Thermal conductance in Ceramics, the science of keeping cool!
The use of ceramics in high thermal conductivity applications is a growing specialist market. The use of oxide ceramics is the most common as the base material cost is cheaper but the majority of materials are limited to 26-30 W/M/K, which when compared to the two most common high thermal conductivity metals, copper around 385 W/M/K and aluminium at 150-185 W/M/K leaves a large gap.
The goal is to increase the thermal conductance whilst retaining the other key feature of ceramic electrical insulation.
Recent developments in Alumina have increased the Thermal conductivity in some grades up to 39 W/M/K, still a long way short of commonly used metals, but a significant increase from its base level.
Materials that are selected to reach toward this higher Thermal conductance are a more specialist group, all have their own strengths and weaknesses, and not all ceramics can be included as some do not provide the other key property electrical insulation.
We are looking at four contenders and assessing these, Boron Nitride, Boron Nitride composites, Beryllium oxide, and Aluminium Nitride.
The first contender is Shapal Hi M soft a composite of Boron Nitride and Aluminium Nitride, this gives a significant increase in Thermal conductivity up to over 93 W/M/K and improves the electrical insulation. The combination of these two materials also brings a second benefit in the material in that it does not require diamond tooling to machine, but in achieving this machinable Aluminium Nitride requires hot pressing, which limits the size of the available material.
Boron Nitride another hot-pressed material is also machinable, but available in larger pieces, there are many grades available, but only the highest purity can match machinable AlN for thermal conductance and in some cases outperform it.
The high Purity grades are mechanically weaker and softer than machinable Aluminium nitride.
Beryllium oxide has been the material of choice for many high-end projects for many years this material has a thermal conductivity of 285 W/M/K, good electrical insulation, and as an oxide ceramic has none of the specialist Nitrogen furnacing of Aluminium nitride.
The main drawback is the health and safety requirements associated with this material meaning that only specialist applications tend to use this in military-type projects.
Aluminium Nitride or AlN, tends to be used mainly in its substrate form as the next step up from Alumina substrate, the thermal conductivity of Aluminium Nitride depends on grade a quality. the industry standard tends to be 170-180 W/M/K with lesser grades down to 150 W/M/K and ultra-high purity up to 220 W/M/K.
Most of the world’s production of Aluminium nitride is in substrate form and mechanical components in whatever geometry are a rarity, which is why many products use machinable Aluminium nitride even though it has a lower thermal conductance in the initial design phases.
If you want the highest thermal conductance and have really deep pockets then at over 2000 W/M/K synthetic Diamond is for you, but your thermal conductance requirement will have to be of an extreme nature.
The below table compares the thermal conductivity of 3 electrical insulators (Aluminium Nitride, Shapal and Alumina) and 2 conductors (Aluminium and Copper):
|Aluminium Nitride||AlN||170 w/m/k|
|Shapal (Machinable AlN)||Al/BN Composite||93 w/m/k|