Investigation of surface exchange and bulk diffusion of oxygen in ceria-based redox materials with isotope exchange experiments utilizing 18O2 and C18O2

Der thematische Schwerpunkt dieser Arbeit liegt auf experimentellen Untersuchungen von Cerdioxid-basierten Redoxmaterialien, die zur zweistufigen solarthermochemischen Spaltung von Kohlendioxid eingesetzt werden. Wesentliche Detailfragen, vor allem zur Reaktionskinetik, zum genauen Einfluss von Dotierungselementen und zur makroskopischen und mikroskopischen Struktur des Redoxmaterials, sowie zur Zyklierstabilität müssen noch beantwortet werden, um eine Skalierung des Redoxprozesses in den großtechnischen Maßstab zu ermöglichen. Oberflächenaustauschreaktionen und die Diffusion von Sauerstoff in Cerdioxid sind hierbei von hoher Relevanz. Daher konzentriert sich diese Arbeit auf die Oberflächenreaktion mit Kohlendioxidreduktion und Sauerstoffaustausch, Sauerstoffdiffusion in Ceroxid-Proben, die phänomenologische Beschreibung (KO, DO), die Interpretation der berechneten Geschwindigkeitskonstanten, den Einfluss der Dotierelemente und den Einfluss des Sauerstoffpartialdrucks der Gasatmosphäre. Der direkteste Ansatz zur Bestimmung von Sauerstoff-Diffusionskoeffizienten sowie von Oberflächenaustauschkoeffizienten in Oxiden ist der Sauerstoffisotopenaustausch in Kombination mit Sekundärionen-Massenspektrometrie (SIMS). Mit der Verwendung von mit Sauerstoffisotopen angereicherten 18O2 - und C18O2 -Gasatmosphären sowie CeO2-δ und Ce0.9M0.1O2-δ (mit M = Y, Sm, Zr) Probenmaterial unter verschiedenen experimentellen Bedingungen stellt dieser Ansatz den Schwerpunkt dieser Arbeit dar. Die experimentellen Ergebnisse sind vielversprechend hinsichtlich der Kohlendioxidpaltung mit dreiwertig dotiertem Cerdioxid, insbesondere Sm-dotiertem Cerdioxid, bei niedrigeren Temperaturen, als bisher für diesen Reaktionsschritt vorgeschlagen wurden. Die Mehrzahl der Isotopenaustauschexperimente zeigte erhöhte Werte für DO und KO bei Sm-dotiertem Cerdioxid, im Vergleich zu Y-dotiertem und Zr-dotiertem Ceroxid, sowie nominell undotiertem Cerdioxid. Die berechneten Aktivierungsenergien sind für Sm-dotiertes Cerdioxid am niedrigsten. Sm-dotierte und Y-dotierte Cerdioxidproben tendieren dazu, das durch den Oberflächenaustausch kontrollierte kinetische Regime bei T ≥ 700 °C zu erreichen. Dies ist auf die Tatsache zurückzuführen, dass dreiwertige Dotierelemente die Konzentration an Sauerstoffleerstellen und damit die Diffusivität des Sauerstoffs im Vergleich zu den Zr-dotierten und nominell undotierten Cerdioxidproben erhöhen. In Anbetracht der Untersuchungen ist aufgrund der schwachen Temperaturabhängigkeit der Parameter KO und DO eine Temperatur von T = 500 °C für solarthermochemische Kreisprozesse realisierbar. Bei konstanten Reaktionsbedingungen sind hierfür höhere Reduktionsgrade δ erforderlich. Eine Untersuchung der optimalen Sauerstoffleerstellenkonzentration des Redoxmaterials mit systematischer Variation der Sauerstoff-Substöchiometrie im Reduktionsschritt wird vorgeschlagen.The focus of this thesis is the experimental investigation of ceria-based redox materials, which are associated to two-step solar thermochemical splitting of carbon dioxide. Important detailed questions regarding the reaction kinetics, the exact influence of doping elements and the macroscopic and microscopic structure of the redox material as well as the cyclic stability have to be answered in order to achieve a scaling of the redox process to the technical scale. Surface exchange reactions and bulk transport of oxygen in ceria are of high relevance. Therefore, this work focusses on the surface reaction with carbon dioxide reduction and oxygen exchange, oxygen diffusion in bulk ceria samples, the phenomenological description (KO, DO), the interpretation of the calculated rate constants, the influence of the doping elements and the influence of the oxygen partial pressure of the gas atmosphere. The most direct method of measuring oxygen diffusivities and surface exchange coefficients in oxides is oxygen isotope exchange followed by Secondary Ion Mass Spectrometry (SIMS). By utilizing oxygen isotope enriched 18O2 and C18O2 gas atmospheres as well as CeO2-δ and Ce0.9M0.1O2-δ (with M = Y, Sm, Zr) samples under various experimental conditions this well-established approach represents the core of the experimental work of this thesis. The experimental results are promising in terms of CO2 splitting with trivalent-doped ceria, especially Sm-doped ceria, at lower temperatures than the proposed conditions for this reaction step. The majorities of isotope exchange experiments show enhanced values of DO and KO for Sm-doped cerium dioxide, in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. For the determinable apparent activation energies, the values for Sm-doped ceria are lowest. Sm-doped and Y-doped ceria samples tend to reach the surface exchange controlled kinetic regime at T ≥ 700 °C. This is attributed to the fact that the trivalent dopant increases the concentration of oxygen vacancies, and hence the diffusivity of oxygen in comparison to the Zr-doped ceria and nominally undoped ceria samples. Considering the investigations, a temperature of T = 500 °C is feasible for solar thermochemical cycling due to the weak temperature dependency of the parameters KO and DO . Higher reduction extents δ are therefore necessary, given constant reaction conditions. An investigation of the optimal oxygen vacancy concentration of the redox material with systematic variation of the oxygen non-stoichiometry in the reduction step is suggested

