Mechanism of Stored-Energy Release at 200° C in Electron-Irradiated Graphite

Mitchell, Emma; Taylor, M.R.S.

doi:10.1038/208638a0

Cited by 44 publications

(32 citation statements)

References 13 publications

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“…Finally, an important question is related to the stability of such a defect. In graphite, it has been proposed [13] that this defect is responsible for the Wigner energy release peak at 200 o C, associated with an experimental barrier [16] of approximately 1.4 ± 0.4 eV, which agrees nicely with the calculated barrier [13,14] of 1.3 eV for the defect recombination. To estimate the barrier, we linearly interpolate the coordinates of the displaced atom between its position in the pure bundle ( Fig.…”

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confidence: 76%

Bundling up Carbon Nanotubes through Wigner Defects

2005

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We show, using ab initio total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right besides a vacancy, which are present in irradiated graphite can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizeable amount, clearly indicating its importance for the mechanical properties of nanotube bundles.Single-walled carbon nanotubes (SWNT) have attracted an enormous amount of attention since its discovery[1] about ten years ago. No doubt this is related to all their extraordinaire materials properties and potential for applications [2]. From a mechanical point of view, SWNT have exceedingly large elastic modulus [3] around 1 TPa, which would make them perfect for lowdensity high-modulus fibers. When in bundles, however, a much worse performance is usually obtained, mostly related to the weak interaction between tubes, which makes it relatively easy for them to slide against each other [4]. A breakthrough has been recently obtained [5] using electron-beam irradiation to generate crosslinks between the tubes. This increased the shear modulus by a factor of approximately 30 thus indicating a new way to fabricate stronger fibers.Irradiation by electrons or ions is being used by many groups [7] as an important tool to modify the structure and properties of nanotubes. As examples, we can mention the welding of tubes [6], and the dramatic increase of shear modulus for a bundle of nanotubes [5]. In order to have a better control of the nanotube engineering, it is fundamental to have a detailed microscopic understanding of the defects that are created upon irradiation. Properties like formation energy, geometry, stability and electronic structure are critical in determining what type of defects will be produced and how they will affect the mechanical behavior of the carbon fiber. Some theoretical works have investigated the effects of electron or ion irradiation on SWNT and SWNT bundles [8,9]. These studies consisted of Molecular Dynamics simulations with the interaction between the atoms being described by empirical potentials. Even though these works are relevant and indicative of the processes that may occur, they may miss important aspects since empirical potentials cannot describe appropriately features such as re-hybridization, rebonding or charge redistribution, all of them quite important when chemical bonds are being broken and made, specially in stressed configurations that may result from the irradiation procedure. Moreover, recombination barriers and formation energies are usually poorly described by these potentials. Yet another difficulty with these potentials is the important fact that carbon atoms have many possible hybridi...

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confidence: 76%

Bundling up Carbon Nanotubes through Wigner Defects

2005

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“…It should be noted that the experimental value is determined indirectly from differential-scanning calorimetric data [7] and can vary somewhat depending on the exact choice of prefactor to first order kinetics of release. The peak is relatively broad with a spread of 0:4e V [8], which is commensurate with our energy saving when the defect sits in regions of sheared crystal. Hence the peak width may be due to the varying local environment of the intimate I-V pairs.…”

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confidence: 55%

“…An energy release event known as the 200 C release peak [8] has received particular attention due to its prominence in many irradiated graphite systems. It is associated with an activation barrier of 1.38 eV, and its nature has previously been open to speculation.…”

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confidence: 99%

“…The characteristics of energy release vary depending on the dose, flux, and irradiation temperature, but generally a complex defect system develops with a wide range of length scales and energy barriers. Numerous defect schemes have been proposed to explain barrier mechanisms to energy release, for example, the breakup of loose aggregates of interstitial pairs into isolated interstitial pairs and their subsequent annihilation with di-vacancies [7], detrapping of interstitials from grain boundaries, dislocations, or impurities [8], and also recrystallization of irradiation-amorphized regions [9].…”

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confidence: 99%

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Metastable Frenkel Pair Defect in Graphite: Source of Wigner Energy?

et al. 2003

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“…Early work, by Mitchel et al (1965), investigated the effects of electron irradiation through stored energy release following irradiation at high temperature [8]. In 1972 the effects of electron irradiation to graphite were examined by Ohr et al for the first time, who reported a displacement threshold accelerating voltage of below 120 kV [9].…”

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confidence: 99%

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

Mironov

Freeman

Brown

et al. 2015

Carbon

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Structural and chemical bonding changes in nuclear graphite have been investigated during in-situ electron irradiation in a transmission electron microscope (TEM); electron beam irradiation has been employed as a surrogate for neutron irradiation of nuclear grade graphite in nuclear reactors. This paper aims to set out a methodology for analysing the microstructure of electron-irradiated graphite which can then be extended to the analysis of neutron-irradiated graphites. The damage produced by exposure to 200 keV electrons was examined up to a total dose of approximately 0.5 dpa (equivalent to an electron fluence of 5.6x 10 21 electrons cm -2 ). During electron exposure, high resolution TEM images and electron energy loss spectra (EELS) were acquired periodically in order to record changes in structural (dis)order and chemical bonding, by quantitatively analysing the variation in phase contrast images and EEL spectra.

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Mechanism of Stored-Energy Release at 200° C in Electron-Irradiated Graphite

Cited by 44 publications

References 13 publications

Bundling up Carbon Nanotubes through Wigner Defects

Bundling up Carbon Nanotubes through Wigner Defects

Metastable Frenkel Pair Defect in Graphite: Source of Wigner Energy?

Electron irradiation of nuclear graphite studied by transmission electron microscopy and electron energy loss spectroscopy

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