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Extended defects in Ga and Al nitrides
Vacek, Petr ; Holec, David (oponent) ; Hospodková,, Alice (oponent) ; Gröger, Roman (vedoucí práce)
III-nitrides crystallize in the hexagonal (wurtzite) structure, whereas the cubic (zincblende, sphalerite) structure is metastable with only slightly higher energy. Their physical properties are strongly affected by the presence of extended defects that are of different kinds in the two structures. In wurtzite III-nitrides, these are primarily threading dislocations, some of which are known to generate deep defect states in the bandgap, through which they affect the electrical and optoelectronic properties of devices. On the other hand, zincblende III-nitrides contain a large density of stacking faults that facilitate local transformations into the more stable wurtzite structure. The aim of this work is to characterize the extended defects in both crystal structures using a combination of electron microscopy, atomic force microscopy, and X-ray diffraction. We demonstrate that (0001)-oriented samples of GaN/AlN and AlN grown on Si (111) substrate by metal-organic chemical vapor deposition contain a large density of threading dislocations. Their Burgers vectors are mostly parallel to the a-direction of the wurtzite lattice, followed by the Burgers vectors parallel to the a+c-direction, whereas the dislocations with Burgers vectors parallel to the c-direction are relatively rare. The probable origin of threading dislocations is discussed according to the type of the film growth. Prismatic stacking faults were found in thin AlN nucleation layers but were not present in the thicker layers. Amorphous layer composed of SiNx and partially of AlN was found at the AlN/Si interface. We propose that this amorphous layer could have a major role in the relief of misfit strain. Analysis of electrical activity of extended defects in AlN was done using electron beam induced current technique. We have found that threading dislocations cause a weak drop of induced current. However, due to their high density and uniform distribution, they have larger impact on electrical properties than V-defects and their clusters. The topographical and crystallographic defects were studied in as-grown and annealed nucleation layers of zincblende GaN grown on 3C-SiC (001) / Si (001) substrate. The sizes of surface features on as-grown samples increase with the thickness of the nucleation layer and are enhanced by annealing. The surface coverage of GaN with the thinnest nucleation layers is reduced after annealing due to diffusion and desorption (or etching by reactor atmosphere). The stacking faults found in GaN near its interface with SiC were mostly of the intrinsic type bounded by Shockley partial dislocations. The origin of these stacking faults was discussed as well as the impact of partial dislocations on the strain relief. Due to the abundance of stacking faults, their interactions were studied in detail. Based on our findings, we have developed a theoretical model of stacking fault annihilation in zincblende GaN films. This model is shown to be able to predict the decrease of the stacking fault density with increasing film thickness.
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Modelování a analýza vlivu ABS na chování vozidla
Holec, Dominik ; Šimek, Václav (oponent) ; Strnadel, Josef (vedoucí práce)
Bakalárska práca sa zaoberá brzdovým systémom ABS a jeho vplyvom na brzdnú dráhu vozidla. Práca obsahuje návrh, popis vývoja a testovanie simulačného modelu vo verifikačnom prostredí UPPAAL. Výsledný model je systémom navzájom komunikujúcich časovaných automatov. Analýza vlastností modelu je založená na metóde štatistického overovania modelu. Model umožňuje skúmať najmä priebeh a dĺžku brzdnej dráhy. Ostatné premenné ovplyvňujúce chovanie modelu, ktoré je možné sledovať sú sklz, koeficient trenia, odpor vzduchu, brzdná sila a rýchlosť vozidla. Je možné taktiež zistiť pravdepodobnosť, že vozidlo za zvolených podmienok zastaví do určitého počtu metrov a skúmať vplyv systému ABS na tieto výsledky.
