Národní úložiště šedé literatury Nalezeno 3 záznamů.  Hledání trvalo 0.00 vteřin. 
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.
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.
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.

Viz též: podobná jména autorů
1 Holec, Dominik
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