Copper vs. Aluminum

[GooeyGus]

[quote author=GooeyGus link=1220828298/0#10 date=1220913837]
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'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.[/quote]

I guess I should have phrased what I said differently. I didn't necessarily mean the active radiation of the heat, but more about the capacitance for heat. Copper simply holds more heat, and will thus carry more heat than aluminum. Aluminum draws heat quickly but just cant hold as much as the copper can, so it will generally stay a bit cooler in a 'block' configuration. Copper draws heat away faster, which is why it's great for 'finned' setups.

I hope that makes more sense.... :-[

 

[Switch]

About the cooling through radiation, it's only gonna get as cold as the air surounding it, that is also radiating and is also directly transfering heat to the heatsink, right? OR is the anodized heatsink always be cooler than the ambient air because of superior emissivity?Then again, it also has superior absorbance so..... :-/ I dunno, I'm guessing it's just gonna find a balance and achieve the same temperature as the air around it.

Also another question related to this: Would an object found in a perfectly empty space with no matter or radiation going through eventually reach 0K because of radiating all the energy out as heat? And as for the temperature in a vaccum , I assume that we can't even talk about temperatur in an empty space, right? :-/

 

[pullbangdead]

About the cooling through radiation, it's only gonna get as cold as the air surounding it, that is also radiating and is also directly transfering heat to the heatsink, right? OR is the anodized heatsink always be cooler than the ambient air because of superior emissivity?Then again, it also has superior absorbance so..... :-/ I dunno, I'm guessing it's just gonna find a balance and achieve the same temperature as the air around it.

Also another question related to this: Would an object found in a perfectly empty space with no matter or radiation going through eventually reach 0K because of radiating all the energy out as heat? And as for the temperature in a vaccum , I assume that we can't even talk about temperatur in an empty space, right? :-/

A "perfect" vacuum, in theory, would be absolute zero, but there's no such thing as a perfect vacuum (not even interstellar space, there's always a pressure, always SOMETHING there, so there's always a temperature as well), and there's no such thing as actually reaching absolute zero.  That's the 3rd Law of Thermodynamics, "You can't get out of the game".  

1st: You can't win.  Energy is conserved, can never be created or destroyed, only changed forms.  Since you can't make more, you can't win.

2nd: You can't break even.  Entropy always increases in a closed system, unless you input more energy.  Since Entropy always increases, you can never be 100% efficient.  Since you can't be 100% efficient and must input more energy, you can't break even.

3rd: You can't get out of the game.  The only way to remove entropy is to get to absolute zero, where temperature and entropy will both be zero.  This is approached asymptotically, so it can never be reached.  Since you can never get to absolute zero, you will always have entropy, and you can't get out of the entropy game.

And yes, a heatsink without any active cooling or active heating will eventually come to equilibrium with its surrounding environment and be the same temperature.  This can be proven with the laws above and some more work.  When the heatsink is receiving thermal energy from whatever you're trying to cool (in this case the LD), however, there will be a temperature gradient across it.  This gets a lot more complicated, but you're pretty much right about heatsinks, they can never get colder or hotter than their surroundings.

Also, at normal pressures, radiation of heat is miniscule compared to conduction.  In any real-world case, the heat dissipation from the heatsink is through conduction directly to gas molecules that collide with the surface of the heatsink and take some of its thermal energy.  

 

[Switch]

Also, at normal pressures, radiation of heat is miniscule compared to conduction. In any real-world case, the heat dissipation from the heatsink is through conduction directly to gas molecules that collide with the surface of the heatsink and take some of its thermal energy.

I always thought the same. Though I have heard that black plastic , for instance will get hotter and melt faster than white plastic when placed near a heat source, like a fire or something. :-/

 

[pullbangdead]

Fires or lightbulbs or things in the hundreds of degrees C can give off a TON of radiation. Fires give off a lot of heat in the form of radiation. A piece of metal at 30C or even 40C doesn't give off much radiation at all, really not much more than the same piece of metal at room temperature, 25C.

It's all about specific regimes. You get that piece of metal glowing red hot, above half of its melting temperature, and radiation can become significant. But at heatsink temperatures here (still cool engouh to touch is less than 50C-60C), radiation is pretty close to irrelevant compared to conduction.

 

[Bionic-Badger]

I always thought the same. Though I have heard that black plastic , for instance will get hotter and melt faster than white plastic when placed near a heat source, like a fire or something. :-/

This is why people color their match heads black to cause them to burn more easily with lasers, and why you don't want your mug for hot drinks to be black.

 

[Krutz]

semi off-topic:

when thinking about which heatsink you want, the "connections" between parts is much more important than the choice of metal. air in between, and you could almost leave the heatsink off altogether. bad/too much/too little thermal compound in between, and you lose (or "gain") many degrees easily!

my choice: copper.
simply because i solder my laserdiodeholders into it, for good conductivity. i use the aizix brass ones, but all holders should work except aluminium ones.

i tried to solder aluminium once, with special AL solder and flux. no way. impossible. dont even try, if it cant be avoided!

manuel