Radiation damage of graphite - degradation of material parameters and defect structures

Based on our previous works, we propose a new model for damage structure of graphite based on sp² to sp³ transition of carbon atoms that can explain the change of the materials' parameters observed both in damaging and annealing processes. In early stage of the irradiation, defects are produced in a basal plane and/or in-between the basal planes (in-plane or two dimensional defects) accompanying sp² to sp³ transition of some carbon atoms and losing their graphitic bonding. They play a critically important role on the reduction of thermal conductivity, increase of electrical resistivity, increase of hydrogen retention and so on. They also cause a dimensional change through the increase of lattice spacing between the basal planes. Such in-plane defects are rather easily annealed out and the materials' parameters for lightly irradiated graphite recover their initial values by annealing far below the graphitization temperature. However, there is no way to avoid their production under neutron irradiation, particularly at low temperatures. After high dose irradiation, the defects grow into three dimensional structures probably including the formation of sp³ clusters. Accordingly, the basal planes lose their ordering resulting in turbulence and bending, and remaining open spaces. They play an important role on the volume expansion, which cannot be caused totally by the lattice expansion. Once graphite loses its 2 dimensional structure, the recovery to the initial structure is very difficult.