The influence of boron on the clustering of radiation damage in graphite

Brown, L. M.; Kelly, Anthony; Mayer, Rayner M.

doi:10.1080/14786436908216330

Cited by 164 publications

(21 citation statements)

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“…They considered an alternative postulate ͑by Reynolds and Thrower 6 ͒ that reconciled the activation energy with a low migration energy, by the formation of less mobile C 2 units. Brown et al 5 explicitly ruled this out as incompatible with their observed kinetics. Telling and Heggie 7 pointed out that a much more natural assumption was that the measured activation energy was the effective migration energy and this is compatible with the covalently bonded model as discussed by Iwata et al Note that, in recent times there have been high-quality DFT calculations by Ma, which are interpreted to give a relatively low migration energy of 0.4 eV for an isolated self-interstitial atom moving in the basal plane.…”

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confidence: 69%

“…At the time, the noncovalently bonded model was assumed, and hence migration energies were expected to be low ͑Շ0.1 eV͒. In this context, Brown et al 5 hypothesized the ability of boron to trap interstitial atoms during their migration. Thus, they arrived at a very convincing functional analysis of disk growth as a function of ͓B͔, arguing that the largest part of the migration energy was the B i binding energy.…”

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confidence: 99%

“…The assumption made by Brown et al was that radiation led to a homogeneous nucleation of a certain concentration of interstitials and vacancies which did not annihilate with one another. 5 Each one either met with another of its own kind, nucleating an aggregate, or met with the aggregate and grew it. For interstitials it was believed the aggregate was a disk and for vacancies a line.…”

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Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

et al. 2010

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Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite ͓T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 ͑2010͔͒ assumes that self-interstitial atoms exist as clusters of nearly free C 2 molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise. DOI: 10.1103/PhysRevB.82.056101 PACS number͑s͒: 65.40.Ba, 61.72.JϪ, 61.80.Hg, 71.20.Ϫb A recent study by Iwata and Watanabe 1 showed that neutron irradiation of graphite produces an enhancement of the material's low-temperature specific heat. They conclude from these measurements that the hindered rotation of interstitial C 2 molecules is responsible for this effect. These are elegant, precise experiments which provide important evidence about the nature of such materials. Nevertheless, we profoundly disagree with the analysis of results that they put forward because it contains unjustified assumptions which can be refuted in a number of different ways, particularly with regard to the C 2 entities invoked.First, recent calculations reported by us, based on densityfunctional theory ͑DFT͒, using large supercells ͑with four graphite layers and up to 290 atoms͒ conclusively demonstrate that the binding energy between pairs of interstitial atoms to yield C 2 is large ͑3 eV͒, and hence their formation must be irreversible at the temperatures of irradiation cited by Iwata and Watanabe ͑333 K͒.2 It is clear from the covalently bonded structures illustrated in Ref. 2 that any migration path for C 2 is unlikely to exist with as low an energy as suggested by Iwata and Watanabe or that any free rotation could occur. Isolated self-interstitial atoms also form covalent bonds with the host crystal, according to our calculations, and in agreement with others. Certainly, none of the structures obtained give rise to c : a aspect ratios or formation volumes comparable with those inferred in their paper.Our calculations were not based on the assumption of a model, either covalent or noncovalent. They are only conjugate-gradient geometry optimizations starting from various initial positions for two C atoms placed between perfect graphitic planes, as in Iwata and Watanabe's C 2 diagram in their Fig. 5. No special distortions or atomic displacements were applied before optimization. There is nothing to prevent the formation of freely floating C 2 units, if this is a valid outcome. The optimization algorithm cannot traverse any energy barrier it experiences; it only proceeds downhill.However, in every case the system spontaneously reorganized into fully covalently bonded structures, integrated with the host lattice, either in the same layer or between layers. Inde...