Transportní a optické vlastnosti monokrystalů CdTe/CdZnTe

Název práce: Transportní a optické vlastnosti monokrystalů CdTe/CdZnTe Autor: Štěpán Uxa Katedra: Fyzikální ústav UK Vedoucí disertační práce: doc. Ing. Eduard Belas, CSc., Fyzikální ústav UK Abstrakt: Práce je zaměřena na studium vnitřního elektrického pole a teplotní závislosti absorpční hrany vzorků CdTe a CdZnTe. V první části práce je studováno elektrické pole rovinných detektorů záření pomocí metody přechodných proudů (TCT). Je předsta- vena ucelená teorie propojující TCT měření s měřeními účinnosti sběru náboje (CCE) studovaného detektoru, vedoucí ve výsledku k vyvinutí dvou nových iteračních metod pro zpracování experimentálních dat. Tyto metody mohou být použity vždy, kdy lze vnitřní elektrické pole v detektoru aproximovat lineárních profilem. V druhé části práce jsou před- stavena vysokoteplotní měření propustnosti tenkých vzorků CdTe, z nichž je vyhodnocena teplotní závislost šířky zakázaného pásu. Je to zároveň poprvé, kdy byla taková měření provedena ve zvýšeném tlaku kadmiových par, což významně snižuje sublimaci vzorků během měření. Klíčová slova: CdTe, metoda přechodných proudů, elektrické pole, absorpční hrana, šířka zakázaného pásuTitle: Transport and optical properties of CdTe/CdZnTe single crystals Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Eduard Belas, PhD, Institute of Physics of Charles University Abstract: The thesis is focused on a study of internal electric field and temperature dependence of the absorption edge of CdTe and CdZnTe samples. In the first part of the thesis the transient-current technique (TCT) is used for investigations of electric fields within planar radiation detectors. The original comprehensive theory that links together TCT measurements with measurements of detector's charge-collection efficiency (CCE) is presented. This approach results in the development of two new iterative methods for processing of experimental data which can be used in any situation when the internal electric field can be approximated by a linear profile. In the second part of the thesis high temperature measurements of transmittance of thin polished CdTe samples are presented, leading to the estimation of the temperature dependence of the bandgap energy from calculated spectra of absorption coefficient. This is for the first time when measurements in a Cd overpressure have been performed which significantly reduces sample sublimation. Keywords: CdTe, transient-current technique,...Fyzikální ústav UKInstitute of Physics of Charles UniversityMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Comparison of Individual Methods of Valuation of Apartment-type Real Estate in the Locality of Plzeň - Bolevec

Diplomová práce analyzuje vybrané používané metody ocenění bytů a jejich vzájemné srovnání. Dále práce definuje základní pojmy a zásady, s kterými problematika ocenění bytů souvisí. Oceňované byty jsou v Plzni v městské části Bolevec. Byly vybrány byty s různým dispozičním řešením.This dissertation analyse selected used methods of appraising of flats and their reciprocal comparing. Then the dissertation defines basic concepts and percepts, with them the matters of appraising of flats hang together. Priceds flats thein are in Pilsen in the in area Bolevec. There were selected flats with differeent layout.

