| 11. |
- Damas, Giane B.
(författare)
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Atomic Scale Modelling in Photoelectrocatalysis : Towards the Development of Efficient Materials for Solar Fuel Production
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Using sunlight to produce valuable chemicals has been pointed out as an interesting alternative to deal with the well-known environmental problem related to the use of fossil fuels for energy generation. Thus, it is crucial for this field the development of novel photocatalysts that could drive the uphill reactions with high efficiency while presenting low price and toxicity. In this context, conjugated polymers with a donor-acceptor architecture have shown good photoactivity for the hydrogen evolution reaction (HER) due to their advantageous properties, including a broad UV-Vis absorption spectrum and thermodynamic driving force to carry out the charge transfer processes. In this thesis, a series of fluorene- and benzothiadiazole-based polymers are evaluated by means of ab initio methods as potential candidates for photocatalytic HER. A set of small-molecules with well-defined molecular weight have also been considered for this application. In general, tailoring a chemical unit has enabled an improvement of the absorption capacity in benzo(triazole-thiadiazole)-based polymers and cyclopentadithiophene-based polymers, with a higher impact exhibited upon acceptor tailoring. On the other hand, all systems under investigation present favorable thermodynamics for proton reduction or hole removal by an appropriate sacrificial agent. In particular, it is demonstrated the active role played by nitrogen atoms from the acceptor units in the hydrogenation process, whose binding strength is significantly decreased in benzo(triazole-thiadiazole)-based polymers. Furthermore, the extension of the electron-hole separation has been assessed through the calculation of the exciton binding energies, which are diminished with an improvement in the donating ability on cyclopentadithiophene-based materials.In another approach to deal with the aforementioned problem, it has been considered the direct conversion of carbon dioxide into formic acid, an important chemical that finds applications in fuel cells, medicine and food industries. In this thesis, such electrocatalytic process has been investigated by using Sn-based electrodes and Ru-complexes. In the former case, a solid-state modelling approach based on slab geometries to model surface states has been employed to explore the reaction thermochemistry. The outcomes support the reaction mechanism where the carbon dioxide insertion into the Sn-OH bond is a thermodynamically favorable step prior to reduction, which has a redox potential in fair agreement with the measurements carried out by our collaborators. In a Ru-complex, the reaction mechanism is likely to follow the route with natural production of CO due to ligand release after the first reduction process, which is further protonated to originate the active species. In this case, the insertion occurs at the Ru-H bond to generate a carbon-bound species that is the intermediate in the formic acid production after the second protonation step. Finally, it has been studied the physical adsorption of carbon dioxide in metal-organic frameworks with a varying metallic center in a theoretical point of view.
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| 12. |
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| 13. |
- Edin, Emil, 1987-
(författare)
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Computational Insights into Atomic Scale Wear Processes in Cemented Carbides
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As Ti-alloys become more and more utilized the need for efficient and robust manufacturing of Ti-alloy components increase in importance. Ti-alloys are more difficult to machine than e.g. steel, mainly due to their poor thermal conductivity leading to rapid tool wear. The atomic scale processes responsible for this wear is not well understood. Here the focus is turned to the effects of C diffusion out of the tools as a source of the observed wear. A combination of Density Functional Theory (DFT) making use of Harmonic Transition State Theory (HTST), classical Molecular Dynamics (MD) and kinetic Monte Carlo (kMC) is used to investigate C diffusion into and within experimentally observed WC/W interfaces that exists as a consequence of the C depletion. Further, tools are built and used to evaluate interface parameters for large sets of interfaces within the WC/W system to determine which are energetically preferred. The results from the DFT study show stable interfaces with large differences in activation energy between the two most prominent surfaces found in WC materials, namely the basal and prismatic surfaces. Within the WC/W interfaces the diffusion barriers are similar between the two. The classical MD simulations support the view of stable interfaces at the early stages of C depletion. As C is removed this picture shifts to one in which the diffusion barriers are substantially decreased and the difference between the basal and the prismatic interfaces vanish pointing to a process which starts out slow but accelerates as C is continually removed. From the kMC simulations the overall diffusion pre-factor and activation energy is estimated to be D0=1.8x10-8 m2/s and dE=1.24 eV for the investigated [10-10]-I/[100] interface, the kMC simulations also confirm previous results indicating that the diffusion is restricted to the interface region. The investigation and screening of properties for WC/W interfaces show a preference for the W terminated [10-10]-I/[110] and [0001]/[110] interface combinations based on the interfacial energy.
