The performance of a heatsink with a black surface

[mikeselectricstuff]

I would imagine a side by side comparison (properly) would show that bare metal is ever so slightly better.

The data sheet I posted seems to suggest the reverse is true but so far only two people (apart from me) viewed it. Of course, I'm not saying this is always the case. It probably depends on the type of anodising, the shape and whether it relies purely on convection or forced cooling.

That graph is meaningless without reference to the absolute temperature - heat loss from radiation is proportional to the fourth power of absolute temperature. You won't see any difference in a fan-cooled heatsink as the airflow is the dominant factor.

[ejeffrey]

Interestingly enough, the ceramic that is formed when aluminium is anodized is an excellent thermal insulator.

I design and build jet engines and did a lot of experimentation on the thermal insulation capabilities of such a ceramic layer.

In one engine we had an aluminium plate at the end of a combustion chamber that was directly exposed to combustion gases.  The rest of the chamber was made from a high temperature nickel alloy but had no ceramic layer.

When operating, the external temperature of the aluminium plate never exceeded 200 deg C, whereas the hi-temp nickel-alloy would glow red hot, at temperatures of around 950 deg C.

A bit of math makes this suspect.  The thermal conductivity of alumina I find listed as around 20 watt / meter*K.  This is about 10% the thermal conductivity of aluminum metal, which is among the most conductive metals.  A really thick hard anodizing layer tops out at around 100 micron I think -- so it has the same thermal impedance of an extra mm of aluminum.

High temperature alloys on the other hand, are generally rather poor thermal conductors.  I just found a data sheet for "ATI 625HP" a nickel-based high temperature alloy.  The thermal conductivity is listed as 10-25 W/(meter*Kelvin) over the temperature range -- about the same as alumina.  For many high temperature applications you want low thermal conductivity: the hot side temperature is fixed and you want to limit heat flow across the material in order to insulate the surrounding structure.  In an engine, you want to contain the heat so that you can generate power from it, not dissipate it through conductive materials.

So what I suspect was happening here is a combination of two things.  First, the anodized aluminum may have been in a part of the engine where the combustion gases were not as hot, or they may have been otherwise protected from the full heat load.  Second, the excellent thermal conductivity of the aluminum, including the oxide layer, allows it to be cooled much more effectively than the low conductivity nickel parts.  There is no way the oxide layer posed a serious thermal impedance that insulated the aluminum.  The reason for the very thick hard anodization layer is for passivation.  It chemically protects the underlying aluminum from attack by oxygen and other combustion gases.  In a high temperature environment you need a thicker layer because gases can diffuse through the oxide more readily at higher temperature.

[Zero999]

That graph is meaningless without reference to the absolute temperature - heat loss from radiation is proportional to the fourth power of absolute temperature.

Normally standard conditions should be assumed, if the temperature is not specified.

You won't see any difference in a fan-cooled heatsink as the airflow is the dominant factor.

I agree, I already said that.

[RCMR]

A bit of math makes this suspect.  The thermal conductivity of alumina I find listed as around 20 watt / meter*K.  This is about 10% the thermal conductivity of aluminum metal, which is among the most conductive metals.  A really thick hard anodizing layer tops out at around 100 micron I think -- so it has the same thermal impedance of an extra mm of aluminum.

Yes, we were also surprised at the results -- but they speak for themselves.

High temperature alloys on the other hand, are generally rather poor thermal conductors.  I just found a data sheet for "ATI 625HP" a nickel-based high temperature alloy.  The thermal conductivity is listed as 10-25 W/(meter*Kelvin) over the temperature range -- about the same as alumina.  For many high temperature applications you want low thermal conductivity: the hot side temperature is fixed and you want to limit heat flow across the material in order to insulate the surrounding structure.  In an engine, you want to contain the heat so that you can generate power from it, not dissipate it through conductive materials.

This is all true.

An un-anodized aluminium plate however was showing signs of ablation and showed a significantly higher (300 deg C) temperature when compared to the anodized one (< 200 deg c).

Another factor in explaining the temperature differences are (as you point out) the thermal conductivity of the two different materials plus the fact that the hi-temp alloys rapidly develop a dark patina which contributes to the transfer of heat from the combustion gases to the metal.  The aluminium remains comparatively bright (especially the anodized surface).

However, the ceramic did (in this instance) make a significant contribution to reducing operating temperatures in this part of the engine -- perhaps due to a combination of preserving reflectivity, maintaining a reduced surface area (ablation roughened the surface) and reducing the rate of thermal transfer.

And, as  you correctly summised, this part of the engine does run cooler than the higher-temperature regions - but we were still able to obtain a significant  operating temperature reduction -- due to the combination of effects outlined.  In the end we also added a second ceramic (refractory) layer and dropped the operating temperature even lower -- but the anodizing certainly had a significant and measurable effect.

[johnwa]

Another factor to consider is radiation from external sources. If your heatsink is exposed to direct sunlight, a black one may actually be worse off than a shiny one. As stated above, radiation is proportional to the fourth power of the source temperature, and the sun is probably a fair bit hotter than your heatsink!