Minulé a současné charakteristiky a formy vázané na permafrost a činnou vrstvu jako indikátory pozdně kvartérních změn přírodního prostředí

Late Quaternary has seen numerous major permafrost expansions and retreats associated with alternating glacial and interglacial periods as well as stadials and interstadials, the research of which is necessary to understand the past environmental evolution, but also provides useful analogues for its present-day and future behaviour. How- ever, observations of permafrost and active-layer phenomena are still limited, and sometimes misleading, even in many present-day permafrost regions, and naturally less comprehensive evidence is available from areas where permafrost existed in the past. The thesis provides comprehensive information on the distribution and morphology of mostly relict patterned ground and rock glaciers in the High Sudetes Mts. and in the Western and High Tatra Mts., respectively, which are the most widespread permafrost features that occur in these Central European mountain ranges situated north of the Alps. It shows that the landforms are closely related to increased severity of climates and/or sparser vegetation at higher elevations and as such they attest to the environmental conditions, which prevailed there towards the end of the Last Glacial Period to the early Holocene, but also to their current states. Similar elevation trends in the pattern morphology are also documented for...V pozdním kvartéru došlo v d̊usledku sťrídání glaciálních a interglaciálních období i stadiál̊u a interstadiál̊u k ̌cet- ným rozší̌rením a ústup̊um permafrostu, jehož výzkum je nezbytný pro pochopení vývoje p̌rírodního prosťredí v minulosti, ale poskytuje cenné informace i z hlediska jeho soǔcasné a budoucí dynamiky. Pozorování charak- teristik a forem vázaných na permafrost a ̌cinnou vrstvu jsou však stále nedostatěcná a ňekdy také zaváďející i v mnoha oblastech se soǔcasným výskytem permafrostu a mnohem méňe informací je k dispozici z region̊u, kde se permafrost nacházel v minulosti. Disertǎcní práce poskytuje ucelené informace o rozší̌rení a morfologii p̌revážňe reliktních strukturních p̊ud a kamenných ledovc̊u ve Vysokých Sudetech a Západních a Vysokých Tatrách, jež jsou nejrozší̌reňejšími formami vázanými na permafrost, které se v ťechto sťredoevropských pohǒrích severňe od Alp vyskytují. Je ukázáno, že tyto tvary reliéfu mají ťesnou vazbu na zvyšující se drsnost klimatických podmínek a ubývání vegetace sm̌erem do vyšších nadmǒrských výšek a jako takové sv̌eďcí o p̌rírodních podmínkách, které zde panovaly ke konci posled- ního glaciálu a na pǒcátku holocénu, jakož i o jejich soǔcasném stavu. Obdobné výškové trendy v morfologii struk- turních p̊ud jsou dokumentovány také pro...Department of Physical Geography and GeoecologyKatedra fyzické geografie a geoekologiePřírodovědecká fakultaFaculty of Scienc

