Heatsink emissivity and surface treatment

[awhiteguy]

From the little bit I know about heat transfer, two equally sized heatsinks of the same material will perform differently because of the surface coating. The higher emissivity factor will provide better cooling.

I'm sure you have seen this table before:
Polished aluminium 0.05
Polished copper 0.07
Rolled sheet steel 0.66
Oxidised copper 0.70
Black anodised Al 0.70
Black enamel 0.85
Dark varnish 0.89
Black oil paint 0.92

So here is my question. I have some uncoated aluminum heatsinks that I got on ebay and I would like to increase the emissivity by coating. I could easily spray them with black enamel to get 0.85. Also, I know a guy that does powder coating for pretty cheap. I think that would look much better.

Are the powder coated heatsinks going to be in the upper emissivity range? The coating is black, so I suppose it should be at least 0.70 unless the powder coating introduces an insulating factor. Is anyone familar with the powder coating process, or tried it with heatsinks?

awhiteguy

[dlharmon]

I think the emmissivity factor would mean the amount of energy radiated, not conducted. The majority of the heat is conducted to the air so you would not want to do anything that would insulate much. Anodizing does look good and shouldn't hurt anything. The paint would insulate some but I suspect it would make little or no difference.

Darrell Harmon

[Thomas]

Please read Rod Elliot's article on heatsinks, it's very good and explains all you ever wanted to know about heasinks.

http://sound.westhost.com/heatsinks.htm

[dlharmon]

Great link. I had never seen that article before. Anodizing definitely would be best. It would be interesting to paint one sink and leave the other, apply equal loads and measure temperature.

Darrell Harmon

[Lenin]

Darrell,

Much better to order black anodised and plain aluminium heatsinks from your preferred supplier.

Or (here's an idea) look at the spec sheets on your suppliers site. Aavid's site is excellent here.

Identical plain and black anodised finned heatsinks - sourced for my current project (approx 300 x 40 x 50mm) have temp rise figures of 0.65deg/C per W and 0.55deg/C per W respectively - a difference of almost 20%.

[dlharmon]

Originally posted by Lenin
Darrell,

Much better to order black anodised and plain aluminium heatsinks from your preferred supplier.

Or (here's an idea) look at the spec sheets on your suppliers site. Aavid's site is excellent here.

Identical plain and black anodised finned heatsinks - sourced for my current project (approx 300 x 40 x 50mm) have temp rise figures of 0.65deg/C per W and 0.55deg/C per W respectively - a difference of almost 20%.

I meant the paint, not anodizing. I was thinking about testing something like 2 really small cheap sinks and some power resistors. The paint would act as insulation more than anodizing.

Anodizing isn't much of an option since most everyone here buys surplus heatsinks.

Darrell Harmon

[dkemppai]

Anodizing isn't really that hard to do. I did a little research on the subject, and it's not much more complicated than etching a board. I'm going to get around to trying it at home at some point... ... I have too many projects.

I had a good link to a page about how to do it at home, but have lost it, so if anyone researches it...

Anyway, from what I remember, you need....

a power supply - 12volt battery or similar....
chunk of lead - for anode/cathode (I don't remember which)...
aluminum wire - antenna ground wire or similar from TV shop...
sulfuric acid solution about - (sounds like battery acid will work just fine)...
organic dye - (cloth dye, or similar, or can buy on line)...
A cool place to work...

Basically, anodizing is the creation of an oxide layer on the aluminum, which is porous. You create the oxied by running current through the peice while it is in a water/sulfuric acid bath. The oxde itself is non conductive, so it's easy to cover odd shapes (unlike electroplating...) Once the oxide layer has formed, the anodized part is then dipped in a dye solution (pores suck up the dye), and sealed. Now, sealing in the neat part, sounds like you dunk it in boiling water, or steam it, and the oxide layer closes up. (Steam is preferred) That's all there is to it. Keep the workpeice cool when working with it, and keep the anodizing tank cool as well (or start with cool fluid). The dye can be any color... ...some work better than others, and there are commercially avaliable dyes, but Black should be easy.

-Dan

[Keld]

I had a good link to a page about how to do it at home, but have lost it, so if anyone researches it...

Some reading:

http://sites.gulf.net/riders/anodize.htm

http://www.focuser.com/atm/anodize/anodize.html

DVC Keld

[dkemppai]

Originally posted by Keld


Some reading:

http://sites.gulf.net/riders/anodize.htm

http://www.focuser.com/atm/anodize/anodize.html

DVC Keld

Yep, these are the pages I had looked at!

-Dan

[vpharris]

If you really want to look at some emissivity data, I found that this is a good source for it.
http://www.tdmginc.com/pages_e/emissivities.html

Notice that color isn't much of an indicator for what an emissivity is likey to be. There's no guarantee that black is better than white. But generally, polished surfaces have lower emissivities than unpolished ones. There's one misleading entry in the table for Anodized Aluminum. The one that says "Plain" is certainly for UNANODIZED aluminum. Anodized clear is in the table, and the emissivity isn't very different than any other color.

Near the end of the Technical paper on Aavid Thermalloy's website, "how to select a heatsink" (only so-so) it mentions that radiation heat transfer is important and can be responsible for up to 25% of the total heat dissipation. Notice that they say "up to."
I think that 25% is the max and usually it's lower.

Paint will have some insulating properties to it. Thermal conductivity is different in paint than it is in aluminum, of course. But I think if the coating can be kept thin, it will have little effect.

Back to the original question: What about the powder coatings? Will they enhance the emissivity of an otherwise raw piece of aluminum? Probably, but I think that they may also insulate it more than standard paint. I did a little search for "powder coating" and "emissivity" and came across one thing that, if correct, tells us some. It said:

Tim, you're mixing together different properties. Powder coating is very much an organic coating, with all the attributes (e.g. flexibility) and disadvantages (e.g. inability to withstand high temperatures) that implies. Just because powder coatings are applied using heat does not in any way make them equivalent to ceramics; which also are applied using heat (although, in their case, using much higher temperatures). The thickness of a particular coating material, its thermal conductivity, and its emissivity all determine its usefulness as a coating for an air-cooled cylinder. A standard black paint and a powder black paint will have roughly the same thermal conductivity and emissivity. However, powder coatings are typically much thicker (~0.015" vs. ~0.002"), which reduces the heat flow across the paint, and thus increases the temperature gradient as compared to the case with a "standard" paint. What that means is that the cylinder will run hotter, as Tim said. However, the reason isn't quite the one he gave.

I still can't imagine it being responsible for more than a couple of percent reduction in effectiveness of the sink though.

This topic has been discussed here before, with some incorrect conclusions drawn. I didn't weigh in 'cause I didn't want to throw a monkeywrench into the works, but the big thing I remember is that some concluded that any color other than black was a waste of a heatsink and that radiation might be responsible for 50% of the heat loss. Both of those are wrong.

What do I know? I spent 15 years in an industrial ceramics kiln design /build business and developed a pretty good understanding of what happens when you heat things up. We did lots of heat transfer calculations, and occasionally had to get into some details like this. The big majority of heat transfer is via convection until something starts to glow. Then you start to cross the line. Above about 1600 deg F or so (say 900 C) radiation becomes much more important.

Check your local anodizing shops, and get your front panels done at the same time. Maybe it won't be a bad deal. I have some raw aluminum heatsinks and that's what I plan to do for some Alephs I'm building.