Transmission electron microscopy, Raman and X-ray photoelectron spectroscopy studies on neutron irradiated polycrystalline graphite

• Neutron irradiated polycrystalline graphite was studied.
• Irradiated graphite was characterized for sp3 and sp2 contents.
• On irradiation, sp3 content was increased to 40.11% from initial value of 12.44%.
• A significant average structural disordering was observed at <1 dpa.

Polycrystalline NBG-18 graphite irradiated by low fluence neutron was studied using transmission electron microscopy, Raman and X-ray photoelectron spectroscopy. The results showed distinct average chemical modification and structural disordering even at low neutron irradiation dose of ~6.12×10−3 displacement per atom (dpa) under ambient conditions. A direct evidence of structural modification and evolution of sp3 C−C bond in contrast to sp2 C−C bond in irradiated graphite samples are elucidated. The sp3 bonding content in un-irradiated graphite was 12.44%, which increased to 40.11% in irradiated sample. The rise in sp3 C−C bond, as well as the disappearance of −COH and reduction in −CO bond intensity peaks in irradiated condition, is possibly thermal annealing induced through thermalisation from the energetic neutron collision cascades effect. The complex graphite structural disordering, at a relatively low irradiation dose are quite distinct and attributed to carbon atom displacements resulting in vacancies–interstitials pairs via the formation of dislocation loops. The change in the characteristics of Raman spectra in irradiated graphite is discussed in correlation to generation of point defects in the lattice, which could be ascribed irradiation-induced damaging effect, evolving the graphite microstructure that deteriorates the physical and mechanical properties, eventually leading to structural failure.