The c-axis thermal expansion of carbons and graphites

The lattice thermal expansion of a number of C atoms and graphites has been studied over the temperature range −265 to 2600°C, and is discussed taking account of other diffraction data published recently. It is shown that for the temperature range 0 to 2600°C, the increase in the interlayer spacing for a given temperature rise is not significantly dependent on stacking disorder. Hence the mechanism whereby the expansion takes place is essentially the same for all C atoms, implying that a van der Waals interlayer bonding prevails in all carbon materials. The degree of directional orientation of a particular carbon is shown to have no effect on its thermal expansion behaviour. At sub-ambient temperatures the thermal expansion is found to be significantly dependent on stacking disorder and hence carbon materials of different degrees of graphitic perfection have different limiting distances of closest approach between the atom layers as the temperature is lowered towards 0°K. At room temperature the coefficient of expansion α_c decreases with increasing stacking order. At higher temperatures the situation is more complex. The variations with temperature of the interlayer binding energies and compressibilities have been calculated. It is shown that C atoms representing the extremities of structural perfection have interlayer binding energies which differ by ~0.76 kcal.mole⁻¹ at 0°C. For an increment of 500°C the binding energy change is ~0.33 kcal.mole⁻¹.