The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been invest... more The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been investigated by Density Functional Theory (DFT) calculations performed with the SIESTA code. The migration energies are found to be systematically lower by ∼ 0.15 eV within the basal plane than out of the basal plane. The electronic origin of this significant contribution to diffusion anisotropy is evidenced by the analysis of the local electronic densities of states and by a comparison with and empirical potential. The average value of the migration energy is in very good agreement with available experimental data in Zr. The activation energies for self-diffusion obtained assuming a vacancy mechanism are in good agreement with experiments in Zr and Hf, although slightly too small, but a significant discrepancy is observed in Ti.
Density functional theory calculations are performed to investigate the impact of magnetism on th... more Density functional theory calculations are performed to investigate the impact of magnetism on the energetics of low-index Cr surfaces and Fe/Cr interfaces, that is, Cr(100), Cr(110), Fe/Cr(100), and Fe/Cr(110). We have also determined the stability of various Cr magnetic structures, particularly the spin-density waves, in the presence of these surfaces and interfaces. We show that the most stable structure of the spin-density wave is mainly dictated by the subtle balance between bulk and surface/interface influences, and strongly dependent on the surface/interface orientation. Regarding the Cr surfaces, we confirm the role of magnetism to lower the surface energy of Cr(100) with respect to Cr(110). Among all the possible orientations of the wave vector, only the out-of-plane wave is found to be stable near Cr(100) surfaces with the high-moment sites located at the surface layer. At variance, the in-plane wave is shown to be the most stable one, consistent with experimental data for very thin Cr(110) films. Concerning the Fe/Cr interfaces, magnetic frustrations are identified to be responsible for a higher formation energy of Fe/Cr(110) compared to that of Fe/Cr(100). This unusual anisotropy of interface energies is clearly different from the corresponding interfaces between Cr and a nonmagnetic element, Cu. Two ways are suggested to relax partially the magnetic frustrations at the (110) interface and to lower its formation energy. Noncollinear magnetic configurations can be developed where local moments of Fe and Cr atoms are perpendicular to each other. Also, in order to preserve phase coherence, in-plane spin-density waves show a very stable magnetic structure with the nodes at the interface layer. The presence of low-moment sites at Fe/Cr(110) offer another way to relax the magnetic frustrations and lower the interfacial energy.
By means of ab initio calculations combined to statistical mechanics, we provide new evidence tha... more By means of ab initio calculations combined to statistical mechanics, we provide new evidence that an experimentally undetectable tiny amount of impurities can be responsible for drastic changes in vacancy concentrations ([V]), inducing large deviations from an Arrhenius law even at low temperature. It is the case of O and N in α-Fe. The present finding is fully compatible with existing experiments, and changes the previous common vision that C has the dominant effect. This study provides a route for bridging the longstanding theoretical-experimental gap on the prediction of [V] in metals.
We have performed long time scale tight-binding molecular dynamics (MD) simulations to study dime... more We have performed long time scale tight-binding molecular dynamics (MD) simulations to study dimer¯ippings on perfect and defective Si(0 0 1) surfaces. The constant temperature MD simulations have been performed for temperatures ranging from 800 to 1300 K. From them we measured the average residence time between¯ips and assuming an Arrhenius equation we obtain a dynamical energy barrier for¯ipping. Our results extrapolate to a dimer¯ipping frequency of 3 Â 10 6 Hz at 300 K in agreement with the symmetric appearance of surface dimers using STM. We have found that the correlated¯ippings of two neighbouring dimers are as important as single dimer¯ippings. The ratio of correlated¯ips with respect to the total number of¯ips is of the order of 0.3 at 800 K and decreases to 0.2 at 1200 K. In the case of surfaces with a single dimer vacancy, the neighbouring dimers to the vacancy have three stable positions instead of two as in perfect surfaces. The dimers on the same row as the defect¯ip faster than those of the other row, leading to their appearance as symmetric dimers in STM measurements at temperatures even below the order±disorder transition temperature. Ó
Classical molecular dynamics and density functional theory calculations are performed to study th... more Classical molecular dynamics and density functional theory calculations are performed to study the impact of two distinct Fe grain boundaries (GBs) on the clustering properties of helium (He) and the possible He effect on GB decohesion. Several He concentrations are considered. Common properties of He clustering are found for the both GBs, which are visibly different from the bcc bulk. In particular, He clusters in the GBs are always elongated in the directions parallel to the interface and contracted in the direction normal to the GB plane, while they are isotropic in the bcc bulk. When the He number in the clusters is sufficiently large, the strong local pressure promotes the occurrence of loop punching, which is easier to trigger in the GBs than in the bulk, resulting in a lower He-to-vacancy ratio in the GB clusters. The emitted self-interstitial atoms (SIAs) can more easily dissociate from the clusters in the GBs than in the bulk, leading to relatively lower local pressures around the clusters in the GBs, and facilitating the clusters growth. He is found to decrease GB cohesion, and the embrittling effect of He increases with its concentration. But interestingly, this effect decreases with He clustering. The present findings are fully compatible with existing experimental evidence, for instance, for a stronger GB embrittlement due to He at rather low temperatures than at higher temperatures.
