Цель работы – установление влияния нейтронного облучения на стабильность структуры и фазового состояния ферритно-мартенситной стали ЭП823. Данная работа посвящена анализу влияния нейтронного облучения на фазовые и структурные составляющие стали. Исследования проводились на образцах оболочек твэлов, облученных в составе материаловедческой сборки в реакторе БН-600 при температурах 570, 600, 660 ˚С до повреждающих доз 51,3, 75,1 и 81,1 сна. Определялись методами металлографии и стереометрического анализа характеристики структурных составляющих, вторичных фаз, образовавшихся в процессе облучения, оценивались характеристики радиационной пористости, плотность дислокаций. Исследования были выполнены на сканирующем электронном микроскопе MIRA3 FEG-SEM, оснащенном детекторами вторичных и отраженных электронов и приставкой энергодисперсионного анализа характеристического рентгеновского излучения x-Act 6 фирмы Oxford Instruments. Так же проводились исследования на просвечивающем электронном микроскопе фирмы JEM-2000EX при ускоряющем напряжении 100 кВ в просвечивающем режиме. Количественная обработка изображений для получения размерных характеристик выделений проводилась с использованием программного обеспечения «Цифровая фотолаборатория SIAMS Photolab», а также при помощи программного обеспечения сканирующего электронного микроскопа MIRE3 FEG-SEM. Проведенное исследование стали Х12НМВБФСР в исходном состоянии показало, что ее структура состоит из мартенсита, остаточного феррита, крупных карбидов на основе Nb и по границам ферритных зерен и мартенситных реек, образуются карбиды типа М23С6. После нейтронного облучения содержание карбидов типа М23С6 увеличивается, в зернах феррита образуется χ-фаза, у которой с повышением температуры облучения понижается концентрация и увеличивается в размерах, по границам обнаружена мелкодисперсная α-фаза, обогащенная хромом, которая при повышенных температурах исчезает, так же по границам зерен мартенсита наблюдается небольшое количество радиационных пор. Прошедшие изменения микроструктуры под воздействием нейтронного облучения не привели к существенным изменениям механических свойств. / The aim of the work is to establish the effect of neutron irradiation on the stability of the structure and phase state of the EP823 ferrite-martensitic steel. This work is devoted to the analysis of the effect of neutron irradiation on the phase and structural components of steel. The studies were carried out on samples of fuel rod shells irradiated as part of a materials science assembly in the BN-600 reactor at temperatures of 570, 600, 660 ℃ to damaging doses of 51.3, 75.1, and 81.1 dpa. The characteristics of structural components and secondary phases formed during irradiation were determined by metallography and stereometric analysis, and the characteristics of radiation porosity and dislocation density were evaluated. The studies were performed using a MIRA3 FEG-SEM scanning electron microscope equipped with secondary and reflected electron detectors and an x-Act 6 energy dispersive analysis of characteristic X-ray radiation from Oxford Instruments. Studies were also carried out on a transmission electron microscope of the JEM-2000EX company at an accelerating voltage of 100 kV in the transmission mode. Quantitative image processing to obtain the dimensional characteristics of the secretions was carried out using the software "Digital Photo Laboratory SIAMS Photolab", as well as using the software of the scanning electron microscope MIRE3 FEG-SEM. The study of steel X12NMVBFSR in the initial state showed that its structure consists of martensite, residual ferrite, large carbides based on Nb and along the boundaries of ferritic grains and martensitic rails, carbides of the M23C6 type are formed. After neutron irradiation, the content of carbides of the M23C6 type increases, a χ-phase is formed in the ferrite grains, in which the concentration decreases and increases in size with an increase in the irradiation temperature, a fine alpha-phase enriched in chromium is found along the boundaries, which disappears at elevated temperatures, as well as a small number of radiation pores are observed along the boundaries of martensite grains. The past changes in the microstructure under the influence of neutron irradiation did not lead to significant changes in the mechanical properties.

