
AB INITIO STUDY OF SILVER NANOPARTICLES, GRAIN BOUNDARIES AND THEIR \nQUADRUPLE JUNCTIONS
Polsterová, S. ; Všianská, Monika ; Friák, Martin ; Pizúrová, Naděžda ; Sokovnin, S. ; Šob, Mojmír
Motivated by our experimental research related to silver nanoparticles with various morphologies, we have employed quantummechanical calculations to provide our experiments with theoretical insight. We have computed properties of a 181atom decahedral silver nanoparticle and two types of internal extended defects, 5(210) grain boundaries (GBs) and quadruple junctions (QJs) of these GBs. We have employed a supercell approach with periodic boundary conditions. Regarding the thermodynamic stability of the decahedral nanoparticle, its energy is higher than that of a defectfree facecentered cubic (fcc) Ag by 0.34 eV/atom. As far as the 5(210) GB is concerned, its energy amounts to 0.7 J/m2 and we predict that the studied GBs would locally expand the volume of the lattice. Importantly, the system with GBs is found rather close to the limit of mechanical stability. In particular, the computed value of the shearrelated elastic constant C66 is as low as 9.4 GPa with the zero/negative value representing a mechanically unstable system. We thus predict that the 5(210) GBs may be prone to failure due to specific shearing deformation modes. The studied GBs have also the value of Poisson’s ratio for some loading directions close to zero. Next, we compare our results related solely to 5(210) GBs with those of a system where multiple intersecting 5(210) GBs form a network of quadruple junctions. The value of the critical elastic constant C66 is higher in this case, 13 GPa, and the mechanical stability is, therefore, better in the system with QJs.


DFT calculations of graphene regarding to biosenzoric applications
Špaček, Ondřej ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
This diploma thesis is focused on calculation of both structure and electronic properties of the graphene after the adsorption of atomic and molecular oxygen and urea using the Density Functional Theory (DFT). The influence of van der Waals interactions on the structure and adsorption energy is studied, as well as influence of the thermal corrections, the charge density spatial distribution and the electronic doping of graphene after the adsorption of the adsorbant on the graphene.


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 aluminiumgermanium alloy, while the other examined alloys (AlPb, AgGe and AgPb) 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.


Quantummechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
Slávik, Anton ; Miháliková, Ivana ; Friák, Martin ; Všianská, Monika ; Šob, Mojmír
The L21structure Fe2AlTi intermetallic compound is one of the two phases identified in FeAlTi superalloy nanocomposites. Experimental data related to lowtemperature magnetic properties of this Heusler compound indicate that magnetic moment is about 0.1 Bohr magneton per formula unit. In contrast, previous quantummechanical 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 quantummechanical fixspinmoment calculations and compared our results with those for nonmagnetic state. It turns out that the total energy of the nonmagnetic 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 nonmagnetic state appears at nonzero 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 compressiontension asymmetry with respect to these strains and, for example, the strain 0.08 destabilizes the spinpolarized state, leaving the nonmagnetic state as the only stable one.


Firstprinciples study of interface energies in FeAlbased superalloy nanocomposites
Miháliková, Ivana ; Slávik, Anton ; Friák, Martin ; Všianská, Monika ; Koutná, N. ; Holec, David ; Šob, Mojmír
FeAlbased nanocomposites with a superalloytype 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. Intermetallicscontaining nanocomposites, such as those with the Fe3Al compound being one of the phases, may open a way towards future automotive and energyconversion technologies with lower fuel consumption and reduced environmental impact. We employ quantummechanical calculations to analyze relations between ordering tendencies of Al atoms in the disordered Fe18.75at.%Al phase on one hand and thermodynamic, structural and magnetic properties of FeAlbased nanocomposites on the other. When comparing supercells modeling disordered FeAl phase with different atomic distribution of atoms we find out that the supercell without 1st and 2nd nearest neighbor AlAl pairs has a lower energy than that mimicking a perfect disorder (a special quasirandom structure, SQS). Further, coherent interfaces with (001), (110) and (110) crystallographic orientations between Fe3Al compound and SQS FeAl phase have higher energies than those exhibiting atomic distribution without 1st and 2nd nearest neighbor AlAl pairs.


