- Email: [email protected]
56. The dilation of neutron irradiated graphite exposed to liquid sodium
MECHANICAL
PROPERTIES
AND
IRRADIATION
211
definition of the released energy profile between 40 and 670°K. They show an influence of graphitization degree. The three first stages of stored energy are analyzed in connection with the electrical resistivity antiannealing stages. An evaluation of Frenkel’s pair formation energy is given for both types of irradiations. 54.
The
intlnence
of boron
on the nucleation
of radiation
damage
in graphite.
R. M. Mayer (Department of Metallurgy, University of Cambridge, Cambridge, England). Electron microscopy observations and X-ray lattice parameter measurements were presented to show that the addition of boron will alter the nucleation pattern of radiation damage in graphite at irradiation temperatures of 650°C and higher. Crystals were doped with either boron 10 or boron 11 in the range 10 ppm to 1 per cent and then irradiated to 7.6 x lOi n/cm2 (Ni). A simple way of looking at the nucleation pattern was suggested, which makes it possible to include the influence of boron on the nucleation of the damage. 55.
Vacancy and interstitial annealing.
loops
in graphite
produced
by high temperature
irradiation
and
P. A. Thrower (Atomic Energy Research Establishment, Harwell, England). Samples of pyrolytic graphites and single crystals of graphite have been simultaneously irradiated at 1350°C to a dose of 11.7 x lOa neutrons cm+ and the effects studied by transmission electron microscopy. A pyrolytic graphite containing a twist boundary every lo-12 layer planes contained defects showing marked deviations from single crystal behaviour, which is as previously reported. In the twist boundaries large interstitial loops are formed, while in the crystallites themselves only a dense population of small vacancy loops are observed. An analysis of this behaviour gives values for the migration energies of a vacancy in the basal plane (3 * 1 eV) and of an interstitial perpendicular to the basal plane (2 - 8 eV). 56.
The
dilation
of neutron
irradiated
graphite
exposed
to liquid
sodium.
J. E. Brocklehurst (U.K. Atomic Energy Authority, Reactor Materials Laboratory, Culcheth, Nr. Warrington, Lanes., England). Unirradiated graphite specimens exposed to liquid sodium at 600°C show a dilation - 1 per cent depending on the thermal expansion coefficient. Specimens irradiated to fast neutron doses in the range lOzO-10aa n.cm-a and then exposed to liquid sodium, show dilations which are dependent on the irradiation history of the specimen and are much greater than those for unirradiated graphite. The dilation increases with the fast neutron dose the specimen has received, and at high doses complete disintegration occurs. Specimens irradiated at temperatures of about 300°C show much greater sodium dilations than those irradiated to the same dose at temperatures > 350°C. 57.
Acoustic emission
in graphite
under
stress.
K. Gilchrist (U.K. Atomic Energy Authority, Reactor Materials Laboratory, Culcheth, Nr. Warrington, Lams., England) and D. Wells (U.K. Atomic Energy Authority, Atomic Weapons Research Establishment, Foulness, Essex, England). The noise emitted by graphite samples when stressed in compression has been examined in a specially designed “quiet” room. Two types of nuclear graphite were examined, one composed of isotropic particles and the other of anisotropic particles. It was observed that the former remained quiet until the fracture stress was . . approached but the latter showed noise emission well below the fracture stress. Measurements of strain conducted in the same experiments showed that Poison’s ratio increased markedly with stress in the anisotropic material, but not in the more isotropic structures. 58.
High strength
and high modulus
carbon
fibers.*
R. Bacon and W. A. Schalamon (Union Carbide Corfioration, Carbon Products Division, Parma Technical Center, Parma, Ohio). High strength and high modulus carbon fibers have been produced by heating carbon yarn at elevated temperatures under an applied tensile load. The section of yam within the hot zone of the furnace stretched during this treatment, causing the graphite layers to become aligned parallel with the fiber axis. X-ray diffraction patterns showed that high degrees of graphite layer alignment were produced. The method proved capable of producing fibers with tensile strengths ranging from 100,000 to 530,000 lb/in” and Young’s properties correlated very well with the modulus ranging from 9 x lo8 to over 90 x 106lb/ ina. Mechanical degree of applied stretch. * This research was sponsored in part by the Air Force Materials Laboratory, Non-Metallic Materials Division, Fibrous Materials Branch.
