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Showing 161 to 169 of 169 (0.033 seconds)Spelling suggestions: "subject:"phosphor"" "subject:"posphor""
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Atomic Scale Investigation of Pressure Induced Phase Transitions in the solid StateBoulfelfel, Salah Eddine 27 November 2009 (has links)
In this work, atomic scale investigation of pressure-induced transformations in the solid state have been carried out. A series of compounds including GaN, ZnO, CaF2, and AgI, in addition
to elemental phosphorus have been studied. The corresponding transition mechanisms have been
elucidated with a clear description of atomic displacements and intermediate structures involved
therein.
In the first group of compounds, the long standing debate on the transition path of the
wurtzite(WZ)-to-rocksalt(RS) transition in semiconductors, GaN and ZnO was resolved using geometrical
modeling combined with molecular dynamics (MD) simulations conducted in the frame
of transition path sampling (TPS) method. In GaN, a two-step mechanism through a metastable
intermediate phase with a tetragonal structure iT has been revealed from simulations. In ZnO,
the tetragonal intermediate structure was kinetically less stable, although still part of the real
transition mechanism. It appeared at the interface between WZ and RS as consequence of a layers
shearing. The transition regime in ZnO was characterized by a competition between iT structure
and another hexagonal intermediate with hexagonal symmetry iH. Although possible, the latter
is not functional for the transition.
In both cases, GaN and ZnO, two points of agreement with experiments have been revealed.
The tilting of structures after transition, and the phonon mode softening associated with atomic
displacements leading to the tetragonal structure iT
In the second group of compounds, the investigation of transitions in superionic conductors,
CaF2 and AgI, demonstrated a different and particular behavior of atomic motion under pressure.
The solid-solid reconstruction of CaF2 structure was shown to be initiated and precedented by high
disorder of the anionic sublattice. The percolation of fluoride ions through voids in the fluorite
structure created a thin interface of liquid like state. The sparce regions caused by the departure
of anions facilitates the cation sublattice reconstruction.
In AgI, ion diffusion during the wurtzite/zincnlende(ZB)$rocksalt transition was more pronounced
due to the extended stacking disorder WZ/ZB. The Ag+ ions profited not only from the
structure of the interface but used the combination of interstitial voids offered by both phases,
WZ and ZB, to achieve long diffusion paths and cause the cation sublattice to melt. Clearly, a
proper account for such phenomena cannot be provided by geometry-designed mechanisms based
on symmetry arguments.
In phosphorus, the question of how the stereochemically active lone pairs are reorganized during
the orthorhombic (PI) to trigonal (PV) structural transition was answered by means of simulations.
Computation was performed at different levels theory.
First, the mechanism of the transition was obtained from TPS MD simulations. MD runs
were performed within density functional tight binding method (DFTB). The analysis of atomic
displacements along the real transformation path indicated a fast bond switching mechanism.
In a second step, the nature of the interplay between orbitals of phosphorus during the bond
switching was investigated. A simultaneous deformation of lone pair and P−P bond showed
a mutual switching of roles during the transformation. This interplay caused a low dimensional
polymerization of phosphorus under pressure. The corresponding structure formed as zigzag linear
chain of fourfold coordinated phosphorus atoms (· · ·(P(P2))n · · ·) at the interface between PI and
PV phases.
A further result of this work was the development of a simulation strategy to incorporate
defects and chemical doping to structural transformations. On top of the transition path sampling
iterations, a Monte Carlo like procedure is added to stepwise substitute atoms in the transforming
system. Introducing a chemically different dopant to a pure system represents a perturbation to
the energy landscape where the walk between different phases is performed. Therefore, any change
in the transition regime reflects the kinetic preference of a given structural motif at times of phase
formation.
This method was applied to the elucidation of WZ-RS transition mechanism in the series of
semiconducting compounds AlN, GaN, and InN. Simulations showed that In atoms adopt the
same transformation mechanism as in GaN and favor it, while Al atoms demonstrated a significant
reluctance to the path going through tetragonal intermediate iT. The difference between transition
regime in mixed systems InxGa1−xN and AlxGa1−xN is in agreement with experiments on high pressure
behavior of AlN, GaN, and InN. While transitions in GaN and InN are reversible down
to ambient conditions, AlN is stable.