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| 14. |
- Edin, Emil, et al.
(författare)
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interfaceBuilder, a python tool for building and analyzing interfaces suitable for use in large screening applications
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Interfaces in materials are of importance in a large number of applications. In order to gain a better understanding of interfaces, their structures, their properties, and processes related to them and to be able to predict which interfaces are most likely to form, easy to use tools are needed for finding, constructing, storing and analyzing interfaces. Presented here is a free python package which allows for the finding and storing of the lowest strain interfaces between base structures at different relative rotations. Further, the package allows for fast and easy analysis of interface datasets by including standardized containers for interface properties and a large number of tools designed for visualization and analysis of geometric parameters as well as properties calculated using third party software and loaded back in to the original dataset. The package is well suited to be part of large scale screening searches of interface parameters which is demonstrated here by the calculation of the work of adhesion for 5000 interfaces, each with three different relative translations, in the interface system made up of the [100]/[110] surfaces of BCC W.
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| 15. |
- Edin, Emil, et al.
(författare)
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Study of C diffusion in WC/W boundaries using Adaptive Kinetic Monte Carlo
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Wear of WC/Co tools during turning of Ti-alloys is a big concern within the tooling industry. The wear is mainly attributed chemical wear in the form of diffusion/dissolution of tool material in the material of the workpiece. The reasons for these processes being so pronounced are in large part due to the low thermal conductivity of Ti which results in very high cutting zone temperatures. Experimental investigations have shown that a layer of BCC W forms on the outermost WC grains in contact with the workpiece. The process by which this layer forms must be through the diffusion of C away from the tool as W is only present in the contact in the form of tool material. The rate of this C diffusion is of importance in understanding the atomic scale processes responsible for the observed wear. In this work Kinetic Monte-Carlo is used to estimate the diffusion rate of C in WC/W interfaces and gain an understanding of the diffusion behaviour within these systems at timescales beyond previous computational investigations. The results show that C diffusion is restricted to the interface region and that it follows the paths set by the bottom surface of the BCC W, within the [10$\bar{1}$0]-I/[100] interface the estimated overall diffusion rate has a barrier of $\Delta E=1.24\pm 0.2$ eV and a pre-factor of $D_0=1.6\times10^{-8} \pm 3.8\times10^{-8}$ m$^{2}$/s estimated from calculations of the Mean Square Displacement at temperatures of 1173 K, 1073 K, 973 K and 873 K.
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| 16. |
- Graça Araújo, Carlos Moysés, 1975-
(författare)
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Hydrogen Storage Materials : Design, Catalysis, Thermodynamics, Structure and Optics
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrogen is abundant, uniformly distributed throughout the Earth's surface and its oxidation product (water) is environmentally benign. Owing to these features, it is considered as an ideal synthetic fuel for a new world energetic matrix (renewable, secure and environmentally friendly) that could allow a sustainable future development. However, for this prospect to become a reality, efficient ways to produce, transport and store hydrogen still need to be developed. In the present thesis, theoretical studies of a number of potential hydrogen storage materials have been performed using density functional theory. In NaAlH4 doped with 3d transition metals (TM), the hypothesis of the formation of Ti-Al intermetallic alloy as the main catalytic mechanism for the hydrogen sorption reaction is supported. The gateway hypothesis for the catalysis mechanism in TM-doped MgH2 is confirmed through the investigation of MgH2 nano-clusters. Thermodynamics of Li-Mg-N-H systems are analyzed with good agreement between theory and experiments. Besides chemical hydrides, the metal-organic frameworks (MOFs) have also been investigated. Li-decorated MOF-5 is demonstrated to possess enhanced hydrogen gas uptake properties with a theoretically predicted storage capacity of 2 wt% at 300 K and low pressure.The metal-hydrogen systems undergo many structural and electronic phase transitions induced by changes in pressure and/or temperature and/or H-concentration. It is important both from a fundamental and applied viewpoint to understand the underlying physics of these phenomena. Here, the pressure-induced structural phase transformations of NaBH4 and ErH3 were investigated. In the latter, an electronic transition is shown to accompany the structural modification. The electronic and optical properties of the low and high-pressure phases of crystalline MgH2 were calculated. The temperature-induced order-disorder transition in Li2NH is demonstrated to be triggered by Li sub-lattice melting. This result may contribute to a better understanding of the important solid-solid hydrogen storage reactions that involve this compound.
