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Electronic structure and mechanical properties of interfaces in solids
Češka, Jakub ; Šesták, Petr (referee) ; Černý, Miroslav (advisor)
In this work I focus on a theoretical study of the properties of transition metal nitrides (TiN, AlN and VN in the rock-salt structure). In addition to the nitrides themselves, I focus on the interface in systems of multilayers consisting of pairs of these nitrides. For nitrides, I predict the lattice parameter and cleavage energy using ab initio calculations. For the interface, the main goal is to predict the cleavage energy of the interface and to predict the influence that an atom substituted in one of layers on the interface has on the value of cleavage energy. According to the calculations performed, an oxygen atom substituted in the interface has a significant influence on the value of cleavage energy. The value of cleavage energy for the interface with an oxygen atom differs significantly from that for the pure interface. Whether there was a decrease or an increase depended on which of the layers at the interface the oxygen atom was substituted in.
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The effect of impurities on the interface cohesion in multilayers in transition metal nitrides
Češka, Jakub ; Zelený, Martin (referee) ; Černý, Miroslav (advisor)
This work deals with the study of transtition metal nitride multilayers using first-principles calculations. Objects of this study are three particular systems AlN / TiN, AlN / VN and TiN / VN. Studied systems are in the B1 structure with an interface along the (001) plane. The main goal is to unravel the effect of impurity on cohesion in these multilayers. The impurity in question is a substitutional O atom replacing N in the lattice. Preferred positions of these substitutions are predicted for three different concentrations of substitution impurity. These predictions are based on the energy balance of substitutions in different positions. Resulting preferred positions within the multilayer may differ depending on the oxygen concentration. In most cases, the preferred position is at the interface between the two nitrides. For such systems with oxygen impurity in the preferred position a cleavage energy along several (001) planes is calculated. The effect of the impurity on the value of cleavage energy depends on its concentration. In the case of AlN / TiN multilayer, a suitable concentration of the impurity may increase the cleavage energy of the weakest link in multilayer compared to clean multilayer. In other cases the presence of impurity either causes a decrease in the cleavage energy or does not significantly affect its value.
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Computer modeling of high-entropy alloys
Papež, Pavel ; Jan, Vít (referee) ; Zelený, Martin (advisor)
This Master’s thesis is focused on theoretical study of the high entropy alloy CoCrNi using ab initio calculations. The focus was on the effect of short range order on the relative stability of FCC and HCP structures and the value of stacking fault energy.The results show increase of stability in both types of structures wtih decreasing number of Cr-Cr nearest neighbours. The effect of the number of Cr-Cr nearest neighbours on the stacking fault energy previously shown in literature was not observed. However the strong dependency was found on the change of short range order caused by the shift of (1 1 1) planes after the transformation from the FCC to HCP structure. The effect of interstitial atoms C a N was also studied. Both these interstitials stabilise FCC structure and thus cause the increase of stacking fault energy. Both interstitials prefer octahedral positions with higher amount of Cr in their nearest neighbour shell.
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Electronic structure and mechanical properties of interfaces in solids
Češka, Jakub ; Šesták, Petr (referee) ; Černý, Miroslav (advisor)
In this work I focus on a theoretical study of the properties of transition metal nitrides (TiN, AlN and VN in the rock-salt structure). In addition to the nitrides themselves, I focus on the interface in systems of multilayers consisting of pairs of these nitrides. For nitrides, I predict the lattice parameter and cleavage energy using ab initio calculations. For the interface, the main goal is to predict the cleavage energy of the interface and to predict the influence that an atom substituted in one of layers on the interface has on the value of cleavage energy. According to the calculations performed, an oxygen atom substituted in the interface has a significant influence on the value of cleavage energy. The value of cleavage energy for the interface with an oxygen atom differs significantly from that for the pure interface. Whether there was a decrease or an increase depended on which of the layers at the interface the oxygen atom was substituted in.
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Computer modeling of twin-boundaries in shape memory alloys
Heczko, Martin ; Pokluda, Jaroslav (referee) ; Zelený, Martin (advisor)
This Master‘s thesis is focused on theoretical study of twinning in magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concentration of manganese at the expense of gallium was studied on total energy and stress profiles along different deformation paths in the (10-1)[101] shear system of non-modulated martensite. Further, this work deals with the effect of the concentration of manganese on the energy of planar fault caused by presence of partial dislocation due to motion of twin boundary. The results show that the shear modulus in studied shear system increases with the increasing concentration of manganese as well as energy barrier and deformation characteristics along shear deformation paths increases, which makes the shear more difficult in Mn-rich alloys. Increasing concentration of manganese also leads to rising the planar fault energy. All these effects can be responsible for lower mobility of twin boundaries in alloys with higher concentration of manganese.
