Metal alloys for heat sinks

Heat sink tutorial

Thermal conductivity

Usually a heat sink is used to spread the heat away from a thermal source in order to reduce the temperature of the source. Typical sources in my world are power semiconductors and power resistors.

This means that thermal conductivity is the most important property of the heat sink. Thermal conductivity of a heat sink is a function of cross section and the material property (thermal conductivity) of the metal.

In metals heat is transferred though conduction electrons and phonons. The phonon (lattice vibration) contribution is similar to that of an insulator with the same weight atoms, so the phonons are not insignificant. For example the thermal conductivity of aluminum oxide is 30, versus aluminum metal at 220, so about 10-15% of the thermal conductivity is due to the phonons. But for most of the thermal conductivity the electrons are doing most of the heavy lifting.

Thermal conductivity of a metal depends on how far conduction electrons, can travel without scattering. Conduction electrons can be scattered by phonons, which is why thermal conductivity gets worse as the temperature goes up. The more phonons traveling through the metal, the higher the probability of having them run into a conduction electron.

Conduction electrons and phonons are also scattered by impurities. An electron traveling through an aluminum crystal will be scattered if it runs into an impurity atom in the lattice, such as zinc or copper. They can also be scattered by defects in the aluminum crystal. So if an aluminum atom is missing, or is displaced from the crystal lattice it will scatter electrons.

Conduction electrons and phonons are also scattered by interfaces. Interfaces include the edges of the metal, but also crystal domain edges.

Here I should point out that the same thing that makes a metal strong also lowers its thermal conductivity. A pure crystal is soft because it is easy to slide the crystal layers over one another. Impurities will "pin" the crystal layers to keep them from sliding. Crystal defects introduced by deforming the metal (work-hardening) strengthen the metal by using crystal defects to keep the atomic layers from sliding. And will also create scattering centers to lower the thermal (and electrical) conductivity.

So the highest thermal conductivity metals will be pure and annealed with large crystal domains.