Thermal conductivity of Aluminum nitride: Things to know

Aluminium nitride has spurred its engagement in science with the advancement in ceramic. AlN is excellent when it comes to thermal conductivity and electrical insulation. AlN also holds merit of having no health hazard to workmen during machining. Hence, AlN is considered to be more superior than other competitive ceramics.

Properties of AlN: A quick guide to refer

Here is a quick guide to learn all the properties of aluminium Nitride. The table will help you assess the context of using AlN.

How is Aluminium Nitride synthesized?

Aluminium nitride can be produced by the direct nitridation of Alumina. The other way of production includes alumina carbo thermal reduction. The covalent bonds in the AlN are very strong that makes it harder for sintering. For manufacturing AlN through sintering, the material needs special additives in liquid form.

There are several methods to produce advanced ceramics. The global production of nitride ceramic is basically done by isostatic pressing or dry pressing. The other methods include injection moulding involving ceramic or low pressure-based injection moulding. Sintering method of AlN considers spark plasma sintering, pressure less sintering and microwave sintering

The forming process of the nitride ceramic are of two types. The dry and wet type. Dry forming can be either ordinary and cold isostatic. Wet forming is used to produce complex shapes. The methods include Slip, tape gel casting and injection moulding.

Detailing on physical properties of Aluminum Nitride

Aluminum nitride as the name suggests stands for nitride in its solid form made up of Alumina. It has wide bandgap.  The small atomic mass and higher interatomic bonds makes the ceramic strong. It generally follows wurtzite crystal structure and also has a metastable cubic phase.

The conductivity of heat of Aluminum ranges from 150 - 320 W/m K. The Aluminum Nitride electrical conductivity is around 10-11 to 10-13. When doped the conductivity gets reduced to 10-5 or 10-6. Generally, Aluminum in its wurtzite phase has applications pertaining to optical electronics.

AlN is stable at high temperature of around 2200 degC. The morphology consists of oxide layers on the surface that protects the material at a working temperature of 1370 degC. Talking about corrosion resistance, it is susceptible to attacks caused by alkalies and mineral acids.  However, the nitride ceramic is resistant to attacks from molten salts.

AlN properties: A glance

Here is a quick summary of the major properties of AlN

• High temperature of operation

• Excellent thermal conductivity which is 5 times larger than Alumina

• The CTE value of Aln is low

• The dielectric constant value is less

• Good electrical resistivity and insulation properties

• Good Compressive resistance

• Good Corrosion resistance except acids and alkalies

• The Purity of AlN is on higher side

Aluminum Nitride CTE

Coefficient of thermal expansion gives us a vivid idea of the temperature behaviour of the material. Advanced ceramics are known for their lesser thermal expansion. The value of CTE will let us know how the material expands under applied temperature. However, Under the application of heat, the atomic structure stays stable when the material expands.

When compared to metals the advanced ceramics such as AlN has half the thermal expansion coefficient of a metal. The lesser value of CTE makes them retain the mechanical strength under compression. In terms of thermal expansion, Zirconia and Alumina ranks the highest whereas Silicon Nitride and Carbide ranks the lowest.

 Aluminum Nitride CTE is in the range of 5.3 to 4.2 (x 10-6) K-1 along x and z axes at the working temperature range of 300 K. The Aluminum Nitride CTE is considered to be equivalent to Silicon.

Coefficient of thermal expansion and the efficiency of ceramic substrate

Although AlN excels in properties of good temperature transfer and electrical resistance that aids microelectronics. Let us see what impact CTE has on the working of Aln based  ceramic substrates.

Impact of Thermal expansion in DBC substrate

The DBC substrate has Aln or Silicon Nitride bonded to copper. The thermal expansion differences between Aln, Cu and SiN (4.5 ppm, 17 ppm and 3 ppm) causes high mechanical stress. The stress induces cracks on the geometry. The cracks are formed as a result of shock cycles caused during heating and cooling of the ceramic.

The management of the CTE is hence crucial for the long-term working of ceramic substrate.  A comparison of the different CTE of various ceramic material has been given above.

Thermal Conductivity of Aluminum Nitride

Thermal conductivity speaks about how well a material allows the heat energy to pass through anything. AlN thermal conductivity is high. In addition, they have high electrical resistance which helps them being useful as excellent heat dissipating material. The ceramic conductivity is influenced by various properties.

The presence of impurity can impact the heat transfer to a greater extend. With the increase in the impurity of Oxygen, the conductivity is said to increase for AlN. Further the number of vacancies present in the crystal lattice or Silicon and carbon impurities decreases thermal conductivity.

The heat properties of AlN can be controlled during the production process itself. The changes in the microstructure and the different conditions during processing helps to vary the thermal flow. The usage of high purity powders and the methodology adopted in the sintering significantly increases the thermal conductivity.

Thermal conductivity of AlN, why it is preferred over BeO?

BeO and AlN both the ceramics have high thermal conductivity. So as, they can be used in high temperature application.  Let’s explore in what way AlN surpasses BeO

Beryllium Oxide (BeO)

The thermal Conductivity of Beo is in the range of 330 W/Mk. They are basically employed in situation where heat management is crucial. Beryllium oxide due to their competitive conductivity is used as substrates and insulators. The other application includes manufacture of tubes, in microwave where thermal stability is important.

The problem with Beo is its toxicity. Beryllium oxide is hazardous and needs careful handling, The recycling and reuse of the ceramic is quite problematic when it comes to handling BeO.

Aluminum Nitride (AlN)

The conductivity of Aluminum Nitride is comparatively lower than BeO in the range of 170- 210 W/mK. Where non toxicity is preferred, Nitride ceramic finds its way of usage. ALN ceramic is used in LED and semiconductor electronics to dissipate heat. AlN is preferred in power electronic applications over BeO due to the delineated merits.

Aln has high electrical resistance and heat stability that makes them better insulating material. Aluminum Nitride is preferred in high temperature applications and is used in consumer based electronic supply to retard any unsafe handling situations.

Conductivities of various ceramic: an Overview

A comparison between the conductivity of different ceramic material is given in the graph above. As per the general rule, a material with lesser conductivity is preferred as an electrical insulator. From the given table, Zirconia has lower conductivity that makes it suitable as an insulator. The thermal conductivity of AlN is very high so as the heat dissipation.

One such prominent use of the conductivity of Aln is in high-power LED’s and also diodes. These instruments generally generate heat. The heat dissipation property of AlN helps in preventing overheating. This increases the reliability and ensures longer lifespan.

The popular two techniques used to measure the AlN thermal conductivity are the hot wire method and transient hot strip method.

Industrial use of AlN

The popular industrial usage of Aluminum Nitride ceramic is been provided below:

• Refractory: AlN is a good refractory. It is used as crucible lining or for providing structural support for casting moulds and related accessories.

• Electronic substrate:  AlN substrates are used as military multi-chip modules.  They function as heat dissipation boards in power electronics, laser diodes and LED heat sink.  The capacity of heat dissipation is attributed to their better conductivity.

• Automobiles: Aluminum Nitride is used in thermal and gas-based hybrid automobile ignition engines. They are stable at high temperature.

• Packaging material: Aluminum nitrides are good packaging materials that are heat resistant and durable.

Conclusion

The introduction of Aluminum Nitride has granted best functionality in high temperature engineering application. The superior thermal conductivity over counterparts has resulted in producing efficient heat dissipation entities in Power electronics. In short, there is an increase in lifespan and performance of materials which is a merit of Aluminum Nitride.