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. 1. 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. Since the fire of Windscale nuclear reactor occurred in 1957 [1] due to a spontaneous release of stored energy from irradiated graphite, radiation damage has been recognized to be important problems to be solved for irradiated graphite. Also, dimensional changes of growth parallel to c-axis and contraction within the basal planes decrease in c-axis electrical resistivity and so forth occur as serious problems. In spite of a lot of investigations on irradiated graphite [2–33], 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]. Raman spectroscopy has been used for the characterization of graphite and investigations on the graphitization process and on the change of graphite structure under irradiation. The advantages of this method for the examination of graphite are a selective sensitiveness to the structural
References
[1]
L. Arnold, Windscale, 1957: Anatomy of a Nuclear Accident, Palgrave Macmillan, London, UK, 1995.
[2]
W. N. Reynolds, Radiation Damage in Graphite, Pergamon, New York, NY, USA, 1965.
[3]
P. A. Thrower, Chemistry and Physics of Carbon, vol. 5 of edited by P. L. Walker, Dekker, New York, NY, USA, 1969.
[4]
P. A. Thrower and R. M. Mayer, “Point defects and self-diffusion in graphite,” Physica Status Solidi A, vol. 47, no. 1, pp. 11–37, 1978.
[5]
B. T. Kelly, Physics of Graphite, Applied Science, London, UK, 1981.
[6]
B. S. Elman, M. S. Dresselhaus, G. Dresselhaus, E. W. Maby, and H. Mazurek, “Raman scattering from ion-implanted graphite,” Physical Review B, vol. 24, no. 2, pp. 1027–1034, 1981.
[7]
T. Iwata, “Fine structure of Wigner energy release spectrum in neutron irradiated graphite,” Journal of Nuclear Materials, vol. 133-134, pp. 361–364, 1985.
[8]
K. Niwase, M. Sugimoto, T. Tanabe, and F. E. Fujita, “Electron microscope study of radiation damage in graphite produced by D+ and He+ bombardment,” Journal of Nuclear Materials, vol. 155–157, no. 1, pp. 303–306, 1988.
[9]
K. Niwase, K. Nakamura, I. Tanaka, Y. Miyamoto, and T. Tanabe, “Laser Raman microprobe analysis of the modified surface of He+-irradiated graphite,” Journal of Nuclear Materials, vol. 179–181, no. 1, pp. 214–217, 1991.
[10]
K. Niwase, T. Tanabe, and I. Tanaka, “Annealing experiment of ion-irradiated graphite by laser Raman spectroscopy,” Journal of Nuclear Materials, vol. 191–194, pp. 335–339, 1992.
[11]
K. Nakai, C. Kinoshita, and A. Matsunaga, “A study of amorphization and microstructural evolution of graphite under electron or ion irradiation,” Ultramicroscopy, vol. 39, no. 1–4, pp. 361–368, 1991.
[12]
T. Tanabe, S. Muto, and K. Niwase, “On the mechanism of dimensional change of neutron irradiated graphite,” Applied Physics Letters, vol. 61, no. 14, pp. 1638–1640, 1992.
[13]
K. Nakamura and M. Kitajima, “Ion-irradiation effects on the phonon correlation length of graphite studied by Raman spectroscopy,” Physical Review B, vol. 45, no. 1, pp. 78–82, 1992.
[14]
M. S. Dresselhaus and R. Kalish, Ion Implantation in Diamond, Graphite and Related Materials, Springer, Berlin, Germany, 1992.
[15]
K. Niwase and T. Tanabe, “Defect structure and amorphization of graphite irradiated by D+ and He+,” Materials Transactions, vol. 34, no. 11, pp. 1111–1121, 1993.
[16]
B. T. Kelly, Materials Science and Technology, vol. 10, VCH, New York, NY, USA, 1994.
[17]
J. Koike and D. F. Pedraza, “Dimensional changes in highly oriented pyrolytic graphite due to electron-irradiation,” Journal of Materials Research, vol. 9, no. 7, pp. 1899–1907, 1994.
[18]
K. Niwase, “Irradiation-induced amorphization of graphite,” Physical Review B, vol. 52, no. 22, pp. 15785–15798, 1995.
[19]
K. Niwase, “Erratum: irradiation-induced amorphization of graphite,” Physical Review B, vol. 56, no. 9, pp. 5685–5685, 1997.
