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172. Halogenation of graphite
405
ABSTRACTS
the diamagnetic susceptibility, and the electron spin resonance g value. In one specimen of highly turbostratic pyrolytic graphite formed at 24OO”C, the diamagnetism was 50 % greater and theg shift 30 % greater than in pure single crystal graphite. The effect of turbostratic stacking is due in part to a decrease in interaction energy between atoms in adjacent planes because the layers are further apart and are randomly rotated. On the other hand, the interplane interactions responsible for the g shift are enhanced in turbostratic material, presumably due to the lower symmetry. *The subject research was sponsored in part by Aeronautical United States Air Force.
171. Kinetics
of bromine
exchange
in graphite-bromine
Systems Division, Air Force
Systems Command,
lamellar compounds*
Seymour Aronson (Rmokhaeten National Laboratory, Upton, New York). A kinetic study of the exchange of normal and radioactive bromine in g~phite-bromine lamellar compounds has been made at tempemtures of 30-50°C. Two sieve fractions of a natural graphite powder and one sieve fraction of an artificial graphite powder were investigated. The compositions studied were CBr.,, and CBr.,, in the natural powders and CBr.,, and CBr.,, in the artificial powder. Exchange data were obtained in the following manner: A graphite sample was exposed to radioactive bromine (atomic number 82) in the vapor phase until bromination was complete. Normal bromine at the bromination pressure was passed through the powder and was collected by condensation in tubes immersed in an ice bath. The activities of the collected bromine samples were measured in a gamma counter. Two alternative mechanisms for the exchange, volume diffusion and surface exchange, were considered. The data were in better agreement with the diffusion mechanism. Diffusion coefficients of 1O-Q-1O-”cm2/sec and an activation energy of 11-14 kcal/mole were calculated for the natural graphite powders. The diffusion coefficients increased with increasing bromine content. Reversibly adsorbed bromine exchanged more rapidly than irreversibly adsorbed bromine. “This work was performed under the auspices of the US.
Atomic Energy Commission.
172. Halogenation of graphite W. T. Eeles and J. A. Turnbuil
(Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire, England). After outgassing brominated or iodised graphite to 10-4mm Hg a
residue of the halogen remains in the lattice. This can be seen in the electron microscope as loops or extended areas, the boundaries of which exhibit diffraction contrast. Selected area diffraction from these regions yields patterns corresponding to lattices which are multiples of the graphite lattice. These can be shown to be compatible with previously unpublished X-ray data (RUDORFF,1941) but not with previously proposed structure. Tentative new structures are discussed. 173. Electron microscope study of g~phite~bro~~e
lamellar compounds
M. Heerschap and P. Delavignette (boded State Physics ~e~art~~t, S.C.K.42E.N. MOL, ~e~i~~). Graphite readily forms lamellar compounds with bromine by direct addition from the vapour. The crystals lose most of the intercalated bromine molecules upon standing at room temperature or in vacuum. A proportion of the additive is retained however, the amount of which depends on the original structure of the graphite. Evidence is given of a mechanism by which at least part of the residue may be retained. The mechanism consists of the splitting off of bromine loops from dislocations bounding the intercalated bromine layers and moving parallel to the c-planes. It is thought that the carbon hexagon networks on either side of a bromine layer are in a over a stacking. Evidence is further given of the blocking of these moving boundary dislocations by normal dislocation ribbons; this process might affect the limiting compositions that could be formed. Presented and discussed are interactions between boundary dislocations, bromine loops and normal dislocation ribbons. The opening up of dislocation ribbons as observed is thought to be caused by the presence of intercalated bromine proceeding along the dislocations by preferential diffusion.
ABSTRACTS
the diamagnetic susceptibility, and the electron spin resonance g value. In one specimen of highly turbostratic pyrolytic graphite formed at 24OO”C, the diamagnetism was 50 % greater and theg shift 30 % greater than in pure single crystal graphite. The effect of turbostratic stacking is due in part to a decrease in interaction energy between atoms in adjacent planes because the layers are further apart and are randomly rotated. On the other hand, the interplane interactions responsible for the g shift are enhanced in turbostratic material, presumably due to the lower symmetry. *The subject research was sponsored in part by Aeronautical United States Air Force.
171. Kinetics
of bromine
exchange
in graphite-bromine
Systems Division, Air Force
Systems Command,
lamellar compounds*
Seymour Aronson (Rmokhaeten National Laboratory, Upton, New York). A kinetic study of the exchange of normal and radioactive bromine in g~phite-bromine lamellar compounds has been made at tempemtures of 30-50°C. Two sieve fractions of a natural graphite powder and one sieve fraction of an artificial graphite powder were investigated. The compositions studied were CBr.,, and CBr.,, in the natural powders and CBr.,, and CBr.,, in the artificial powder. Exchange data were obtained in the following manner: A graphite sample was exposed to radioactive bromine (atomic number 82) in the vapor phase until bromination was complete. Normal bromine at the bromination pressure was passed through the powder and was collected by condensation in tubes immersed in an ice bath. The activities of the collected bromine samples were measured in a gamma counter. Two alternative mechanisms for the exchange, volume diffusion and surface exchange, were considered. The data were in better agreement with the diffusion mechanism. Diffusion coefficients of 1O-Q-1O-”cm2/sec and an activation energy of 11-14 kcal/mole were calculated for the natural graphite powders. The diffusion coefficients increased with increasing bromine content. Reversibly adsorbed bromine exchanged more rapidly than irreversibly adsorbed bromine. “This work was performed under the auspices of the US.
Atomic Energy Commission.
172. Halogenation of graphite W. T. Eeles and J. A. Turnbuil
(Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire, England). After outgassing brominated or iodised graphite to 10-4mm Hg a
residue of the halogen remains in the lattice. This can be seen in the electron microscope as loops or extended areas, the boundaries of which exhibit diffraction contrast. Selected area diffraction from these regions yields patterns corresponding to lattices which are multiples of the graphite lattice. These can be shown to be compatible with previously unpublished X-ray data (RUDORFF,1941) but not with previously proposed structure. Tentative new structures are discussed. 173. Electron microscope study of g~phite~bro~~e
lamellar compounds
M. Heerschap and P. Delavignette (boded State Physics ~e~art~~t, S.C.K.42E.N. MOL, ~e~i~~). Graphite readily forms lamellar compounds with bromine by direct addition from the vapour. The crystals lose most of the intercalated bromine molecules upon standing at room temperature or in vacuum. A proportion of the additive is retained however, the amount of which depends on the original structure of the graphite. Evidence is given of a mechanism by which at least part of the residue may be retained. The mechanism consists of the splitting off of bromine loops from dislocations bounding the intercalated bromine layers and moving parallel to the c-planes. It is thought that the carbon hexagon networks on either side of a bromine layer are in a over a stacking. Evidence is further given of the blocking of these moving boundary dislocations by normal dislocation ribbons; this process might affect the limiting compositions that could be formed. Presented and discussed are interactions between boundary dislocations, bromine loops and normal dislocation ribbons. The opening up of dislocation ribbons as observed is thought to be caused by the presence of intercalated bromine proceeding along the dislocations by preferential diffusion.