Heatpipes are among THE most important components of a modern heatsink, but not all heatpipes are created equal. Different wick structures affect heatpipe performance and if your heatsink has cheap heatpipes, the rest of the heatsink suffers.
What is a heatpipe wick? Heatpipes are hollow copper tubes that conduct heat from one location to another. They operate by means of a small amount of working fluid which is contained in the tube, under a slight vacuum. The vacuum lowers the boiling point of the working fluid so relatively small increases in temperature vaporize the liquid which is then naturally drawn towards the colder end of the heatpipe where it condenses back to a liquid. As the vapour condenses the heat energy it carried is conducted to the surrounding metal. An internal wick structure coats the inside of the sealed tube and allows that working fluid to return by a capillary action.
It's these properties that make heatpipes much more efficient at conducting heat from one location to another than say, a solid bar of copper of equal size.
The crux of the situation is that some heatpipe wick structures are more efficient than others and some have limitations with respect to orientation and gravity. The three main types of wick structure used in commercial CPU heatsinks are sintered metal powder, grooved and metal mesh wick. The sintered copper powder type is expensive to produced but the performance is generally considered very good.
The point of this short article is not to judge heatpipe wick structures, but rather see if the type specified on the packaging is actually used in the product. Frostytech picked a Zalman CNPS11X Extreme heatsink because it's about the only manufacturer that comes to mind which discloses the type of heatpipe wick structure used.
Let's get to it!
Heatpipe Wick Structures - The Unknown Factor
The problem for consumers is that you can't see what's inside a sealed copper tube embedded in a heatsink. You simply have to trust what the manufacturer says is true, assuming the maker even discloses what kind of heatpipe wick structure is used.
Far too few heatsink manufactures specify the wick structure of heatpipes used in their pricey performance CPU coolers. However, if you think thermal compound is important to achieving the best cooling possible, you ought to know what kind of heatpipe wick structure is used for the performance heatsink you're considering.
For a collection of its performance heatsinks, Zalman specify a type of composite copper heatpipe. This 6mm diameter heatpipe is supposed to contain a dual wick structure of grooved and sintered metal that makes it "more efficient at conducting heat than a typical metal mesh wick." There's no way to see the wick structure on the inside of heatpipes without destroying a rather expensive heatsink in the process, so Frostytech did just that.
A 1" diamond saw disk will cut through a fragile hollow copper heatpipe tube without damaging the internal characteristics. Here's what Frostytech found after removing a small 37mm long section of heatpipe from the Zalman CNPS11X Extreme heatsink and splitting it down the middle. The section of heatpipe is shown next to the heatsink it was cut from.
What you see here is a section of 6mm diameter copper heatpipe with fine axial triangular grooves covering the interior surface of the tube, on top of that is a thin layer of sintered copper powder. The sintered copper powder layer is about 0.4mm thick. The outside of the copper heatpipe is nickel plated, hence the silver colour.
The longitudinal cracks you see in the sintered copper power are from bending the sides of the copper tube down to expose the insides. A section of sintered copper power wick (at left) was mechanically removed for this photograph in order to reveal the grooves below. The distortion is from the microscope lens.
The dissected composite heatpipe Zalman use in the CNPS11X Extreme is indeed comprised of a grooved and sintered metal wicks as you can plainly see. Good to know Zalman stands by its marketing materials.
The big question remains, what about the heatsink you're using right now? Frostytech would really like to see more performance heatsink makers start listing the type of heatpipe wick structure used in their CPU coolers. What do you think?
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