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Conductivity across materials

post #1 of 9
Thread Starter 
I can find the thermal conductivity sample values in this table

http://upload.wikimedia.org/wikipedia/commons/1/1e/Thermal_conductivity.svg

I don't know however, how should be measured the thermal conductivity across materials?
For an example does the thermal grease conduct heat to the copper sink with the value for copper, or the value for thermal grease (lower)?

Also, in a cooling system of consequently related elements (CPU, grease, sink, air) is the overall conductivity equal to the lowest value (like in electricity resistance) ? Or is there some formla of calculating that?
(which is practically the same question smile.gif )
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post #2 of 9
Heat transfer is often modeled as circuits, believe it or not, and I think this visualization method is the most straightforward one for describing it. A resistor of value 12 ohms puts up less resistance than a resistor of 50 ohms.

Material 1 (CPU): 50 ohms (COMPLETELY arbitrary, just for sake of argument.)
Material 2 (grease): 20 ohms (ditto)
Material 3 (HSF): 30 ohms (ditto)

The energy flowing through the composite material has an easier time going through the grease than through the HSF or the CPU. You get the idea.

Now, this surely isn't the full story, but I hope this helps describe that one needs to examine the heat transfer through each individual media to understand what kind of energy is being transferred to the adjacent media. Each Material X is going to be a set of equations all its own. Once you solve one layer, you can move on to the next.

Heat transfer was arguably the most difficult and wide-spectra course in the mechanical engineering department at my university, so a sufficiently able person could go into hours worth of explanation on this one simple example.

The gist of it is: overall conductivity can't really be coalesced into a single value - at least not without some rudimentary analysis, i.e., it isn't as simple as treating the lowest value as your k-value.

There are formulas, but if I showed them to you, you might vomit. Perhaps a more able mind on here can show you an easy way of looking at them, but my professor did his best to keep it on the brain-melting level. Thus I'm pretty awful at being able to explain things like this.

This is an excellent resource if one of your buddies has one on hand.

Hope this helps. smile.gif

Edit: I forgot to mention a higher k-value allows for more heat transfer. It's kind of an "I knew that" sort of thing, but just to clear up any ambiguities. More Watts/(kelvin*surface area.) The idea behind thermal grease is to maximize surface area between the CPU and HSF without restricting heat flow, since at the microscopic level they're not actually flat. Hence, lapping.

/lecture tongue.gif
Edited by Aeschylus - 4/13/11 at 1:32am
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post #3 of 9
Thread Starter 
Quote:
Originally Posted by Aeschylus;13107461 
Heat transfer is often modeled as circuits, believe it or not, and I think this visualization method is the most straightforward one for describing it. A resistor of value 12 ohms puts up less resistance than a resistor of 50 ohms.

Material 1 (CPU): 50 ohms (COMPLETELY arbitrary, just for sake of argument.)
Material 2 (grease): 20 ohms (ditto)
Material 3 (HSF): 30 ohms (ditto)

The energy flowing through the composite material has an easier time going through the grease than through the HSF or the CPU. You get the idea.

Now, this surely isn't the full story, but I hope this helps describe that one needs to examine the heat transfer through each individual media to understand what kind of energy is being transferred to the adjacent media. Each Material X is going to be a set of equations all its own. Once you solve one layer, you can move on to the next.

Heat transfer was arguably the most difficult and wide-spectra course in the mechanical engineering department at my university, so a sufficiently able person could go into hours worth of explanation on this one simple example.

The gist of it is: overall conductivity can't really be coalesced into a single value - at least not without some rudimentary analysis, i.e., it isn't as simple as treating the lowest value as your k-value.

There are formulas, but if I showed them to you, you might vomit. Perhaps a more able mind on here can show you an easy way of looking at them, but my professor did his best to keep it on the brain-melting level. Thus I'm pretty awful at being able to explain things like this.

This is an excellent resource if one of your buddies has one on hand.

Hope this helps. smile.gif

Edit: I forgot to mention a higher k-value allows for more heat transfer. It's kind of an "I knew that" sort of thing, but just to clear up any ambiguities. More Watts/(kelvin*surface area.) The idea behind thermal grease is to maximize surface area between the CPU and HSF without restricting heat flow, since at the microscopic level they're not actually flat. Hence, lapping.

/lecture tongue.gif

Thanks a lot.

On the Edit- yes, I knew that, but still thanks wink.gif

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post #4 of 9
Thread Starter 
Got another Q, not related to the OP:
Why do they insert thermal compound inside the heat pipes, if conductivity of copper itself is ~1000 times better than any liquid? (0.5 for water, 400 for copper) (thermal grease is just a bit more conductive than water)
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post #5 of 9
Quote:
Originally Posted by ronnin426850;13107589 
Got another Q, not related to the OP:
Why do they insert thermal compound inside the heat pipes, if conductivity of copper itself is ~1000 times better than any liquid? (0.5 for water, 400 for copper) (thermal grease is just a bit more conductive than water)

There is no thermal compound in heat pipes. A solid copper pipe would rely only on conduction. Heat pipes use evaporation/convection/condensation to transport heat.

Convection is a much more efficent method of heat transportation. A 4mm copper pipe cannot transfer that much heat. A heat pipe can transfer a few magnitude more energy.
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post #6 of 9
Quote:
Originally Posted by ronnin426850;13107589 
Got another Q, not related to the OP:
Why do they insert thermal compound inside the heat pipes, if conductivity of copper itself is ~1000 times better than any liquid? (0.5 for water, 400 for copper) (thermal grease is just a bit more conductive than water)

Perhaps unrelated, but conduction =/= convection!
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post #7 of 9
Thread Starter 
Thanks to both smile.gif
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post #8 of 9
Quote:
Originally Posted by ronnin426850;13110658 
Thanks to both smile.gif
Think about this....


If you're sweating, would you rather stand in front of a fan or lie down on a slab of aluminum? wink.gif
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post #9 of 9
Thread Starter 
Quote:
Originally Posted by DuckieHo;13111605 
Think about this....


If you're sweating, would you rather stand in front of a fan or lie down on a slab of aluminum? wink.gif

biggrin.gif Great comparisson, I get it now.

I wanted to make some mods to my sink, and I want to make them myself, not buy them smile.gif That's why I was asking. Thanks
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