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Burnt pancake displacer (carbon foam)

Posted: Tue Oct 04, 2016 2:38 pm
by Tom Booth
Recently I've been re-researching what sturdy lightweight materials might be used for an LTD displacer. The engine I got from amazon had some black foam rubber type displacer that shrunk and warped when I tried to run it on top of my wood stove for a few minutes just to see how fast it might go. The plexiglass ring also melted.

So I've been looking for replacements.

I came across some articles on carbon foam. Then saw a YouTube video by a guy who made carbon foam from a slice of wonder bread.

The method is generally to cook the bread in argon gas to prevent oxygen from just burning it up.

The goal is to keep oxygen away. But I've made bio-char without argon gas and charcloth for a fire piston just by heating the material in a sealed can to keep out most of the air.

I decided to try just wrapping several layers of aluminum foil around half a hamberger bun (the flat bottom half) crimping all the edges over to seal it up as tight as possible and with as little air as possible then set it on the hot coals in my wood stove, covered it with twigs and opened up the draft.

After 20 minutes or so when the fire died down I opened the stove and to my amazement, there was the pyrolized "toast" glowing red hot on top of the coals. The aluminum foil had mostly all melted away. I took out the toast and it felt like lightweight ceramic and had a kind of metalic pinging sound to it when I droped it on the floor.

It seemed quite strong and hard. Out of curiosity I tried biting off a corner and chewing it to see if it tasted like burnt toast but it was virtually tasteless. It felt like shards of glass or sharp sand in my mouth as I tried to chew it up. It did not dissolve but stayed very hard. I had to spit it out.

The bun was a little too small to actually be used. Also it had bent and cracked a little. But this stuff was pretty strong, hard and practically weightless and apparently quite heat resistant if not entirely fireproof. It certainly was able to withstand the heat of a blazing wood fire hot enough to melt aluminum.

Next I'm going to try making some special "pancakes" and cook them up in a sealed cookie tin in an oven for more control.

I'll be trying various "pancake" recepies (not necessarily palatable) and yeast cakes and different heating methods untill I cook something up that comes out nice and uniform without warping or cracking with a good cell structure.

Apparently the bun was able to carbonize before the aluminum got hot enough to melt. It was rather amazing to open the wood stove and to see that bun still sitting there glowing red hot even after all the twigs had burned up and even the aluminum foil it was wraped in melted away.

Apparently getting "burned" or heated hot enough to burn, but without enough oxegen to actually burn causes some change that makes the carbonized bread impervious to the hottest fire my wood stove could generate. ... bread.html

Re: Burnt pancake displacer (carbon foam)

Posted: Wed Oct 05, 2016 9:13 pm
by Bumpkin
Hey Tom, thanks for posting this. Is the toast foam permeable/flow-through as in a Stirling regenerator matrix? Very interesting stuff.


Re: Burnt pancake displacer (carbon foam)

Posted: Thu Oct 06, 2016 10:19 am
by Tom Booth
The white bread hamberger bun was not very permeable. Not much air could get through. Some made out of a whole grain slice of bread ended up with very open pores that air could very easily flow through.

It does also seem that various similar carbon fiber/mesh materials have been used as regenerators so perhaps this stuff could serve both purposes.

Researching various bread cooking sites, If you want to avoid holes or too large uneven pores 1: don't use too much yeast. 2: don't let the dough rise too much and/or punch it down to eliminate large bubbles more than usually called for. 3: make the dough mixture relatively dry. 4: Try adding salt and/or sugar and/or oil. 5: Keep the dough relatively cool when rising..

As bread rises it starts out with tiny even bubbles then as it continues to rise the small bubbles break and combine making larger bubbles that end up as holes so for a fine even texture slow down fermentation and arrest it early.. (bake the bread before it can rise too much)

Also rice flour may make for a lighter more even cell structure than wheat flour.

If you want a more open airy material to use as a regenerator I suppose the opposite of the above might be in order to some extent.

Though not very far along in experimenting yet. Perhaps a cake like angelfood/sponge cake. Adding some cream of tartar to whipped egg white is supposed to make the resulting foam more resilient so the bubbles don't collaps when the meringue is folded into tha batter.

Not sure how any cake type concoctions using whipped egg white as leaven instead of yeast might turn out. I havn't tried any of these things yet.

What I notice so far is the white bread hamberger bun turned out rather like melted volcanic glass, not leaving any carbon powder on your fingers when handling it. The whole grain bread turned out rather chaulk like.

I'm thinking gluten vs starch.

Starch can be removed by kneading bread dough under water. The starch is water soluble while gluten is not. This should result in a very high gluten low starch dough which I think may be desireable for our purposes. The dough will break up as it is washed then tend to glue back together as the starch is eliminated.

But I have yet to try any of this. Just sharing various clues I've picked up that might be worth experimenting with.

Re: Burnt pancake displacer (carbon foam)

Posted: Thu Oct 06, 2016 1:58 pm
by Tom Booth
One other thing.

It's quite possible that starch is not the culprit as far as chalkiness goes. Possibly the opposite as some bread recipies call for added corn starch to make the dough more plastic and give the bread a better texture. The corn starch heated with some water seperately to make a goo which is then added or the other ingredients added to the plasticy goo.

Also corn starch or other starch is used for making bio-plastic. (Cooked up with some oil or glycerin and vinegar).

Perhaps some sort of carbonized bioplastic foam could be concocted. At any rate I'll be trying eliminating starch on the one hand and adding extra starch on the other and possibly also seeing if some bioplastic might be made into a foam one way or another and carbonized without bread flour.

Re: Burnt pancake displacer (carbon foam)

Posted: Thu Oct 06, 2016 6:02 pm
by Tom Booth
Some interesting info regarding the heat conductivity of various carbon materials.

Very variable depending on how it is made. Interesting that pyrolytic carbon is or can be MORE heat conductive than copper by a factor of 5X. ... ivity.html

Re: Burnt pancake displacer (carbon foam)

Posted: Thu Jan 03, 2019 11:08 am
by Bumpkin
I know this is an old thread but here's another recipe that makes me wonder just how far we can take easy home cookery towards producing useful Stirling components:


Re: Burnt pancake displacer (carbon foam)

Posted: Fri Jan 04, 2019 3:17 am
by Ian S C
Thanks for that Bumkin, as a first thought it could be used to line the furnace around the hot cap of a Stirling Engine. Also the blackened stuff (what ever you want to call it) could be wrapped round the cool end to dissipate heat. Something to think about.

Ian S C

Re: Burnt pancake displacer (carbon foam)

Posted: Fri Jan 04, 2019 7:19 am
by MikeB
I can see this kind of material being useful for displacers, particularly if you are using an inert gas, but surely not very useful for "open air" applications (such as round the hot cap, as suggested). Basically, what you have is open-cell graphite, so given enough heat, and any amount of oxygen, it _will_ deteriorate over time, and is perfectly capable of burning well if enough oxygen is available - it is basically just charcoal after all.