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This wikipedia article states that emissivity of polished copper is 0.04, and emissivity of oxidized copper is 0.87 - more than 20x of the polished copper.

So my question is - why are all copper heatsinks shiny and polished?

Wouldn't oxidized copper heatsink be much more effective in radiating the heat away from the source?

Searching for "oxidized copper heatsink" I find people asking and giving advice on how to remove oxidation from the heatsink because it makes the heatsink less effective - seemingly contradicting the information presented on given wikipedia article?

Is there something I am misunderstanding about emissive properties of a material?

Is there a way I could oxidize a copper heatsink in order to make it more effective?

XIII
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    My guess would be that the thermal conduction (of heat to the surface) is much more important than the emissivity. In addition I would guess that more heat is lost to the air (or a cooling fluid) than by radiation – doetoe May 14 '18 at 15:51
  • @doetoe But wouldn't the oxidation only be the most outer layer of the heatsink? The internal material wouldn't be oxidized and would retain the same thermal conduction properties?

    Isn't the emission/radiation of the material a property of "losing the heat" to air or water surrounding it? Which would be the desired effect.

    Take the heat - conduct it to a larger area of heatsink - and radiate the heat to the air surrounding it. So wouldn't it be better to have copper conduct the heat, and the oxidized surface radiate it 20x better?

     – XIII May 14 '18 at 16:03
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    I am by no means an expert, so I am only speculating. I would think that the outermost layer serves as a (relative) insulator, so that even if it cools fast by itself it takes long to heat up again. – doetoe May 14 '18 at 16:06
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    As for the radiation, I think air absorbs very little thermal radiation, and it gets mostly heated by conduction. – doetoe May 14 '18 at 16:08
  • @doetoe I see. With my limited understanding it somehow seemed logical that heat is radiated to the air, rather than conducted. But oxidation acting as kind of an insulator makes sense. Apparently copper oxide thermal conductivity is about 20x less than that of copper.

    Thank you for commenting my on question. It also seems your undertanding is confirmed by niels nielsen.

     – XIII May 14 '18 at 16:20
  • If you really wanted to increase the emissivity, don't bother with oxidation, just spray it with flat black paint. – user71659 May 15 '18 at 04:11
  • There are some black CPU coolers out there. I’m not sure if it’s paint or an oxidation layer (i.e. anodized aluminium). I guess it’s only for aesthetics though. – Michael May 15 '18 at 06:08
  • It should be added that you ask about heat sinks operated in atmosphear, operated somewhat around room temperature (250..400K). If you look at cryogenic things like a He-cryostat, they are always gold plated to not catch unwanted radiation. – dani May 15 '18 at 20:09

3 Answers3

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Radiative heat transfer is not dominant at the temperatures at which computer/electronic heatsinks operate, so the emissivity of the heatsink fin surfaces is not important for their operation.

Conduction of heat from the copper to the air, and then convection driven either by buoyancy or mechanical ventilation, is the dominant heat transfer mode that heatsinks exploit.

This makes the cleanliness of the fins far more important than their emissivity, which means anything that maintains them in a dust-free state will enhance their operation. This is why heatsink fins are made as smooth as possible, and not rough, and why the fan intake will have a lint filter on it.

The oxide tarnish that naturally forms on exposed copper fins at near-room temperatures is far thinner than a thousandth of an inch and therefore has a negligible effect on heat transfer. People who polish the oxides off to "improve" heat transfer are misinformed.

niels nielsen
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Although you make a good argument for the efficiency of emissivity (radiation), it seems you forgot about conduction and convection.
In the environment and the temperature surrounding electronic circuits, conduction and convection, are the dominant means of heat transfer. To maximize these methods, a clean and smooth surface is required between the hot object and its heat-sink, as well as between the heat-sink and the fluid flowing between its fins.

Although maximizing the conduction efficiency reduces the radiation efficiency, the gain in conduction more than offsets the reduction.

Guill
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the purpose of a heatsink is to conduct heat, to transfer it, away from a component as fast as possible, to a radiator(where emissivity is more important) that is usually made of anodized Aluminum(better emissivity & convection properties than rusty copper).

Ynnek
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