Stirlingengineforum.com

Many small model Stirling engines, especially those of the LTD coffee cup type, come with foam displacers.

While this is fine if the engine is only going to be run on a cup of coffee, or on ice.

One time, many years ago, I sent for a Stirling engine of this type, with the idea of generating power to charge batteries to run an inverter. So I put the little engine on top of my wood stove, which was not operating particularly hot at the time.

The engine ran vigorously for a minute or so before the foam displacer started shriveling up from the heat and the plastic ring serving as the engines displacer cylinder softened and curled up at the bottom edge breaking the seal.

But, by this sacrifice of one small toy engine, I did learn that there was no real reason why an LTD engine couldn't run at high temperature other than some of the material it was constructed from; foam and plastic.

Recently, again, I've encountered the same issue with my efforts at restoring the Brent Van Arsdell Limited Edition engine. Though otherwise of good construction and fine workmanship, had the same type of foam displacer and clear plastic displacer chamber as any $20 coffee cup engine.

The idea, is, of course, that displacers need to be as light as possible, in particular if they are used in an engine intended for demonstrating that a Stirling can run on the heat of your hand, and while that is all well and good, the ability for an LTD flat style engine to run at higher temperatures in some situations could, I think, be a very worthwhile feature..

So, to that end, over the years, I've experimented with various light weight but heat resistant displacer materials, none of which turned out to be ideal.

Too heavy, too brittle, too conductive, transferring excess heat, not really as heat proof as I would like, etc.

Recently I've stepped up my efforts to find or manufacture some displacer material suitable for a high temperature LTD of conventional design, but of high temperature material.

Towards that end, I have sent for a tube of this material;

https://www.ceramaterials.com/product/c ... dable-mix/

The reason being, there are some lightweight ceramic fiberboard insulating materials that look promising, but IMO are prohibitively expensive, airspace type stuff for those not living on a budget I suppose.

Anyway, in my quest, I came across this relatively inexpensive high temperature ceramic fiber board material that is available in small caulking tube quantities.

There is also relatively cheap "glass bubbles" or super lightweight glass or ceramic microspheres that could possibly be mixed with the ceramic fiber board caulking, if it is not light enough on its own.

I did attempt a search of the forum for "high temperature displacer" but the search algorithm returned the message that all three "common" terms were ignored.

Anyway, I cannot recall a specific thread ever having been devoted to the subject of high temperature displacers, so I though it might be worthwhile to start one, as this seems to be an ongoing, unresolved problem.

The problem of the plastic displacer cylinder is not really an issue. It could be made of metal or glass and the additional weight is not any real issue.

There was the "burnt pancake" displacer material thread a while back. The problem there was as the common bread or dough material was pyrolyzed it would shrink and warp and become misshapen. Otherwise, it seemed like a pretty good, hard, strong, very lightweight material.

Just do it in metal. Like from old candy box. Or just two pieces of 0.1mm ss foil with rolled sides and some tack welding.

Whatever the material, (preferably porous to act as a regenerator) It could be stronger (or lighter for the same strength) if it is formed into a slight cone shape — with chamber ends to match of course. I’ve mentioned somewhere before about carbon fiber felt being an interesting possibility, it’s already the preferred black color and light and strong. If there is some commonly available high-temp epoxy/stiffener that would leave it porous I think it could be a great answer.

Bumpkin

The following I found to be very intriguing, probably because of the potential for mishap. Don't try this at home. Zero safety is implied.


Foam Concrete, Wikipedia:

https://en.m.wikipedia.org/wiki/Foam_concrete



Foam Generator, Honey Do Carpenter:

https://m.youtube.com/watch?v=aSLJCPR1Prw



Ultimate Wood Stove, Honey Do Carpenter:

https://m.youtube.com/watch?v=e9kHjci9S2k



Make your own Water Glass, Honey Do Carpenter:

https://m.youtube.com/watch?v=wzUXVwf_hOA




Born of fire diy refractory from Tim Tinker:

https://timtinker.com/diy-refractory/

Thanks,

I think I've tried, over the years, all the options mentioned. As far as displacer material.

Carbon fiber seems, by itself, impervious to flame, but I have not yet been able to find it without it being sealed in epoxy, so...

If anyone knows of a source for thin carbon fiber board that is NOT sealed in an epoxy binder, I can see how that works.

