| 1. |
- Arhammar, C., et al.
(författare)
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A theoretical study of possible point defects incorporated into alpha-alumina deposited by chemical vapor deposition
- 2013
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Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 133:2, s. 1433-
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Tidskriftsartikel (refereegranskat)abstract
- The energetics and electronic structure of carbon, chlorine, hydrogen, and sulfur in alpha-Al2O3 was investigated by first principles and thermodynamical calculations. These species are present in the gas phase during the synthesis of alpha-Al2O3 by chemical vapor deposition (CVD) but little is known of their solubility in this compound. The heat of formation from standard reference states of the elements varying the chemical potential of each element was calculated. An attempt to model the actual conditions in the CVD process was made, using the species and solid compounds present in a common CVD process as reference states. Our calculations suggest that sulfur from the catalyzing agent H2S will not solve in alpha-Al2O3 during deposition by CVD. It is found that the neutral chlorine and hydrogen interstitial defects display the lowest heat of formation, 281 and 280 kJ/mol, respectively, at the modeled CVD conditions. This energy is too high in order for neutral defects to form during CVD of alpha-Al2O3 at any significant amounts. The charged defects and their compensation were studied. Carbon substituting oxygen is found to be energetically favored under the modeled CVD conditions, considering carbon dioxide as competing species to solid solubility in alpha-Al2O3 at an energy of -128 kJ/mol. However, care needs to be taken when choosing the possible competing carbon-containing phases. Compensation of carbon substituting for oxygen by oxygen vacancies takes place at 110 kJ/mol from standard reference states, graphite, fcc-Al and O-2. The carbon solubility in Al2O3 is difficult to measure with standard analysis techniques such as X-ray diffraction and energy dispersive X-ray spectroscopy, but several stable compounds in the Al-C-O are available in the literature.
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| 2. |
- Blomqvist, A., et al.
(författare)
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Understanding the catalytic effects of H(2)S on CVD-growth of alpha-alumina : Thermodynamic gas-phase simulations and density functional theory
- 2011
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 206:7, s. 1771-1779
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic effect of H(2)S on the AlCl(3)/H(2)/CO(2)/HCl chemical vapor deposition (CVD) process has been investigated on an atomistic scale. We apply a combined approach with thermodynamic modeling and density functional theory and show that H(2)S acts as mediator for the oxygenation of the AI-surface which will in turn increase the growth rate of Al(2)O(3). Furthermore we suggest surface terminations for the three investigated surfaces. The oxygen surface is found to be hydrogenated, in agreement with a number of previous works. The aluminum surfaces are Cl-terminated in the studied CVD-process. Furthermore, we find that the AlClO molecule is a reactive transition state molecule which interacts strongly with the aluminum and oxygen surfaces.
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| 3. |
- Blomqvist, A., et al.
(författare)
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Understanding the catalytic effects of H2S on CVD-growth of α-alumina : Thermodynamic gas-phase simulations and density functional theory
- 2011
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Ingår i: Proceedings of the 38th International Conference on Metallurgical Coatings and Thin Films (ICMCTF) — ICMCTF 2011. - : Elsevier BV. ; 206:7, s. 1771-1779
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Konferensbidrag (refereegranskat)abstract
- The catalytic effect of H2S on the AlCl3/H2/CO2/HCl chemical vapor deposition (CVD) process has been investigated on an atomistic scale. We apply a combined approach with thermodynamic modeling and density functional theory and show that H2S acts as mediator for the oxygenation of the Al-surface which will in turn increase the growth rate of Al2O3. Furthermore we suggest surface terminations for the three investigated surfaces. The oxygen surface is found to be hydrogenated, in agreement with a number of previous works. The aluminum surfaces are Cl-terminated in the studied CVD-process. Furthermore, we find that the AlClO molecule is a reactive transition state molecule which interacts strongly with the aluminum and oxygen surfaces.
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| 4. |
- Huang, L. M., et al.
(författare)
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Tuning magnetic properties of In2O3 by control of intrinsic defects
- 2010
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 89:4
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and magnetic properties of In2O3 with four kinds of intrinsic point defects (O vacancy, In interstitial, O interstitial, and In vacancy) have been theoretically studied using the density functional theory. The defect energy states of the O vacancy and In interstitial are close to the bottom of conduction band and act as shallow donors, while the defect energy states of the In vacancy and O interstitial are just above the top of the valence band and act as shallow acceptors. Without addition of any magnetic ions, all the hole states are completely spin polarized, while the electron states display no spin polarization. This implies that semiconducting In2O3 can display magnetic ordering, purely due to the intrinsic defects. However, the formation energies for neutral p-type defects are too high to be thermodynamically stable at reasonable temperatures. Nevertheless, it is shown that negative charging can greatly decrease the formation energies of p-type defects, simultaneously removing the local magnetic moments. We conlcude that V-In''' and O-I '' will be the dominant compensating defects as In2O3 is doped with TM ions, such as Sn, Mo, V and Cr. This result is consistent with the general view that the p-type defect is a key feature to mediate ferromagnetic coupling between transition metal ions of dilute concentration in metal oxides.