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Extended defects in Ga and Al nitrides
Vacek, Petr ; Holec, David (oponent) ; Hospodková,, Alice (oponent) ; Gröger, Roman (vedoucí práce)
III-nitrides crystallize in the hexagonal (wurtzite) structure, whereas the cubic (zincblende, sphalerite) structure is metastable with only slightly higher energy. Their physical properties are strongly affected by the presence of extended defects that are of different kinds in the two structures. In wurtzite III-nitrides, these are primarily threading dislocations, some of which are known to generate deep defect states in the bandgap, through which they affect the electrical and optoelectronic properties of devices. On the other hand, zincblende III-nitrides contain a large density of stacking faults that facilitate local transformations into the more stable wurtzite structure. The aim of this work is to characterize the extended defects in both crystal structures using a combination of electron microscopy, atomic force microscopy, and X-ray diffraction. We demonstrate that (0001)-oriented samples of GaN/AlN and AlN grown on Si (111) substrate by metal-organic chemical vapor deposition contain a large density of threading dislocations. Their Burgers vectors are mostly parallel to the a-direction of the wurtzite lattice, followed by the Burgers vectors parallel to the a+c-direction, whereas the dislocations with Burgers vectors parallel to the c-direction are relatively rare. The probable origin of threading dislocations is discussed according to the type of the film growth. Prismatic stacking faults were found in thin AlN nucleation layers but were not present in the thicker layers. Amorphous layer composed of SiNx and partially of AlN was found at the AlN/Si interface. We propose that this amorphous layer could have a major role in the relief of misfit strain. Analysis of electrical activity of extended defects in AlN was done using electron beam induced current technique. We have found that threading dislocations cause a weak drop of induced current. However, due to their high density and uniform distribution, they have larger impact on electrical properties than V-defects and their clusters. The topographical and crystallographic defects were studied in as-grown and annealed nucleation layers of zincblende GaN grown on 3C-SiC (001) / Si (001) substrate. The sizes of surface features on as-grown samples increase with the thickness of the nucleation layer and are enhanced by annealing. The surface coverage of GaN with the thinnest nucleation layers is reduced after annealing due to diffusion and desorption (or etching by reactor atmosphere). The stacking faults found in GaN near its interface with SiC were mostly of the intrinsic type bounded by Shockley partial dislocations. The origin of these stacking faults was discussed as well as the impact of partial dislocations on the strain relief. Due to the abundance of stacking faults, their interactions were studied in detail. Based on our findings, we have developed a theoretical model of stacking fault annihilation in zincblende GaN films. This model is shown to be able to predict the decrease of the stacking fault density with increasing film thickness.
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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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Modelování a analýza vlivu ABS na chování vozidla
Holec, Dominik ; Šimek, Václav (oponent) ; Strnadel, Josef (vedoucí práce)
Bakalárska práca sa zaoberá brzdovým systémom ABS a jeho vplyvom na brzdnú dráhu vozidla. Práca obsahuje návrh, popis vývoja a testovanie simulačného modelu vo verifikačnom prostredí UPPAAL. Výsledný model je systémom navzájom komunikujúcich časovaných automatov. Analýza vlastností modelu je založená na metóde štatistického overovania modelu. Model umožňuje skúmať najmä priebeh a dĺžku brzdnej dráhy. Ostatné premenné ovplyvňujúce chovanie modelu, ktoré je možné sledovať sú sklz, koeficient trenia, odpor vzduchu, brzdná sila a rýchlosť vozidla. Je možné taktiež zistiť pravdepodobnosť, že vozidlo za zvolených podmienok zastaví do určitého počtu metrov a skúmať vplyv systému ABS na tieto výsledky.
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Quantum-mechanical study of tensorial elastic and high-temperature thermodynamic properties of grain boundary states in superalloy-phase 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. Quantum-mechanical 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 elasticity-based 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's-function 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 79-81% of the bulk value) is predicted for nanometer-size grains.
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Theory-guided design of novel Fe-Al-based 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 high-temperature applications in automotive and energy-producing industrial\nsectors. Iron-aluminides are known to possess excellent oxidation and sulfidation\nresistance as well as sufficient strength at elevated temperatures. New Fe-Al-based\nmaterials will have to meet multiple casting, processing and operational criteria\nincluding high-temperature creep strength, oxidation resistance and room-temperature\nductility. Such desirable combination of materials properties can be achieved in multi-phase\nmulti-component superalloys with a specific type of microstructure (the matrix contains\ncoherent particles of a secondary phase - a superalloy microstructure). In order to design\nnew Fe-Al-based superalloys, we employ a state-ofthe-art theory-guided materials design\nconcept to identify suitable combinations of solutes.
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Redistribuce napětí a její vliv na precipitaci v mikrostruktuře
Holec, David ; Dlouhý, Antonín
Práce studuje dva mechanismy lokální redistribuce napětí v mikrostruktuře konstrukčních materiálů. První model uvažuje anisotropii koeficientu teplotní roztažnosti a predikuje stav napětí v blízkosti hranice mezi dvěma různě orientovanými zrny po změně teploty. Druhý model studuje distribuce napětí v polykrystalech, které mají anisotropické elastické vlastnosti a které jsou podrobeny stárnutí za asistence vnějšího aplikovaného napětí. Je demonstrován vliv redistribuce napětí na selektivní precipitaci krystalografických variant fáze Ni4Ti3 v mikrostruktuře tvarově paměťových slitin NiTi.
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