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During recent decades metallic nanoparticles have been found very interesting due to their unique characteristics which make them suitable for different applications. In this research, for the very first time, we tried to perform selective surface photo activation chemistry on the targeted facets of nanoparticles while they are in suspension. This technique enabled us to form desired assemblies of nanoparticles. We focused on elongated shaped gold nanorod due to its unique surface plasmon resonance and probable biomedical applications. In this research we formed a dumbbell shape assembly of nanoparticles in suspension. A probable application for these assemblies can be in vivo imaging. Initially, we reproduced gold nanorods using existing techniques in prior papers and optimized them according to our research needs. A low rpm centrifugal separation technique was developed to efficiently separate synthesized gold nanorods from other shapes. Several characterization techniques were utilized to characterize nanoparticles at each step including UV-absorbance, zeta potential, and dynamic light scattering. Different generations of oligomers were synthesized to be used as gold nanorods coating, and each coating was tested and characterized using appropriate techniques. Our two-step coating replacement method using one of these photocleavable oligomers enabled us to achieve, for the very first time, selective UV photo activation of gold nanorod tips. The photo activated tips were then exposed to oppositely charged gold nanospheres to form dumbbell shape assemblies of gold nanorods and nanospheres. Furthermore, dumbbell shape assembly of nanoparticles was investigated and characterized. / Master of Science

<p dir="ltr">For reliable, facile, and user-friendly, solution-based synthesis of materials, the colloidal nanocrystal route has proven to be the method of choice for so many. The tunability that this process renders its users---from choice of precursors, solvent systems, and reaction conditions including temperature, pressure, and precursor addition order---is truly second to none. In their simplest form, these nanomaterials are usually comprised of an inorganic core of the desired material and an outer layer of surface-stabilizing molecules called ligands. These ligands provide colloidal stability and allow for the solution-processing of these materials for downstream usage in devices such as light-emitting diodes and photovoltaics, for example. In this thesis, the study and use of colloidal nanomaterials of Cu(In,Ga)(S,Se)₂ (CIGSSe), IIA-IVB-S₃ (including BaZrS₃ and SrZrS₃), alkaline earth polysulfides (IIAS_x; IIA = Sr, Ba; x = 2, 3), and other materials like Cu₂GeS₃ and Cu₂BaSnS₄, for studies into the formation, colloidal stability, and fabrication into solar cells was performed.</p><p dir="ltr">More specifically, an experimental protocol was developed to fabricate high-quality CIGSSe nanoparticles with carbonaceous residues that are substantially reduced from traditional pathways. Traditional methods for synthesizing colloidal CIGS NPs often utilize heavy, long-chain organic species to serve as surface ligands which, during annealing in a Se/Ar atmosphere, leave behind an undesirable carbonaceous residue in the film. In an effort to minimize these residues, N-methyl-2-pyrrolidone (NMP) was used as an alternative surface ligand. Through the use of the NMP-based synthesis, a substantial reduction in the number of carbonaceous residues was observed in selenized films. Additionally, the fine-grain layer at the bottom of the film, a common observation of solution-processed films from organic media, was observed to exhibit a larger average grain size and increased chalcopyrite character over those of traditionally prepared films, presumably as a result of the reduced carbon content, allowing for superior growth. As a result, a gallium-free CuIn(S,Se)₂ device was shown to achieve power-conversion efficiencies of over 11% as well as possessing exceptional carrier generation capabilities with a short-circuit current density (J_SC) of 41.6 mA/cm², which is among the highest for the CIGSSe family of devices fabricated from solution-processed methods. It was shown that pre-selenized films of sulfide nanoparticles instead of selenide nanoparticles performed better as solar cells. While the exact mechanism is still under debate, it appears that the growth phase during selenization, which varies depending on the chalcogen present in the starting material plays an important role.