The work presented in this thesis constitutes the seed of new perspectives in the understanding
of pressure-induced phase transformations in the solid state, where the physics and the chemistry
are brought together by means of computer simulations.
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Люминесцентные свойства и фотометрические характеристики наноструктур с квантовыми точками InP/ZnS : магистерская диссертация / Luminescent properties and photometric characteristics of nanostructures with InP/ZnS quantum dotsСавченко, С. С., Savchenko, S. S. January 2016 (has links)
В работе проведено исследование оптических характеристик коллоидных квантовых точек (КТ) InP/ZnS различных размеров (QD-1, QD-2, QD-3) и композитных наноструктур анодированного оксида алюминия (AAО) с КТ методами спектрофотометрии и люминесцентной спектроскопии.
Выполнен литературный обзор, касающийся электронных состояний в идеальном нанокристалле (НК), синтеза КТ на основе InP, использования НК для создания нанокомпозитов и расчёта цветовых характеристик излучателей. Описаны методики подготовки образцов и проведения измерений спектров оптического поглощения (ОП) и фотолюминесценции (ФЛ).
По анализу спектров ОП КТ определены значения энергий оптических переходов. Полосы с наименьшей энергией соответствуют первому экситонному пику поглощения ядра InP. Другие могут быть приписаны оболочке из ZnS. По синему сдвигу осуществлена оценка размера ядер образцов КТ. Для QD-1 исследована температурная зависимость первого экситонного пика поглощения. Спектры ФЛ позволяют предположить, что полосы свечения формируются как экситонными переходами, так и дефектами кристаллической решётки ядра InP.
Синтезирован ряд структур нанопористого оксида алюминия, отожженного при различных температурах, с осаждёнными КТ и исследована их ФЛ. Показано, что после осаждения в AAО НК InP/ZnS, сохраняют свои флуоресцентные свойства, следовательно, можно говорить об успешном создании композитных люминофоров InP/ZnS@AAO. Обсуждаются цветовые характеристики изучаемых образцов. / This study deals with the investigation of optical characteristics of differently sized InP/ZnS colloidal quantum dots (QD-1, QD-2, QD-3) and composite nanostrucrures of anodic aluminum oxide with QDs by means of spectrophotometry and luminescence spectroscopy techniques.
The literature review concerning electronic states in an ideal nanocrystal (NC), synthesis of InP-based QDs, use of NCs for creating nanocomposites and calculating color characteristics of emitters was carried out. The methods of sample preparation and measurements of optical absorption (OA) and photoluminescence (PL) spectra are described.
Values of optical transition energies are determined according to the analysis of QD OA spectra. The bands with the lowest energy correspond to the first exciton absorption peak of the InP core. The other transitions can be attributed to the ZnS shell. The core size of the QD samples was evaluated using the blue shift. The temperature dependence of the first exciton absorption peak was investigated for the QD-1. PL spectra of QDs indicate that the emission bands are formed by exciton transitions and defects of the InP crystal lattice.
A series of structures of nanoporous aluminum oxide, annealed at different temperatures, with deposited QDs were synthesized and their PL were studied. Fluorescent properties of the QDs are found to be retained after the deposition, therefore, InP/ZnS@AAO composite phosphors were successfully created. Сolor characteristics of the samples under study are discussed.
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Preparation and Reactions of Compounds with Heavier Group 14 Elements in Low Oxidation States / Synthese und Reaktionen von Verbindungen der Schweren Elemente der 14. Gruppe in niedrigen OxidationsstufenSen, Sakya Singha 04 October 2010 (has links)
No description available.
