Photonic crystals for blocking heat
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Art Carlson
- Posts: 794
- Joined: Tue Jun 24, 2008 7:56 am
- Location: Munich, Germany
"scientists have found a material even less able to conduct heat" than a vacuum?
Ho hum.
http://en.wikipedia.org/wiki/Multi-layer_insulation
Ho hum.
http://en.wikipedia.org/wiki/Multi-layer_insulation
That is why the sides of a thermos bottle are painted silver, to reduce black body radiation losses. Aluminum mirrors are ~89-94%(single or multicoated) reflective. If they have improved the reflectivity of 'photonic crystals' to 99.99999% efficiency. This would reflect 
a ~ 5-10% improvement in radiative losses per surface*. Actually, since there might be four surfaces (each side of the inner and outer wallthe net difference may add up to ( 0.9 * 0.9 * 0.9 *0.9 ) or as much as ~ 63% reflectivity. Not quite 50% but close, especially if the reflectivity of comenly used aluminum coatings is less at infared wavelengths. A more meaningfull way of looking at this is that each surface will absorb 10% of the heat from one side and reemit 10 of that heat on the otherside, etc. That means the material will heat up untill a balance between absorption and emission is reached. I'm not sure how this would work out, but any improvement in reflectivity would presumably help. Which would be better - a silver surface on the cold side and a black surface on the hot side, visa versa? Or highy reflective surfaces on both sides? I suspect the latter as that seams to be the layout of the vacuum thermos bottles that I have broken.
*A more acurate assesmant would require concideration of secondary, tertiary, etc. reflections. In telescopes and other optics these subsequent reflections can degrade contrast. That is why non reflective coating are used on all the surfaces the elements of a good refractive lens. Uncoated glass will reflect ~ 5% of the light that hits it perpendicular to the surface.
Dan Tibbets
a ~ 5-10% improvement in radiative losses per surface*. Actually, since there might be four surfaces (each side of the inner and outer wallthe net difference may add up to ( 0.9 * 0.9 * 0.9 *0.9 ) or as much as ~ 63% reflectivity. Not quite 50% but close, especially if the reflectivity of comenly used aluminum coatings is less at infared wavelengths. A more meaningfull way of looking at this is that each surface will absorb 10% of the heat from one side and reemit 10 of that heat on the otherside, etc. That means the material will heat up untill a balance between absorption and emission is reached. I'm not sure how this would work out, but any improvement in reflectivity would presumably help. Which would be better - a silver surface on the cold side and a black surface on the hot side, visa versa? Or highy reflective surfaces on both sides? I suspect the latter as that seams to be the layout of the vacuum thermos bottles that I have broken.*A more acurate assesmant would require concideration of secondary, tertiary, etc. reflections. In telescopes and other optics these subsequent reflections can degrade contrast. That is why non reflective coating are used on all the surfaces the elements of a good refractive lens. Uncoated glass will reflect ~ 5% of the light that hits it perpendicular to the surface.
Dan Tibbets
To error is human... and I'm very human.