Carbon reinforced refractory

[martyn]

There are now a few scientific papers available like this one www.researchgate.net/publica...ent_composites
There are other folk experimenting on a more basic level using a basic mix of one part fondu cement, four parts crushed fire brick, one percent carbon fibres (20mm) one percent burn out fibre.
More exciting experiments incorporating individual long stands of carbon look very promising too.
The basic mix as above is used to line a mold half full, then the long strips of fibres are laid in place and the mold toped up and vibrated.
It seems carbon fibre does not really have a melt temperature but decomposes around 3500c.

I have build several  pizza oven components using the above mix that do not show any sighs of even small cracks but temperatures  are limited to around 550c .

[Vortex]

So the carbon fibers are solely for added strength? I use the same mix and method but with very light grade chicken wire instead of the carbon fibers.

[martyn]

Yes the fibres are used to prevent cracking in high heat areas, unlike metal they will not expand and contract.
There are lots of potential benefits like thiner casements and edge durability, leaner mixes and therefor more tolerance to heat expansion and spot heating.
The nylon burn out fibres offer water and steam outlets as they melt away a complex matrix is formed within the end product.
One % of either fibre is in effect many millions on fibres when casting 25kg
I have been using fibergalss fibres in the past but they are only tolerante to 600-800c
It seems experimenting is still in its infancy but even with DIY projects the results seem to be outstanding … if you believe the non scientific reports!
I think it might mean something like a J tube could be cast in one piece rather than two or four pieces .

[Vortex]

Aug 23, 2021 22:51:45 GMT -8 martyn said:

I have build several  pizza oven components using the above mix that do not show any sighs of even small cracks but temperatures  are limited to around 550c .

Where did you get the carbon fibres you used?

[martyn]

You just cut then yourself from carbon fibre weave (mating) available from many outlets in various weights.
I just use scissors to cut a 20mm strip a 1mt roll and the individual fibres come apart but you can buy small pieces from ebay if you want to experiment .
Have not found anywhere that just sells ready cut fibres.
I have read some people have super heated the fibres with a blow torch as they mix in to the refractory cement easier but I just mixed them in gradually,
That seems like the biggest issue to me is that they have a tendency ball up when mixing.

[fuegos]

"I think it might mean something like a J tube could be cast in one piece rather than two or four pieces "
Now that would be something.imagine the benefits of having cheap , mass produced cores available in developing countries.

[martyn]

Yes it would Fuegos but like many small scale operations they need real time and investment rather than a few guys experimenting in their  back yard as such.
It is very interesting to see proper test papers available but there are quite difficult to decipher into real world circumstances.
I am looking forward to doing a bit more experimenting myself, I thought I might make a little 4” vortex stove …..

[Forsythe]

I know carbon-composite ceramics work well in high-temp applications... and in at least that field, the thermal stability of the carbon is dependent on being both encased in surrounding refractory molecules, and —to some degree— chemically bonded with them.

It makes me wonder what the actual chemical composition of the carbon fiber is.

Because carbon by itself is not refractory, and instead becomes a fuel source when in the presence of heat and oxygen. I mean...that's what charcoal is. (and it's the reason that Starlight puffed-carbon insulation doesn't work as the hot-face in fireboxes. It has to be utilized as a back-up lining insulator where it's isolated from the oxygen-rich and highest-temp combustion zone(s).)

...so I would think that adding burn-out materials to promote porosity would work *against* the entrained carbon fibers, allowing them to gasify and burn-out themselves, given enough time and prolonged use.

That is, unless the carbon fibers were molecularly "alloyed" with something to prevent them from oxidizing, which is... well... burning — into either carbon monoxide or carbon dioxide.

[Forsythe]

Another example would be insulating firebricks, where anthracite coal (which is almost purely compressed carbon) is used as a burnout material to create the insulative pores in the brick.

[martyn]

I have not followed up the latest results on the pizza oven threads but there had been several full sized, one piece domes cast the last time I searched.
With my own experiments, the main issue I encountered was getting the fibers to mix in with the cement without balling up.
It seems the longer the individual strands the more effective the mix becomes but in turn the longer the fibre the more difficult it is to mix!
In fact it was extremely difficult to mix anything longer than a few mm, even in a small bucket of dry ingredients the fibers are so fine you just end up with a twisted mess of balled up fibers.
I was using standard carbon fiber matting and cutting it up with a craft knife but the fibers seem to be too fine once separated from the weave and almost impossible to mix!
I think you would need some way to keep the strands together rather than breaking down to microscopic individual fibers?
I would like to do more experimenting, perhaps laying some long strands into a part filled mold and encasing them with a top layer ?