Regarding electrical insulation of anodising, I am looking at using this in a design with some TO-220s, but they will be clamped rather than screwed to the heatsink. I presume a 0.02-0.03mm hard anodise would be appropriate - does anyone else have any experience with this? I guess I can always go back to silicone pads if it doesn't work out.

[Alex]

Regarding electrical insulation of anodising, I am looking at using this in a design with some TO-220s, but they will be clamped rather than screwed to the heatsink. I presume a 0.02-0.03mm hard anodise would be appropriate

This is a very unreliable insulation method. If the heatsink is fully enclosued in the device then it can be live. Otherwise you must use a proper insulation method or a TO220 with a plastic tab.

[johnwa]

OK, fair enough. This is actually an automotive application, there are no high voltages, and I am only looking for functional insulation rather than for safety. But perhaps I would be better off using an insulating washer anyway.

[SgtRock]

Greeting EEVbees:
--Two questions:
1) Why do Bedouin Arabs wear black?
2) If your were trying to cool off at night, would you wear black or white clothing?
Clear Ether

[Kiriakos-GR]

Greeting EEVbees:
--Two questions:
1) Why do Bedouin Arabs wear black?  [/quote]

Black paint on discount ? 

2) If your were trying to cool off at night, would you wear black or white clothing?
Clear Ether

None I would be naked. LOL

[Zero999]

Greeting EEVbees:
--Two questions:
1) Why do Bedouin Arabs wear black?
2) If your were trying to cool off at night, would you wear black or white clothing?
Clear Ether

1) I don't know: it looks good or for religious reasons?

2) Black because it improves the transfer of thermal energy from your body to the outside environment, assuming the black dye used is not reflective as mid to far infra-red wavelengths of course.

Incidentally, under a dark clear sky, an infra-red absorbent surface exposed to the sky can become much colder than the air temperature due to radiative cooling which is why there can be frost on the ground despite the air temperature being above freezing. This is more common at higher altitudes because there's less of an atmosphere to reflect the infra-red radiation back to earth.

[IanB]

Going back to the beginning of this thread, radiative heat transfer at low temperatures (<100°C) can be significant under some circumstances. Examples include the silvering of Dewar (vacuum) flasks as mentioned above, and British home heating systems. In Britain, home central heating by hot water to radiators is very common. Central heating radiators do heat the air by convection of course, but radiation is also a big component. If you sit near to an operating radiator at approximately 60°C you will soon start to feel rather warm.

With heat sinks it is all about the air flow. Forced (fan driven) air flow makes a huge difference to the heat removal in a heat sink. If the heat sink is in still air, and if it perhaps is in an unfavorable orientation, or inside an enclosure, then convective cooling will be severely restricted. Under these circumstances the proportion of cooling by radiation may be significant and a dull black surface will certainly be a better radiator than a shiny metallic surface.

[ejeffrey]

Greeting EEVbees:
--Two questions:
1) Why do Bedouin Arabs wear black?

I am not certain, but I think from the physics point of view (rather than cultural or whatever) that the black cloth is less transparent than white, so the sun heats your clothing more than you.  This would only work if the clothes were very loose and flowing.  If you have fitted clothes, you want white because it reflects more of the sunlight.

2) If your were trying to cool off at night, would you wear black or white clothing?

Wouldn't matter.  The emissivity of black and white fabric is essentially the same in the far infra-red that is relevant for black-body radiation at human body temperature.  The color only makes a difference in sunlight because a lot of the sun's power is in the visible spectrum and most of the mid- to far-IR is blocked by the atmosphere i.e., by greenhouse gases.

[Time]

Analytically, the coating reduces the thermal resistance between the air and heatsink.  If you look at a good heatsink company (like aavid thermalloy) you can see specs on their products.  Different extrusions with different heatsink/air thermal resistances based on geometries and coatings.  They even offer a proprietary coating with better than normal coating heat transfer characteristics.

[IanB]

Analytically, the coating reduces the thermal resistance between the air and heatsink.  If you look at a good heatsink company (like aavid thermalloy) you can see specs on their products.  Different extrusions with different heatsink/air thermal resistances based on geometries and coatings.  They even offer a proprietary coating with better than normal coating heat transfer characteristics.

The Aavid website seems to be a good resource for technical information. Here is a note for example that explains why anodizing of heat sink surfaces can be useful:

http://www.aavidthermalloy.com/product-group/extrusions-na/anodize

As to reducing the heatsink/air thermal resistance, that really depends only on surface roughness and the turbulence of the air flow. Greater roughness and greater air flows both will increase the film coefficient at the surface. But with low air flows and natural convection, the radiation effects are much more important than the roughness effects.

[Frangible]

1) Why do Bedouin Arabs wear black?

