The coefficient of thermal expansion (CTE) as measured for natural graphite and HOPG is temper­ature dependent (Figure 2) and the data from a number of authors has been collated by Kelly.6 The room temperature values of CTE are about 27.5 x 10—6K—1 and —1.5 x 10—6K—1 in the ‘c’ and V directions, respectively.

4.11.2.2 Modulus

The crystal elastic moduli6 are Cn (parallel to the basal planes) = 1060.0 x 109Nm—2, C12 = 180.0 x 109Nm—2, C13 = 15.0 x 109Nm—2, C33 (perpendicular to the basal planes) = 34.6 x 109Nm—2, and C44 (shear of the basal planes) = 4.5 x 109Nm—2 as defined by the orthogonal co-ordinates given below:

[1]

The strength of the crystallite is also directly related to the modulus, that is, the strength along the basal planes is higher than the strength perpendicular to the planes, and the shear strength between the basal panes is relatively weak.

4.11.2.3 Thermal Conductivity

The thermal conductivity of graphite along the basal plane ‘a’ direction is much greater than the thermal conductivity in the direction perpendicular to the basal plane ‘c.’ At the temperature of interest to the nuclear reactor engineer, graphite thermal con­duction is due to phonon transport. Increasing the temperature leads to phonon-phonon or Umklapp scattering (German for turn over/down). Imperfections in the lattice will lead to scattering at the boundaries.