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The degree of phosphorus saturation of agricultural soils in Brazil and Germany: New approaches for risk assessment of diffuse phosphorus losses and soil phosphorus managementFischer, Peter 30 November 2018 (has links)
Diffuse Phosphor (P)-Austräge aus der Landwirtschaft tragen zur Gewässereutrophierung bei. Der Phosphorsättigungsgrad (DPS) ist ein etablierter Parameter, um das P-Austragsrisiko aus Böden zu erfassen. Ein bodentypunabhängiger Ansatz, der die Abschätzung des DPS durch eine einfache Standardmethode wasserlöslichen P (WSP) ermöglicht (WSP-DPS-Ansatz), wurde an europäischen Böden entwickelt. In der Dissertation wurde dieser Ansatz erstmalig: i) an tropischen Böden getestet und ii) dazu verwendet P-Austragsrisiken von Boden-P-Monitoringdaten und von landwirtschaftlichen Institutionen empfohlenen P-Gehalten abzuleiten. Neben dem DPS wurde der Einfluss der in Brasilien gängigen anorganischen Oberflächendüngung auf das Austragsrisiko mittels Laboranalysen und Feldstudien erfasst. Die Bodentypunabhängigkeit des WSP-DPS-Ansatzes wurde für Böden Brasiliens bestätigt. Infrarotspektroskopische Analysen lieferten eine Erklärung für relativ niedrige gelöste P-Konzentrationen im Oberflächenabfluss von Oxisols. Pedotransferfunktionen zwischen WSP und Methoden, die in Brasilien und Deutschland zur Abschätzung pflanzenverfügbaren P verwendet werden, ermöglichten die Berechnung von DPS-Werten aus Monitoringdaten. Erste DPS-Karten zeigten relative geringe Austragsrisiken für das Untersuchungsgebiet in Brasilien und hohe Risiken für Deutschland, die teilweise durch unterschiedliche empfohlene Boden-P-Gehalte erklärbar waren. Um mit einer einfachen und kosteneffizienten Methode sowohl die landwirtschaftliche Produktion als auch den Gewässerschutz zu berücksichtigen, wurden die Wasser- und CaCl2-Methode zur Abschätzung von pflanzenverfügbarem P mit dem WSP-DPS-Ansatz kombiniert. Dieser Ansatz könnte helfen die Herausforderungen zu lösen mit denen die Menschheit in den nächsten Jahrzehnten bezüglich P in der Landwirtschaft konfrontiert sein wird: Einer effizienten Nutzung der limitierten Ressource P und dem Schutz der Gewässer vor diffusen P-Einträgen. / Diffuse phosphorus (P) losses from agriculture contribute to the eutrophication of surface waters. The degree of P saturation (DPS) is an established parameter for assessing the risk of P loss from agricultural soils. A soil type-independent approach for estimating the DPS by a simple standard method of water-soluble phosphorus (WSP; the WSP-DPS approach) was developed on European soils. In the thesis, the WSP-DPS approach was for the first time: i) tested on tropical soils and ii) used to derive P loss risks from soil P monitoring data and from recommended soil P levels by agricultural institutions. In addition to DPS, laboratory analyses and field studies were combined to assess the risk of P loss associated with the superficial application of inorganic fertilizer, which is commonly used in Brazil. The soil type-independency of the WSP-DPS approach was confirmed for soils of Brazil. Infrared spectroscopic analyses provided an explanation for the relatively low dissolved P concentrations in the surface runoff of Oxisols. Pedotransfer functions were determined between WSP and methods used to estimate plant-available P in Brazil and Germany and allowed for the transformation of soil P monitoring data into DPS values. The first DPS maps revealed relatively low P loss risks for the investigation area in Brazil and high risks for Germany. This difference was partly explainable by the recommended soil P levels in the two countries. To consider both agricultural production and the protection of surface waters in soil P management with a simple and cost-effective method, the soil test methods of using water and CaCl2 to estimate plant-available P and the WSP-DPS approach were combined. This approach could help to solve the challenges humanity faces regarding P in agriculture in the coming decades: An efficient use of the limited resource P and the protection of surface waters from diffuse P losses.
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Relationships between soil chemical properties and forest structure, productivity and floristic diversity along an altitudinal transect of moist tropical forest in Amazonia, Ecuador. / Beziehungen zwischen bodenchemischen Eigenschaften und Waldstruktur, Produktivität und floristischer Diversität tropischer Regenwälder Amazoniens entlang eines Höhengradienten in Ecuador.Unger, Malte Arne 30 April 2010 (has links)
No description available.
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Design of Phosphorus Centered Janus Head Ligands / Design phosphorzentrierter Januskopf-LigandenObjartel, Ina 31 October 2011 (has links)
No description available.