Concrete foam, promising but not really as light as foam / styrofoam type material. Could work in a bigger engine probably. Also, I wasn't able to get a mix that was not rather fragile in thin layers.

I tried plaster foam as well.

A kind of epoxy foam I made seemed great once hardened, forget how exactly I made it. Fiberglass resin and shaving cream mixed together or some such thing. Very light, very strong, rather heat resistant, but not as heat proof as I'd like. Maybe it was epoxy and sawdust?

What I'm working on at the moment is a lightweight porcelain.

I just mixed up some porcelain "slip" (liquid clay) with about equal parts, maybe more, added to make a thick paste -- glass microspheres.

The mix itself seemed rather light, similar to the lightweight spackle "Red Devil" stuff,

(maybe what I tried before was epoxy and lightweight spackle, not sure).

The lightweight spackle is also SOMETHING? mixed with the same sort of glass microspheres I'm trying again now, but without the plastic, or whatever polymer binder.

I tried the lightweight spackle by itself. Too flexible, not very strong. Maybe if I let it harden longer, but anyway not fireproof.

So I just finished pouring, spreading, and vibrating out the big bubbles in this porcelain + glass bubbles slurry in a stainless steel canister lid.

The mix had to be thin enough to get out the bubbles in the clay, but not so thin that the glass bubbles float to the top like a layer of cream.

Don't know how long it will take to dry in a metal container. The usual practice for clay slip would be to use a plaster mould, then the plaster of Paris absorbs the water out of the clay, but the canister lid was just about the right size.

If this works out I may take the time to make a plaster mould.

Hopefully the kitchen countertop is fairly level.

At this point it's just a trial.

After it dries, I may just try firing the "greenware" in the Chubby coal stove since I don't have the kiln set up yet.

Anyway, lightweight high temperature ceramic seemed like a good idea to me. As one of the above videos says,. "Born in fire".

Porcelain is, I think, about the hardest of the common clay/ceramics. It can be very thin but strong. Pretty resilient. But not light enough on its own I don't think, for a little LTD engine.

So, now I just wait and find something else to do for a while.
If the clay paste seems light now, being mostly water, it should be very light once dry.

Tom said: “Carbon fiber seems, by itself, impervious to flame, but…” Hey Tom, I mentioned carbon fiber felt, but I meant to say carbon felt. It’s different. Google it up - it might be of interest. Looking at that, I ran across carbon foam again (like the burnt pancake,) and it looked like there might be some new insights available since the last time I looked into it. It’s been a few years and one of these nights I’ll have to dive back in to that rabbit-hole.

Bumpkin

From Tom:

Concrete foam, promising but not really as light as foam / styrofoam type material. Could work in a bigger engine probably. Also, I wasn't able to get a mix that was not rather fragile in thin layers.

If glass or carbon fiber is added to foam cement it may help. Also possible is pre-stressing the reinforcement glass or steel bars.

That last link, "Tim Tinker", has many formulas and encourages experimentation, even for fired foam ceramics. None of which are cement. Most are a refractory with silica binder in the form of "water glass" sodium silicate, (cat litter water and lye).

I'm sure there are a number of different options that could work.

I'm not too enthused about having to make foam, or water glass, especially involving caustic chemicals.

I'm hopeful that the same, or a similar material to ceramic foam with a water glass binder can be achieved by simply mixing glass bubbles with clay. Both are pretty inert and easy to handle.

This mixture, with a little water, can be made any consistency and shaped and molded without limitation and no necessity of working with potentially dangerous chemicals. Though the glass microbubbles in dry form can get into the air.

The pan of this porcelain clay, glass microbubbles and water mixture has been drying very slowly, but has thickened. I don't know if the inclusion of the glass spheres in the clay slip is itself increasing the drying time.

Any clay greenware is rather fragile until fired, so I'm not rushing anything. Once it has air dried enough, I may try putting it by the coal stove so it can get good and dry before attempting to remove it from the stainless Steel pan.

I guess if the glass bubbles were wetted the powder would have less tendency to become airborne. Dry It is a bit like confectioners sugar or baking powder in consistency.

Well, unfortunately, this method is not working out, the clay is adhering to the stainless steel too much and the mixture is developing cracks as it dries.

If it doesn't crack much more it could likely be repaired.

