Inventor and main developer of the ReaxFF reactive force field method - which enables large scale (>>1000 atoms) fully dynamical, fully reactive simulations on complex materials/molecules and their interfaces. Supervisors: Jan de Leeuw, Steve Larter, and William Goddard
We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive fo... more We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for ∼3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including details of crack dynamics for loading in the [110] orientations, such as dynamical instabilities with increasing crack velocity. We also observe formation of secondary microcracks ahead of the moving mother crack. We conclude with a study of Si(bulk)-O2 systems, showing that Si becomes more brittle in oxygen environments, as known from experiment.
We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes ... more We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes on Al(111) surface. Our simulations show that, on the Al(111) surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al(111) surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al(111) surface. This is in excellent agreement with the experimental works of Go et al. [E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 (1999)]. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al2H6 is 2.99 kcal/mol while ...
We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes (MWC... more We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes (MWCNTs). The as-grown MWCNTs show continuous walls that do not provide sites for hydrogen storage under ambient conditions. However, after treating the nanotubes with oxygen plasma to create nanopores in the MWCNTs, we observed the appearance of a new hydrogen desorption peak in the 300–350 K range. Furthermore, the calculations of density functional theory and molecular dynamics simulations confirmed that this peak could be attributed to the hydrogen that is physically adsorbed inside nanopores whose diameter is approximately 1 nm. Thus, we demonstrated that 1 nm nanopores in MWCNTs offer a promising route to hydrogen storage media for onboard practical applications.
Since iron is the primary constituent of the Earth's core, understanding its properties at h... more Since iron is the primary constituent of the Earth's core, understanding its properties at high pressures and temperatures is of great geophysical importance. In particular, its melting temperature, along with those of its alloys, constrains the composition and temperature at the inner-core, outer-core boundary. We present findings on the melting behavior of Fe from molecular dynamics (MD) simulations. Our description of iron begins with the ReaxFF (reactive force field), whose parameters are derived from quantum mechanics calculations for various phases of Fe at pressures from 150 to 350 GPa. With ReaxFF it is practical to carry out MD on thousands of atoms as a function of pressure and temperature. Melting curves are computed via two methods: 1) Application of single-phase heating and cooling of 256-atom systems at 150-350 GPa, combined with solving the Clausius-Clapeyron equation, over time scales of 10-100 picoseconds. The effects of heating and cooling rates, as well as vacancies, are examined. Additionally, cool-down simulations yielded crystallized structures with stacking faults; upon reheating, these structures had lower melting temperatures than perfectly crystalline initial structures. 2) Computation of the vibrational density of states from analysis of the velocity autocorrelation function to obtain the entropy and free energy of both solid and liquid phases of Fe. Among the wide range of published melting curves for Fe in the literature, our results are consistent with the highest temperature, theoretical and shockwave-based curves, rather than the much lower curves based on extrapolation of diamond-anvil cell observations. By computing x-ray diffraction patterns during melting, as well as running simulations of surface melting, we explore some of the ways diamond-cell experiments determine the onset of melting.
Langmuir : the ACS journal of surfaces and colloids, Jan 12, 2016
Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find... more Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find the atomistic mechanisms for tribo-chemical reactions occurring at the sliding interface of fully-hydroxylated amorphous silica and oxidized silicon as a function of interfacial water amount. The ReaxFF-MD simulations showed a significant amount of atom transfers across the interface occurs during the sliding. In the absence of water molecules, the interfacial mixing was initiated by dehydroxylation followed by the Si-O-Si bond formation bridging two solid surfaces. In the presence of sub-monolayer thick water, the dissociation of water molecules can provide additions reaction pathways to form the Si-O-Si bridge bonds and atom transfers across the interface. However, when the amount of interfacial water molecules were large enough to form full monolayer, the degree of atom transfer was substantially reduced since the silicon atoms at the sliding interface were terminated with hydroxyl g...
We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive fo... more We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for ∼3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including details of crack dynamics for loading in the [110] orientations, such as dynamical instabilities with increasing crack velocity. We also observe formation of secondary microcracks ahead of the moving mother crack. We conclude with a study of Si(bulk)-O2 systems, showing that Si becomes more brittle in oxygen environments, as known from experiment.
