Structural change in graphite under electron irradiation at low temperatures M. Takeuchi a , S. Muto b, * , T. Tanabe b , H. Kurata c , K. Hojou c a Department of Nuclear Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan b Center for Integrated Research in Science and Engineering (CIRSE), Nagoya University, 1-1 Furo-cho, Chikusa-ku, Nagoya 464-01, Japan c JAERI, Tokai, Naka, Ibaraki 319-11, Japan Abstract Changes in crystallographic and electronic structure of electron-irradiated graphite were examined at low temper- atures by means of transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The long range order within the basal planes at the low temperatures was lost more quickly than at room temperature, which hardly aected the continuous changes in electronic and short range structures. This was mainly caused by the faster fragmentation of the crystal into pieces with the local structure maintained. The basal plane buckling and lattice di- lation in the c-direction, similar to the previous results obtained at room temperature, suggest the local formation of non-hexagonal atomic rings, on the analogy of fullerenes, by the displacement damage incorporated with electronic excitations. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Carbon-based materials have been utilized as repre- sentatives of low-Z materials in the nuclear environment [1]. Their usefulness, from the practical point of view, probably consists in the ¯exibility in their properties, such as light but hard enough for use as structural ma- terials, high-corrosion resistance, high or moderate heat and electrical conductivities even under high-energy particle irradiation. These practically utilized carbon materials are often mixtures of disordered graphite and other impurities partly because homogeneous and iso- tropic properties are required. On the other hand, continuous eorts to understand the physical and chemical properties of carbon materials in well-de®ned structures (i.e. crystalline states) have been made from the fundamental point of view. In this respect graphite shows a variety of interesting characters because of its uniaxial anisotropy where the covalent bonding within the graphitic sheet (basal or c-plane) and the van der Waals interaction between them. If limiting the subjects to radiation eects in graphite, there still remain a number of issues to be understood, for in- stance, anomalous swelling in the c-direction, drastic reduction of heat and electrical conductivities, increase in resistance for cleavage, etc [2]. These eects cannot be fully understood by applying the conventional concept of crystal defect physics, i.e. the formation and annihi- lation of point defects and their agglomeration, which has been successful for most metals and alloys. Present authors' group has claimed that these above mentioned radiation eects above are partly ascribed to a transition of the local chemical bonding from the sp 2 to sp 3 hybridization to construct 3-dimensional bridging between the c-planes [2]. An attempt to take such eects of covalent bonding and its highly anisotropic character into account has recently started, primarily using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) under electron irradi- ation [3±7]. The main results obtained at room temper- ature are summarized as follows: (i) The [0 0 0 1] electron diraction pattern turned to complete halo rings at the electron dose of about 1 dpa (displacements per atom) [5]. (ii) High-resolution electron microscopy Journal of Nuclear Materials 271&272 (1999) 280±284 * Corresponding author. Tel.: +81-52 789 5200; fax: +81-52 789 3791 or 5177; e-mail: muto@cirse.nagoya-u.ac.jp. 0022-3115/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 1 1 5 ( 9 8 ) 0 0 7 1 4 - 4