What material retains heat the best?

I thought of something I could make for an Instructable. To keep the idea until I publish, I'll exclude the actual idea. I need a material for this project that will retain heat from a microwave for ≥20 to ≥30 minutes. It will be about 8x8x1 inches. What can I use for this?

Here are some things I found to be helpful:
http://en.wikipedia.org/wiki/Heat_capacity
http://www.extremetech.com/extreme/130523-zeolite-thermal-storage-retains-heat-indefinitely-absorbs-four-times-more-heat-than-water
http://www.fraunhofer.de/en/press/research-news/2012/june/compact-and-flexible-thermal-storage.html
http://www.igb.fraunhofer.de/en/press-media/press-releases/2014/sorption-energy-storage.html

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There's more than one way to store heat. I can think of roughly three ways: heat capacity (also called specific heat), phase change, and chemical change.

Usually phase change and chemical change can give you more heat per gram of material, compared to just using the heat capacity of a material.

However for a phase change, the heat tends to be delivered at a particular temperature. For example cooling molten lead delivers its heat at around 330 C. Molten water delivers its heat at around 0 C. So for that, you kind of want to pick a material with a melting point that is at the temperature you want to deliver your heat at. For example sodium sulfate decahydrate has a melting point of around 32 C (90 F) which is convenient for heating a room. I mean this is a temperature humans consider warm. More on this here:

http://en.wikipedia.org/wiki/Sodium_sulfate#Therma...

and here:

http://en.wikipedia.org/wiki/Phase-change_material


This zeolite stuff you linked to, is a different kind of mojo. I would tend to think of this as a chemical change, almost. I mean you use heat energy to separate water from the zeolite. Then the heat can be stored indefinitely, as long as the zeolite is kept dry (uncombined with water). Heat is evolved when the zeolite recombines with water, and theoretically you could allow that to happen as slowly as you wanted, by way of plumbing. That is to say there is sealed container of zeolite, and a different sealed container of liquid water and water vapor, and these containers are connected by a pipe with a valve in it. The valve allows water vapor to flow from the water container to the zeolite container, quickly, or slowly, depending on how open, or closed, the valve is.

The articles you link to suggest this kind of zeolite is maybe something new, which maybe means it is hard to find. Honestly, I don't know how hard or easy, it would be to find. However, I have seen materials like it, classified as drying agents. For example I found a big table of drying agents here:

http://www.erowid.org/archive/rhodium/chemistry/eq...

I am ?guessing? these drying agents will work in a manner similar to the zeolite you linked to. Well, that's kind of a big guess. However, some of the materials in that table I linked to, are cheap and easy to find. For example in stores in the Former US, dry calcium chloride, is sold under the brand name DampRid (r). Also I have seen silca gel sold as a brand of cat litter, Mimi Litter (r). Not sure how well regenerating either of those with microwaves will work. Probably further experimentation is required.

Use a liquid like water or oil and keep it well insulated. But after about 2 or 3 minutes in a standard microwave the liquid will be plenty hot. Water will be boiling over and the oil could catch file if left any longer than that.

You can buy microwavable heating pads. There are instructables for home made versions as well.

Rice!? The heat storage material in homemade microwave heating pads is typically rice? I didn't see that one coming.

Or wheat.

Wheat!? I wasn't expecting that one either.

But I guess now I'm totally prepared in case someone else mentions dried corn, or beans.
;-)

How about lavender flowers?

OK. That one did not totally catch me by surprise, because I already read mention of "essential oil", things that smell nice, in one of these 'ibles. I think it was this one:

https://www.instructables.com/id/Making-A-Microwave...

although I guess I didn't read it too closely, or else I would have noticed "wheat" as a suggested filling, also:

{feed corn, buckwheet hulls, barely oatmeal, beans, flax seed, cherry pits}

I dunno. I think some of these filling materials might attract mice, or fungi, or microbes. I know I've seen oatmeal turn moldy.

Vyger2 years ago

If you want something to retain heat then you insulate it. The amount of calories a certain mass can hold will not necessarily mean that it can retain that amount of heat. And retaining heat will depend on things like the ambient temperature and whatever contact it might have with other materials that it can exchange heat with. If you put a hot mass of anything on an insulated pedestal and isolated in a vacuum then it will stay at its current temperature for a long time. Although the heat will eventually be radiated away by things like infrared radiation.

Temperature control is one of the big problems for the Space Station because it can't just dump heat into the surrounding air since there is none. And when it gets in direct sunlight much of the light converted into heat stays with whatever it is shining on. Hope this makes sense to you but I think you are actually thinking about something other than heat retention which is in effect insulating. There is also heat transfer rates involved and the actual calories a mass can contain.

rickharris2 years ago

Like always it depends:

On the temperature you need to maintain

On the weight you can use

On the cost of the material

Water will retain it's heat fairly well, even longer if insulated

Anything with a large thermal mass and the hotter you can get it the hotter it will be 20 mins down the line.

A lump of steel for example.

JM19992 years ago

Kiln bricks?

I haven't checked out the links to see if they are one of the results but I know kiln bricks retain heat extremely well.