Copper vs. Aluminum

FYI: Cu = copper, Al = aluminum

I have a Cu heatsink that I could use that would be about 10-25% (I'm not a good estimator when it comes to cylindrical objects; it may not even be 10%, I'm really not sure) smaller than an available Al heatsink. I read a bit online about Cu vs. Al in CPU heatsinks. It said that Cu is better, but only worth the extra cost if there's 800 ft.^3 moving over the fins (which doesn't really apply to this application) and/or (it wasn't terribly specific) there's 25% or less of the surface area being used. (I'm not sure how I would apply that last one since there's multiple surfaces. It mentioned that Cu can be up to 3x more effective than Al. So, which would be better? A slightly smaller Cu heatsink, or a slightly larger Al heatsink?

Thanks!
-Bearded_Galaxy

If your comparing two exact same sized peices of material, the copper is going to be better. However it really comes down to surface area. If your aluminum has a larger surface area exposed to air for wicking the heat away, the aluminum is going to be better. Copper transfers heat faster than aluminum. Silver is better than copper, and diamond is better than silver.

I don't know too much about the difference, but I do know that Cu isn't necessarily better than Al.
Cu will absorb more heat because it has a higher capacity for heat. However, Cu cannot transfer this heat away into the air as effectively as Al. This means that the Cu heatsink will absorb a lot more heat but will hold on to it longer than an Al heatsink would.


EDIT: Diamond as a heatsink? I've never heard of that... Would you mind explaining, Kenom?

Wow, never knew that diamond was good for that. It appears carbon nanotubes are also good. Once those can be constructed in mass quanities, maybe we'll see carbon nanotube heatsinks.

RA_pierce: you can read the wikipedia article for some light information on the topic.

Long ago, I knew a guy who was experimenting on diamond substrates for high power IC's. He was paid by an employer but I don't know if he was ever able to achieve the coating process.

Mike

Diamonds?
Wow diamonds have so many uses.. I would highly doubt that they would be used for heatsinks though.. But thinking about it you would have a nice looking laser *goes off to buy a diamond heatsink*

They actually have diamond thermal compound you can buy for your CPU. You've got artic silver, and then sure would be cool, arctic diamond. it's rare and expensive but they do make it. Not thermaltake though.

http://hackedgadgets.com/2007/12/05/diamond-thermal-grease/

Metals transfer heat mostly through electron transport. Diamond transfers heat purely through phonon transport. Metals also transfer some heat through phonons, but the atoms in a metal are not tightly bonded and metals have grain boundaries and dislocations that scatter phonons.

Diamond, however, has a very tightly-bonded lattice (almost purely covalent bonds, rigid lattice) and as a single crystal, doesn't have nearly the density of scattering centers as a metal. So diamond has an extremely high thermal conductivity due to it's high rate of phonon transport.

However, it's not just thermal conductivity that makes a good heatsink. I don't remember the exact formulations for what makes the best heatsink, but you have to bring in not only thermal conductivity, but also heat capacity. Again, I don't remember exactly, but I think thermal effusivity may be the parameter that best fits a materials heatsink-ness. The thermal effusivity is the square-root of the product of the thermal conductivity and volumetric heat capacity. If someone finds a table listing the thermal effusivity of different materials, that would be awesome.

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Trivia question for the day: as a metal gets HOTTER, does it conduct heat better or worse, all else being equal?

Answer for "TRIVIA QUESTION" After giving this some deep thought, I figure that a metal conducts heat BETTER as it gets hotter. Reason: As the metal heats up, it also expands, increasing it's surface area, thereby conducting heat better, due to that increase in surface area. just my nickel's worth :-/ rob

phoenix77 said:

Answer for "TRIVIA QUESTION" After giving this some deep thought, I figure that a metal conducts heat  BETTER as it gets hotter. Reason: As the metal heats up, it also expands, increasing it's surface area, thereby conducting heat better, due to that increase in surface area. just my nickel's worth :-/ rob

Click to expand...

Sorry, I'll clarify the question: I mean within the metal, heat moving from one end to the other within a chunk.

Good reasoning for heat leaving the metal to its environment, though.

RA_pierce said:

I don't know too much about the difference, but I do know that Cu isn't necessarily better than Al.
Cu will absorb more heat because it has a higher capacity for heat. However, Cu cannot transfer this heat away into the air as effectively as Al. This means that the Cu heatsink will absorb a lot more heat but will hold on to it longer than an Al heatsink would.

Click to expand...

Bingo.

As far as I understand it, Al will be better for the type of heat sinks we are using. Copper can hold much more heat, therefore it has a harder time radiating that heat out into the air. Al cant hold nearly as much heat so it radiates it out into the air. Copper is great in a 'fin' design when there is air blowing over it, as the transfer from metal to air is much better.

I think aluminum will dissipate heat to the ambient air faster.
I would select copper if it was used to transfer that heat through a
medium such as water.

GooeyGus said:

As far as I understand it, Al will be better for the type of heat sinks we are using. Copper can hold much more heat, therefore it has a harder time radiating that heat out into the air. Al cant hold nearly as much heat so it radiates it out into the air. Copper is great in a 'fin' design when there is air blowing over it, as the transfer from metal to air is much better.

Click to expand...

I've seen that argument before, and it is simply incorrect. How well a substance radiates heat is dependent on its emissivity, which is a surface property of the object. For example, ideal matte black, the kind we strive for on our thermopiles, has an emissivity coefficient of 1.0 (the highest). It absorbs, and radiates heat extremely effectively. However, aluminum foil has 0.04, and polished copper 0.023-0.052, which would make them better for blocking out radiated heat in a vacuum. This is why you don't want to use black colored mugs for hot drinks.

Copper is indeed great for heatsinks involving active airflow, because it has superior thermal conduction between the CPU and the air that passes through it. Aluminum is used because it is cheap, light, and a decent conductor; it has nothing to do with "radiative" properties. Cooling by radiation did have a part to play in the old days when we didn't use fans on computer heatsinks. Most heatsinks were anodized or painted black in order to help them radiate heat more effectively. However, as the need for active cooling arose, radiation as a cooling method became a very small contributor to overall cooling, and now most heatsinks are not colored.

So if you want your laser barrels to radiate heat more effectively, you ought to have them coated in black. Still, more of the heat will be dissipated through conduction, and hence the fins on the new Kryton barrels and the larger mass.

I believe the answer to the trivia question is worse. Since the atoms inside the block are moving faster it is harder for the heat to get sent to the others. (your example with the crystals in the diamond.) If it were the opposite then the hotter something gets, in this example I'll use a cpu heatsink, the more efficient it is going to be at heatsinking. And as we all know, there is a peak or plateau that can be reached but not exceeded by heatsinks in it's ability to transfer heat. so, I'm going to guess worse.

phoenix77 said:

Answer for "TRIVIA QUESTION" After giving this some deep thought, I figure that a metal conducts heat  BETTER as it gets hotter. Reason: As the metal heats up, it also expands, increasing it's surface area, thereby conducting heat better, due to that increase in surface area. just my nickel's worth :-/ rob

Click to expand...

who said anything about nickel?