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| 17. |
- Huang, Lunmei, 1978-
(författare)
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Computational Material Design : Diluted Magnetic Semiconductors for Spintronics
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis deals with the application of ab-initio electronic structure calculations based on density functional theory for material design.The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation U of d electrons, which could change the energy levels of TM d band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe3C, has also been studied in a wide pressure range.We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.
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| 18. |
- Hussain, Tanveer, 1981-
(författare)
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Computational Insights on Functional Materials for Clean Energy Storage : Modeling, Structure and Thermodynamics
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The exponential increase in the demands of world’s energy and the devastating effects of current fossil fuels based sources has forced us to reduce our dependence on the current sources as well as finding cleaner, cheaper and renewable alternates. Being abundant, efficient and renewable, hydrogen can be opted as the best possible replacement of the diminishing and harmful fossil fuels. But the transformation towards the hydrogen-based economy is hindered by the unavailability of suitable storage medium for hydrogen. First principles calculations based on density functional theory has been employed in this thesis to investigate the structures modelling and thermodynamics of various efficient materials capable of storing hydrogen under chemisorption and physisorption mechanisms.Thanks to their high storage capacity, abundance and low cost, metal hydride (MgH2) has been considered as promising choice for hydrogen storage. However, the biggest drawback is their strong binding with the absorbed hydrogen under chemisorption, which make them inappropriate for operation at ambient conditions. Different strategies have been applied to improve the thermodynamics including doping with light and transitions metals in different phases of MgH2 in bulk form. Application of mechanical strain along with Al, Si and Ti doping on MgH2 (001) and (100) surfaces has also been found very useful in lowering the dehydrogenation energies that ultimately improve adsorption/desorption temperatures.Secondly, in this thesis, two-dimensional materials with high surface area have been studied for the adsorption of hydrogen in molecular form (H2) under physisorption. The main disadvantage of this kind of storage is that the adsorption of H2 with these nanostructures likes graphane, silicene, silicane, BN-sheets, BC3 sheets are low and demand operation at cryogenic conditions. To enhance the H2 binding and attain high storage capacity the above-mentioned nanostructures have been functionalized with light metals (alkali, alkaline) and polylithiated species (OLi2, CLi3, CLi4). The stabilities of the designed functional materials for H2 storage have been verified by means of molecular dynamics simulations.
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| 19. |
- Hussain, Tanveer, 1981-, et al.
(författare)
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Transition Metals Doped MgH2 for Hydrogen Storage: A Hybrid Density Functional Calculations
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this study, we have investigated the structural, electronic and thermodynamicproperties of MgH2 doped with selected transition metals (TMs) by means of hybriddensity functional theory (PBE0). On the structural side, the calculated latticeparameters and equilibrium volumes increase in case of Sc, Zr and Y opposite to allthe other dopants indicating volumetrically increased hydrogen density. Except Fe, allthe dopants improve the kinetics of MgH2 by reducing the heat of adsorption with Cu,Nb, Ni and V proving more efficient than others studied TM’s. The electronicproperties have been studied by density of states and correlated with hydrogenadsorption energies.
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| 20. |
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