What it really takes to be black

Does it reflect RADAR?

I didn't see any mention of that.

I was wondering about solar collectors, all the light is reflected around in the nanotubes until all is absorbed and... turned to heat. Can we get all the heat to the back plate? They say it is very expense.

Mikek

On Fri, 4 Mar 2016 19:16:49 -0600, amdx Gave us:

You racy pig! :-)

Depends on the backing. The absorber has to be several wavelengths deep. Old UHF Russian radars saw some of our early attempts at stealth.

That's easy. You can't see a laser pointer on an LCD screen, either.

It would be good for making steam from cold water using sunlight.

Great, if they could eliminate the gigantic parabolic reflectors and Fresnel lenses.

Yep, political correctness sucks!

Mikek

Years ago one of my kids science class was doing solar energy projects. I came in with a Fresnel lens from a large screen TV. I had built it into a frame on legs. I started wrong, I melted a penny, the teacher got extremely nervous.

Mikek

That is what 100% efficiency solar cell would look like.

Would converting that heat to electricity make the collector more efficient than standard solar?

On Sat, 05 Mar 2016 10:46:43 -0800, Robert Baer Gave us:

Peltier has that covered. Needs power robbing air handling for heat removal from the cool side to work well.

Of course. It is illegal to deface US currency.

No, for solar thermal collectors you want a material that is black in the wavelengths produced by the sun (including visible) (like vantablack), but you don't want to lose that energy by radiating it off in the far-infrared (e.g. 10um wavelength). Therefore you want the material to be shiny (low emissivity) in the far-infrared at the same time.

You can already buy such a material, it is sold for making solar thermal collectors.

The extra tiny amount that you could collect by using a blacker material (vantablack) would be more than lost due to the vantablack probably also being too black in the far-infrared, and radiating away your power.

On Sun, 6 Mar 2016 21:20:06 +1100, Chris Jones Gave us:

It would likely make a very nice uniformly heated black body source for instrument calibration.

A thin covering of it over an Aluminum ingot would likely be better than the current ingot sprayed with matte black "IR Paint" method.

I used to make just such "ovens" for the NIST and other such customers for IR instrument calibration. Most have a 6.5" diameter ingot about an inch behind a 6 inch hole in the chassis to keep room air currents from messing with the surface temp uniformity (as much). The ingot surface had concentric pyramidal rings cut into it.

This stuff could simply be applied to a flat ingot surface.

I think a hole looking into a hollow sphere is nearly a perfect black body if I remember correctly from a physics book 35 years ago. That kinda makes sense.

On Sun, 6 Mar 2016 03:35:35 -0800 (PST), snipped-for-privacy@gmail.com Gave us:

heaters formed around a basketball with an alumina tube to site into it through that were that.

But for low temp stuff (relatively speaking) between ambient and about

and used by the NIST as well.

Colder stuff used water baths as a source.

Seems to be specifically allowed, in the case of pennies and nickels.

On Sun, 06 Mar 2016 07:34:09 -0500, Spehro Pefhany Gave us:

Cannot apply to pennies anymore at all, since the friggin things are electroplated zinc.

It's more of a thought experiment, and doesn't involve any actual geometrical calculations of the resonator. The shape isnt mentioned, just that it's a container filled with thermal photons, and there exists a hole which looks into that container.

A cube would probably be easier, if you wanted to include actual calculations of modes and reflections. If you like, suppose the opening is perfectly focused with telescopic optics, and angled at a transcendental angle (i.e., tan(angle) is not a rational number) so it undergoes infinite reflections within the box.

That's a very roundabout way of simply willing it into being: that there is some sort of chamber, which exhibits a series of modes, and that the viewport somehow experiences the thermal equilibrium of those modes. Physicists are great at making up s%!t like that. :-)

Yup. The best "blacks" take advantage of the residual reflectance of the surface, using the "stack of razor blades" effect to trap glancing reflections ever deeper within the surface. And then making a cavity out of that, so there really isn't hardly anything that's getting back out in a non-thermal manner.

The ideal cavity has a fractal structure, something like a Koch snowflake with a bit steeper angles on the faces to better trap the radiation. Of course, the nano scale "black" materials do this to the lowest of scales, so they're quite good even without the larger structure.

Tim