- Email: [email protected]
185. Threshold energy for the displacement of atoms in graphite
CARBON
380
tures of irradiation and are less severe in the case of the graphite containing carbon black. These effects are discussed in the light of current hypotheses relating to the mechanism of irradiation damage in graphite. 185. Threshold
energy for the displacement
of atoms in graph%@
G. L. Montet (Argonne National Laboratory, Argonne, Illinois). The energy required to displace a carbon atom from its normal lattice site has been determined directly by detection of the lattice vacancies resulting from the displacement process. Annealed natural crystals of graphite containing less than 10-i’ vacancies per carbon atom were irradiated at room temperature with electrons of selected energies in the range from 100 to 500 KeV. These crystals were then cleaved to a thickness of a few hundred ~~~o~, etched in a mixture of oxygen and chlorine to enlarge the lattice vacancies present in the surface planes, decorated by deposition of a thin layer of gold, and examined in an electron microscope. The vacancies then appear as rings of gold particles. Examination of crystals irradiated with electrons of energies less than 140 KeV showed that these irradiations produced no vacancies; whereas, examination of crystals irradiated with electrons of energies exceeding 150 KeV revealed a production of vacancies, the concentration increasing with the electron energy. From these observations it may be concluded that the displacement energy in graphite is approximately 30 eV. This value is in rough agreement with that found by Qgen, (‘) but is in disagreement with the value recently reported by Lucas and Mitchell(2) It may be that the orientation of the crystal with respect to the electron beam and its temperature during irradiation are important factors in the displacement process; perhaps the discrepancies can be explained by investigating the influence of these factors on the displacement energy. *Based on work performed under the auspices of the U.S. Atomic Energy Commission. 1. EGGEN D. T., Report MAA-SR-69 (1955). 2. LUCAS M. W. and MITCHELL E. W. J., Carbon 1, 345 (1964).
186. Deformation of graphite, lattices by interstitial carbon atoms C. A. Coulson (MuthematicaZ Institute, Oxford, England); S. Senent, M. A. Herraez,* (Mrs.) M. Leal$ and E. Santosf (~~v~~ de Va~ado~id, Spain). Calculations are made of the self-energy of an interstitial carbon atom placed between the layers of an otherwise perfect graphite crystal. The position of minimum energy is confirmed to be directly above an atom in the one plane, and centrally placed below three atoms in the other plane. It is essential to allow for deformation of the layers up to two on each side of the interstitial. The maximum displacement of an atom near to the interstitial is about 0.6A. The most uncertain feature of the analysis is the form of the carbon-carbon repulsion potential. Various alternatives are compared, and the choice of a function due to Crowell is made. With this function the selfenergy of an interstitial (i.e. energy difference between the interstitial situation and a perfect lattice with the extra atoms carefully removed) is about 2.32 eV, of which about 0.12 eV comes from the deformation of second neighbour planes. The migration energy of an interstitial is estimated to be about 0.14 eV, and comparison is made with available experimental measurements. *Present address: IPresent address:
Universidad U&e&dad
de Santiago de Composteela, Spain. de Costa Rica, Costa Rica, America Central.
187. The effect of the random formation of vacancy complexes on the irradiation induced properties of graphite P. Horner (Central Electricity Gemrat& Board, Berkeley Nuclear Laboratories, Be&&y, Gloucestershire, England). The concept of collapsed lines of vacancies formed by the random agglomeration of irradiation produced point defects was introduced to explain certain observed irradiation properties of graphite. In the present paper this concept is examined theoretica~y in more detail and a kinetic irradiation damage model is developed. This model, besides accounting for the random formation of the vacancy complexes, makes allowance for the nucleation of interstitial clusters and their subsequent
380
tures of irradiation and are less severe in the case of the graphite containing carbon black. These effects are discussed in the light of current hypotheses relating to the mechanism of irradiation damage in graphite. 185. Threshold
energy for the displacement
of atoms in graph%@
G. L. Montet (Argonne National Laboratory, Argonne, Illinois). The energy required to displace a carbon atom from its normal lattice site has been determined directly by detection of the lattice vacancies resulting from the displacement process. Annealed natural crystals of graphite containing less than 10-i’ vacancies per carbon atom were irradiated at room temperature with electrons of selected energies in the range from 100 to 500 KeV. These crystals were then cleaved to a thickness of a few hundred ~~~o~, etched in a mixture of oxygen and chlorine to enlarge the lattice vacancies present in the surface planes, decorated by deposition of a thin layer of gold, and examined in an electron microscope. The vacancies then appear as rings of gold particles. Examination of crystals irradiated with electrons of energies less than 140 KeV showed that these irradiations produced no vacancies; whereas, examination of crystals irradiated with electrons of energies exceeding 150 KeV revealed a production of vacancies, the concentration increasing with the electron energy. From these observations it may be concluded that the displacement energy in graphite is approximately 30 eV. This value is in rough agreement with that found by Qgen, (‘) but is in disagreement with the value recently reported by Lucas and Mitchell(2) It may be that the orientation of the crystal with respect to the electron beam and its temperature during irradiation are important factors in the displacement process; perhaps the discrepancies can be explained by investigating the influence of these factors on the displacement energy. *Based on work performed under the auspices of the U.S. Atomic Energy Commission. 1. EGGEN D. T., Report MAA-SR-69 (1955). 2. LUCAS M. W. and MITCHELL E. W. J., Carbon 1, 345 (1964).
186. Deformation of graphite, lattices by interstitial carbon atoms C. A. Coulson (MuthematicaZ Institute, Oxford, England); S. Senent, M. A. Herraez,* (Mrs.) M. Leal$ and E. Santosf (~~v~~ de Va~ado~id, Spain). Calculations are made of the self-energy of an interstitial carbon atom placed between the layers of an otherwise perfect graphite crystal. The position of minimum energy is confirmed to be directly above an atom in the one plane, and centrally placed below three atoms in the other plane. It is essential to allow for deformation of the layers up to two on each side of the interstitial. The maximum displacement of an atom near to the interstitial is about 0.6A. The most uncertain feature of the analysis is the form of the carbon-carbon repulsion potential. Various alternatives are compared, and the choice of a function due to Crowell is made. With this function the selfenergy of an interstitial (i.e. energy difference between the interstitial situation and a perfect lattice with the extra atoms carefully removed) is about 2.32 eV, of which about 0.12 eV comes from the deformation of second neighbour planes. The migration energy of an interstitial is estimated to be about 0.14 eV, and comparison is made with available experimental measurements. *Present address: IPresent address:
Universidad U&e&dad
de Santiago de Composteela, Spain. de Costa Rica, Costa Rica, America Central.
187. The effect of the random formation of vacancy complexes on the irradiation induced properties of graphite P. Horner (Central Electricity Gemrat& Board, Berkeley Nuclear Laboratories, Be&&y, Gloucestershire, England). The concept of collapsed lines of vacancies formed by the random agglomeration of irradiation produced point defects was introduced to explain certain observed irradiation properties of graphite. In the present paper this concept is examined theoretica~y in more detail and a kinetic irradiation damage model is developed. This model, besides accounting for the random formation of the vacancy complexes, makes allowance for the nucleation of interstitial clusters and their subsequent