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QUANTUM-MECHANICAL STUDY OF INTERNAL STRUCTURAL TRANSFORMATIONS IN Pb-SUPERSATURATED Pb-Sn ALLOYS
Friák, Martin ; Čípek, Petr ; Pavlů, J. ; Roupcová, Pavla ; Miháliková, Ivana ; Msallamová, Š. ; Michalcová, A.
Motivated by a decades-long controversy related to the crystal structure of Pb-supersaturated solid solutions of Pb in Sn, we have performed a quantum-mechanical study of these materials. Focusing on both body-centred-tetragonal beta-Sn and simple-hexagonal gamma-Sn structures, we have computed properties of two alloys with the chemical composition Pb5Sn11, i.e. 31.25 at. % Pb, which is close to the composition of the experimentally found alloy (30 at. % Pb). The 16-atom computational supercells were designed as multiples of the elemental beta- and gamma-Sn unit cells, where the Pb atoms were distributed according to the special quasi-random structure (SQS) concept. Full structural relaxations of both beta- and gamma-phase-based alloys resulted in very significant re-arrangements into structures which do not exhibit any apparent structural features typical for the original alloys, and are, therefore, difficult to classify. The formation energies of the beta- and gamma-phase-originating equilibrium phases are 50 meV/atom and 53 meV/atom, respectively. Therefore, they are not stable with respect to the decomposition into the elemental lead and tin. Moreover, our calculations of elastic constants of both phases revealed that they are close to mechanical instability. Our results indicate that the studied Pb-supersaturated Pb-Sn solid solutions may be prone to structural instability, transformations into different phases and decomposition. Our findings may contribute into the identification of the reason why the subsequent experimental studies did not reproduce the initial published data.
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QUANTUM-COMPUTING STUDY OF THE ELECTRONIC STRUCTURE OF CRYSTALS: THE CASE STUDY OF SI
Ďuriška, Michal ; Miháliková, Ivana ; Friák, Martin
Quantum computing is newly emerging information-processing technology which is foreseen to be exponentially faster than classical supercomputers. Current quantum processors are nevertheless very limited in their availability and performance and many important software tools for them do not exist yet. Therefore, various systems are studied by simulating the run of quantum computers. Building upon our previous experience with quantum computing of small molecular systems (see I. Mihalikova et al., Molecules 27 (2022) 597, and I. Mihalikova et al., Nanomaterials 2022, 12, 243), we have recently focused on computing electronic structure of periodic crystalline materials. Being inspired by the work of Cerasoli et al. (Phys. Chem. Chem. Phys., 2020, 22, 21816), we have used hybrid variational quantum eigensolver (VQE) algorithm, which combined classical and quantum information processing. Employing tight-binding type of crystal description, we present our results for crystalline diamond-structure silicon. In particular, we focus on the states along the lowest occupied band within the electronic structure of Si and compare the results with values obtained by classical means. While we demonstrate an excellence agreement between classical and quantum-computed results in most of our calculations, we further critically check the sensitivity of our results with respect to computational set-up in our quantum-computing study. A few results were obtained also using quantum processors provided by the IBM.
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MINOR PHASES IN TIN-RICH HISTORICAL MATERIALS
Michalcová, A. ; Msallamová, Š. ; Fink, D. ; Friák, Martin
The historical organ pipes used to be manufactured from tin and tin-lead alloys. The tin-rich organ pipes usually contained copper as alloying elements, which make them similar to modern lead free soldering materials. The historical organ pipe materials can be used as model materials for natural ageing of soldering materials. Tin pest is a term for modification from white tin to grey tin, which leads to degradation of tin or tin-rich alloys. The influence of individual alloying elements has not been reliably described, yet. Not only on the concentration of the alloying element, but also the distribution may play the important role. In dependence on heat-treatment, the Cu can be in the Sn matrix present in these forms: i) as large intermetallic particles after casting (Cu6Sn5), ii) as solid solution after homogenization annealing and iii) as fine precipitates after ageing. The microstructure of the real historical organ-pipes material was studied and it was proven that contradictionary to the equilibrium phase diagram the Cu enriched was observed.
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First-Principle Study of Electronic Properties of Ultrathin Layers
Nezval, David ; Vázquéz, Hector (oponent) ; Friák, Martin (oponent) ; Bartošík, Miroslav (vedoucí práce)
This work characterizes the structural properties of adsorbed gallium atoms and water molecules on graphene. It also investigates the changes in the electronic properties of graphene caused by the adsorption of each adsorbent. The density functional theory (DFT) calculations are the perfect tool to investigate and explain the physical and chemical processes that occur during adsorption. The electronic properties are studied using band structure calculations and the Bader charge analysis. Recent experimental findings have revealed that in low concentration the Ga atoms negatively dope (n-doping) graphene. This doping effect is reduced at higher Ga concentrations when the clustering of Ga atoms occurs. This work presents the adsorption of individual Ga atoms and the Ga clusters. While single atoms n-dopes graphene with 0.64 electrons, atoms bound in clusters interact with each other and thus weaken the doping of graphene. Cluster formation is fundamentally affected by the diffusion of Ga atoms over graphene. Therefore, a section is devoted here to calculations of the diffusion barrier energy and how this barrier can be affected by the charging of graphene. The experimental observations indicate positive doping (p-doping) of graphene exposed to water molecules. However, these observations were not supported by DFT calculations. This thesis investigates the effect of multilayer water on the electronic properties of graphene. Attention has been paid to the influence of the water molecule orientation in the first layer toward graphene on its doping properties. The presented results show p-doping of graphene when 6 or more layers of water are oriented by oxygen to graphene.
