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Atomistic Computer Simulations of Melting, Diffusion and Thermal Defects in High Pressure Solids
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. (Applied Material Physics)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present work describes the use of atomistic computer simulations in the area of Condensed Matter Physics, and speci cally its application to the study of two problems: the dynamics of the melting phase transition and the properties of materials at extremely high pressures and temperatures, problems which defy experimental measurements and purely analytical calculations.

A good sampling of techniques including classical and rst-principles Molecular Dynamics, and Metropolis Monte Carlo simulation have been applied in this study. It includes the calculation of melting curves for a wide range of pressures for elements such as Xe and H2, the comparison of two di erent models for molecular interactions in ZrO2 with respect to their ability to reproduce the melting point of the stable cubic phase, the study of the elastic constants of Fe at the extreme conditions of the Earth's inner core, and the stability of its crystalline phases. One of the most interesting results in this work is the characterization of di usion and defects formation in generic models of crystalline solids (namely Lennard-Jones and Embedded-atom) at the limit of superheating, including the role they play in the triggering of the melting process itself.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , 82 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-11027ISBN: 978-91-7415-407-8 (print)OAI: oai:DiVA.org:kth-11027DiVA: diva2:234462
Public defence
2009-09-18, F3, Lindstedstvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100708Available from: 2009-09-10 Created: 2009-09-08 Last updated: 2011-04-19Bibliographically approved
List of papers
1. Properties of the fcc Lennard-Jones crystal model at the limit of superheating
Open this publication in new window or tab >>Properties of the fcc Lennard-Jones crystal model at the limit of superheating
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2007 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, 064121- p.Article in journal (Refereed) Published
Abstract [en]

The face-centered-cubic (fcc) Lennard-Jones (LJ) model can be considered as a representative model of a simple solid. We investigate the mechanism of melting at the limit of superheating in the fcc LJ solid by means of the procedure recently developed by us [Phys. Rev. B 73, 012201 (2006)]. Insight into the mechanism of melting was gained by studying diffusion and defects in the fcc LJ solid by means of molecular dynamics simulations. We found that the limit of superheating achieved by us is likely to be the highest so far. We also found that the size of the cluster which ignites the melting is very small (down to five to six atoms, depending on the size of the supercell) and closely correlates with the linear size of a supercell when the number of atoms varies between 500 and 13 500.

Keyword
STABILITY LIMIT, MOLECULAR-DYNAMICS, CATASTROPHE, LINDEMANN, SURFACE, STATE
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11017 (URN)10.1103/PhysRevB.76.064121 (DOI)000249155200036 ()2-s2.0-34548437497 (Scopus ID)
Note
QC 20100708Available from: 2009-09-08 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
2. Xenon melting: Density functional theory versus diamond anvil cell experiments
Open this publication in new window or tab >>Xenon melting: Density functional theory versus diamond anvil cell experiments
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2006 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 74, 054114- p.Article in journal (Refereed) Published
Abstract [en]

We performed two-phase ab initio density functional theory based molecular dynamics simulations of Xe melting and demonstrated that, contrary to claims in the recent literature, the pressure dependence of the Xe melting curve is consistent with the corresponding-states theory as well as with the melting curve obtained earlier from classical molecular dynamics with a Xe pair potential. While at low pressure the calculated melting curve is in perfect agreement with reliable experiments, our calculated melting temperatures at higher pressures are inconsistent with those from the most recent diamond anvil cell experiment. We discuss a possible explanation for this inconsistency.

Keyword
MOLECULAR-DYNAMICS, EARTHS CORE, CORRESPONDING STATES, HIGH-PRESSURES, MGO, IRON, KRYPTON, PHASE, TEMPERATURES, SIMULATIONS
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11020 (URN)10.1103/PhysRevB.74.054114 (DOI)000240238400030 ()2-s2.0-33748197844 (Scopus ID)
Note
QC 20100708Available from: 2009-09-08 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
3. Origin of the Low Rigidity of the Earth's Inner Core
Open this publication in new window or tab >>Origin of the Low Rigidity of the Earth's Inner Core
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2007 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 316, 1603- p.Article in journal (Refereed) Published
Abstract [en]

Earth's solid-iron inner core has a low rigidity that manifests itself in the anomalously low velocities of shear waves as compared to shear wave velocities measured in iron alloys. Normally, when estimating the elastic properties of a polycrystal, one calculates an average over different orientations of a single crystal. This approach does not take into account the grain boundaries and defects that are likely to be abundant at high temperatures relevant for the inner core conditions. By using molecular dynamics simulations, we show that, if defects are considered, the calculated shear modulus and shear wave velocity decrease dramatically as compared to those estimates obtained from the averaged single-crystal values. Thus, the low shear wave velocity in the inner core is explained.

