Measurement of disorder in turbostratic carbons by small-angle X-ray scattering

The variation of the apparent porosity and density of a turbostratic carbon has been measured in the temperature range 1000 to 2800°C from absolute intensity small-angle scattering curves. There is agreement with bulk density values at 2800°C, but a difference develops at progressively lower temperatures. This indicates the presence of disordered regions with relatively low electron density, surrounding the turbostratic layer planes and cross-linking neighbouring crystallites. The average density of the porous/non-crystalline phase has been evaluated and is closely identified with an atomic arrangement which gives rise to diffuse scattering in high-angle X-ray patterns. The values of the weight fraction of this component, derived from both diffraction domains, are in agreement. A gradual reduction of disorder takes place with increase in heat treatment, so that a strictly porous carbon is formed by 2800°C. This variation follows the loss of impurity nitrogen atoms, most probably located in distorted or buckled layers and at layer plane edges as an integral part of the disordered phase. For the turbostratic carbon fibres used in these experiments, the mean sizes of the inhomogeneities in electron density, located between more crystalline regions and expressed by radius of gyration, mean chord intercept length and integral parameters, follow a linear relationship with L_c, the crystallite size along the c axis.