| 1. |
- Bloch, J., et al.
(författare)
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Prediction and Hydrogen Acceleration of Ordering in Iron-Vanadium Alloys
- 2012
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:21, s. 215503-
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations of binary metallic systems often predict ordered compounds in contrast to empirical reports of solid solutions or disordered phases. These discrepancies are usually attributed to slow kinetics that retains metastable structures at low temperatures. The Fe-V system is an example of this phenomenon, in which we predict two ordered stable ground states, Fe3V and FeV3, whereas a disordered sigma phase is reported. We propose to overcome this difficulty by hydrogen absorption, which facilitates metal atom mobility through vacancy formation and separation between the two elements due to their opposite affinities towards it, thus accelerating transformation kinetics. Hydrogen also increases the relative stability of the ordered structures compared with that of the sigma phase without affecting the shape of the phase diagram. The hydrogen-induced formation of the ordered structures is expressed by a reversible decrease of the electrical resistivity with increasing hydrogen pressure. Such behavior has not been reported before in thin H absorbing films. Formation of the ordered structures is further substantiated by the kinetics of the resistivity changes upon variation of the hydrogen pressure, where two stages are distinguished: a fast initial stage and a much slower subsequent process in which the resistivity changes direction, associated with hydrogen dissolution and phase transformation, respectively.
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| 2. |
- Bolton, Kim, et al.
(författare)
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Carbon Nanotube Growth Mechanisms
- 2007
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Ingår i: Proceedings of Diamond 2007, the 18th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide.
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Konferensbidrag (refereegranskat)abstract
- We have used a variety of computational methods to study key aspects of single-walled carbon nanotube (SWNT) growth. Molecular dynamics (MD) studies based on an empirical force field showed; for example; why SWNT growth occurs in a temperature window and why; for 1-2 nm catalyst particles; the SWNT diameter varies linearly with the size of the particle. In addition; the liquid or solid phase of the catalyst particle is strongly dependent on particle size; and smaller particles (< 1.5 nm) are liquid at typical chemical vapor deposition temperatures whereas larger particles (> 5 nm) are solid. The phase of particles of intermediate sizes depends on the exact temperature and on their carbon content. The effect of substrates on metal-carbide properties and SWNT growth has been studied by combing density functional (DFT) and MD methods. A major effect of flat; inert substrates is to flatten the catalyst particles thereby increasing their melting points. DFT has also been used to study the catalyst-SWNT interaction which is critical for the growth of long SWNTs; and is also being used to study the importance of the SWNT cap structure on its chirality. This knowledge is important; for example; when using SWNTs as seeds for the growth of longer nanotubes.
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| 3. |
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| 4. |
- Ding, Feng, 1970, et al.
(författare)
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Modeling the melting of supported clusters
- 2006
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Ingår i: APPLIED PHYSICS LETTERS. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:13
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations have been used to study the structural and dynamic changes during melting of free and supported iron clusters ranging from 150 to 10 000 atoms. The results reveal a method for determining effective diameters of supported metal clusters, so that the melting point dependence on cluster size can be predicted in a physically meaningful way by the same analytic model used for free clusters.
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| 5. |
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| 6. |
- Duan, Haiming, et al.
(författare)
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Initial growth of single-walled carbon nanotubes on supported iron clusters: a molecular dynamics study
- 2007
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Ingår i: EUROPEAN PHYSICAL JOURNAL D. - : Springer Science and Business Media LLC. - 1434-6060 .- 1434-6079. ; 43:1-3, s. 185-188
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations were used to study the initial growth of single-walled carbon nanotubes (SWNTs) on a supported iron cluster (Fe-50). Statistical analysis shows that the growth direction of SWNTs becomes more perpendicular to the substrate over time due to the weak interaction between carbon nanotube and the substrate. The diameter of the nanotube also increases with the simulation time and approaches the size of the supported iron cluster.
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| 7. |
- Duan, Haiming, et al.
(författare)
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Size dependent melting mechanisms of iron nanoclusters
- 2007
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Ingår i: CHEMICAL PHYSICS. - : Elsevier BV. - 0301-0104 .- 1873-4421. ; 333:1, s. 57-62
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations were used to study the change in the mechanism of iron cluster melting with increasing cluster size. Melting of smaller clusters (e.g., Fe-55 and Fe-100) occurs over a large temperature interval where the phase of the cluster repeatedly oscillates between liquid and solid. In contrast, larger clusters (e.g., Fe-300) have sharper melting points with surface melting preceding bulk melting. The importance of the simulation time, the force field and the definition of cluster melting is also discussed.
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| 8. |
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| 9. |
- Harutyunyan, Avetik, et al.
(författare)
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Hidden features of the catalyst nanoparticles favorable for single-walled carbon nanotube growth
- 2007
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Ingår i: APPLIED PHYSICS LETTERS. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:16
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Tidskriftsartikel (refereegranskat)abstract
- Combining in situ studies of the catalyst activity during single-walled carbon nanotube (SWCNT) growth by mass spectrometry with differential scanning calorimetry and Raman spectroscopy results, the authors expose the favorable features of small catalyst for SWCNT growth and their relationship with synthesis parameters. The sequential introduction of C-12 and C-13 labeled hydrocarbon reveals the influence of catalyst composition on its lifetime and the growth termination path. Ab initio and molecular dynamics simulations corroborate "V"-shape liquidus line of metal-carbon nanoparticle binary phase diagram, which explains observed carbon-induced solid-liquid-solid phase transitions during nanotube growth.
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| 10. |
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