</p><p dir="ltr">The IIA-IVB-S₃ system is just beginning to emerge as a material system shown to be capable of solution-based synthesis methods. This is primarily due to the extremely high oxophilicity of the IVB elements, Ti, Zr, and Hf, necessitating that extreme care and judicial use of inert environments be used to synthesize these materials via solution-based methods. In the IIA-IVB-S₃ system exists some of the chalcogenide perovskites, including BaZrS₃, which are expected to have similar electronic properties to the well-known, high-performing halide perovskites, albeit much more stable, making them attractive prospects as novel semiconductor materials for optoelectronic applications. This work builds upon recent studies to show a general synthesis protocol, involving the use of carbon disulfide insertion chemistry to generate highly reactive precursors, that can be used towards the colloidal synthesis of numerous nanomaterials in the IIA-IVB-S₃ system, including BaTiS₃, BaZrS₃, BaHfS₃, α-SrZrS₃ and α-SrHfS₃. Additionally, we establish a method to reliably control the formation of the BaZrS₃ perovskite, a complication seen in previous literature where BaZrS₃ appears to exist as two different phases when synthesized via colloidal methods. The utility of these nanomaterials is also assessed via the measurement of their absorption properties and in the form of highly stable colloidal inks for the fabrication of homogenous, crack-free thin films of BaZrS₃. In addition to the chalcogenide perovskites, the IIA-S system was also explored to better understand the solution-based formation of these materials and how the control of IIA polysulfides can be achieved. We show that the synthesis of these materials is strongly correlated to the reaction temperature and that the length of the S_n^2- oligomer chain is the dependent variable. We also report on the synthesis of a previously unreported polymorph of SrS₂ which appears to take on the <i>C2/c</i> space group, the same as BaS₂.</p><p dir="ltr">Finally, some discussion is also provided on the use of transmission electron microscopy (TEM) to analyze the crystal structure of materials. Some tips and techniques used throughout this thesis are summarized in this section.</p>

Die untersuchten Ferekristalle sind neuartige Verwachsungs-Schichtverbindungen aus m Monolagen von Niobdiselenid (NbSe2), die wiederholt mit n atomaren Bilagen von Bleiselenid (PbSe) oder Zinnselenid (SnSe) geschichtet sind. Niobdiselenid als Volumenmaterial besitzt eine Schichtstruktur und ist ein Supraleiter. Aufgrund ihrer gezielt einstellbaren atomar geschichteten Struktur können Ferekristalle als Modellsysteme für geschichtete Supraleiter dienen. In dieser Arbeit werden ihre strukturellen und elektrischen Eigenschaften untersucht. Mittels Transmissionselektronenmikroskopie wird ihre turbostratisch ungeordnete, nanokristalline Struktur nachgewiesen. Die atomare Struktur innerhalb der einzelnen Schichten ist ähnlich wie in den Volumenmaterialien NbSe2, PbSe und SnSe, wobei die kristallographischen c-Achsen parallel zur Stapelrichtung der Ferekristalle zeigen. Eine quantitative Analyse unter Verwendung eines Zwei-Schicht-Modells für den spezifischen Widerstand, Hall-Koeffizienten und Magnetwiderstand liefert ähnliche Ladungsträgersorten, -dichten und –beweglichkeiten in den NbSe2-Schichten, wie sie für isolierte Einzellagen von NbSe2 berichtet wurden. Diese unterscheiden sich von denen des Volumenmaterials NbSe2. Erstmals wurde ein Übergang der Ferekristalle in den supraleitenden Zustand nachgewiesen. Die Sprungtemperaturen sind dabei in etwa auf die Hälfte der Sprungtemperaturen der jeweiligen nicht turbostratisch ungeordneten Misfit-Schichtverbindungen reduziert. Diese Reduzierung kann der turbostratischen Unordnung der Ferekristalle zugeordnet werden. Das Verhältnis zwischen der schichtsenkrechten Ginzburg-Landau-Kohärenzlänge und dem Abstand zwischen den supraleitenden Schichten ist bei den Ferekristallen kleiner als bei den nicht ungeordneten Misfit-Schichtverbindungen, was Ferekristalle zu vielversprechenden Kandidaten für (quasi-)zweidimensionale Supraleiter macht. / The investigated ferecrystals are novel layered intergrowth compounds consisting of m monolayers of niobium diselenide (NbSe2) stacked repeatedly with n atomic bilayers of lead selenide (PbSe) or tin selenide (SnSe). Bulk NbSe2 is a layered compound showing superconductivity. Due to their artificially atomic-scale layered structure, which is tunable on the atomic scale, ferecrystals can serve as model systems for layered superconductors. In this study, their structural and electrical properties are investigated. Using transmission electron microscopy their turbostratically disordered, nanocrystalline structure is revealed. The atomic structure within the individual layers is similar as for bulk NbSe2, PbSe and SnSe, with the crystallographic c-axes parallel to the stacking direction in the ferecrystals. A quantitative analysis using a two-layer model fit for the electrical resistivity, Hall coefficient and magnetoresistance yields a similar carrier type, density and mobility in the NbSe2 layers as reported for isolated NbSe2 monolayers. These values differ from those of bulk NbSe2. For the first time, a normal-to-superconducting transition has been detected in ferecrystals. The transition temperatures of the ferecrystals are reduced to about a half of those of analogous non-disordered misfit layer compounds. This reduction in transition temperature can be correlated to the turbostratic disorder in ferecrystals. The ratio between the cross-plane Ginzburg-Landau coherence length and the cross-plane distance between the NbSe2 layers for the ferecrystals is lower than for non-disordered misfit layer compounds, making ferecrystals promising candidates for (quasi-)two-dimensional superconductors.