Magnetooptické vlastnosti polovodičových kvantových struktur

Název práce: Magnetooptické vlastnosti polovodičových kvantových struktur Autor: Štěpán Uxa Katedra: Fyzikální ústav Univerzity Karlovy Vedoucí diplomové práce: doc. RNDr. Roman Grill, CSc. E-mail vedoucího: [email protected] Abstrakt: V této teoretické práci je představena podrobná studie optických vlastností excitonů v dvojitých kvantových jamách umístěných v magnetickém a electrickém poli. Vycházejíce ze známých Luttingerových vztahů popisujících reálnou strukturu valečního pásu polovodičových sloučenin typu III-V, rozvinuli jsme efektivní způsob řešení Schrödin- gerovy rovnice pro coulombicky vázaný pár elektron-díra tvořící exciton. Odvozené vzta- hy byly ilustrovány na odpovídajících obrázcích, dávajíce nám možnost lépe porozumět efektům vnějších polí na studovaný systém. Byly spočteny disperzní relace, posun energe- tických hladin v elektrickém poli, absorpční a luminiscenční spektra i rozložení nábojové hustoty, a dosažené výsledky byly podrobně diskutovány. Klíčová slova: exciton, dvojitá kvantová jáma, vázané kvantové jámy, Luttingerův (-Kohnův) hamiltonián, míchání stavů ve valenčním pásuTitle: Magnetooptical Properties of Semiconductor Quantum Structures Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Roman Grill, PhD Supervisor's e-mail address: [email protected] Abstract: In this theoretical work, a detailed study of optical properties of excitons in double quantum wells subject to magnetic and electric fields is presented. Starting from the well-known Luttinger formulae describing the real valence-band structure of III-V semi- conductor compounds, we developed an efficient way to solve the Schrödinger equation of a coulombically-bound pair electron-hole forming an exciton. Derived formulae were illus- trated on relevant figures, giving us an opportunity to better understand the effects of external fields on the studied system. Dispersion relations, the shift of energy levels in an electric field, absorption and photoluminescence spectra, and charge density distribution were calculated and achieved results were discussed in detail. Keywords: exciton, double quantum well, coupled quantum wells, Luttinger (-Kohn) hamiltonian, valence-subband mixingFyzikální ústav UKInstitute of Physics of Charles UniversityFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Lithium-ion diffusion in near-stoichiometric polycrystalline and monocrystalline LiCoO2

Lithium-metal-oxide-based cathode materials like LiCoO2 are an essential part of lithium-ion batteries, which are intensively researched and continuously improved. For a basic understanding of kinetic processes that control lithium incorporation and removal into/from electrodes, the lithium-ion transport in the cathode material is of high relevance. This concerns lithium diffusivities, as well as defect structures, transport mechanisms, and confined diffusion paths, such as grain boundaries. In the present study, lithium tracer self-diffusion is investigated by means of isotope exchange and secondary ion mass spectrometry in polycrystalline sintered bulk samples of stoichiometric LiCoO2 with an average grain size of about 70 nm and in single crystalline LiCoO2 in the ab-plane and c-axis in the temperature range between 200 and 700 °C. For the polycrystals, we found an activation enthalpy of ΔH = 0.75 eV. In the single crystal, the lithium-ion diffusivities along the ab-plane are identical to the diffusivities in polycrystalline LiCoO2. This indicates that diffusion along grain boundaries is similar to bulk diffusion and does not play a dominating role for the overall lithium-ion migration. Along the c-axis, diffusivities are some orders of magnitude lower, but only a slightly higher activation energy of 0.94 eV is found. This provides the experimental evidence of the often-claimed sluggish lithium diffusion along the c-axis. We suggest that lithium diffusion along the c-axis is most likely determined by fast diffusion in the ab-plane and a slow transfer of lithium ions across the CoO2 layers

Synthesis, powder-metallurgical production and lithium tracer diffusion in LiNi0.33Mn0.33Co0.33O2 cathode material for lithium-ion batteries

The Li(Ni0.33Mn0.33Co0.33)O2 (NMC333) compound is one of the most promising cathode materials for Li-ion batteries. In the following, the solid-state reaction in a two-step synthesis scheme of this material is examined, as well as the powder-metallurgical production of sintered bulk samples. Long range Li tracer self-diffusion is investigated in polycrystalline sintered bulk NMC333 samples with an average grain size of about 50 nm. For analysis, stable 6Li tracers are used in combination with secondary ion mass spectrometry. At a temperature of T = 300 °C the determined Li diffusivity yields DLi = 4.9 ∙ 10-16 m2/s

Coadministration of Atorvastatin Prevents Nitroglycerin-Induced Endothelial Dysfunction and Nitrate Tolerance in Healthy Humans

