I just wanted to see if there are any concerns I should look into with my setup or any improvements I can make.

 

[btrueblood]

Mo is a conductive metal just like W and Ta. Having the wires lay in a groove in the Mo cylinder is going to cause a short circuit. Consider using supports/spacers for the heating wires made from suitable refractory ceramics. Boron nitride, alumina, or silica are possibilities. Making an electrical pass-thru that will withstand the temperature and maintain high vacuum is your next problem. Don't try to make a fluxed braze joint from a copper conductor to the tungsten work in high vacuum, you will spend days chasing various volatile oxides. Look into a furnace brazed (hyrogen or vacuum atmosphere) connection using a gold or gold/nickel alloy. Why not make the "oven" from ceramics?

Ta is more likely to evaporate away at 1200 C than pure tungsten wire, also it is much more reactive to residual oxygen than tungsten. Mo wire would be a better choice than Ta. Ta is somewhat flexible at room temp, moly somewhat less so. Mo/Re alloys exist with somewhat better RT ductility, but increasing Re content increases the wire evaporation rate. W/Re alloys, same thing.

W wire can be wound hot (800 to 900 F minimum, or dull red heat), heating with a reducing flame or via resistance heating. Low-sag, potassium-doped tungsten works best for heating wires that reach incandescense.

The CRC handbook has data regarding vapor pressure vs. temperature for various refractory metals and their oxides. It also has very good data for the resistivity, emissivity, vapor pressure and evaporation rate for tungsten wire.

Enjoy.

 

[Hellomyfriend]

Thanks trueblood, this is exactly what I wanted to know about the wiring options. I was going to shoot for alumina beads for electrical insulation, but I didn't think about the connection or the longevity at all though, thanks for letting me know. I’m going to start looking up them up and hopefully I have all my questions figured out!

 

[btrueblood]

If you want some help finding a source for wound/formed filaments, holler back.

 

[saplanti]

I went through the same exercise almost 15 years ago for 1500 deg C heater/pressure vessel for a research lab. The heating wires are more expensive than the gold wires. I would not use them in this kind of process, very costly.
I would consult a local representative of Kanthal. They do work in this area very efficiently, and their handbooks might be very useful for you.

http://www.kanthal.com/

Hope it helps

Ibrahim Demir

 

[saplanti]

The attached catalog shows some kilns that use Kanthal A1 product for 1300 deg C heating.

http://downloads.hilldav.com.au/Hilldav_TetlowKilns.pdf

Ibrahim Demir

 

[Hellomyfriend]

Hmm, that's good to know Ibrahim, thanks. There is specific geometry I need the oven to have for evaporation, but if dropping the geometry in a kiln is cheaper I'll have to look into it.

I got back to finalizing the design and I just want to finish it. I had two final questions I came across when reviewing the heating.

If I do go with resistance wires covered in alumina beads, did I need to be concerned with the thermal contact of the beaded wire and the body? Or in a UHVacuum would radiative heating be sufficient with little reflection from the alumina beads?

If so, are there any improvements I can make? Or does anyone know of any soft metals or thermal interfaces that would work at these temperatures/pressure?


Secondly, I was going to use a series of heat shields (tantalum foil). I’m just unsure about mounting the heat shields. I was thinking of using a support system made out of zirconium oxide ceramic with minimal contact with the heat shields (how crucial is actually minimalizing contact between the ceramic supports and the shields? Instead of having a simplified support system).

Are there better ceramics I should use? Or any concerns/improvements I should consider/make?


Thanks again for all the help you guys have been, I really appreciate it.

 

[saplanti]

I have attached the links of this kind of refractory providers/manufacturers below.

http://www.cof.com.au/

http://www.zircarceramics.com/

I think you need to talk to them or similar supplier/manufacturer in your local area to make sure you are on the right way. They have plenty experience on similar applications to yours. You can find conductivity of this kind of ceramic refractories in their web sites or catalogues.

I still would not recomment very expensive wires for your application, if you do not want to hide your oven in a safe.
To understand the cost involvement you need to go through what I had done almost 15 years ago. To be able to find the resistant wire you neet to go through at least two safe doors just to discuss the cost involvement for these materials if you are accepted. Unfortunately, I can not drive you what materials are required for wires at the moment. You need to search some heating applications books for the application temperature and calculate the required diameter and the length of the wires before discussion with anyone.

Talk to KAnthal for your applications. I believe they will be very happy to advise you. I am not a Kanthal related person, I have used their service once.

Regards,

Ibrahim Demir

 

[saplanti]

Have a look at this. I finally found one catalogue.

 

[saplanti]

This is another one for radiant tubes.