Quantummechanical study of tensorial elastic and hightemperature thermodynamic properties of grain boundary states in superalloyphase Ni3Al
Friák, Martin ; Všianská, Monika ; Holec, D. ; Šob, Mojmír
Grain boundaries (GBs), the most important defects in solids and their properties are crucial for many materials properties including (in)stability. Quantummechanical methods can reliably compute properties of GBs and we use them to analyze (tensorial) anisotropic elastic properties of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al. Selecting the Sigma 5(210) GBs as a case study because of its significant extra volume, we address the mechanical stability of the GB interface states by checking elasticitybased Born stability criteria. One critically important elastic constant, C 55, is found nearly three times smaller at the GB compared with the bulk, contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals. Next, comparing properties of Sigma 5(210) GB state which is fully relaxed with those of a Sigma 5(210) GB state when the supercell dimensions are kept equal to those in the bulk we conclude that lateral relaxations have only marginal impact on the studied properties. Having the complete elastic tensor of Sigma 5(210) GB states we combine Green'sfunction based homogenization techniques and an approximative approach to the Debye model to compare thermodynamic properties of a perfect Ni3Al bulk and the Sigma 5(210) GB states. In particular, significant reduction of the melting temperature (to 7981% of the bulk value) is predicted for nanometersize grains.


Theoryguided design of novel FeAlbased superalloys
Friák, Martin ; Holec, D. ; Jirásková, Yvonna ; Palm, M. ; Stein, F. ; Janičkovič, D. ; Pizúrová, Naděžda ; Dymáček, Petr ; Dobeš, Ferdinand ; Šesták, Pavel ; Fikar, Jan ; Šremr, Jiří ; Nechvátal, Luděk ; Oweisová, S. ; Homola, V. ; Titov, Andrii ; Slávik, Ondrej ; Miháliková, Ivana ; Pavlů, Jana ; Buršíková, V. ; Neugebauer, J. ; Boutur, D. ; Lapusta, Y. ; Šob, Mojmír
Our modern industrialized society increasingly requires new structural materials\nfor hightemperature applications in automotive and energyproducing industrial\nsectors. Ironaluminides are known to possess excellent oxidation and sulfidation\nresistance as well as sufficient strength at elevated temperatures. New FeAlbased\nmaterials will have to meet multiple casting, processing and operational criteria\nincluding hightemperature creep strength, oxidation resistance and roomtemperature\nductility. Such desirable combination of materials properties can be achieved in multiphase\nmulticomponent superalloys with a specific type of microstructure (the matrix contains\ncoherent particles of a secondary phase  a superalloy microstructure). In order to design\nnew FeAlbased superalloys, we employ a stateoftheart theoryguided materials design\nconcept to identify suitable combinations of solutes.


Firstprinciples studies of the electronic and structural properties of Lead Zirconate Titanate (PZT)
Planer, Jakub ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
This work is focused on Density Functional Theory (DFT) calculations of oxygen vacancy diffusion barriers in mixed perovskite lead zirconate titanate and its pure counterparts. We found out that barrier heights are different in lead titanate and lead zirconate caused by the different localization of the excess electrons due to the oxygen vacancy formation. Diffusion barriers were also determined for titaniumrich mixed phases and compared to experimental values. This work contributes to clarify unusually low experimentally measured diffusion coefficients in PZT. We found out that the induced vacancy states are forming localized bonds to the lead atoms which causes the oxygen vacancies to become immobile due to the increase of the activation energy of the diffusion process.

 

Qantummechanical study of structural stability of Ni4N allotropes
Hemzalová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
Parameterfree density functional theory (DFT) calculations of Ni4N in eight crystallographic phases were performed using the pseudopotential approach implemented in the VASP code; the exchangecorrelation energy was evaluated within the generalized gradient approximation (GGA). In agreement with experiments, the cubic structure with Pearson symbol cP5, space group Pm3m (221), has been found to be the most stable. It is also the only thermodynamically stable structure at T=0 K with respect to decomposition into elemental Ni crystal and N2 gas phase. We determine structural, thermodynamic, electronic, magnetic and elastic properties of all eight Ni4N allotropes studied. The thermodynamic stability and bulk modulus is found to be anticorrelated. For the cubic allotropes, we predict a complete set of singlecrystalline elastic constants, directional dependence of the singlecrystalline Young modulus and homogenized polycrystalline elastic moduli.
