The dimensional changes produced in graphite by absorption of bromine

Abstract

It is well known that, in general, polycrystalline graphites have substantially lower volume coefficients of thermal expansion than the crystallites of which they are composed. It has previously been suggested that this is because microporosity, orientated parallel to the layer planes, absorbs part of the c-axis expansion of the crystallites. If this is true then the volume expansion of polycrystalline graphite when it absorbs bromine should also be lower than that of the crystallites and there should be a correlation between linear thermal expansion and initial linear growth per unit bromine concentration. The dimensional changes of a wide variety of polycrystalline graphites, when they absorb bromine, have been investigated and the correlation between initial growth per unit concentration of bromine and thermal expansion coefficient has been demonstrated. It has also been observed that when the strain of the crystallites in the c-axis direction becomes large the orientated microporosity fills up. When this happens the volume growth of the polycrystalline graphites per unit concentration of bromine approaches that of the crystallites. At even higher crystallite strains in the c-axis direction, anisotropic polycrystalline materials tend to expand faster than the crystallites and this is considered to be caused by the generation of additional porosity. This effect is less marked in the isotropic polycrystalline graphites. The relation between growth due to bromination and dimensional changes produced by fast neutron irradiation is discussed. Data on the effect of graphitization temperature on the subsequent dimensional changes produced by bromination are also described.