By John Clifford, Technical Specialist
As the electronics industry expands, so too does the need for effective heat dissipation. A key example of this is the emergence of the electric vehicle (EV) industry, where thermally conductive adhesives, gap fillers, and potting compounds are crucial to performance and safety of the vehicles.
One common way to improve the thermal management of electronics is with the use of composites containing thermally conductive fillers. Many conductive fillers exist on the market today and choosing one with the optimal properties for an application is crucial. Below is a list of five common thermally conductive fillers, as well as description of some of their pros and cons.
1. Alumina – Thermal Conductivity: 36 W/mK 
As a filler material, alumina can increase the thermal conductivity of epoxies effectively at a low cost, as low as $2/kg . It also has a relatively low electrical conductivity making it an excellent fit for electrically sensitive applications, such as thermal interface materials or potting compounds. Alumina also has a very high melting point (>2000°C) and is fairly chemically inert. Alumina also benefits from having a shorter settling velocity than other
Alumina has a relatively low thermal conductivity compared to other filler material options. Likewise, its spherical shape contributes to high surface area which can inhibit the flowability for large-scale applications . Alumina, like many nanoparticles, suffers from agglomeration, whereby the nanoparticles in suspension will come together. This increases the density and causes particles to settle; this causes inhomogeneity in the mixture and will offer poor thermal performance if not properly mixed .
An example of how alumina can be used to raise the thermal conductivity of thermoplastic composites can be found in a paper from the University of Shanghai, here.
2. Copper – Thermal Conductivity: 400 W/mK 
Copper has a long history of being used in engineering applications due primarily to its impressive thermal and electrical conductivity. This enables it to be used as a filler for materials that need both strong electrical and thermal performance characteristics. It can be combined with later entries on this list, further improving the composite’s thermal performance . Unlike some additives, copper is a naturally occurring metal meaning that it does not require extensive synthesis techniques, reducing the cost of production.
Copper has weak mechanical properties, particularly at high temperatures. The usual solution to this is to use copper alloys; however, the thermal and electrical performance suffers from this substitution . Furthermore, while its conductivity is well suited for large appliances and radiators, it is not ideal when being applied to advanced flexible electronics. Copper also suffers from agglomeration, even more than alumina. This is due to their metallic bonds, which attract one another leading to higher particle sizes and a worse particle distribution 
A paper from the UK studies the effect of combining copper with other filler materials, in order to account for copper’s material weakness and improve the composite performance. Read it here.
3. Boron Nitride – Thermal Conductivity: 751 W/mK 
BN has a high thermal conductivity while acting as an electrical insulator, meaning it is well suited in materials that are designed to cool sensitive electronics. It is non-toxic and possesses excellent chemical and thermal stability, making it easy to work with compared to some other filler materials . Depending on the form used, BN can exhibit less agglomeration than other thermally conductive fillers, provided that the synthesis process contains few impurities 
One downside with using BN is that it is relatively brittle in its cubic form, making it difficult to incorporate into non-rigid materials, such as flexible PCBs . Cubic BN is produced in a similar manner as synthetic diamond, meaning it requires high temperature and pressure to synthesize, and as such it can be more expensive than other fillers .
A study in the Journal of Material Science examines the impact BN additives have on the thermal conductivity of polymer blends, read it here.
4. Carbon Nanotubes – Thermal Conductivity: 2000-5000 W/mK 
Carbon nanotubes (CNTs) have an exceptional thermal conductivity due to the rigid carbon-carbon bonds. Depending on form, thermal conductivity can reach as high as 5000 W/mK along the tube. This exceptional thermal performance is related to a CNT property known as “ballistic conduction” which is typically observed in quasi-1D structures. CNTs can also have a remarkably high electrical conductivity, making them very efficient in small sensitive circuits . There have even been reported claims of intrinsic superconductivity; however, these claims are not as widely supported due to the lack of consistent evidence in literature.
Due to the equipment reagents required for synthesis, CNTs are very expensive to produce, which can be prohibitive for use in mass production. Furthermore, the many carbon-carbon bonds tend to agglomerate together due to the high influence of Van der Waal’s forces . SWCNTs are insoluble in many common solvents like water, and since it is a nanomaterial, it inherently poses certain health risks, mainly towards the lungs .
A paper out of Brazil describes how the aspect ratio of CNTs impacts the thermal conductivity of the end composite, as well as its mechanical strength for use in concrete, read it here.
5. Graphene – Thermal Conductivity: 5300 W/mK 
Like CNTs, graphene benefits from a remarkably high thermal conductivity. Whereas CNTs are tubular in nature, graphene consists of a single layer of atoms in a two-dimensional honeycomb lattice nanostructure . The large surface area allows heat and electricity to transfer very efficiently along the monolayer . Graphene is also the strongest material ever tested, about 100 times stronger than the strongest steel would be at the same thickness. The material also strongly absorbs light of all visible wavelengths which accounts for its black colour, however due to its extreme thinness a single graphene sheet is nearly transparent . It is commonly used in polymer materials or as a flexible conductor for electrical applications such as phones or solar cells .
Despite these remarkable properties, graphene does have some downsides. For one, the carbon atoms at the edge of the sheet can be chemically reactive, which is enhanced by any defects present in the sheet . It has also been shown that single-layer graphene supported on an amorphous substrate suffers from a reduction in thermal performance by 500-600 W/mK dye to scattering of the lattice waves by the substrate. Similar to CNTs, graphene suffers from a high cost due to a combination of extreme synthesis conditions, and high competition in the market. Furthermore, the production of graphene requires hazardous chemicals which pose a safety and environmental concerns . Like CNTs, graphene also suffers from the influence of Van der Waal’s forces, promoting the accumulation of particles. This is amplified by its large surface area .
In an effort to improve its thermal conductivity, electrical performance, and strength, graphene was added to rubber in different mass fractions to determine its effectiveness as an additive. Read it here.
- Thermal Conductivity Mapping and Filler Settling Detection in Polymer Composites with MTPS
- Thermally Conductive Fillers in Polymer Composites
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About the Author
John Clifford is a marketing intern at C-Therm. He is currently in his third year of his Chemical Engineering degree at the University of New Brunswick in Fredericton.