Molecular Dynamics Simulations of Cascades in Nuclear Graphite H. J. Christie, D. L. Roach, D. K....
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Transcript of Molecular Dynamics Simulations of Cascades in Nuclear Graphite H. J. Christie, D. L. Roach, D. K....
![Page 1: Molecular Dynamics Simulations of Cascades in Nuclear Graphite H. J. Christie, D. L. Roach, D. K. Ross The University of Salford, UK I. Suarez-Martinez,](https://reader036.fdocuments.us/reader036/viewer/2022062516/56649e315503460f94b22180/html5/thumbnails/1.jpg)
Molecular Dynamics Simulations of Cascades in Nuclear Graphite
H. J. Christie, D. L. Roach, D. K. RossThe University of Salford, UK
I. Suarez-Martinez, M. Robinson, N. Marks Curtin University, Perth, Western Australia
A. McKenna, M. Heggie Surrey University, UK
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• Motivation
• Background
• Methodology
• Results:• Graphite
• Carbon Materials
• Conclusions and Further Work
Outline
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• Show how graphite behaves extremely differently to other carbon materials
Motivation
• Create quality simulations using molecular dynamics in graphite
• Extend the life-span of current nuclear reactors
• Crucial information for next generation of nuclear reactors
• Understanding of processes occurring in irradiated graphite
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• Molecular Dynamics (MD) and Monte Carlo have a heritage that extends back to the Manhattan project (1946)
• Virtually no MD simulations of radiation damage in graphite
Background
WHY?
Difficult to use MD in Carbon based materials due to its hybridized states and anisotropic layers
• Only in the last ten years or so have suitable MD potentials for Carbon been developed
•Previous work – Nordlund et al., Smith, Yazyev et al.
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Methodology
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Swift Heavy Ions Cascades Defects
Primary Knock-On Atom passes straight through transferring energy to the surrounding atoms
Primary Knock-On Atom (denoted in blue) passes through the cell colliding with atoms. Displaced atoms can then collide with other atoms in the cell
Primary Knock-On Atoms now has a low energy but can still collide with atoms. Displaced atoms can make interstitials. Vacancies are created when an atoms is displaced.
Methodology
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START
Calculate Forces on all atoms using
Chosen Potential
Update Positions and Velocities
Initialise Positions and Velocities
Analyse Data
Many Potentials for Carbon:
• Tersoff & Brenner (1988) – short-ranged potentials inverts the density relationship between graphite and diamond
• Adaptive Interaction REBO (2000) – extension of Brenner potential. Long-ranged interactions between sp2 sheets described using Lennard- Jones interaction
• Environment Dependent Interaction Potential – atom centred bond order was employed drawing on an earlier Silicon EDIP method
Molecular Dynamics (MD) - a simulation of the movement of atoms
Methodology
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MethodologyThe Environment Dependent Interaction Potential
• Developed for Pure Carbon Systems (Marks, 2000)
• Interactions vary according to the environment
• Accurate description of bond-making and breaking
U U2 ( rij ,Zi) U3 (rij ,rik ,,Zi)
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MethodologyThe Ziegler-Biersack-Littmarck Potential
• Universally employed in ion implantation simulations
• Screened Coulomb potential
• High accuracy at small bond lengths
)(1
4 0
2
21 rr
ezzVzbl
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Thermostats
Fixed atoms
PKA region
Thermostats
Methodology
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Methodology
Thomson Problem
• Randomise initial direction of PKA
• Eliminate Human Bias
• Substantial number of results
• Produces 1400 cascades
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Methodology
Left: 20 directionsToday: 10 directions
• Up to 160, 000 atoms
• Side length of 105Å
• Variable time-step
• Edge thermostat
• Follows 5ps of motion
• Uniform sample of the unit sphere
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Results – 250eV Cascade
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Results – 1000eV Cascade
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Results – 1000eV Cascade
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Results
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Single interlayer Interstitial
Bi-pentagon I2 grafted intralayer bridge
Grafted Interstitial
α-β I2 interlayer bridge
Stone-Wales
β-β I2 bent interlayer bridge
Latham, JP 20, 395220 (2008)
Latham, JP 20, 395220 (2008)
Latham, JP 20, 395220 (2008)
El-Barbary, et al, PRB 68, 144107 (2003)
Telling & Heggie, Phil Mag. 87, 4797 (2007)
Telling & Heggie, Phil Mag. 87, 4797 (2007)
Latham, JP 20, 395220 (2008)
Vacancy
Latham, JP 20, 395220 (2008)
Split Interstitial
Results
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Results: Diamond
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Ef = 7.33 eV
Point defect: (100) split interstitial
The cascade in diamond produces the (100) split interstitial which has the lowest formation energy ~ 7eV.
Mainwood, Solid-state Electronics, 21 1431(1978)
32768 atomsPKA energy 1KeV
Results: Diamond
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Results: Glassy Carbon
• 100% sp2 bonded
• High temperature resistance and high purity
• Low density and low electrical resistance
• Very hard material
• Low thermal resistance to chemical attack and impermeability to gases and liquids
Properties:
Atoms can travel further without causing collisions because of the large number of vacant spaces. This causes a large number of atoms to be displaced over a greater distance.
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Results: High Density Amorphous Carbon
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ResultsLow Den-Amor-Carbon High Den-Amor-Carbon Graphite
Graphite is Directionally Dependent
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Summary
Remarkable Result!
Graphite does not behave like any other material
• Even at high energies – little damage to final cell
• Directionally dependent – each cascade unique
• Graphite behaves completely differently to other carbon materials highlighting it’s uniqueness
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Further Work
• Further analysis of material after cascade
• High energy cascades for graphite (several MeV)
• Complete Thomson directions
• Comparison of different materials
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Acknowledgements
This work was completed under the auspices of the Fundamentals of Nuclear Graphite Project, funded by the UK Engineering and Physical Science Research Council, Grant EP/I003312.
The Authors would like to gratefully acknowledge the financial support of EPSRC during this work.