Thermosetting Hardener or other solution required.

[jb1008]

Hi All,

I am not entirely sure this is the right forum to be asking, but I am trying to find a liquid hardener type material or other similar chemical that I can use for my application.

I need to find a fluid that I can fill with 90%+ MgO or BN and pressure fill into a long narrow chamber.

Ideally I would like something with a 12+ hour working life or better still would stay viscous until cured at 300°C+.

I need to keep the thermal and dielectric properties of the MgO/BN as much as possible.

The maximum operating temperature of the hardened compound would be 800°C indefinitely.

I am sure you will need to ask some questions in order to make a recommendation so please do not hesitate to ask.

Many Thanks

 

[Compositepro]

I believe that normal practice is simply use high pressure to compact the materials. Boron oxide may work as a binder/fluxing agent. It has a low melting point (for a glass) and when it melts it will dissolve the MgO and form a higher melting glass. But glass will be much more brittle than a compacted solid, and will not have the thermal shock resistance.

 

[btrueblood]

I know of parts that are made with MgO in (water or water/alcohol?) slurry form, which are then slowly dried. I assume your assembly long in length/diameter ratio, and thus you are concerned about voiding and/or incomplete drying of your assembly? How much voiding could you tolerate?

I don't know of any hardenable fluid/resin that wouldn't break down and potentially react with BN or MgO at 800 C. Your best bet is to use a fluid which is then removed by evaporation. I don't think forming BN in-situ would work, as the precursor is boric acid or trioxide, and thus the precursor filler would (I think) shrink as it is reacted, not what you want. The heat of reaction of Mg to its oxide precludes that approach too.

A possibility is to use a silicone rubber (two-part platinum cure) mixed with a high-as-possible percentage of the filler you want, pour/inject the mix into the device and cure it. You'd need to then oxidize out the silicone rubber at high temperature, about 600 C would do it (or it will do it on its own at the operating temp), producing silicon dioxide (SiO2) residue, and probably leaving some voids behind (I'm assuming SiO2, being an electrical insulator, would be ok for your application). The oxidation of the silicone will produce a fair bit of gaseous reaction products, though, in the form of water, carbon mon- and di-oxide, formaldehyde, formic acid, and methanol. Dunno if any of that worries you.

 

[unclesyd]

This dovetails with the information posted above.

You might want to contact Aremco about the inorganic binders.
We have this type material for making finish rolls. I have used porcelain glazes for a similar applications.

http://www.aremco.com/product/a11/

 

[Compositepro]

Another binder that you can consider is sodium metasilicate, also known as water glass. It is water soluble and will need to be dried. After firing it has little solubility.

 

[unclesyd]

What is the size of your proposed tube as the a re allot of tubes

out there. I setting here with a MgO kiln stilt 4" x 3/8" x 3/32" id.

One process to make MgO tubes is to use 2% P.E. in paraffin as a

binder. The material is then calcined and sintered.
Another approaches might be through Magnesium Oxychloride
cement.

Depending on the end product you may be able to use plastic
MgO refractories. Plibrico should have some information about
same.

http://www.premierchemicals.com/

http://www.morgantechnicalceramics.com/

 

[jb1008]

Hi All,

Thank you all for your replies, I wasn't expecting so many in such a short amount of time, so that's great!

I shall look into some of your solutions, but would also like to narrow down your ideas.

Obviously I cannot get you all to sign an NDA so there are some points that will be a bit vague.

The parts in question are hairpin heating elements. These are currently made by welding an incoloy800 (or SS) bar into a tube to your desired length. Lets say you want a 4m hairpin element, you would start with a length of tube 6.6m long.

A resistance wire is coiled and welded to inactive terminals which can be up to a metre long.

These are then inserted vertically into the incoloy sheaths and held centrally as MgO powder is shaken into the sheath to encapsulate the resistance wire. When full the elements are rolled which reduces the diameter by about 20%, compressing the MgO to ensure there are no voids while increasing thermal properties. This would increase the 6.6m sheath to the 8m required.

After this they are fired and bent into a hairpin shape before IR testing and sealing.

The solution I am trying to find is to replace the compressed MgO powder. The idea being that I can pressure fill the sheaths from below, they can still be rolled and fired the normal way.

Anything water based can be a problem unless I cannot guarantee complete expulsion as this will greatly reduce the elements IR rating.

The terminal pins can handle a constant 690V, and anything with conductivity greater than your own finger would cause the element to earth.


In the attached drawing, I have tried to make things a little clearer.
A - the terminal pin, usually an M6 thread.
B - the incoloy sheath
C - (yellow) the compressed MgO powder to replace.
D - Coiled resistance wire
The inactive area will not be heated, whereas nearer the middle of the hairpin the element can reach 750°C + and the solution would need to remain a solid at this temperature.

As before, if you have any further questions please ask.

 

[jb1008]

EDIT:

The OD Ø12.5mm

 

[unclesyd]

As starter I would contact Saueresin, Cotronics, or Aremco posted above.
I would also make sure no one makes elements that fir your requirements.

http://www.sauereisen.com/adhesives/adhesives.aspx

http://www.cotronics.com/vo/cotr/