Einzelschichten von Graphen-Nanobänders (GNRs) wurden auf SiC(0001)-Substraten mit zwei unterschiedlichen Fehlschnitten bei Temperaturen von 1410 bis 1460 °C synthetisiert. Das GNR-Wachstum lässt sich bei niedriger Stufenkantenhöhe am besten durch eine exponentielle Wachstumsrate, welche mit der Energiebarriere für die Ausdiffusion von Si korreliert ist. Anderseits wird bei Substraten mit höheren Stufenkanten eine nicht-exponentielle Rate beobachtet, was mit der Bildung von mehrlagigen Graphen an den Stufenkanten in Verbindung gebracht wird. Die Sauerstoffinterkalation von epitaktischen GNRs mittels Ausglühen an Luft von Bändern wird als nächstes untersucht, welche auf unterschiedlichen SiC-Substraten gewachsen wurden. Neben der Umwandlung von monolagigem zu zweilagigem Graphen in der Nähe der Stufenkanten von SiC, führt die Sauerstoffinterkalation zusätzlich zu der Bildung einer Oxidschicht auf den Terrassen des Substrats, was die zweilagigen GNRs elektrisch isoliert voneinander zurücklässt. Die elektrische Charakterisierung der zweilagigen GNRs zeigten dass die Bänder durch die Behandlung mit Sauerstoff elektrisch voneinander entkoppelt sind. Eine robuste Lochkonzentration von etwa 1x10¹³ cm-² und Mobilitäten von bis zu 700 cm²/(Vs) wurden für die GNRs mit einer typischen Breite von 100 nm bei Raumtemperatur gemessen. Wohl definierte Mesastrukturen gebildet mittels Elektronenstrahllithographie auf SiC-Substraten, wurde zuletzt untersucht. Die Charakterisierung des Ladungsträgertransports von GNRs die auf den Seitenwänden der strukturierten Terrassen gewachsen wurden, zeigt eine Mobilität im Bereich von 1000 bis 2000 cm²/(Vs), welche für verschiedene Strukturen auf der gesamten Probe homogen ist, was die Reproduzierbarkeit dieses Herstellungsverfahrens hervorhebt, sowie dessen Potential für die Implementierung in zukünftigen Technologien, welche auf epitaktischgewachsenene GNRs basieren. / Monolayer graphene nanoribbons (GNRs) were synthesized on SiC(0001) substrates with two different miscut angles at temperatures ranging from 1410 to 1460 °C. The GNR growth in lower step heights is best described by an exponential growth rate, which is correlated with the energy barrier for Si out-diffusion. On the other hand, a non-exponential rate is observed for substrates with higher steps, which is associated with the formation of few-layer graphene on the step edges. Oxygen intercalation of epitaxial GNRs is investigated next by air annealing ribbons grown in different SiC(0001) substrates. Besides the conversion of monolayer into bilayer graphene near the step edges of SiC, the oxygen intercalation also leads to the formation of an oxide layer on the terraces of the substrate, leaving the bilayer GNRs electronically isolated from each other. Electrical characterization of bilayer GNRs reveals that the ribbons are electrically decoupled from the substrate by the oxygen treatment. A robust hole concentration of around 1x10¹³ cm-² and mobilities up to 700 cm²/(Vs) at room temperature are measured for GNRs whose typical width is 100 nm. Well defined mesa structures patterned by electron beam lithography on the surface of SiC substrates is lastly researched. Transport characterization of GNRs grown on the sidewalls of the patterned terraces shows a mobility in the range of 1000 – 2000 cm²/(Vs), which is homogeneous for various structures throughout the sample, indicating the reproducibility of this fabrication method and its potential for implementation in future technologies based on epitaxially grown GNRs.