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Anion-exchange enabled tuning of caesium lead mixed-halide perovskites for high-energy radiation detection
Matula, Radovan ; Friák, Martin (oponent) ; Dvořák, Petr (vedoucí práce)
Lead halide perovskites (LHPs) with their unprecedented functional qualities which are only enhanced by the simple band gap tuning, have taken the world of semiconductors by storm. The process of anion exchange, possible even post-synthesis, allows for band gap tuning of LHPs, resulting in lead mixed-halide perovskites (LMHPs), thus expanding their potential for applications, notably in tuneable detectors. The widespread adoption of LMHPs is, however, hindered by their chemical instability, which leads to halide segregation in the material, seriously inhibiting reliable operation of any LMHP-based device. Understanding the kinetics of the halide segregation over extended periods remains a challenge, motivating the use of theoretical simulations like Monte Carlo (MC) methods. Yet, MC simulations rely on well-defined potential energy surfaces (PES), typically derived from computationally intensive density functional theory (DFT) calculations. In this thesis, we propose a novel approach for constructing well-defined PES from high-fidelity DFT data with fraction of the computational load. Utilizing activation-relaxation technique noveau (ARTn) motivated searches for transition points in the PES combined with state-of-the-art machine learning approaches, we aim to to significantly reduce computational costs. Additionally, employing classical theory, we assess the detection capabilities of selected LMHPs.
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Nanoparticels from brake pads wear
Švábenská, Eva ; Roupcová, Pavla ; Friák, Martin ; Schneeweiss, Oldřich
Exposure to particulate air pollution has been associated with a variety of health problems. One of the main\nsources of metal-rich airborne particulate pollution in roadside environments are brake-wear emissions. The \nscanning electron microscopy (SEM) and transmission electron microscopy (TEM), Mössbauer spectroscopy \n(MS), X-ray powder diffraction (XRD) was used to a make quantitative evaluation of wear particles in two \nsamples. Sizes, morphologies, oxidation state, and trace element compositions of wear particles were\ninvestigated using combination of several methods revealed high concentrations of Fe species and phases, \noften associated with carbonaceous material. SEM and TEM observations show that brake wear particles with \na size of tens to hundreds of nm mainly form particle agglomerates. However, ultrafine (<100 nm) particles are \nlikely to pose a health threat after inhalation and/or ingestion.
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Ab-initio výpočty elektronických a strukturních vlastností olovo-zirkonátu-titanátu (PZT)
Planer, Jakub ; Friák, Martin (oponent) ; Bartošík, Miroslav (vedoucí práce)
Tato práce je zaměřena na výpočty difuzních bariér kyslíkových vakancí v olovo-zirkonátu-titanátu a jeho komponent pomocí teorie funkcionálu hustoty. Zjistili jsme, že velikost bariér je různá v olovo titanátu a olovo zirkonátu, což je způsobené rozdílnou lokalizací elektronů pocházejících ze vzniku kyslíkových vakancí. Difuzní bariéry byly nadále určeny pro směs s vysokým podílem titanu a porovnány s experimentálními výsledky. Přínos této práce spočívá v objasnění neobvykle nízkých difuzních koeficientů, které byly experimentálně měřeny na olovo-zirkonátu-titanátu. Zjistili jsme, že elektronové stavy vyvolané přítomností kyslíkových vakancí vytváří lokální vazby mezi atomy olova, což způsobuje, že kyslíkové vakance jsou nepohyblivé v důsledku zvýšení aktivační energie difuzního procesu.
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DFT výpočty grafenu s výhledem na uplatnění v biosenzorech
Špaček, Ondřej ; Friák, Martin (oponent) ; Bartošík, Miroslav (vedoucí práce)
Tato diplomová práce se zaměřuje na výpočet strukturních a elektronových vlastností grafenu po adsorpci atomárního a molekulárního kyslíku a močoviny pomocí teorie funkcionálu hustoty (DFT). Je zde studován vliv van der Waalsovy interakce na strukturu a adsorpční energii, vlivy teplotních korekcí, distribuce hustoty náboje a míra elektronového dopování grafenu po dosednutí adsorbátu na vrstvu grafenu.
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Ab initio výpočty křivek rozpustnosti tuhých roztoků
Fikar, Ondřej ; Friák, Martin (oponent) ; Zelený, Martin (vedoucí práce)
Tato diplomová práce se zabývá teoretickým studiem fázové stability tuhých roztoků ve vybraných slitinách hliníku a stříbra. Ab initio výpočty byly provedeny s využitím projektovaných přidružených vln a teplotní závislosti termodynamických veličin byly získány pomocí fononových výpočtů. Pozornost je v této práci věnována zejména slitině hliníku a germania, zatímco ostatní slitiny (Al-Pb, Ag-Ge a Ag-Pb) byly zkoumány spíše za účelem porovnání predikce rozpustnosti v různých soustavách a její spolehlivosti. Teploty, při kterých dochází ke stabilizaci tuhého roztoku, byly hodnoceny z energetického hlediska, přičemž bylo vyhodnocení provedeno vícekrát se zahrnutím různých příspěvků k celkové energii jednotlivých soustav. Dále byly spočítány elektronové a fononové hustoty stavů pro všechny čisté prvky a tuhé roztoky, porovnány výsledné teploty rozpustnosti s experimentálními daty, které byly zprostředkovány metodou CALPHAD a vyobrazeny jednotlivé příspěvky k celkové energii. Výsledky získané v této práci podhodnocují experimentální hodnoty teplot rozpustnosti jednotlivých tuhých roztoků řádově o stovky Kelvinů.
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