Raman Spectroscopy for Quantitative Analysis of Point Defects and Defect Clusters in Irradiated Graphite

Keisuke Niwase

doi:10.1155/2012/197609

Abstract

We report the development of Raman spectroscopy as a powerful tool for quantitative analysis of point defect and defect clusters in irradiated graphite. Highly oriented pyrolytic graphite (HOPG) was irradiated by 25 keV He+ and 20 keV D+ ions. Raman spectroscopy and transmission electron microscopy revealed a transformation of irradiated graphite into amorphous state. Annealing experiment indicated a close relation between Raman intensity ratio and vacancy concentration. The change of Raman spectra under irradiation was empirically analyzed by “disordered-region model,” which assumes the transformation from vacancy-contained region to disordered region. The model well explains the change of Raman spectra and predicts the critical dose of amorphization, but the nature of the disordered region is unclear. Then, we advanced the model into “dislocation accumulation model,” assigning the disordered region to dislocation dipole. Dislocation accumulation model can simulate the irradiation time dependencies of Raman intensity ratio and the c-axis expansion under irradiation, giving a relation between the absolute concentration of vacancy and Raman intensity ratio, suggesting an existence of the barrier on the mutual annihilation of vacancy and interstitial.

Highlights

Graphite is one of useful materials for thermal nuclear and fusion reactors due to their outstanding nature with respect to heat load, neutron activation, and so forth
Thin foils of highly oriented pyrolytic graphite for transmission electron microscopy (TEM) were irradiated with 25 keV He+ and 20 keV D+ at temperatures ranging from room temperature (RT) to 973 K [8]
Raman spectral changes and associated TEM diffraction changes of highly oriented pyrolytic graphite (HOPG) under 25 keV He+ and 20 keV D+ irradiation have been systematically investigated in terms of irradiation dose and temperature

Summary

Introduction

Graphite is one of useful materials for thermal nuclear and fusion reactors due to their outstanding nature with respect to heat load, neutron activation, and so forth. In spite of a lot of investigations on irradiated graphite [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33], the natures of point defects and defect clusters, and their reaction kinetics are obscure This can be attributed to the anisotropy of the graphite lattice which requires a description of the motion of interstitials and vacancies in terms of activation energies parallel and perpendicular to the basal planes, and the instability of vacancy and interstitial clusters [3, 7]. We show Raman spectroscopy as a powerful tool for the study of radiation damage of graphite

Ion Irradiation to HOPG Foils

Model for the Change of Raman’s Spectra and Irradiation-Induced Amorphization

Simulation

Conclusion