ABSTRACT The interplay between vacancies (V) and interstitial solutes X (X = C, N, and O) and its... more ABSTRACT The interplay between vacancies (V) and interstitial solutes X (X = C, N, and O) and its impact on thermodynamic properties of alpha-Fe solid solutions are studied, starting from first principles calculations. A systematic comparison between the three solutes is performed, investigating X-Fe, X-X, and V -X interactions. In the alpha-Fe lattice, the strength of X-Fe interactions is found to govern the dissolution properties. Next to vacancies, the competition between solute volume effects and X-Fe interactions results in the preference of all the solutes to occupy off-centered sites. Low-energy configurations of small VnXm clusters are calculated for n and m up to 4. They are used to parametrize lattice interaction models at the atomic scale. A detailed analysis of the cluster properties suggests the relevance of many-body terms in these models. The accuracy of the resulting models is verified through their satisfactory prediction of vacancy-solute cluster properties beyond the fitting database. From these models, an entire set of VnXm clusters is generated with a new configurational space exploration method. Statistical treatment of the solid solution including these clusters is then achieved by means of low-temperature expansions, checked against Monte Carlo simulations in some specific conditions. Based on the calculation of equilibrium cluster distributions, it is shown that the solubility limit of oxygen in Fe, hardly measurable experimentally, is largely affected by the presence of small VnOm clusters.
... Hirata Effect of Seawater on Thermal Behavior of Conventional and Nanophased Carbon/Epoxy Com... more ... Hirata Effect of Seawater on Thermal Behavior of Conventional and Nanophased Carbon/Epoxy Composites . . . . . 257 Mohammad K. Hossain, Kazi A. Imran, Mahesh Hosur, and Shaik Jeelani High Temperature ...
Tight binding molecular dynamics simulations, with a non orthogonal basis set, are performed to s... more Tight binding molecular dynamics simulations, with a non orthogonal basis set, are performed to study the fragmentation of carbon fullerenes doped with up to six silicon atoms. Both substitutional and adsorbed cases are considered. The fragmentation process is simulated starting from the equilibrium configuration in each case and imposing a high initial temperature to the atoms. Kinetic energy quickly converts into potential energy, so that the system oscillates for some picoseconds and eventually breaks up. The most probable first event for substituted fullerenes is the ejection of a C2 molecule, another very frequent event being that one Si atom goes to an adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they have four or more atoms, while the smaller ones tend to dissociate and sometimes interchange positions with the C atoms. These results are compared with experimental information from mass abundance spectroscopy and the products of photofragmentation.
The electronic and structural properties of Si-doped fullerenes, obtained from C 60 by replacing ... more The electronic and structural properties of Si-doped fullerenes, obtained from C 60 by replacing up to 12 C atoms with Si atoms, are studied by means of first-principles density-functional theory calculations using numerical atomic orbitals as basis sets. We have ...