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Analyzing Transactions in Linked Value Chains of Wastewater Treatment and Crop ProductionMaaß, Oliver 12 July 2019 (has links)
In dieser Dissertation wird der Einfluss von Transaktionen zur Wiederverwendung von Nährstoffen und gereinigtem kommunalen Abwasser auf die Wertschöpfungsketten der Abwasserbehandlung und Pflanzenproduktion untersucht. Ziel ist es, Kosten und Nutzen sowie die Wertschöpfung von Transaktionen in verknüpften Wertschöpfungsketten der Abwasserbehandlung und Pflanzenproduktion zu analysieren. Darüber hinaus wird untersucht, wie Transaktionen und Interdependenzen zwischen Akteuren in verknüpften Wertschöpfungsketten die lokalen Governance-Strukturen für die Wiederverwendung von Abwasser beeinflussen. Die Analyse wird hauptsächlich durch das Wertschöpfungskettenkonzept, das Konzept der Kreislaufwirtschaft und die Theorie der Transaktionskostenökonomie geleitet. Mit verschiedenen Methoden, wie der Kosten-Nutzen-Analyse, der Wertschöpfungskettenanalyse und der Transaktionskostenanalyse, werden zwei Fallstudien in Deutschland untersucht: (1) die Fällung von Struvit (Magnesium-Ammonium-Phosphat) und dessen Verwendung als Dünger in Berlin-Brandenburg und (2) das Modell der landwirtschaftlichen Abwasserwiederverwendung in Braunschweig. Die Ergebnisse zeigen, dass die Wiederverwendung von Nährstoffen und Abwasser zu geringeren Kosten für die Abwasserbehandlung, höherer Rentabilität und Wertschöpfung in der Pflanzenproduktion und zu einem hohen Anteil an regionaler Wertschöpfung führen. Die Ergebnisse verdeutlichen aber auch, dass die Wiederverwendung von Abwasser zu Einschränkungen, Verdrängungseffekten und Veränderungen in der Verteilung der Wertschöpfung führen kann. Des Weiteren zeigen die Ergebnisse, dass differenzierte Governance-Strukturen erforderlich sind, um den unterschiedlichen Eigenschaften der Transaktionen zwischen Abwasserbehandlung und der Pflanzenproduktion gerecht zu werden. Interdependenzen zwischen Abwasseranbietern und Landwirten erhöhen den Bedarf an hybriden und hierarchischen Elementen in den Governance-Strukuren für die Wiederverwendung von Abwasser. / This dissertation explores the impact of transactions for reusing nutrients and treated municipal wastewater on the value chains of wastewater treatment and crop production. It aims to analyze what costs and benefits and what added-value can result from transactions in linked value chains of wastewater treatment and crop production. Furthermore, it aims to analyze how transactions and interdependences between actors in linked value chains shape the governance structures for reusing wastewater at the local level. The analysis is mainly guided by the value chain concept, the concept of the circular economy and the theory of transaction costs economics. Different methods including cost-benefit analysis, value chain analysis and transaction cost analysis are used to investigate two case studies located in Germany: (1) the precipitation of struvite (magnesium ammonium phosphate) in the wastewater treatment plant in Waßmannsdorf and its application as fertilizer in Berlin-Brandenburg, and (2) the agricultural wastewater reuse scheme of the Wastewater Association Braunschweig. The results show that transactions for reusing nutrients and wastewater result in the development of linked regional value chains with lower costs of wastewater treatment, higher profitability and added-value in crop production, and a high share of regional added-value. However, the results also highlight that the reuse of wastewater can lead to restrictions (e.g., cultivation bans on certain crops), crowding out effects and changes in the distribution of the added-value. Furthermore, the findings suggest that different governance structures are needed to match the different properties of the transactions between wastewater treatment and crop production. Interdependences resulting from transactions between wastewater providers and farmers increase the need for hybrid and hierarchical elements in the governance structures for reusing wastewater.
