Why are top grade heatsinks never BLACK?

[[GSV]Trig]

I've seen lots of black heatsinks...

[Zak33]

you have trig....on graphics cards and on systems where the fan is somewhere else in the case...ie ducted air to the heatsink. But not on high end coolers.

I am sure that painting a copper heatsink black would help disipate heat.

I remember at shool heating up a sheet of copper that was painted black on one side...

the copper side checked out much less heat than the black side. You could feel it with your hand near by.

[Zak33]

White radiators are much less effective than black or whatever ones, but in a lot of cases you actually want to regulate the heat loss even from radiators as the returning water will require less heating.

that is exacty right...its an issue of economy and a gentle heat up for a house, not a speedy heat up where the boiler slogs its arse off and you all boil on your sofa in 30 minutes!

I painted a radiator at home black once...you could feel the heat coming off it everytime you walked last BUT it musthave sent much colder water back to the boiler for reheating.

My Dad painted it white again....something about seeing my blood splattered on a fresh white radiator cheered him up

But for the heatsink in a dark PC, where the air is blowing over it, warm air being blown off the heatsink and cool replacing it iver and over...

I think a copper heatsink, with the main part and fins painted MATT BLACK would rock!

Anyone got the same heatsink as a pair....could paint one with a spray can off matt black and swap it using same fan? to check!

[joshwa]

if you did paint it black you'd have to figure out whether paint (or whatever you used to make it black) was any good at transferring heat?

I guess there's only one way to find out (I've got a can of black spray paint at home, but I fear it would clog up the fins on one of my heatsinks as they're very close together, but I've got another heatsink I can try it on) I could test it by running one full load for 30 minutes (from off), and then spray then run full load for 30 minutes (from off again...)

[Zak33]

JOSH..do it

NOT GLOSS....must be matt black

and obviously not on the underside....not where it touches the cpu! It must conduct heat to the copper like normal.

It'll RADIATE better with black though! I'm sure!

[Zak33]

it needs to be matt black paint such as is used for car body work. It gets sprayed on and then fine fine wet and dry paper is used to see where the rough bits of a panel are sticking up....cos they go silver and the rest stays black.

Black Primer....very thin coat!

[joshwa]

I've got black matt spraypaint So we'll just have to see what happens (when i get home)

[StoneNewt]

The moment you do anything other than a light anodising of Al you lose more than you gain, as for the colour of the coating the actual colour seems to have negligible effect if measurable at all, however long term any anodising helps as it stops oxidisation. Basically what a guy told me that designed heatsinks for >4kw heat sources

[autopilot]

I pretty sure that painting a heatsink black, or any colour, would not help heat dispersion as the paint coating would have an insulating effect. I'm also pretty sure there is no paint (exept maybe the sort of stuff NASA use) that would have the same heat dispersion properties as a metal such as copper, etc, so keeping it as pure as posible helps (afterall, if the paint was better than a metal at losing heat the heatsink could be just made out of a lump of paint). The colouring would have to be added to the metal as an ingredient at fabrication time to be most effective. You have to take into account the heat conducting properties of the paint itself as well as the colour.

However, black and dull surfaces do lose heat quicker. Thats a fact. Come on, this is GCSE stuff! Heat is lost as infrared AND convection to the surounding air (or water). Some people are getting confused between heat radiation and reflection, and that there are two ways which heat is lost. Which is why you get hotter when you are wearing a black t-shirt rather than a white one. It's also to do with the 'shinyness' as well as the actual colour. It's why you never see a white hot plate on a cooker, they are always black (athough black also hides burnt on dirt better too).

As for radiators usually being white in homes, two reasons;

1) Heat regulation is bang on (whoever said that), But there is a bit more to it. Radiators are part of a circular system of heated water. If to much heat is given out by one radiator the the next one down the system would be to cool. As would the next one after that and so on. You have to try and save some heat in the water before you get to the end of the system. However, the colour makes little difference in reality, the main reason is...

