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68 results on '"Grammatikopoulos, Panagiotis"'

53. Hydrogen Flux through Size Selected Pd Nanoparticles into Underlying Mg Nanofilms.

Author

Kumar, Sushant, Pavloudis, Theodore, Singh, Vidyadhar, Nguyen, Hoa, Steinhauer, Stephan, Pursell, Christopher, Clemens, Bruce, Kioseoglou, Joseph, Grammatikopoulos, Panagiotis, and Sowwan, Mukhles

Subjects
HYDROGEN storage, HYDROGEN absorption & adsorption, HYDROGENATION, MAGNESIUM films, PALLADIUM, METAL nanoparticles, NANOFILMS
Abstract

Abstract: The application of Mg for hydrogen storage is hindered due to the slow absorption of hydrogen in Mg films. Herein, the hydrogenation process is explored theoretically using density functional theory calculations, and energy barriers are compared for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e., Pd(H)/Mg(O). Decomposing the mechanism into basic steps, it is shown that Pd undergoes a strain‐induced crystallographic phase transformation near the interface, and indicated that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, it is explained why hydrogen diffusion is practically prohibited through native Mg oxide and seriously suppressed through existing hydride domains. Hydrogen flux is experimentally investigated through the nanoportals in Pd‐nanoparticle decorated Mg films by pressure‐composition isotherm measurements. An r ≈ t1/3 relationship is theoretically calculated for the radial growth of hemispherical hydride domains, and this relationship is confirmed by atomic force microscopy. The diffusion constant of hydrogen in Mg films is estimated as DHfilm ≈ 8 × 10−18 m2 s−1, based on transmission electron microscopy characterization. The unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of hydrogenation in any material. [ABSTRACT FROM AUTHOR]

Published
2018
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64. Geometrical Aspects of Dislocation-Obstacle Interaction in Iron

Author

Grammatikopoulos, Panagiotis, Bacon, David John, and Osetsky, Yuri

Abstract

Experiments and atomic-scale computer simulations have shown that nano-scale voids and copper precipitates can be strong obstacles to the glide of dislocations in neutron-irradiated iron. Simulations have shown that voids are strong obstacles and that an edge dislocation climbs by absorbing vacancies at it breaks away from voids. The obstacle strength of copper precipitates is enhanced by a dislocation-induced structural transformation if they are large enough and the temperature is low enough. Most simulations have the centre of a spherical void or precipitate on the slip plane of an edge dislocation. The present work investigates how the strength of 2 and 4 nm voids and precipitates varies with the distance of their centre from the slip plane at temperatures across the range 0 to 450 K. The strength of voids is highest when their centre coincides with the slip plane, but this is not the case for small precipitates, which do not transform from the bcc structure. The strength of both type of obstacle, and the extent of climb at voids and transformation of large precipitates are not symmetric with respect to the position of their centre from the slip plane. The results are discussed in terms of the atomic mechanisms involved.

Published
2010
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65. Geometrical Aspects of Dislocation-Obstacle Interaction in Iron

Author

Grammatikopoulos, Panagiotis, Bacon, David John, and Osetsky, Yuri

Abstract

Experiments and atomic-scale computer simulations have shown that nano-scale voids and copper precipitates can be strong obstacles to the glide of dislocations in neutron-irradiated iron. Simulations have shown that voids are strong obstacles and that an edge dislocation climbs by absorbing vacancies at it breaks away from voids. The obstacle strength of copper precipitates is enhanced by a dislocation-induced structural transformation if they are large enough and the temperature is low enough. Most simulations have the centre of a spherical void or precipitate on the slip plane of an edge dislocation. The present work investigates how the strength of 2 and 4 nm voids and precipitates varies with the distance of their centre from the slip plane at temperatures across the range 0 to 450 K. The strength of voids is highest when their centre coincides with the slip plane, but this is not the case for small precipitates, which do not transform from the bcc structure. The strength of both type of obstacle, and the extent of climb at voids and transformation of large precipitates are not symmetric with respect to the position of their centre from the slip plane. The results are discussed in terms of the atomic mechanisms involved.

Published
2006
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66. Crystallization of silicon nanoclusters with inert gas temperature control.

Author

Junlei Zhao, Singh, Vidyadhar, Grammatikopoulos, Panagiotis, Cassidy, Cathal, Aranishi, Kengo, Sowwan, Mukhles, Nordlund, Kai, and Djurabekova, Flyura

Subjects
*CRYSTALLIZATION, *MOLECULAR dynamics, *MAGNETRON sputtering, *NUCLEATION, *PROBABILITY theory
Abstract

We analyze the fundamental process of crystallization of silicon nanoclusters by means of molecular dynamics simulations, complemented by magnetron-sputter inert gas condensation, which was used to synthesize polycrystalline silicon nanoclusters with good size control. We utilize two well-established Si interatomic potentials: the Stillinger-Weber and the Tersoff III. Both the simulations and experiments show that upon cooling down by an Ar gas thermal bath, initially liquid, free-standing Si nanocluster can grow multiple crystal nuclei, which drive their transition into polycrystalline solid nanoclusters. The simulations allow detailed analysis of the mechanism, and show that the crystallization temperature is size-dependent and that the probability of crystalline phase nucleation depends on the highest temperature the cluster reaches during the initial condensation and the cooling rate after it. [ABSTRACT FROM AUTHOR]

Published
2015
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67. Perspective of computational modeling of nanomaterials

Author

Kai Nordlund, Grammatikopoulos, Panagiotis, Faculty of Science, Department of Physics, Helsinki Institute of Sustainability Science (HELSUS), and Helsinki Institute of Urban and Regional Studies (Urbaria)

Subjects
Computer science, Perspective (graphical), Monte Carlo method, education, Nanotechnology, Computer simulation, Molecular dynamics, Modelling, Nanomaterials, Nanoscience, Density functional theory, 221 Nano-technology, Monte Carlo
Published
2020

68. Computational modeling of nanoparticles in inert environment

Author

Junlei Zhao, Flyura Djurabekova, Grammatikopoulos, Panagiotis, Department of Physics, and Helsinki Institute of Physics

Subjects
Materials science, Field (physics), business.industry, Monte Carlo method, education, Nanoparticle, Nanotechnology, 02 engineering and technology, 021001 nanoscience & nanotechnology, 01 natural sciences, 114 Physical sciences, Molecular dynamics, Semiconductor, 0103 physical sciences, Cluster (physics), Deposition (phase transition), Density functional theory, 010306 general physics, 0210 nano-technology, business
Abstract

Nanoparticles (NPs), as the most prototypical functional building blocks in nanoscience, exhibit size-, shape-, composition-, and structure-dependent behavior which is different from the corresponding bulk materials. Arguably, the structures and properties of NPs cannot be understood purely from experimental data. Deeper insights can be gained by complementary theoretical or computational modeling. This chapter presents an introduction to simulations of metallic and semiconductor NPs in inert environment using multiscale computational modeling techniques, including density functional theory, classical molecular dynamics, and Monte Carlo methods. We review relevant results obtained with such simulations in the field of cluster beam deposition and comment on several challenges that need to be addressed in future.

Published
2020

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