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This is really unintuitive to me because metal typically heats up more easily than organic materials, and it's really the air spacing in a thermus or other similar storage devices that insulates the heat from entering or exiting.

So why would someone ever want to use a material that heats up more easily to create something that is meant to prevent the transfer of heat as much as possible?

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This is what a cross-section of a thermos looks like: it's not the metal that's stopping heat conduction, it's a vacuum created in between two separate layers of metal making up the thermos. That's why it's also referred to as a vacuum flask.

You're absolutely correct. If you were to take off the outer layer, and only have a steel container, it wouldn't be very useful at all.

I think metal is used simply because it's durable and malleable, but it's the vacuum that prevents the flow of heat in or out of the container.

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    $\begingroup$ You may want to add to the answer that a vacuum inside means a very strong atmospheric pressure on the walls. A container made of a softer material would be crushed by the pressure. Also the reason why a metal is used is that it reflects infrared like a mirror. $\endgroup$ – safesphere Nov 27 '18 at 4:35
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    $\begingroup$ So metal heats up easily...but reflects infrared...which is often a means of transmitting heat. Something isn't adding up here. $\endgroup$ – user608672 Nov 27 '18 at 19:26
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Stainless steel is used for practical considerations, getting a sufficiently effective container for a reasonable cost.

Regardless what material would be ideal, you still have to turn the raw materials into your end product. Stainless steel is a material easy to fabricate with, and its corrosion resistance makes it suitable for many ordinary liquids, such as food and beverage. It also makes the resulting product durable.

Dewar flasks have been made from other materials, including glass, ceramics, plastics etc. If you were building a space probe or a sensitive medical instrument, it would probably be worth the extra cost to use more exotic materials with more challenging fabrication. But if you're trying to build something to sell for $9.99 at a local retailer to store coffee in, those other materials aren't worth the hassle.

Plus, stainless steel isn't all that great of a thermal conductor anyway. I have a stainless steel saucepan with a stainless, uninsulated handle. I can have it on the stove boiling water in it, and I can grab it by the handle barehanded and the handle barely feels warm. Copper comparatively is about 25 times more thermally conductive, aluminum about 10 times.

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    $\begingroup$ Stainless steel is, perhaps surprisingly (given the question) not a particularly good thermal (or electrical) conductor compared with, say, copper. Its strength really helps a thermos survive (many of us older folks remember the sickening sound made by a glass thermos bottle upon impacting the floor). $\endgroup$ – Jon Custer Nov 26 '18 at 17:46
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    $\begingroup$ @JonCuster: good point, SS is a pretty lousy conductor as far as metals go. It's even worse that cast iron! $\endgroup$ – whatsisname Nov 26 '18 at 17:56
  • $\begingroup$ stainless steel is significantly more difficult to machine, shape and form than either plastic or nonstainless grades of steel. It is also significantly more expensive than nonstainless. It gets used in food service because of its corrosion resistance and specifically in thermos bottles because it is unbreakable. $\endgroup$ – niels nielsen Nov 26 '18 at 18:28
  • $\begingroup$ How can stainless steal be cheaper than plastic? Plastic is lighter weight, can be durable depending on what kind you buy, cheaper to manufacture and is also corrosion resistant. $\endgroup$ – user608672 Nov 26 '18 at 22:48
  • $\begingroup$ @user608672: There are plenty of plastic vacuum flasks. They're common in hospitals for beverages, for example. $\endgroup$ – whatsisname Dec 7 '18 at 2:30

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