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Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

et al. 2010

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“…At the beginning of an irradiation, interstitials will tend to either be annihilated at vacancies or be trapped at another interstitial, resulting in the nucleation of a dislocation loop. Toward the end of an irradiation, dislocation loops will grow, as interstitials will tend to add to existing loops rather than nucleate new ones (BROWN et al 1969). Because evaluated irradiation temperatures cause interstitial loops to nucleate and grow rapidly, we will be conducting further irradiations at elevated temperatures.…”

Section: Results Of Electron Beam Irradiationmentioning

confidence: 99%

Determination of transmutation effects in crystalline waste forms. 1998 annual progress report

Strachan¹,

Hess

Fortner³

et al. 1998

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Executive SummaryA team from two national laboratories is studying transmutation effects in crystalline waste forms. Analyses are being done with 18 year old samples of 13'Cs-bearing pollucite (CsAlSi20~~0.5 HZO) obtained from a French company. These samples are unique in that the pollucite was made with various amounts of 13'Cs, which was then sealed in welded stainlesssteel capsules to be used as tumor irradiation sources. Over the past 18 years, the 13'Cs has been decaying to stable Ba in the capsules, i.e., in the absence of atmospheric effects. This material serves as an analogue to a crystalline waste form in which such a transmutation occurs to possibly disrupt the integrity of the original waste form.Work this year consisted of determining the construction of the capsule and state of the pollucite in the absence of details about these components from the French company. We have opened one capsule containing nonradioactive pollucite. The information on the construction of the stainless-steel capsule is useful for the work that we are preparing to do on capsules containing radioactive pollucite. Microscopic characterization of the nonradioactive pollucite revealed that there are at least two compounds in addition to pollucite: a Cs-silicate and a Cs-aluminosilicate (CsAlSiO 4 ). These findings may complicate the interpretation of the planned experiments using X-ray absorption spectroscopy. Electron energy loss spectroscopy and energy dispersive X-ray spectroscopy (flourescence) have been used to characterize the nonradioactive pollucite.We have investigated the stability of the nonradioactive pollucite to p radiation damage by use of 200 keV electrons in a transmission electron microscope. The samples were found to become amorphous in less than 10 minutes with loss of Cs. This is equivalent to many more years of p radiation damage than under normal decay of the 13'Cs. In fact, the dose was equivalent to several thousand years of normal radiation damage from the decay of .13'Cs. Of course, there would not be any 13'Cs remaining after that length of time because the half-life of 13'Cs is 30 y.Preparations have been started to study the radioactive pollucite samples at the Stanford Synchrotron Radiation Laboratory. Our calculations show that by thinning the base of the capsules we should be able to obtain about a factor of ten increase in the fluorescence signal. Procedures for thinning capsules containing the radioactive pollucite and examining the samples at the Stanford synchrotron are in place.

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“…Many experimental and theoretical studies on high energy electron irradiation have been carried out in order to study the growth behaviour of dislocation loop, using respectively the high energy electron microscopy tools [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and the Chemical Reaction Rate Theory (CRRT) [1,4,7,10,[18][19][20]. In the preceding papers [21][22] the present authors have specifically dealt with the atomic scale simulation of the diffusion and agglomeration of point defects under high energy irradiation by using transputer systems of up to 25 processors working in parallel.…”

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confidence: 99%

Atomic Scale Simulation of Dislocation Loops Formation in Thin Foil under High Energy Electron Irradiation

Boucetta¹,

Amghar²,

Idrissi-Saba³

et al. 2014

IJCA

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Using a personnel computer, we have simulated the diffusion and agglomeration of point defects in thin foil under high energy electron irradiation. The physical model has been developed by using the Monte Carlo technique. Four types of reactions are assumed to take place: di-interstitial creation by agglomeration of two free interstitials, vacancy-interstitial annihilation, interstitial trapping by dislocation loops and interstitial annihilation on the sample surfaces. In the simulation only interstitials are mobile and extended defects are assumed to be interstitial type. We have calculated the concentration of point defects, extended defects and the size of the latter. We compared them to the results of the Chemical Reaction Rate Theory (CRRT). It has been found that the dislocation loops are distributed in the center of material leaving areas denuded close to the surface and the loops radius is also strongly dependent on the location of the defect in thin foil with respect to the results of experimental and CRRT. To explain the origin of these phenomena we have exploited the spatial distribution of vacancies close to free surfaces and around dislocation loops. These types of informations are totally missing in the CRRT and experimental. General TermsAtomic scale simulation of point defects diffusion

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The influence of boron on the clustering of radiation damage in graphite

Cited by 164 publications

References 16 publications

Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

Determination of transmutation effects in crystalline waste forms. 1998 annual progress report

Atomic Scale Simulation of Dislocation Loops Formation in Thin Foil under High Energy Electron Irradiation

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