The role of porosity in the accommodation of thermal expansion in graphite

Abstract

Analysis of crystal orientation and the thermal expansion behaviour of two blocks of British reactor grade A graphite has shown that accommodation of thermal expansion is associated with the crystallographic c-axis, some 37 % of the c-expansion contributing to that of the bulk. This result is confirmed by analysis of the thermal expansion behaviour of several different types of reactor graphite and of a pressed natural graphite. It is shown that the thermal expansion behaviour of polycrystalline graphite may be characterised by two basic parameters, one a measure of the effective contribution of lattice c-expansion to that of the bulk material, the other a measure of the preferred orientation of the crystals. These parameters may be derived from measurements of the linear thermal expansion coefficients at 400° C in directions parallel and perpendicular to that of extrusion or pressing. Accommodation of thermal expansion in reactor graphite is attributed to the presence of transcrystalline cleavage cracks, observed by electron microscopy of replicas. These cracks exist in parallel arrays, with individual crack length up to ~ 10 μm, mean width ~ 250 Å, and mean periodicity ~ 2 cracks/μm. Their estimated volume agrees well with that of the closed porosity, 4.3 % relative to the volume of crystalline material, derived from density measurements by X-ray diffraction and helium displacement. The cracks are believed to have been initiated during cooling from the graphitisation temperature, ~ 2700° C, and are thought to have formed in order to relieve internal stresses generated by interaction between the restraining effect of strong, intercrystalline C-C bonds and anisotropic contraction of the crystal lattice. Evidence is given that the marked increase in thermal expansion observed in the course of neutron irradiation of reactor graphite at 200° C is associated with the closing of cleavage microcracks.