handbook of carbon, graphite, diamond and fullerenes

Pyrolytic Graphite 159
Thermal Expansion. The thermal expansion of pyrolytic graphite,
like that of the ideal graphite crystal, has a marked anisotropy. It is low in
the ab directions (lower than most materials) but an order of magnitude
higher in the c direction. The effect of temperature is shown in Fig. 7.10.
Such a large anisotropy may lead to structural problems such as delamination
between planes, especially in thick sections or when the material is
deposited around a sharp bend.
O
c
o
"w
c
TO
Q.
X
111
15 Ea>
s:
c
2
o
O
30
20
10
a Direction
c Direction
500 1000 1500 2000 2000
Temperature, °C
Figure 7.10. Coefficient of thermal expansion (CTE) of pyrolytic graphite as a
function of temperature.I 24 '
Electrical Properties. The electrical properties of pyrolytic graphite
also reflect the anisotropy of the material and there is a considerable
difference between the resistivity in the ab and the c directions. Pyrolytic
graphite is considered to be a good electrical conductor in the ab directions,
and an insulator in the c direction. Its electrical resistivity varies with
temperature as shown in Fig. 7.11.