Evolution with heat treatment of crystallinity in carbons

The variation with heat treatment of the dimensions L(a) and L(c) of the graphite-like crystallites of graphitizable and non-graphitizable carbons is studied. The increases of L(a) and L(c) with heat treatment temperature (HTT) owing to three processes (crystallite growth in-plane, coalescence of crystallites along the c-axis and coalescence of crystallites along the a-axis) are functionally separated. The evolution with HTT of the number of crystallites (N-cr), the mean volume of the crystallites (v(cr)) and the total volume occupied by the crystallites (V-cr) are determined in terms of the changes of L(a), L(c) and d(002). Since among other characteristics the crystallites form the electrical and thermal conducting phase of the carbon, N-cr, v(cr) and V-cr are important parameters in many physical properties of these materials. The developed expressions were applied to a non-graphitizable and to a graphitizable carbon.