[20]
T. Tanabe, T. Maruyama, M. Iseki, K. Niwase, and H. Atsumi, “Radiation damage of graphite: degradation of material parameters and defect structures,” Fusion Engineering and Design, vol. 29, pp. 428–434, 1995.
[21]
T. Tanabe, “Radiation damage of graphite—degradation of material parameters and defect structures,” Physica Scripta T, vol. 64, pp. 7–16, 1996.
[22]
H. Abe, H. Naramoto, A. Iwase, and C. Kinoshita, “Effect of damage cascades on the irradiation-induced amorphization in graphite,” Nuclear Instruments and Methods in Physics Research, Section B, vol. 127-128, pp. 681–684, 1997.
[23]
S. Muto and T. Tanabe, “Damage process in electron-irradiated graphite studied by transmission electron microscopy. I. High-resolution observation of highly graphitized carbon fibre,” Philosophical Magazine A, vol. 76, no. 3, pp. 679–690, 1997.
[24]
S. Muto, S. Horiuchi, and T. Tanabe, “Local structural order in electron-irradiated graphite studied by high-resolution high-voltage electron microscopy,” Journal of Electron Microscopy, vol. 48, no. 6, pp. 767–776, 1999.
[25]
T. Meguro, A. Hida, M. Suzuki et al., “Nanoscale modification of electronic states of graphite by highly charged Ar-ion irradiation,” Journal of Vacuum Science and Technology B, vol. 19, no. 6, pp. 2745–2748, 2001.
[26]
K. Niwase, “Irradiation-induced amorphization of graphite: a dislocation accumulation model,” Philosophical Magazine Letters, vol. 82, no. 7, pp. 401–408, 2002.
[27]
R. H. Telling, C. P. Ewels, A. A. El-Barbary, and M. I. Heggie, “Wigner defects bridge the graphite gap,” Nature Materials, vol. 2, no. 5, pp. 333–337, 2003.
[28]
A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, and S. Iijima, “Direct evidence for atomic defects in graphene layers,” Nature, vol. 430, no. 7002, pp. 870–873, 2004.
[29]
K. Niwase, “Formation of dislocation dipoles in irradiated graphite,” Materials Science and Engineering A, vol. 400-401, no. 1-2, pp. 101–104, 2005.
[30]
A. Asthana, Y. Matsui, M. Yasuda, K. Kimoto, T. Iwata, and K. I. Ohshima, “Investigations on the structural disordering of neutron-irradiated highly oriented pyrolytic graphite by X-ray diffraction and electron microscopy,” Journal of Applied Crystallography, vol. 38, no. 2, pp. 361–367, 2005.
[31]
T. Hirai, J. Compan, K. Niwase, and J. Linke, “Laser Raman microprobe analysis of graphite exposed to edge plasma in the TEXTOR tokamak,” Journal of Nuclear Materials, vol. 373, no. 1–3, pp. 119–122, 2008.
[32]
T. Hirai, J. Compan, and K. Niwase, “Micro-Raman study on structure evolution of graphite exposed to intense thermal shock load,” Advanced Materials Research, vol. 59, pp. 66–70, 2009.
[33]
C. Karthik, J. Kane, D. P. Butt, W. E. Windes, and R. Ubic, “In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite,” Journal of Nuclear Materials, vol. 412, no. 3, pp. 321–326, 2011.
[34]
F. Tuinstra and J. L. Koenig, “Raman spectroscopy of graphite,” Journal of Chemical Physics, vol. 53, no. 3, pp. 1126–1130, 1970.
[35]
K. Niwase, K. G. Nakamura, M. Yokoo, K. I. Kondo, and T. Iwata, “Pathway for the transformation from highly oriented pyrolytic graphite into amorphous diamond,” Physical Review Letters, vol. 102, no. 11, Article ID 116803, 2009.
[36]
D. Ugarte, “Formation mechanism of quasi-spherical carbon particles induced by electron bombardment,” Chemical Physics Letters, vol. 207, no. 4–6, pp. 473–479, 1993.