2) White is a neutral colour. Most people have white or light colour walls and a black radiator would look silly. That said, i have seen black radiators in a couple of pubs and old stately homes (maybe in select rooms).



May i also add (i had to check this out before posting) that most heat is exchanged to the air passing around it. The colour of the surface make no differnece at all in this process.

So to summerise, painting a heatsink is pointless because a) the paint would act as in insulator, and B) the colour makes no differnece in the heat exchance between the air and the surface of the heatsink/radiator, which the the primary mode of heat loss. Colour only effects heat lost via infrared, which is minimal.

[schmunk]

This is from DansData:

"Is black better?

Recently, as part of our electronics course, we learned about the properties of heat sinks. The course notes (and exam mark schemes) claim that to make a heat sink more efficient it should be painted matte black.

I understand that this would make it more efficient, but my friend and I wondered why CPU heat sinks are not painted matte black? Most other heat sinks (attached to amplifiers etc) seem to be painted in this fashion, so why not CPU heat sinks?

Peter

Answer: Your course notes are right, and they're wrong.

A black object will, all things being equal, radiate heat better than one of any other colour. However, painting a shiny heat sink black may do nothing, or less than nothing, for its thermal performance, because the layer of paint acts as an insulator. The black colour must be an integral quality of the heat sink material, or a very thin, thermally conductive layer on the outside; black-anodised aluminium is a perfect example of a good black heat sink material. It's possible to put a useful thermal black patina on copper by putting it in a hot sodium hydroxide and sodium chloride solution bath (also useful for disposing of corpses), but that's neither a quick nor an easy process, so people usually only bother doing that for copper that's being used as a thermal absorber, as in solar water heaters, not on heat sinks.

This is because the colour of the heat sink matters less and less the more air you move over it. If the sink's hanging in vacuum (like the heat radiators on spacecraft that stop their own waste heat from boiling them) then it must be matte black; if it's sitting on earth being cooled by convection then it should be matte black; if it's got a bunch of forced air cooling from an attached fan then it doesn't matter a great deal what colour it is.

Again, all things being equal, a shiny aluminium heat sink with a fan on it won't work quite as well as a black one - but the difference will be small enough that the extra marketability of the shiny heat sink is likely to be the deciding factor.

A shiny fan-cooled copper heat sink, which can't easily be made black without pointless insulative paint, will work better than an aluminium one with the same dimensions, thanks to copper's rather higher thermal conductivity."

[oralpain]

Putting an appreciable layer of anything with less thermal conductivity that the heatsink, on the heatsink, will probably not help.

[autopilot]

Color in of it self has absolutely nothing to do with how an object radiates heat.

Sorry mate, but it does effect heat radiation (infrared).

Colour has no effect at all on how well heat is lost via convection.

However as the vast majority of heat lost by convection to whatever is surrounded by (air, water etc) then colour makes virtually no differnce. In fact actual paint would act as an insulator, so you are right in that respect.

Colour of heatsinks is cosmetic and must be added to the actual ingredients of the metal to be most effective (or least counter productive should i say).

[autopilot]

Hehe, managed to quote you before your edited your post oralpain

[oralpain]

Yeah. I was thinking more of heat conduction to the surrounding air, not the infrared radiation itself.

Actually now that I think of it. Would color really matter with an active heatsink? Most of the heat is going to be cairried away by direct contact with the passing air, not disipating infrared.

Maybe this is why most mondern computer heatsinks are not black? Maybe black is only appreciably different for passive heatsinks?

[rajagra]

The reflectivity of a heatsink in the visible spectrum may have no correlation to its reflectivity in the IR range.
A silver heatsink might appear black to an IR eye/camera, a black heatsink might be bright to an IR eye.

(But the bottom line is that heat radiation is just insignificant compared to conduction/forced air flow in a typical CPU cooling system.)

[[GSV]Trig]

you have trig....on graphics cards and on systems where the fan is somewhere else in the case.

Mite wanna edit the title of the thread then chap

Oh and a socket 7 heatsink on an Amiga 1200 accelerator card is classed as a high spec cooler lol