Evolution of graphite structure under irradiation a new scenario from molecular dynamics simulations
Résumé
Graphite response to irradiations has been widely studied in the past because of its importance for nuclear engineering. Although its behavior under irradiation has been widely investigated, the very details of the underlying mechanisms are still under debate and several scenarios are available [1,2,3]. With molecular dynamics simulations using empirical potentials we investigate in this paper the creation of damages by single irradiation event and the effect of the irradiation dose until complete amorphisation of the graphite structure.Two methodologies are used displacement cascades to investigate the primary damage and Frenkel pair accumulations to study the dose effect. We show that graphite is not amorphised by a direct impact mechanism for initial energies of the primary knocked-on atoms (carbon and chlorine) up to 40 keV. Instead, various defects stabilize and remain after a single irradiation event. However, amorphisation is reached by accumulation of defects. Before amorphisation, we show that graphite follows a three stages evolution characterized by (1) an increase of point defects (2) a pinning and wrinkling of the graphene planes at small amorphous pockets and (3) an amorphisation by percolation of the small amorphous pockets. These three stages all together constitute an alternative scenario to those already existing [4]. We also show the mechanisms of each stage drive the change of lattice dimension of graphite. Interstitials contribute as expected to the swelling of the c-axis, while vacancies link the shrinkage of the basal plane in the first stage. The point defects contribution of the first stage is replaced by the dimension change induced by the rippling of the graphene planes. This topological change is depicted by a power law relation between the c-axis swelling and the basal-plane shrinkage as a function of the irradiation dose. [1] K. Niwase, Phil. Mag. Lett. 82 (2002) 401.[2] M.I. Heggie, I. Suarez-Martinez, C . Davidson, and G. Haffenden, J. Nucl. Mater. 413 (2011) 150.[3] B.J. Marsden, and G.N. Hall, in Comprehensive Nuclear Materials, chap. 4.11 (2012) 325.[4] A. Chartier, L. Van Brutzel, B. Pannier, and P. Baranek, submitted in Carbon (2014)