Liquid Metal

The liquid metal featured inside the Danamics coolers is NaK 78. This is a truly unique metal alloy consisting of 78 Wt % K (Potassium) and 22 Wt.  % Na (Sodium). It is particular useful as a heat transfer medium for several reasons. Firstly it has a very high thermal conductivity of 0,232 W/cm at 100°C. Secondly it is in liquid form in a very large temperaturespread. It remains liquid from -12,6°C all the way to 785°C. Finally this liquid metal has high electrical conductivity making it possible to circulate via an electromagnetic pump.

Both Potassium and Sodium are alkali metals comprising group 1 in the periodic table. This group also counts other alkali metals which are; Rubidium (Rb), Caesium (Cs), Francium (Fr) and Lithium (Li). The last one of these - Lithium - is highly known in commercial consumer products as Lithium is used for high-capacity batteries powering laptops, PDA's, mobile phones, RC units and many other applications.

But why is liquid metal better for cooling when compared with water or other alternatives? Firstly liquid metal has superior thermo physical properties that mean it conducts heat many times better than for instance water. The waste temperature difference needed to push heat from the walls of a cool-plate into the liquid metal or vice verse on the radiator side is minimal. This efficiently helps drive down thermal resistance and therefore lowering overall chip temperature.

Another reason is the electrical properties of liquid metals, which enables efficient, reliable and ultra compact electromagnetic pumping without the use of moving parts.

In order to illustrate how heat transfer via liquid metal differentiates from water we provide the illustration below which show the principal difference. The figures illustrate the temperature drop from a fixed heat source with equal power dissipation through the cool-plate into the cooling medium as a function of time. The flow through the coolplates is equal in both cases.

Left on the figure above is NaK and it can be seen that NaK heats up more through the cool- plate because of the lower specific heat. This is generally seen and is not related to a specific temperature level. As the conductivity of liquid metal is higher, then the cool-plates temperature will be lower. As the figure shows, the temperature is very close to the exit temperature of the medium, meaning that liquid metal is very efficient in transfering the heat from the heat surface compared to for instance water, which is shown on the right. Here you will find that the water heats up less while passing through the cool-plate but note the difference between the surface temperature of the cool-plate and the exit temperature of the fluid is much higher. This also demonstrates that in a system, where the medium is moved rapidly due to high flow, the conductivity is far more important as a physical factor than specific heat.