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Comment by imoverclocked | Hacker News Reader

I’m just waiting for a version of a vacuum thermos that keeps my drink hotter, longer. Put this on the inner side of the outside surface and paint the inner side of the inside surface with the blackest black available. Almost like a thermal diode. Reverse the sides and you have a cold-cup instead…

I’d buy two: one for hot and one for cold!

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This would not work in your thermos! This material is reflective in the visible but emissive in the infrared. The only thing that matters in the thermos is being reflective in the infrared.

Thermodynamics does not permit a thermal diode to exist.

I think it's even simpler than that. Any black body that is colder than its surroundings heats up. Any black body warmer than its surroundings will cool down.

This thermos would only keep your coffee hot if you dropped it into a furnace.

You could make a colored flask which absorbs visible light (from the sun) but reflects thermal wavelengths (from the drink). But due to infrared leakage and thermal conduction, I don't know if it would keep drinks above 100 degrees F for anywhere near as long as a standard vacuum flask.

What about real materials, e.g. not a black body?

The neat thing about wearing white in the desert is that it keeps you cool during the day and warm at night. Wearing black does the opposite.

A perfect black surface on a spacecraft can do more than charcoal black paint, but only by a matter of degrees. If memory serves its less than a 2x multiplier between off the shelf and exotic materials. But when ounces cost thousands of dollars, you’re going to go exotic.

"The Second Law of Thermodynamics: Heat cannot, of itself, pass from one body to a hotter body."

https://www.youtube.com/watch?v=bYZyipx01tc

My "solution" is a $5 mug heating pad that pulls 16W. It only heats when there's a weight placed on. Probably accounts to 10kWh for the whole year to keep a warm cup of coffee ready for the whole day.

Every random thermos out there already has a reflective surface for insulation.

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