PROPERTIES
AND
IRRADIATION
211
definition of the released energy profile between 40 and 670°K. They show an influence of graphitization degree. The three first stages of stored energy are analyzed in connection with the electrical resistivity antiannealing stages. An evaluation of Frenkel’s pair formation energy is given for both types of irradiations. 54.
The
intlnence
of boron
on the nucleation
of radiation
damage
in graphite.
R. M. Mayer (Department of Metallurgy, University of Cambridge, Cambridge, England). Electron microscopy observations and X-ray lattice parameter measurements were presented to show that the addition of boron will alter the nucleation pattern of radiation damage in graphite at irradiation temperatures of 650°C and higher. Crystals were doped with either boron 10 or boron 11 in the range 10 ppm to 1 per cent and then irradiated to 7.6 x lOi n/cm2 (Ni). A simple way of looking at the nucleation pattern was suggested, which makes it possible to include the influence of boron on the nucleation of the damage. 55.
Vacancy and interstitial annealing.
loops
in graphite
produced
by high temperature
irradiation
and
P. A. Thrower (Atomic Energy Research Establishment, Harwell, England). Samples of pyrolytic graphites and single crystals of graphite have been simultaneously irradiated at 1350°C to a dose of 11.7 x lOa neutrons cm+ and the effects studied by transmission electron microscopy. A pyrolytic graphite containing a twist boundary every lo-12 layer planes contained defects showing marked deviations from single crystal behaviour, which is as previously reported. In the twist boundaries large interstitial loops are formed, while in the crystallites themselves only a dense population of small vacancy loops are observed. An analysis of this behaviour gives values for the migration energies of a vacancy in the basal plane (3 * 1 eV) and of an interstitial perpendicular to the basal plane (2 - 8 eV). 56.
The
dilation
of neutron
irradiated
graphite
exposed
to liquid
sodium.
J. E. Brocklehurst (U.K. Atomic Energy Authority, Reactor Materials Laboratory, Culcheth, Nr. Warrington, Lanes., England). Unirradiated graphite specimens exposed to liquid sodium at 600°C show a dilation - 1 per cent depending on the thermal expansion coefficient. Specimens irradiated to fast neutron doses in the range lOzO-10aa n.cm-a and then exposed to liquid sodium, show dilations which are dependent on the irradiation history of the specimen and are much greater than those for unirradiated graphite. The dilation increases with the fast neutron dose the specimen has received, and at high doses complete disintegration occurs. Specimens irradiated at temperatures of about 300°C show much greater sodium dilations than those irradiated to the same dose at temperatures > 350°C. 57.
Acoustic emission
in graphite
under
stress.
K. Gilchrist (U.K. Atomic Energy Authority, Reactor Materials Laboratory, Culcheth, Nr. Warrington, Lams., England) and D. Wells (U.K. Atomic Energy Authority, Atomic Weapons Research Establishment, Foulness, Essex, England). The noise emitted by graphite samples when stressed in compression has been examined in a specially designed “quiet” room. Two types of nuclear graphite were examined, one composed of isotropic particles and the other of anisotropic particles. It was observed that the former remained quiet until the fracture stress was . . approached but the latter showed noise emission well below the fracture stress. Measurements of strain conducted in the same experiments showed that Poison’s ratio increased markedly with stress in the anisotropic material, but not in the more isotropic structures. 58.
High strength
and high modulus
carbon
fibers.*
R. Bacon and W. A. Schalamon (Union Carbide Corfioration, Carbon Products Division, Parma Technical Center, Parma, Ohio). High strength and high modulus carbon fibers have been produced by heating carbon yarn at elevated temperatures under an applied tensile load. The section of yam within the hot zone of the furnace stretched during this treatment, causing the graphite layers to become aligned parallel with the fiber axis. X-ray diffraction patterns showed that high degrees of graphite layer alignment were produced. The method proved capable of producing fibers with tensile strengths ranging from 100,000 to 530,000 lb/in” and Young’s properties correlated very well with the modulus ranging from 9 x lo8 to over 90 x 106lb/ ina. Mechanical degree of applied stretch. * This research was sponsored in part by the Air Force Materials Laboratory, Non-Metallic Materials Division, Fibrous Materials Branch.