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Topological band theory of relativistic spintronics in antiferromagnets
Šmejkal, Libor ; Jungwirth, Tomáš (advisor) ; Kuneš, Jan (referee) ; Shick, Alexander (referee)
Nanoelectronics and spintronics are concerned with writing, transporting, and reading information stored in electronic charge and spin degrees of freedom at the nanoscale. Past few years have shown that two spintronics effects discovered in the 19th century, namely anisotropic magnetoresistance and anomalous Hall effect, can be used also for sensing antiferromagnetism which opened the field of antiferromagnetic spintronics. The more than a century of controversial studies of these effects have shown their relativistic spin-orbit coupling and spin-polarisation symmetry breaking origin. However, a complete understanding of these effects and a fully predictive theory capable of identifying novel suitable antiferromagnetic materials are still lacking. Here, by extending modern symmetry and topology concepts in condensed matter physics, we have further developed the theory of anisotropic magnetoresistance and spontaneous Hall effect. Our approach is based on magnetic symmetry and topology analysis of antiferromagnetic energy bands, Bloch spectral functions, and Berry curvatures calculated from the state-of-the- art first-principle theory. This guided us to the prediction of two novel, previously unanticipated effects: relativistic metal-insulator transition from antiferromagnetic Dirac fermions, and crystal Hall...
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Computational prediction of solubility limits in solid solutions
Fikar, Ondřej ; Friák, Martin (referee) ; Zelený, Martin (advisor)
This diploma thesis is focused on a theoretical study of the phase stability of solid solutions in selected aluminium and silver alloys. The ab initio calculations were performed using projected augmented waves method and the thermal dependencies of thermodynamic quantities were acquired using phonon calculations. The main focus of this work is the of aluminium-germanium alloy, while the other examined alloys (Al-Pb, Ag-Ge and Ag-Pb) serve the purpose of comparison of the solid solubility prediction and its reliability in systems with different composition. The temperatures, at which the solid solutions become stable, were evaluated using the energy difference between possible states and this evaluation was carried out for different contributions to the total energy. Also the electronic and phonon densities of states were calculated for all pure elements and solid solutions. The temperatures of solubility were compared to the experimental ones provided by the CALPHAD method and the individual contributions to the total energy were determined and depicted. The results obtained in this work tend to underestimate temperatures of solubility of individual solid solutions by hundreds of Kelvin.
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Quantum-mechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
Slávik, Anton ; Miháliková, Ivana ; Friák, Martin ; Všianská, Monika ; Šob, Mojmír
The L21-structure Fe2AlTi intermetallic compound is one of the two phases identified in Fe-Al-Ti superalloy nanocomposites. Experimental data related to low-temperature magnetic properties of this Heusler compound indicate that magnetic moment is about 0.1 Bohr magneton per formula unit. In contrast, previous quantum-mechanical calculations predicted Fe2AlTi to have much higher magnetic moment, 0.9 Bohr magneton per formula unit. In order to solve this discrepancy between the theory and experiment we have performed a series of quantum-mechanical fix-spin-moment calculations and compared our results with those for non-magnetic state. It turns out that the total energy of the non-magnetic state is only by 10.73 meV/atom higher than that of the magnetic state. When applying Boltzmann statistics to this very small energy difference we predict that the non-magnetic state appears at non-zero temperatures with significant probabilities (for instance, 22.36 % at T = 100 K) and reduces the overall magnetic moment. As another mechanism lowering the magnetization we studied selected shape deformations, in particular trigonal shearing. Fe2AlTi exhibits a compression-tension asymmetry with respect to these strains and, for example, the strain 0.08 destabilizes the spin-polarized state, leaving the non-magnetic state as the only stable one.
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First-principles study of interface energies in Fe-Al-based superalloy nanocomposites
Miháliková, Ivana ; Slávik, Anton ; Friák, Martin ; Všianská, Monika ; Koutná, N. ; Holec, David ; Šob, Mojmír
Fe-Al-based nanocomposites with a superalloy-type of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades in high temperature applications. Intermetallics-containing nanocomposites, such as those with the Fe3Al compound being one of the phases, may open a way towards future automotive and energy-conversion technologies with lower fuel consumption and reduced environmental impact. We employ quantum-mechanical calculations to analyze relations between ordering tendencies of Al atoms in the disordered Fe-18.75at.%Al phase on one hand and thermodynamic, structural and magnetic properties of Fe-Al-based nanocomposites on the other. When comparing supercells modeling disordered Fe-Al phase with different atomic distribution of atoms we find out that the supercell without 1st and 2nd nearest neighbor Al-Al pairs has a lower energy than that mimicking a perfect disorder (a special quasi-random structure, SQS). Further, coherent interfaces with (001), (110) and (1-10) crystallographic orientations between Fe3Al compound and SQS Fe-Al phase have higher energies than those exhibiting atomic distribution without 1st and 2nd nearest neighbor Al-Al pairs.
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Magnetic shape memory alloys - ab initio approach
Heczko, Martin ; Šesták, Petr (referee) ; Zelený, Martin (advisor)
This Bachelor’s thesis is focused on theoretical study of magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concertation of manganese instead of gallium was studied on total-energy and magnetic moment profiles along the tetragonal deformation path between austenite phase with cubic L21 structure and phase of nonmodulated martensite. Further, the effect of manganese atoms distribution within the gallium sublattice was studied as well as changes of this distribution under applied tetragonal deformation. At last but not at least, the elastic constants for austenitic and martensitic structures of studied alloys were calculated. The results show that the non-modulated martensite stabilized with increasing concentration of manganese, because its total energy decreased. The energetic barrier between austenitic and martensitic structures also decreased, which means the metastable austenite will change to unstable.
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