handbook of carbon, graphite, diamond and fullerenes

(a)
Deep trough
Small amplitude
Small dimensional changes
Graphite Structure and Properties 59
Interatomic Spacing, a
(b)
Shallow trough
Large amplitude
Large dimensional changes
Rgure3.11. The energy troughof graphite in fajabdirectionsandftycdirection.l 14 !
As a result, the thermal expansion of the graphite crystal has a marked
anisotropy. It is low in the ab directions (lower than most materials) but
higher by an order of magnitude in the c direction, as shown in Fig.
The increase with temperature is not linear. In the c direction, it
increases slowly and gradually. At 0°C, the coefficient of thermal expansion
averages 25 x lO^C and at 400°C, it reaches 28 x ia«/"C.BJ[ 14 K 17 l
In the ab directions, the thermal expansion is actually negative up to
approximately 400°C with a minimum at 0°C. It is possible that this observed
negative expansion is due to internal stress (Poisson effect) associated with
the large expansion in the c direction and it has been suggested that, if it
were possible to measure the ab thermal expansion of a single atomic
plane, this expansion would be positive.^ 18 '
The large thermal expansion anisotropy often results in large internal
stresses and structural problems such as delamination between planes as
will be seen in Ch. 5. Sec. 3.