The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been invest... more The vacancy properties in group-IV hexagonal close-packed metals (Ti, Zr and Hf) have been investigated by Density Functional Theory (DFT) calculations performed with the SIESTA code. The migration energies are found to be systematically lower by ∼ 0.15 eV within the basal plane than out of the basal plane. The electronic origin of this significant contribution to diffusion anisotropy is evidenced by the analysis of the local electronic densities of states and by a comparison with and empirical potential. The average value of the migration energy is in very good agreement with available experimental data in Zr. The activation energies for self-diffusion obtained assuming a vacancy mechanism are in good agreement with experiments in Zr and Hf, although slightly too small, but a significant discrepancy is observed in Ti.
Density functional theory calculations are performed to investigate the impact of magnetism on th... more Density functional theory calculations are performed to investigate the impact of magnetism on the energetics of low-index Cr surfaces and Fe/Cr interfaces, that is, Cr(100), Cr(110), Fe/Cr(100), and Fe/Cr(110). We have also determined the stability of various Cr magnetic structures, particularly the spin-density waves, in the presence of these surfaces and interfaces. We show that the most stable structure of the spin-density wave is mainly dictated by the subtle balance between bulk and surface/interface influences, and strongly dependent on the surface/interface orientation. Regarding the Cr surfaces, we confirm the role of magnetism to lower the surface energy of Cr(100) with respect to Cr(110). Among all the possible orientations of the wave vector, only the out-of-plane wave is found to be stable near Cr(100) surfaces with the high-moment sites located at the surface layer. At variance, the in-plane wave is shown to be the most stable one, consistent with experimental data for very thin Cr(110) films. Concerning the Fe/Cr interfaces, magnetic frustrations are identified to be responsible for a higher formation energy of Fe/Cr(110) compared to that of Fe/Cr(100). This unusual anisotropy of interface energies is clearly different from the corresponding interfaces between Cr and a nonmagnetic element, Cu. Two ways are suggested to relax partially the magnetic frustrations at the (110) interface and to lower its formation energy. Noncollinear magnetic configurations can be developed where local moments of Fe and Cr atoms are perpendicular to each other. Also, in order to preserve phase coherence, in-plane spin-density waves show a very stable magnetic structure with the nodes at the interface layer. The presence of low-moment sites at Fe/Cr(110) offer another way to relax the magnetic frustrations and lower the interfacial energy.
By means of ab initio calculations combined to statistical mechanics, we provide new evidence tha... more By means of ab initio calculations combined to statistical mechanics, we provide new evidence that an experimentally undetectable tiny amount of impurities can be responsible for drastic changes in vacancy concentrations ([V]), inducing large deviations from an Arrhenius law even at low temperature. It is the case of O and N in α-Fe. The present finding is fully compatible with existing experiments, and changes the previous common vision that C has the dominant effect. This study provides a route for bridging the longstanding theoretical-experimental gap on the prediction of [V] in metals.