That was explained in a Scientific American article years ago.  Evidently the dark coloring does absorb more heat, but since the clothes are loose and flowing, it creates a natural convection of the air next to the wearer's skin, thus creating a sort of "micro climate".  The circulating air dries the sweat on the wearer's skin, cooling it and the air right next to it.

With heat sinks, many are painted or coated in some way in order to increase its surface area, as would sandblasting.  This is especially the case with non-reflective or flat finishes.

[IanB]

With heat sinks, many are painted or coated in some way in order to increase its surface area, as would sandblasting.  This is especially the case with non-reflective or flat finishes.

Indeed, but it is not actually the increase in surface area that is responsible for the improvement, it is the increase in surface roughness. The primary barrier to heat transfer from the surface is the so called "film coefficient", the resistance of a stagnant boundary layer of air touching the surface that the heat has to get through (remember that air is an insulator). Increasing the roughness of the surface will increase heat transfer by inducing air turbulence and reducing the thickness of the stagnant film. Of course to be effective all heat sinks must have good air flow, so increasing the surface roughness without having good air flow will not do much for you.

[SgtRock]

To All:
--Thanks for your informative answers concerning Bedouin Arabs and white vs black Clothing. I think I have a handle on it now.

--As I understand it the mirrored glass, vacuum sealed Dewar Flask tends to keep cold liquids cold and hot liquids hot. My question is: How does it know?
Clear Ether

[Uncle Vernon]

As I understand it the mirrored glass, vacuum sealed Dewar Flask tends to keep cold liquids cold and hot liquids hot. My question is: How does it know?

It done by memory, unfortunately Dewar Flask memory is particularly volatile, with each flask forgetting either hot or cold within a matter of hours. 

[IanB]

As I understand it the mirrored glass, vacuum sealed Dewar Flask tends to keep cold liquids cold and hot liquids hot. My question is: How does it know?

Well, you see, what it actually does is keep the outside at the same temperature regardless of what is going on on the inside. So it doesn't need to know about the inside...

[SgtRock]

Dear Uncle Vernon and IanB:
--Alas, once again I am hoist by my own petard. I am heartened to know that  my humble remarks were accepted in the same spirit as they were given.

--From "The Complete Idiot's Guide to Wilderness Survival":
High Mountain Survival Tip #7 "Don't eat yellow snow"
Best Regards
Clear Ether

[EEVblog]

To conclude, painting heatsinks black does not contribute to their ability to dissipate heat by radiation.

Black anodising of heatsinks does improve the thermal dissipation performance. This is a known and measured fact.

Dave.

[SgtRock]

Here, here! Well said Sir. And just what would be expected from the Stefan–Boltzmann Law for "Black Body" radiators.
Clear Ether

[amspire]

Painting or anodizing definitely improves radiation dissipation, but it doesn't have to be black.  The colour in the visible spectrum has absolutely no correlation with the colour in the IR heat spectrum of a heatsink.  So you can get any colour you want, as long as it is "black" in the IR frequencies. Most standard paint of any colour is fine to use so fluorescent green is as good as black. You don't even have to paint or anodize it. Stick masking tape or electricians tape to the raw aluminium heatsink and you also get radiation cooling close to a black anodised one.

This is not the same as choosing the paint colour for a car as there is a lot of energy from the Sun at much higher frequencies then heatsink IR emission, and definitely a black car gets hotter then a white one. I think the Sun must be a bit hotter then a 100 deg C heatsink.

Unpainted aluminum - shine or rough - has lousy radiation and also doesn't absorb radiation from other heats sources.

This fact is really useful.  If you want to have fan cooled heatsinks in a case, you want them unpainted, so they aren't radiating heat inside the case. Also if you paint the outside of a heatsink but not the inside, it will only radiate significant heat to the outside. Nice!

On the other hand external convection cooled  only work with a height up to about 100mm.  A 200mm high heatsink only gets about 50% more convection cooling as a 100mm high one and over 200mm, you are just wasting expensive aluminium for no good reason.  So if you need a very tall heatsink, forget about the fins. Make it a flat sheet, paint the outside surface any colour you like, and get rid of almost all the heat by radiation.

[N TYPE]

Dragging up an old topic here but..
Say I have a Heatsink which is painted or powdercoated on all sides, and I want to mount my TO220 reg onto the heatsink. Sure I'm gonna use thermal paste + mica pad and a nut & bolt to fasten it but is it necessary/beneficial to scrape the paint from the contact surface of the heatsink to transfer the heat better?

[SnakeBite]

Dragging up an old topic here but..
Say I have a Heatsink which is painted or powdercoated on all sides, and I want to mount my TO220 reg onto the heatsink. Sure I'm gonna use thermal paste + mica pad and a nut & bolt to fasten it but is it necessary/beneficial to scrape the paint from the contact surface of the heatsink to transfer the heat better?

i don't think anodising and powercoating is the same thing.. in powercoating you apply paint on the surface of the heatsink that it's also used as insolator and anodising locks the color inside the metal itself that gives you the better heat transfer.

BUT i'm not sure