Die Synthese kolloidaler Nanopartikel ist daher eines der wichtigsten Forschungsthemen der letzten Jahre. Die gepulste Laserablation in Flüssigkeiten stellt eine Alternative zu den nasschemischen Synthesewegen dar. Merkmale der gepulsten Laserablation in Flüssigkeiten ist der geringe experimentelle Aufwand, die Vielseitigkeit und die Möglichkeit stabilisatorfreie kolloidale Nanopartikel herzustellen. Eine Weiterentwicklung ist die Verwendung von Pulversuspensionen als Ausgangsmaterial. Dies führt zu höheren Produktivitäten und neuen Materialien. Die zugrundeliegenden Prozesse sind allerdings komplex. Um diese Mechanismen aufzuklären, wurde ein chemischer Ansatz gewählt. Als Ausgangsmaterial wurden daher verschiedene Kupferverbindungen (Cu2C2, Cu5Si, Cu3N, Cu(N3)2, Cu3P, Cu2O, CuO, Cu2S, CuS und CuI) verwendet. Die hergestellten Nanopartikel wurden mit Hilfe der analytischen Transmissionselektronenmikroskopie charakterisiert. Dadurch konnten nachgewiesen werden, dass zwei Mechanismen an der Nanopartikelbildung beteiligt sind. Die Laserbestrahlung von Ausgangsmaterialien wie CuO und Cu3N führt zur Bildung von metallischen Kupfernanopartikeln. In dem dabei erzeugten Plasma nukleieren die Kupferatome zunächst zu kleinen primären Nanopartikeln. Diese Partikel koaleszieren anschließend und bilden größere sekundäre Partikel. Im Gegensatz zu dieser reduktiven Ablation, resultiert die Laserbestrahlung von CuI in der Fragmentierung des Materials. In diesem Fall wird kein Plasma erzeugt, sondern der induzierte thermische Stress führt zur Fragmentierung des Kristalls unter Beibehaltung der chemischen Zusammensetzung. Die Frage, welcher der beiden Mechanismen für ein bestimmtes Ausgangsmaterial dominiert, ist entscheidend für potentielle Anwendungen der Methode, da hiervon die chemische Zusammensetzung der erhaltenen Nanopartikel abhängt. Dies wird am Beispiel der Synthese von Bi2Te3 Nanopartikeln diskutiert, die in thermoelektrischen Elementen zur Anwendung kommen können. / The synthesis of colloidal nanoparticles has become a major topic in recent years. The pulsed laser ablation in liquids poses an alternative to the common wet-chemical approaches. Key features of the pulsed laser ablation in liquids are its simple setup, its versatility, and the possibility to generate surfactant-free colloidal nanoparticles. A further development of this technique is the use of suspended powders instead of bulk targets. This leads to higher productivities and even new materials. Although the generation of colloids by irradiating a suspension is straight forward, the underlying mechanisms of the size reduction from micrometer to nanometer sized particles appear to be quite complex. In order to reveal the mechanism a chemical approach was chosen. Hence, various copper compounds (Cu2C2, Cu5Si, Cu3N, Cu(N3)2, Cu3P, Cu2O, CuO, Cu2S, CuS and CuI) were used as a model system in order to investigate the impact of the leaving group on the ablation process. The generated nanoparticles were characterized with analytical transmission electron microscopy. These investigations clearly show that there are two distinct mechanisms involved in nanoparticle formation. The laser irradiation of precursors like CuO and Cu3N results in the formation of metallic copper nanoparticles. In the generated plasma copper atoms nucleate and form small primary particles. These particles later coalesce to larger secondary particles. In contrast to this reductive ablation, the irradiation of CuI follows a fragmentation mechanism. Here, the absorbed power of the laser beam does not produce a plasma but introduces thermal stress leading to fragmentation of the crystal while the chemical composition is preserved. The question which mechanism is predominant is of utmost importance as the chemical composition of the nanoparticles depends on the formation process. This is discussed on the example of the synthesis of Bi2Te3 nanoparticles, which can be used in thermoelectric applications.