We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes ... more We have used reactive force field (ReaxFF) to investigate the mechanism of interaction of alanes on Al(111) surface. Our simulations show that, on the Al(111) surface, alanes oligomerize into larger alanes. In addition, from our simulations, adsorption of atomic hydrogen on Al(111) surface leads to the formation of alanes via H-induced etching of aluminum atoms from the surface. The alanes then agglomerate at the step edges forming stringlike conformations. The identification of these stringlike intermediates as a precursor to the bulk hydride phase allows us to explain the loss of resolution in surface IR experiments with increasing hydrogen coverage on single crystal Al(111) surface. This is in excellent agreement with the experimental works of Go et al. [E. Go, K. Thuermer, and J. E. Reutt-Robey, Surf. Sci. 437, 377 (1999)]. The mobility of alanes molecules has been studied using molecular dynamics and it is found that the migration energy barrier of Al2H6 is 2.99 kcal/mol while ...
We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes (MWC... more We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes (MWCNTs). The as-grown MWCNTs show continuous walls that do not provide sites for hydrogen storage under ambient conditions. However, after treating the nanotubes with oxygen plasma to create nanopores in the MWCNTs, we observed the appearance of a new hydrogen desorption peak in the 300–350 K range. Furthermore, the calculations of density functional theory and molecular dynamics simulations confirmed that this peak could be attributed to the hydrogen that is physically adsorbed inside nanopores whose diameter is approximately 1 nm. Thus, we demonstrated that 1 nm nanopores in MWCNTs offer a promising route to hydrogen storage media for onboard practical applications.
Since iron is the primary constituent of the Earth's core, understanding its properties at h... more Since iron is the primary constituent of the Earth's core, understanding its properties at high pressures and temperatures is of great geophysical importance. In particular, its melting temperature, along with those of its alloys, constrains the composition and temperature at the inner-core, outer-core boundary. We present findings on the melting behavior of Fe from molecular dynamics (MD) simulations. Our description of iron begins with the ReaxFF (reactive force field), whose parameters are derived from quantum mechanics calculations for various phases of Fe at pressures from 150 to 350 GPa. With ReaxFF it is practical to carry out MD on thousands of atoms as a function of pressure and temperature. Melting curves are computed via two methods: 1) Application of single-phase heating and cooling of 256-atom systems at 150-350 GPa, combined with solving the Clausius-Clapeyron equation, over time scales of 10-100 picoseconds. The effects of heating and cooling rates, as well as vacancies, are examined. Additionally, cool-down simulations yielded crystallized structures with stacking faults; upon reheating, these structures had lower melting temperatures than perfectly crystalline initial structures. 2) Computation of the vibrational density of states from analysis of the velocity autocorrelation function to obtain the entropy and free energy of both solid and liquid phases of Fe. Among the wide range of published melting curves for Fe in the literature, our results are consistent with the highest temperature, theoretical and shockwave-based curves, rather than the much lower curves based on extrapolation of diamond-anvil cell observations. By computing x-ray diffraction patterns during melting, as well as running simulations of surface melting, we explore some of the ways diamond-cell experiments determine the onset of melting.
Langmuir : the ACS journal of surfaces and colloids, Jan 12, 2016
Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find... more Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find the atomistic mechanisms for tribo-chemical reactions occurring at the sliding interface of fully-hydroxylated amorphous silica and oxidized silicon as a function of interfacial water amount. The ReaxFF-MD simulations showed a significant amount of atom transfers across the interface occurs during the sliding. In the absence of water molecules, the interfacial mixing was initiated by dehydroxylation followed by the Si-O-Si bond formation bridging two solid surfaces. In the presence of sub-monolayer thick water, the dissociation of water molecules can provide additions reaction pathways to form the Si-O-Si bridge bonds and atom transfers across the interface. However, when the amount of interfacial water molecules were large enough to form full monolayer, the degree of atom transfer was substantially reduced since the silicon atoms at the sliding interface were terminated with hydroxyl g...
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Papers by Adri van Duin