Keyword
NANOCRYSTALLINE METALS, MOLECULAR-DYNAMICS, IRON, ELASTICITY, STRESS, MODEL, GPA
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11021 (URN)10.1126/science.1141374 (DOI)000247239900037 ()2-s2.0-34250837502 (Scopus ID)
Note
QC 20100708Available from: 2009-09-08 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
4. High-pressure melting curve of hydrogen
Open this publication in new window or tab >>High-pressure melting curve of hydrogen
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2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, 194508- p.Article in journal (Refereed) Published
Abstract [en]

The melting curve of hydrogen was computed for pressures up to 200 GPa, using molecular dynamics. The inter- and intramolecular interactions were described by the reactive force field (ReaxFF) model. The model describes the pressure-volume equation of state solid hydrogen in good agreement with experiment up to pressures over 150 GPa, however the corresponding equation of state for liquid deviates considerably from density functional theory calculations. Due to this, the computed melting curve, although shares most of the known features, yields considerably lower melting temperatures compared to extrapolations of the available diamond anvil cell data. This failure of the ReaxFF model, which can reproduce many physical and chemical properties (including chemical reactions in hydrocarbons) of solid hydrogen, hints at an important change in the mechanism of interaction of hydrogen molecules in the liquid state.

Keyword
density functional theory, equations of state, high-pressure effects, hydrogen, melting, molecular dynamics method
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11022 (URN)10.1063/1.3013704 (DOI)000261141300029 ()2-s2.0-56849103616 (Scopus ID)
Note
QC 20100708Available from: 2009-09-08 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
5. Molecular dynamics simulation of zirconia melting
Open this publication in new window or tab >>Molecular dynamics simulation of zirconia melting
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2010 (English)In: Central European Journal of Physics, ISSN 1895-1082, E-ISSN 1644-3608, Vol. 8, no 5, 789-797 p.Article in journal (Refereed) Published
Abstract [en]

The melting point for the tetragonal and cubic phases of zirconia (ZrO2) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.

Keyword
soft oxide fuel cells, molecular dynamics, melting, zirconia
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11023 (URN)10.2478/s11534-009-0152-3 (DOI)000280377000012 ()2-s2.0-77955050839 (Scopus ID)
Funder
Swedish Research Council
Note
Uppdaterad från manuskript till artikel: 20100708 QC 20100708. Tidigare titel: "Molecular dynamics simulation of zirconia melting: a comparison of two models"Available from: 2009-09-09 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
6. Relative stability of phases of iron in the Earth's inner core
Open this publication in new window or tab >>Relative stability of phases of iron in the Earth's inner core
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We have investigated the phase diagram of iron in a range close to the conditions inside the Earth’sinner core, using molecular dynamics with a semi–empirical, embedded atom interatomic potential.Our calculated melting curves for the body centered cubic (bcc) and hexagonal close packed (hcp)phases of iron are in good agreement with experimental and first–principles data. Interestingly ourmodel suggests evidence of a crossover in relative stability of these crystalline phases: the moststable phase being bcc below 315 GPa, and hcp above that point. The dependence of structuraland dynamical properties of crystallized iron on the temperature, at the pressure of the inner core,were determined.

Identifiers
urn:nbn:se:kth:diva-11024 (URN)
Note
QC 20100708Available from: 2009-09-09 Created: 2009-09-08 Last updated: 2010-07-08Bibliographically approved
7. SearchFill: A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems
Open this publication in new window or tab >>SearchFill: A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems
2011 (English)In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 182, no 5, 1105-1110 p.Article in journal (Refereed) Published
Abstract [en]

We present an implementation of a stochastic optimization algorithm applied to location of atomic vacancies. Our method labels an empty point in space as a vacancy site, if the total spatial overlap of a "virtual sphere", centered around the point, with the surrounding atoms (and other vacancies) falls below a tolerance parameter. A Metropolis-like algorithm displaces the vacancies randomly, using an "overlap temperature" parameter to allow for acceptance of moves into regions with higher overlap, thus avoiding local minima. Once the algorithm has targeted a point with low overlap, the overlap temperature is decreased, and the method works as a steepest descent optimization.

Our method, with only two free parameters, is able to detect the correct number and coordinates of vacancies in a wide spectrum of condensed-matter systems, from crystals to amorphous solids, in fact in any given set of atomic coordinates, without any need of comparison with a reference initial structure.

Keyword
Monte Carlo, Vacancies, Interstitials, Computer simulation, Numerical optimization
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-11025 (URN)10.1016/j.cpc.2010.12.009 (DOI)000288926100001 ()2-s2.0-79952002697 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20100708 Uppdaterad från manuskript till artikel i tidskrift (20110419)Available from: 2009-09-16 Created: 2009-09-08 Last updated: 2017-12-13Bibliographically approved
8. An atomistic model for homogeneous melting
Open this publication in new window or tab >>An atomistic model for homogeneous melting
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The diffusion statistics of atoms in a crystal close to the critical superheating temperature wasstudied in detail using molecular dynamics (MD) and Monte Carlo (MC) simulations. We present ageneral dynamic percolation model for diffusion of atoms hopping through thermal vacancies. Theresults obtained from our model suggest that the limit of superheating is precisely the temperaturefor which dynamic percolation happens at the time scale of a single individual jump. We show thatthis prediction of the critical superheating temperature can give an estimate of the melting pointusing only the dynamical properties of the solid state.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-11026 (URN)
Note
QC 20100708Available from: 2009-09-09 Created: 2009-09-08 Last updated: 2010-07-21Bibliographically approved

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Output format
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