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Potential of purpose-specific fish feeds for aquaponics and circular multitrophic food production systemsShaw, Christopher 27 May 2024 (has links)
Durch die Nutzung des fischfutterbedingten Nährstoffeintrags für die kombinierte Produktion von Fischen und Pflanzen können Aquaponiksysteme eine nachhaltige Erweiterung von Kreislaufanlagen der Aquakultur (RAS) darstellen. Herkömmliche Aquakulturfutter zielen jedoch auf Fischproduktion mit geringer Umweltbelastung ab und sind somit nicht für die Aquaponik optimiert. Daher weist RAS-Wasser häufig Mängel im Profil gelöster anorganischer Pflanzennährstoffe auf. So war es Ziel dieser Arbeit, die Auswirkungen unterschiedlicher Proteinquellen auf die Nährstoffdynamik in RAS durch Fütterungsversuche mit Afrikanischem Raubwels und Nil-Tilapia zu untersuchen, bei denen Wachstum, gelöste anorganische Nährstoffkonzentrationen im RAS-Wasser und die Ausscheidung von Nährstoffen über den Kot verfolgt wurden. Der Fokus lag auf nachhaltigen alternativen Proteinquellen zu marinem Fischmehl und terrestrischen Pflanzenproteinen: Larvenmehl der Schwarzen Soldatenfliege (BSFM), Welsschlachtabfallmehl (CM), Geflügelschlachtabfallmehl (PM) und Geflügelblutmehl (PBM). Experimentalfutter, die phosphorreiches PM und CM enthielten, förderten erhöhte Ausscheidung von löslichem reaktivem Phosphor, erzeugten die besten gelösten N:P-Verhältnisse im RAS-Wasser verglichen mit einer Hydroponik-Nährlösung und ermöglichten in Kombination mit PBM besseres Wachstum beim Wels als ein vergleichbares kommerzielles Futter. In Futtern basierend auf einer einzigen Proteinquelle führte PM bei Wels und insbesondere bei Tilapia zu ähnlichem Wachstum verglichen mit marinem Fischmehl, wohingegen BSFM und PBM bei beiden Arten Wachstumsleistung beeinträchtigte. Meta-Analysen aller Versuche legen nahe, dass höherer Phosphor-, Kalium- und Magnesiumgehalt im Futter erhöhte Ausscheidung dieser Elemente in gelöster Form bedingt, was sie zu Zielnährstoffen in Aquaponikfuttern macht, während die Optimierung des Protein zu Energie-Verhältnisses im Futter die gelösten N:P- und N:K-Verhältnisse im RAS-Wasser verbessern kann. / By using the nutrient input from fish feeds for the combined production of fish and plants, aquaponic systems can be a sustainable extension of recirculating aquaculture systems (RAS). However, conventional aquaculture feeds are optimized for fish production and reduced environmental impact rather than aquaponics. Hence, RAS water is often characterized by deficiencies regarding its dissolved inorganic plant nutrient profile. Therefore, this thesis aimed to explore the effect of purposeful dietary protein choice on nutrient dynamics in RAS through four systematic feeding trials involving African catfish and Nile tilapia in which growth performance, dissolved inorganic nutrient concentrations in RAS water and solid fecal nutrient excretion were tracked. Focus was on sustainable alternative protein sources to marine fish meal and terrestrial plant proteins: black soldier fly larvae meal (BSFM), catfish by-product meal (CM), poultry by-product meal (PM) and poultry blood meal (PBM). Experimental diets including phosphorus-rich PM and CM supported increased excretion of soluble reactive phosphorus, produced the most favorable dissolved N:P ratios in RAS water when compared to a renowned hydroponic nutrient solution, and, combined with PBM, enabled better growth performance in African catfish than a comparable commercial diet. In single protein source diets, PM produced similar growth performance in African catfish and particularly Nile tilapia versus marine fish meal, whereas BSFM and PBM impaired growth performance in both species. Meta-analyses covering all trials suggest that higher dietary phosphorus, potassium and magnesium content leads to their increased excretion in dissolved form, making them target nutrients for aquaponic feed formulation, while the optimization of the dietary protein to energy ratio can further improve dissolved N:P and N:K ratios in RAS water.
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Das Auftreten acidophiler/calcifuger Pflanzenarten in Kalk-Halbtrockenrasen / The occurence of acidophile/calcifuge plant species in semi-arid calcareous grasslandsFühner, Christoph 02 November 2005 (has links)
No description available.
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