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thermal radiation and thermal conductivity

Allright.. I'm having an argument with some friends, and the question that has come up is how copper and aluminum transfer heat to air. Are they correct that although copper can absorb heat better, aluminum can transfer heat to the air better?

What's the difference between thermal radiation and thermal conductivity?

Radiation is the transfer of heat through the release of electromagnetic radiation. CPUs will lose almost no heat this way.

You're thinking of thermal convection, which is the transfer of heat from a solid to a fluid.

As for Al vs. Cu, that road's been covered quite a few times. I'm sure someone else can find a good link.

Alchemy

Specifically, this part is the most relevant:

Quote:

Fact 1: Aluminum has a higher specific heat and lower density than Copper. Specific heat is a measure of how densely a material can store thermal energy. If you add thermal energy to a specific mass of material, it will increase in temperature.

The specific heat of aluminum is 903 joules/kg*K at 300K while copper is only 385. The density of aluminum is 2702 kg/m^3 at 300K while copper’s is 8933. The product of specific heat and density determines how much energy may be stored in a given volume. For a given volume of material, copper will store more energy at a given temperature change than aluminum.

False Perception: Since aluminum stores less energy per volume, it must be more efficient at getting rid of heat.

Truth: During steady-state operation, there is no net energy storage in the heat sink or fins/pins; hence, specific heat plays no part in steady-state performance.

Fact 2: Aluminum has lower density than Copper. Volume for volume, aluminum is much lighter than copper.

False Perception: Weight acts as a “sink” for heat. Since copper is more dense, it absorbs heat well from the die. Since aluminum is light, it gets rid of heat more effectively than copper.

Truth: Density has no direct relationship with steady-state heat transfer.

Fact 3: A small volume of aluminum will cool more quickly than an equal volume of copper once the heat source is gone. This is due to the same reason as fact #1, namely there is less energy stored per unit volume is aluminum than copper. This is, however, a transient condition. Heat transfer from a computer is a steady-state condition where the temperature of the heat sink remains relatively constant. The specific heat of a material partially determines how a material responds to transient conditions but has no effect at all on steady-state operation.

False Perception: Since aluminum cools more quickly once a heat source is removed, it must be more efficient at convection.

Truth: The heat source driving energy into the heat sink remains in effect until you turn off your computer. If you have aluminum pins or fins, congratulations, they will cool off more quickly than copper ones after you shutdown your PC.

Aluminium is better at radiating heat that copper is, but that is not very relevant unless you are planning to have your computer operating in a vacuum.

Quote:

Originally posted by KnightElite

Aluminium is better at radiating heat that copper is, but that is not very relevant unless you are planning to have your computer operating in a vacuum.

Isn't radiation specific to the surface finish of the material?

Meaning black anodised aluminium is a more effective radiator. But I doubth plain aluminium is better at radiation.

Either way, it isn't terribly relevant, unless you're designing a cooling system for a satellite (where heat dissipated is ~100% radiation)

It is relevant, for a passive heatsink, it makes a difference.

Thanks. I actually found that site just after I posted. Dave Smith does a great job explaining things.

My friends were trying to tell me that the most efficient heat sink was one with a copper core, and aluminum fins which I knew to be wrong. In their minds, the aluminum transfered all heat to air better, which I knew to be mostly wrong, but didn't know where the line of truth lay.

Thank you all for your help, you are both pimps and scholors.

Quote:

Originally posted by hara
It is relevant, for a passive heatsink, it makes a difference.

Not really, because most of your heat removal will still be through convection. Even if you have zero _forced_ airflow (fans, etc), you will still have airflow, due to hot air decreasing in density, and flowing upward, which is then replaced by cooler, denser air, and so on and so forth.

As BB2K said, heat removal through radiation is not terribly relevant here on Earth, in most cases. :D

Alright, I'm now breaking out the heat transfer notes from last semester ;).

Yeah, with a passive heatsink you have what is called free convection. Forced Convection, with a fan, can be up to 100 times more efficient than free convection. From my textbook:

Code:

Type of Convection                          Convection Coefficient (W/(m^2 * ºC))

--------------------------------            -------------------------------------

Free Convection: Atmospheric air            5-25

Free Convection: Water                      400-1000

Forced Convection: Air                      15-500

Forced Convection: Water                    1000-15000

Forced Convection: Engine Oil               1000-2000

Those represent some typical values.

As for radiation, you are correct. I was misguided, because the table lists aluminium foil ;). Black paint has the best emissivity, followed by red brick and asbestos.