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parsson

Best material to absorb heat and then slowly radiate that heat

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I am looking to build a "fire place" of sorts. I need to use a solid material around the heating unit/fire that can absorb heat and slowly radiate it for a long period of time. I know there are masonry heaters that does this well. They mostly use refactory bricks (fire bricks). Is there any other material that would be even better? Any particular stone, or stone composite that would be better? What about the possibility of manufacturing a stone composit with better properties than those already in ecistence?

 

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If the goal is to absorb as much heat as possible and then to slowly release it over time, you might look at the properties of materials that undergo reversable endothermic phase transitions (solid to liquid and then back to solid). The fire will melt the material, and once the fire dies down the material will return to its solid phase, in so doing releasing its stored heat into the room. The material could be sealed into cylinders and the cylinders embedded into the walls of the "fireplace".

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If you are using ceramic or stone, then what's commercially available is probably the best combination of ease of use and cost, etc. Bill's suggestion is a good one; there are materials used to keep beverages warm that exploit this. So a low melting-point metal that has a decent latent heat of fusion might work. Tin or possibly zinc. (lead has a really low heat of fusion)

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You have to decide whether heat is to be radiated by the material or brought to the users by blown air. In the first case you need a refractory, in the second case a melting paraffin brings excellent capacity (but may burn with a wick).

 

The second factor of choice is material cost. Just a block of aluminium for instance is probably too expensive for a heater. This usually drives the choice to non-technical ceramics.

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If the goal is to absorb as much heat as possible and then to slowly release it over time, you might look at the properties of materials that undergo reversable endothermic phase transitions (solid to liquid and then back to solid). The fire will melt the material, and once the fire dies down the material will return to its solid phase, in so doing releasing its stored heat into the room. The material could be sealed into cylinders and the cylinders embedded into the walls of the "fireplace".

Sodium Sulphate:

 

from http://en.wikipedia.org/wiki/Sodium_sulfate

 

The high heat storage capacity in the phase change from solid to liquid, and the advantageous phase change temperature of 32 °C (90 °F) makes this material especially appropriate for storing low grade solar heat for later release in space heating applications. In some applications the material is incorporated into thermal tiles that are placed in an attic space while in other applications the salt is incorporated into cells surrounded by solar–heated water. The phase change allows a substantial reduction in the mass of the material required for effective heat storage (the heat of fusion of sodium sulfate decahydrate is 25.53 kJ/mol or 252 kJ/kg[19]), with the further advantage of a consistency of temperature as long as sufficient material in the appropriate phase is available.

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Parson,

 

I think the material you are looking for is SoapStone. It's commonly used in the same type of application you specified for same reasons you mention.

 

Here is a video that i came across that describes and demonstrates it's ability to hold and radiate heat for an extended period of time. Traditonaly folks used to put these in their bed to keep warm through the night in the old days.

 

 

Hope this helps!

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