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Thermally Conductive Carbon Fibers Expand Composites Markets

23 March 2003

Historically, use of carbon fiber-reinforced polymer matrix composites (PMCs) has been driven by their outstanding strength and stiffness. However, carbon fibers also have unique physical properties that are opening up a broad range of new applications. In particular, some carbon fibers made from pitch precursor materials have extremely high thermal conductivity and stiffness, combined with low coefficient of thermal expansion (CTE) and density. This makes it possible to use composites in applications for which carbon fibers made from polyacrylonitrile (PAN), which have low thermal conductivities, don’t work.

The unique properties of pitch-based carbon fibers include:
- Thermal conductivities up to 1100 W/m-K, compared to 400 for copper and 8 for standard modulus PAN fibers
- Moduli up to 965 GPa (140 Msi), compared to 230 GPa (34 Msi) for standard modulus PAN-based fibers
- Low CTEs
- Low densities: 1.9 – 2.2 g/cc (0.069 – 0.079 pci)

As for PAN-based carbon fibers, the first applications of thermally conductive pitch-based carbon fibers (which we call pitch fibers for simplicity) were in the aerospace industry: spacecraft radiators and battery sleeves, avionics printed circuit board heat sinks (also called cold plates and thermal planes) and honeycomb used in aircraft engine nacelle sandwich structures.

Pitch fibers are now finding their way into commercial PMC applications. A major breakthrough is the development of injection molding compounds reinforced with discontinuous fibers that have much higher thermal conductivities than traditional reinforced thermoplastics. This opens up an unlimited range of new applications which are now made of various metals. Current production parts include commercial electronic packaging components, such as microprocessor heat spreaders and heat sinks. In addition, there are countless other potential industrial and consumer applications. For example, the figure shows an injection molded combination enclosure and heat sink for motor integrated controller electronics that previously was made of aluminum.

Pitch fiber-reinforced metal matrix composites and carbon/carbon composites are being used in production aerospace electronic packaging and other applications. Ceramic matrix composites are under development.

The author is conducting a study of pitch fiber applications for the U.S. Navy, and would welcome information from anyone working in this area.

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