I think it will be necessary to either use a plaster mold or alternatively, maybe make the mixture much thicker/drier and roll it out like a pie crust.

Regardless if this is repairable or not, if I can get it out of the pan, I may just try firing up the pieces to see how it comes out.

Not looking great.

The stuff stuck to the stainless steel like glue.

Not worth trying to repair I don't think. Dumped out into another canister lid it broke up into dozens of pieces.

One good thing though, it does seem quite lightweight and with a little water this should reconstitute back into a putty, so it won't all go to waste.

I'll toss the biggest chunk into the coals and see how it cooks up.


More good news. The chunk did not immediately explode or anything, and no popping noise from all the glass bubbles exploding.

The manufacturer representative told me on the phone that that could not happen as the spheres contained only a vacuum, not any air or gas.

He did say the glass would melt though. Which is actually what I wanted. A light ceramic foam with glass reinforced hollow voids. The glass melting and fusing with the ceramic should be a good thing.

Five hours later

One surface is covered with a shiny glass glaze.

I have to conclude that the glass spheres floated out of the too thin mix, (or perhaps sunk to the bottom?) as it was drying, sitting around in the pan.

The sample also ended up with some curvature. Not sure if that is due to the unintentional surface "glaze", uneven consistency due to separation while drying or uneven firing temperature, or just gravity (slumping during the firing , from too hot a firing temperature.)

Otherwise, I would say, all in all I'm not entirely displeased with the product. It basically survived a sustained white hot temperature in the heart of a coal fire for several hours, and seems super hard, impossible to break with just my hands.

I think it is probably worth trying a thicker/dryer consistency mixture to start with. As I said, rolled out like a pie crust.

I suppose it is possible the glass bubbles came to the surface during the firing process itself, but I rather doubt it, but who knows?

Hopefully a thick dry mixture will produce a more uniform result.

Tom, what if you put a piece of fiberglass or carbon fiber cloth in the mix to act as a reinforcement, to keep it from cracking? Sort of like wire mesh they put in concrete slabs.
It looks like it held up to the heat, as setting on the coals possibly got up to 1500 degrees or more.
To help with the sticking to the stainless, try coating the stainless with wax or silicone. The problem may be that the silicone may contaminate the ceramic. The wax could be crayoned onto the metal and after the ceramic sets up slight heating would release it.
Looking forward to seeing what works for you, I love learning about new processes and materials. Keep up the good work!

skypupbob wrote: ↑Sat Feb 26, 2022 9:14 am Tom, what if you put a piece of fiberglass or carbon fiber cloth in the mix to act as a reinforcement, to keep it from cracking?

Well, I've actually done a lot of ceramic work and I think the main reason for the shrinking and cracking is just too thin, or too watery a mix.

Also a steel form for clay is just not the way it is ordinarily done. I never had a problem with a plaster mold.

Since the stainless pan was just the right size, I could use that to make the plaster mold.

There is something else I want to try first though.

I picked up various long, thin wooden coffee stir sticks and barbecue skewers, candy apple sticks, etc.

I've actually had these kicking around for about a year, with just this in mind, but never got around to it.

The idea being; to make the displacer lighter by rolling out the clay, then layering the skewers in between another layer, criss crossing that with another layer of sticks etc. to the desired thickness, pressing it all together.

Then when fired, the skewers should burn up and turn to ash, leaving behind voids - air channels or pockets

Something like this:

Coating the metal might work, but really the clay needs to be able to dry. Metal just doesn't have the porosity of the plaster, (the standard material for ceramic slip molds)

I think I may also try adding some perlite, I think that may have more stability than the glass. Possibly vermiculite, basically mica. But as this displacer is so thin, those will need to be sifted to get out the larger pieces.

I might just try a batch with sawdust mixed in. Corn flower?

There is also some perforated paper available, used for cross stitch. Some of that layered in might create voids.

I'm afraid, or I think rather, that the porcelain might be fine by itself without the glass bubbles, which at this point, just seem to melt and add weight, though, maybe not with a drier mix.

Still lots of experimenting to do, but so far, I do like the porcelain. It seems incredibly hard after firing. I'm not sure how much that might be due to the addition of the glass though.

This might be worth fooling with:

https://www.researchgate.net/publicatio ... _technique

Liquid clay/slip, corn starch and yeast.

Look up paper clay process.