ObjectivesWe aimed to assess whether concurrent administration of atorvastatin would modify the development of tolerance and endothelial dysfunction associated with sustained nitroglycerin (GTN) therapy in humans.BackgroundAnimal studies have demonstrated that administration of 3-hydroxy-3 methylglutaryl coenzyme A reductase inhibitors can protect against GTN-induced endothelial dysfunction and tolerance, likely through an antioxidant mechanism.MethodsThirty-six healthy male volunteers were randomized to receive continuous transdermal GTN (0.6 mg/h) and placebo, atorvastatin (80 mg/day) alone, or continuous transdermal GTN (0.6 mg/h) with concurrent atorvastatin (80 mg/day), all for 7 days. On the second visit, forearm blood flow was measured with venous-occlusion strain gauge plethysmography in response to incremental infusions of acetylcholine (7.5, 15, and 30 μg/min). Acetylcholine infusions were coinfused first with saline, and repeated during the coinfusion of vitamin C (24 mg/min). Blood pressure responses to sublingual GTN (400 μg) were assessed on both visits.ResultsAcetylcholine responses in the GTN plus placebo group were significantly attenuated versus those in the GTN plus atorvastatin and atorvastatin groups (p < 0.01). Coinfusion of vitamin C completely restored acetylcholine responses in the GTN plus placebo group (p < 0.01 vs. saline coinfusion), but caused no change in either the atorvastatin or the GTN plus atorvastatin groups. Blood pressure responses to sublingual GTN did not significantly change between visits in subjects receiving GTN plus atorvastatin and atorvastatin alone, but were significantly blunted in the GTN plus placebo group (p < 0.05).ConclusionsThe present findings demonstrate, for the first time in humans, that atorvastatin prevents both GTN-induced endothelial dysfunction and nitrate tolerance, likely by counteracting the GTN-induced increase in oxidative stress

Impact of a quality improvement intervention on neonatal mortality in a regional hospital in Burkina Faso

The neonatal period is the most vulnerable time in terms of a child's survival, with mortality during this period accounting for approximately half of the deaths before the age of 5 years. The Neonatal Essential Survival Technology (NEST) project is a program aiming to reduce mortality by improving the quality of neonatal care in sub-Saharan Africa. This study presents the evaluation of the first phase of the NEST intervention program at Saint Camille Hospital Ouagadougou (HOSCO), Burkina Faso, in terms of the reduction in neonatal mortality.This is a retrospective analysis, based on "pre-intervention" data collected in 2015, and "post-intervention" data collected in 2018, including all infants admitted to the neonatal unit of HOSCO. The intervention period (2016 and 2017) comprised a structured quality improvement process conducted by a multidisciplinary working group that focused on improving infrastructure, equipment, training and use of clinical protocols, team working within the neonatal unit and with other hospital departments, and communication with referring healthcare facilities. Mortality data were compared pre- vs. post-intervention using a logistic regression model.The analysis included 1427 infants in the pre-intervention period, and 819 post-intervention. In both time periods, more than 75% of admissions were infants with low birth weight, and nearly 50% were very low birth weight. Post-intervention, while there was a decrease in overall admission, the proportion of multiple births increased from 20% to 24% (The first phase of the NEST quality improvement program was associated with a decrease in mortality in outborn infants admitted to the neonatal unit at HOSCO. Long-term assessment is expected to provide a more comprehensive evaluation of the program in a low-income setting

New methods to reconstruct clast transport history in different glacial sedimentary environments: Case study for Old Red sandstone clasts from polythermal Hrbyebreen and Bertilbreen valley glaciers, Central Svalbard

The objective of this study were Little Ice Age (LIA) to recent subglacial tills, glaciofluvial sediments of proglacial sandur, esker sediments and sediments of moraine-mound complexes of Hrbyebreen polythermal valley glacier and LIA to recent sediments of lateral moraine, frontal moraine, ice-cored moraine and glaciofluvial sediments of proglacial sandur of Bertilbreen polythermal valley glacier. Fossil (probably early Holocene) subglacial and supraglacial tills and sediments of coarse-grained glaciomarginal delta of Bertilbreen have also been studied. The research focused on Old Red sandstone clasts, for which roundness, shape and striation presence have been investigated. The results from this research led to the proposal of new methodological approaches. It is mainly the covariant plot of striations and RA (the share of striated clasts versus the share of very angular and angular clasts), which effectively differentiate subglacial tills from glaciofluvial sediments and allows for identification of the source material of moraine-mound complexes, especially the oldest, more degraded parts near the maximum LIA glacier extent. The second is the covariant plot of distance and RS index (the distance from the starting point versus the share of subangular and subrounded clasts) to present downstream roundness trends in proglacial glaciofluvial sediments. The research confirmed the crucial role of lithological properties of thinly bedded rocks on the shape of these rock clasts in sediment and a minimum impact of passive and active transport on the clast shape modification