Heat transfer compounds are the pastes, greases, and adhesives intended to increase the thermal conductivity across a joint between two solid objects. The idea is that the compound will fill irregularities in the joint between two imperfectly flat smooth clean surfaces, and will do much better than the air that would otherwise fill those irregularities. They used to be fairly obscure industrial products, but then powerful microprocessors with heat sinks and cooling systems brought these materials to the attention of people who work with the hardware inside their computers. And I fear something has gone wrong.
The thermal conductivity of the compound itself is a bulk property. It pretty much depends on how high one can get the volume fraction of finely powdered filler that is added to the pure oil or grease or polymer that forms a vehicle for the compound. At some point there’s not enough oil (or whatever) left to fill the voids between the little particles of silver (or whatever), and the compound gets crumbly.
But the problem is that this is only relevant for small areas, like what you typically see on computer microprocessors. We still use these compounds on large things, too, in various industries. I’ve been trying to get good thermal contact with things that are around 100 to 200 mm in diameter, and the problem I have seems to be that I can’t get the surfaces close to one another because heat transfer compounds are too stiff. The compound isn’t supposed to behave as a rigid mechanical component that can support force on its own, but in larger areas that’s exactly what’s happening. The forces that would extrude all but the last compound out of such a big joint are destructive.
I can’t find anything out there about the compromise between high loading and good flow. Oh, sure, one can use a liquid per se - I’ve got a bottle of glycerine on my desk right now (and it’s wonderful versatile stuff). But I think there should be a range of loading, and application guides about how to choose between them. And I can’t find it out there.
What know the Dopers?