We have performed long time scale tight-binding molecular dynamics (MD) simulations to study dime... more We have performed long time scale tight-binding molecular dynamics (MD) simulations to study dimer¯ippings on perfect and defective Si(0 0 1) surfaces. The constant temperature MD simulations have been performed for temperatures ranging from 800 to 1300 K. From them we measured the average residence time between¯ips and assuming an Arrhenius equation we obtain a dynamical energy barrier for¯ipping. Our results extrapolate to a dimer¯ipping frequency of 3 Â 10 6 Hz at 300 K in agreement with the symmetric appearance of surface dimers using STM. We have found that the correlated¯ippings of two neighbouring dimers are as important as single dimer¯ippings. The ratio of correlated¯ips with respect to the total number of¯ips is of the order of 0.3 at 800 K and decreases to 0.2 at 1200 K. In the case of surfaces with a single dimer vacancy, the neighbouring dimers to the vacancy have three stable positions instead of two as in perfect surfaces. The dimers on the same row as the defect¯ip faster than those of the other row, leading to their appearance as symmetric dimers in STM measurements at temperatures even below the order±disorder transition temperature. Ó
Classical molecular dynamics and density functional theory calculations are performed to study th... more Classical molecular dynamics and density functional theory calculations are performed to study the impact of two distinct Fe grain boundaries (GBs) on the clustering properties of helium (He) and the possible He effect on GB decohesion. Several He concentrations are considered. Common properties of He clustering are found for the both GBs, which are visibly different from the bcc bulk. In particular, He clusters in the GBs are always elongated in the directions parallel to the interface and contracted in the direction normal to the GB plane, while they are isotropic in the bcc bulk. When the He number in the clusters is sufficiently large, the strong local pressure promotes the occurrence of loop punching, which is easier to trigger in the GBs than in the bulk, resulting in a lower He-to-vacancy ratio in the GB clusters. The emitted self-interstitial atoms (SIAs) can more easily dissociate from the clusters in the GBs than in the bulk, leading to relatively lower local pressures around the clusters in the GBs, and facilitating the clusters growth. He is found to decrease GB cohesion, and the embrittling effect of He increases with its concentration. But interestingly, this effect decreases with He clustering. The present findings are fully compatible with existing experimental evidence, for instance, for a stronger GB embrittlement due to He at rather low temperatures than at higher temperatures.
ABSTRACT The interplay between vacancies (V) and interstitial solutes X (X = C, N, and O) and its... more ABSTRACT The interplay between vacancies (V) and interstitial solutes X (X = C, N, and O) and its impact on thermodynamic properties of alpha-Fe solid solutions are studied, starting from first principles calculations. A systematic comparison between the three solutes is performed, investigating X-Fe, X-X, and V -X interactions. In the alpha-Fe lattice, the strength of X-Fe interactions is found to govern the dissolution properties. Next to vacancies, the competition between solute volume effects and X-Fe interactions results in the preference of all the solutes to occupy off-centered sites. Low-energy configurations of small VnXm clusters are calculated for n and m up to 4. They are used to parametrize lattice interaction models at the atomic scale. A detailed analysis of the cluster properties suggests the relevance of many-body terms in these models. The accuracy of the resulting models is verified through their satisfactory prediction of vacancy-solute cluster properties beyond the fitting database. From these models, an entire set of VnXm clusters is generated with a new configurational space exploration method. Statistical treatment of the solid solution including these clusters is then achieved by means of low-temperature expansions, checked against Monte Carlo simulations in some specific conditions. Based on the calculation of equilibrium cluster distributions, it is shown that the solubility limit of oxygen in Fe, hardly measurable experimentally, is largely affected by the presence of small VnOm clusters.
... Hirata Effect of Seawater on Thermal Behavior of Conventional and Nanophased Carbon/Epoxy Com... more ... Hirata Effect of Seawater on Thermal Behavior of Conventional and Nanophased Carbon/Epoxy Composites . . . . . 257 Mohammad K. Hossain, Kazi A. Imran, Mahesh Hosur, and Shaik Jeelani High Temperature ...
Tight binding molecular dynamics simulations, with a non orthogonal basis set, are performed to s... more Tight binding molecular dynamics simulations, with a non orthogonal basis set, are performed to study the fragmentation of carbon fullerenes doped with up to six silicon atoms. Both substitutional and adsorbed cases are considered. The fragmentation process is simulated starting from the equilibrium configuration in each case and imposing a high initial temperature to the atoms. Kinetic energy quickly converts into potential energy, so that the system oscillates for some picoseconds and eventually breaks up. The most probable first event for substituted fullerenes is the ejection of a C2 molecule, another very frequent event being that one Si atom goes to an adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they have four or more atoms, while the smaller ones tend to dissociate and sometimes interchange positions with the C atoms. These results are compared with experimental information from mass abundance spectroscopy and the products of photofragmentation.
The electronic and structural properties of Si-doped fullerenes, obtained from C 60 by replacing ... more The electronic and structural properties of Si-doped fullerenes, obtained from C 60 by replacing up to 12 C atoms with Si atoms, are studied by means of first-principles density-functional theory calculations using numerical atomic orbitals as basis sets. We have ...
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