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| 5. |
- Ramzan, Muhammad, et al.
(författare)
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Electronic Structure from First-Principles of LiBH(4)center dot NH(3), Sr(NH(2)BH(3))(2), and Li(2)Al(BH(4))(5)center dot 6NH(3) for Hydrogen Storage Applications
- 2011
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:40, s. 20036-20042
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Tidskriftsartikel (refereegranskat)abstract
- A series of compounds containing boron, nitrogen, and hydrogen atoms, namely, LiBH(4)center dot NH(3), Sr(NH(2)BH(3))(2), and Li(2)Al(BH(4))(5)center dot 6NH(3), were synthesized recently, and found to be promising for hydrogen storage. We have conducted the corresponding ab initio calculations using density functional theory and analyzed the corresponding electron density as well as derived properties. Also, molecular dynamics simulations were performed to study the diffusion of hydrogen in this series of compounds. We found that despite having widely different crystal structures and difference in their chemical composition, these compounds share common features at the level of their electronic structure. Moreover, diffusion of hydrogen appears to be more favorable in LiBH(4)center dot NH(3) than in Sr(NH(2)BH(3))(2) and Li(2)Al(BH(4))(5)center dot 6NH(3).
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| 6. |
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| 7. |
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| 8. |
- Silvearv, Fredrik, 1973-, et al.
(författare)
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An ab-initio study of (Mn,Al) doped ZnO including strong correlation effects
- 2012
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : Elsevier BV. - 1386-9477 .- 1873-1759. ; 44:6, s. 1095-1097
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Tidskriftsartikel (refereegranskat)abstract
- (Mn,Al)-codoped ZnO has been investigated by means of first-principles calculations using density-functional theory within the generalized-gradient approximation plus on-site Coulomb interaction (GGA + U) to take into the strong correlations on the Mn site. It is found that for several Al-Mn distances, the system shows always an antiferromagnetic behavior. This is explained by the fact that even with the correct electronic structure described with the GGA + U scheme, the extra electron brought by Al is localized in the host conduction band, and therefore does not modify the occupation of the Mn atoms, suggesting that the antiferromagnetic interaction is still dominant.
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| 9. |
- Silvearv, Fredrik, et al.
(författare)
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Establishing the most favorable metal-carbon bond strength for carbon nanotube catalysts
- 2015
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 3:14, s. 3422-3427
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Tidskriftsartikel (refereegranskat)abstract
- We have studied a wide range of transition metals to find potential carbon nanotube (CNT) catalysts for chemical vapor deposition (CVD) production. The adhesion strengths between a CNT and a metal cluster were calculated using first principle density functional theory (DFT) for all 1st, 2nd and 3rd row transition metals. We have developed the criterion that the metal-carbon adhesion strength per bond must fulfill a Goldilocks principle for catalyzing CNT growth and used it to identify, besides the well known catalysts Fe, Co and Ni, a number of other potential catalysts, namely Y, Zr, Rh, Pd, La, Ce and Pt. Our results are consistent with previous experiments performed either in a carbon arc discharge environment or by a CVD-process with regard to CNT catalyst activity
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| 10. |
- Silvearv, Fredrik, 1973-
(författare)
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First Principle Studies of Functional Materials : Spintronics, Hydrogen Storage and Cutting Tools
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The properties of functional materials have been studied with density functional theory. The first type of materials that have been investigated is the so called diluted magnetic semiconductors. It is a new class of materials that could offer enhanced functionality by making use of spin in addition to the charge of the electron. (Mn,Al) co-doped ZnO has been investigated regarding the Al significance on ferromagnetic behavior using density functional theory within the generalized-gradient approximation plus on-site Coulomb interaction. Despite the presence of Al the system always shows antiferromagnetic behavior. The role of intrinsic defects on ferromagnetism in pure and Cr doped In2O3 was also studied. For pristine In2O3, In vacancy and O interstitial states are completely spin polarized. Moreover, these hole states will create Cr ions in mixed valence state, giving rise to a strong ferromagnetic coupling. The second type of functional materials studied are hydrogen storage materials for mobile applications. These materials are considered as alternative if hydrogen is to replace fossil fuels as a energy carrier. In the view of this a series of compounds containing boron, nitrogen and hydrogen has been examined with respect to electronic structure, dehydrogenation energy and hydrogen diffusion properties. One compound, NH3BH3, has many desirable properties as a hydrogen storage material. In an effort to improve those properties, one of the H atoms in the NH3 group was replaced by Li, Na or Sr. The calculated hydrogen removal energies of the hydrogen release reactions were found to be significantly improved. Finally, a coating material, Al2O3, for wear resistant coatings on high performance cemented carbide cutting tools has been investigated. Chemical vapor deposition grown Al2O3 has been used for decades by the industry. To improve the growth process H2S is added to the gas mixture. The catalytic effect of H2S on the AlCl3/H2/CO2/HCl chemical vapor deposition process has been investigated on an atomistic scale. By applying a combined approach of thermodynamic modeling and density functional theory it seems that H2S acts as mediator for the oxygenation of the Al-surface which will in turn increase the growth rate of Al2O3.
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