Pyrolytic graphite – thermal performance by structure

Continuing our discussion of different forms of carbon, this column addresses the topic of pyrolytic graphite, a material that opens up many interesting applications not only for cooling but also for scientific measurement equipment.

Pyrolytic graphite – the finest forms of which are often called Highly Ordered Pyrolytic Graphite (HOPG) – is manufactured through a pyrolysis reaction. Here hydrocarbon gas is decomposed at 3300 K in a vacuum. The carbon atoms form again into a structure that in one direction consists of planar layers of hexagonally arranged carbon atoms (the a b layer), and in the other perpendicular direction consists of randomly oriented atoms (c direction). The manufacturing method gives an almost perfect lattice of this kind, whereas natural graphite always has more or less defects in its crystal structure.

The highly anisotropic lattice structure, with a strong atomic binding in the planar layers and a weak one in the perpendicular direction, gives pyrolytic graphite extraordinary properties. For example, the thermal conductivity in the direction of the planar layers is one of the highest, but in the other direction it is lower than that of alumina. Also the mechanical strength, electrical conductivity, magnetic properties, and thermal expansion reflect this anisotropy. The degree of anisotropy depends on how parallel the planar layers are. This is characterized by a term called mosaic spread angle, and as the number becomes smaller, it gives a higher parallelism and thus a more perfect lattice.

Application areas for pyrolytic graphite are manifold: The extremely smooth surface together with low thermal expansion coefficient makes it a good material for scientific mirrors. It consists only of carbon atoms, which makes it a good substrate for elemental analysis measurements. The lattice type lends itself to application for filters in x-ray and neutron diffractometry. For its thermal conductivity properties, pyrolytic graphite was formerly exploited only in aerospace applications, but lately it is also being considered for use for electronics cooling applications. However, compared to natural graphite, the more perfect lattice comes at a higher price.

Some typical values of different properties of pyrolytic graphite
Density   2.2 g/cm3
Thermal expansion coefficient (a b)
(c)
0.5 x 10-6 1/K
6.5 x 10-6 1/K
Thermal conductivity (a b)
(c)
400 – 1700 W/m�K
3.5 W/m�K
Electrical resistance (a b)
(c)
0.05 ohm/m
50 ohm/m
Flexural strength (a b) 120 MPa
Tensile strength (a b) 80 MPa
Compressive strength (a b) 100 MPa
Young’s modulus (a b) 20,000 MPa