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buzzp (Electrical) (OP)
24 Feb 05 13:56
I am an electrical guy and hope someone can tell me or point me to some discussions on the best materials that are able to not only absorb IR heat but also the best at retaining this heat. Think solar IR waves here. Thanks in advance.
Helpful Member!  EdStainless (Materials)
24 Feb 05 14:03
Does heat transfer matter?
Are you looking for a metal?
What product form (tube, plate or ?)

Large solar collectors use tubes of Cu, Al or stainless steel with black coatings.  Surface conversion coatings, or even oxides, work better than paints or applied coatings.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

Helpful Member!  IRstuff (Aerospace)
24 Feb 05 14:05
Depends on what you're trying to achieve.  As a general rule, there's not much that is one-way in radiative terms, unlike diodes.  

E.g., a high absorbance surface will radiate quite nicely as well.  The only plus side is that you might possibly be able to tailor the surface to be low emissivity in the LWIR regime to minimize radiative loss.  

Again, depending on complexity, you may need to reconfigure your outer surface depending on the time of day, i.e., absorbing during the day and reflective at night.

TTFN

Helpful Member!  BillPSU (Industrial)
24 Feb 05 18:04
Aluminum primary heat loss method when cooling from a molten state is conduction and gives off little IR. While Iron alloys give off plenty of IR.
buzzp (Electrical) (OP)
25 Feb 05 11:49
I think Ed gave some 'terms' to search for.
Forgive me but I do not want to divulge all the info as this might be against a non-disclosure agreement my friend has with a company. I am not doing the design just interested in what they are trying to achieve.  

Let me ask another question if I may:
As I understand it, if you had two identical pieces of material (dimensions and construction) and you painted one black and one white, the black one would abosrb more heat than the white one but at the same time cool faster than the white one if the heat source is removed. I believe this is basically what IRstuff was referring too.

I believe for what we are trying to achieve, that if what I said is true, the black material would definately be best for absorbing the heat but not sure if we want the heat to radiate away quickly. Is there a color that might sacrifice some absorbtion for better ability to retain the heat (grey)? (Heat transfer is a huge deal as well). If you guys have any links to educate me that would be great too.  

The ideal material would be inexpensive (of course) so no elaborate materials would be good. I like the idea of using copper or aluminum. I believe a plate shape would be best for this application.  

Thanks to all. I will do some research on the net this weekend and probably have more questions.

IRstuff (Aerospace)
25 Feb 05 12:44
Bear in mind that Wien's Displacement Law places the peak of solar radiated power around 500nm, but a room temperature blackbody peak radiation is around 9um.  

If you arbitrarily pick 3um as the cut-off, ~80% of the solar energy is below 3um, while >99% of the room temperature radiation is above 3um

TTFN

Helpful Member!  rnd2 (Materials)
4 Mar 05 3:08
What about glass that darkens upon exposure to sunlight?
There are two I can think of straight away: spectacle lens that go from clear to dark in a few seconds and battery-operated welder's lens that go from clear to dark in a mili-second. Make two panels so air-space reduces the overall loss of convected heat from retained radiated heat.
buzzp (Electrical) (OP)
4 Mar 05 13:26
Thats a good idea rnd2. I like the idea of using 'two' layers.
Would some of the light be reflected if I use black glass? Seems some of it would, or at least more than I would like.
IRstuff (Aerospace)
4 Mar 05 18:21
If you want to go that route, you could simply build a hothouse over your equipment

TTFN

buzzp (Electrical) (OP)
4 Mar 05 19:23
That'd be too expensive, at least for now. Thanks
rnd2 (Materials)
4 Mar 05 22:44
Black absorbs and radiates heat energy faster than any other colour. If you have ever had to walk barefoot on a black bitumen road on a hot day and then stepped onto grey concrete sidewalk you will have had first hand experience of this phenomenon.
The point is when the radiation source is "on" eg daytime black absorbs heat energy relatively faster than any other colour. Conversely, when the radiation source is "off" black will also radiate retained heat energy faster than any other colour. The "logic" if that's what it is, is to devise a strategy to change the  black colour to something that reduces radiation loss. Black to clear may be beneficial and the technology to do it is commercially available. Another option is to have something like venetian blinds that are white or silver on one side and black on the other.
IRstuff (Aerospace)
4 Mar 05 23:06
I don't think it'll be that bad; it could essentially be a sheet of plexiglass on standoffs.  if you make the standoffs around 1/4" and seal the plexiglass, you'll mimic a double pane window and snuff the convective heat transfer.  

The plexiglass will allow sunlight in, but will not transmit the radiated LWIR.  If the inside box surface is blackened, that'll maximize the sunlight absorption.

This should work like the spectrally selective absorption that I was talking about earlier

TTFN

buzzp (Electrical) (OP)
8 Mar 05 13:38
Thanks for all the suggestions/help. You have given me a lot to pass along.

I like your final suggestion IR, I was thinking of a 'building' on your other post. Also like the tinted glass idea.
Helpful Member!  unclesyd (Materials)
10 Mar 05 9:06
You should give Tiodize Technical a call as they have done a lot of work in this field, absorptivity/emissivity.  I had several papers concerning essentially the same problem  you presented.   If the windmills are in synch I think the numbers were very low and could be tailored to some value.  I know they work with Ti & Al.

Still looking.

http://www.tiodize.com/emissivity.html  

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