In dieser Arbeit untersuchen wir ultradünne (In,Ga)N Quantentöpfe (QW) in Form von kurzperiodischen Übergittern auf (0001) GaN. Wir charakterisieren dieser Strukturen mit verschiedenen Methoden, d.h.: die hochauflösende Transmissionselektronenmikroskopie, die Rastertransmissionselektronenmikroskopie, Röntgenbeugung und die hochenergetischer Refeflexionselektronenbeugung an Oberflächen, sowie die Photolumineszenz (PL) und die Kathodolumineszenz. Wir fokussieren uns dabei auf die Quantifizierung des Indiumgehaltes solche ultradünnen Schichten und diskutieren über grundlegende optische Eigenschaften dieser Übergitter. Wir finden, dass: 1. Der Indiumeinbau in GaN unter Exposition von In und N-Fluss ist selbst-begrenzend auf eine Zusammensetzung von 25% und eine Schichtdicke von einer Monolage. Die Variation der Wachstumsbedingungen führen weder nicht zu einer Höhung des Indiumgehalts noch der Schichtdicke. Diese Selbstbegrenzung ist im Ergebnis auf die Unterschiede in der Bildungsenthalpie von InN und GaN und auf die hohe Gitterfehlanpassung des Systems. Die niedrigste Energiekonfiguration ist einer (2»3×2»3)R30° Oberflächenrekonstruktion. 2. In diesen polaren In0.25Ga0.75N Übergitter Polarisationsfelder, Dickenfluktuationen oder Kompositionsschwankungen keine wesentliche Rolle spielen. Unsere optischen Studien in Kombination mit DFT-Berechnungen zeigen, dass der Rekombinationsprozess durch den Einschluss der Lochwellenfunktion in den Monoschichten gesteuert wird, dass mit abnehmender Barrieredicke verändert werden können. Im Gegenteil, ist die Elektronenwellenfunktion immer delokalisiert. Unsere Übergitter Phänomene sind als in konventionellen QWs, z.B. den nichtexponentiellen Abfall der PL-Intensität, die spektrale Abhängigkeit der PL Lebensdauer und eine S-förmige Temperaturabhängigkeit des Emissionspeaks. Die letzte lassen sich durch das Zusammenspiel von Ladunsgträgerlokalisation und nicht-strahliger Rekombination erklären. / In this work we investigate ultra-thin (In,Ga)N quantum wells (QWs) grown on (0001) GaN in the form of short-period superlattices (SLs). We perform a comprehensive study of these structures via various methods, i.e.: high resolution transmission electron microscopy, scanning transmission electron microscopy, x-ray diffraction and reflection high-energy electron diffraction, as well as photoluminescence (PL) and cathodoluminescence. We focus on the quantification of In incorporation and study basic optical properties of these SLs. The main results of our investigations are: 1. The In incorporation into GaN under exposure of In and N flux is self-limited to a composition of 25% and a layer thickness of one monolayer. Varying growth conditions do not increase the In content or the layer thickness. This self-limitation is a result of the differences in formation enthalpy of InN and GaN and the high lattice mismatch of the system. The lowest energy configuration that sets maximum In concentration to a fundamental limit of 25%, stable under various growth regimes, is the one with (2»3×2»3)R30° surface reconstruction. 2. Our polar In0.25Ga0.75N SLs serve as model system for recombination process in (In,Ga)N since their recombination is not suffering from polarization fields, well-width or high compositional fluctuations. The optical studies combined with DFT calculations show that the recombination process is governed by the confinement of the hole wavefunction in the QWs, that can be substantially weakened by decreasing barrier thickness. This leads to an increase of non-radiative recombination in the barriers. In the opposite, the electron wave function is always delocalized. Our SLs show common phenomena observed in conventional QWs or bulk alloys like a non-exponential decay of the PL intensity, spectral dependence of the decay time and S-shape temperature dependence. The latter can be explained by the interplay of carrier localization and non-radiative recombination.