Dissertations / Theses on the topic 'Quenching'

The main objective of the present work was to investigate the opportunity for the use of light hydrocarbons as quenching agents to quench high temperature gas streams. When light hydrocarbon, for example propane, is introduced into a hot gas, for example synthesis gas, the cooling of hot gas will occur both by dilution and by endothermic reactions of hydrocarbon. Thus, the hot gas may be quenched to a lower temperature. At the same time, thermal energy present in the hot gas may be recovered in the form of desired products that are produced during hydrocarbon cracking. This type of quenching is called for chemical quenching. A proof-of-concept study would be performed, but was not completed because of problems with a realization of experimental conditions and problems with equipment.Methane pyrolysis experiments were performed at high temperatures 1400 and 1450 °C. Short residence times and high dilution with hydrogen were applied in order to minimize coke formation. Methane conversions, selectivities and yields of products for different experimental conditions were calculated from the gas chromatographic analysis. The aim of pyrolysis experiments was to validate that the experimental set-up Pyrolyserigg works properly. This was done by comparison of experimental results with results that were obtained before on the same set-up and by comparison with a literature data. After the work of Pyrolyserigg was verified, it was possible to modify existing cooling/quenching system. A new quencher part that gives opportunity to introduce a cold gas into hot gas, heated by high temperature furnace, was designed. The quencher part was designed in a way that the temperature of hot gas entering the quencher and temperature of cooled gas mixture leaving the quencher could be measured. Thus, quenching effect (temperature drop) could be measured.Three types of experiments were performed after modification of cooling system: experiments with only hot inert gas (N2), experiments with a hot nitrogen gas «quenched» by methane, and hot nitrogen gas quenched by propane. Hot gas temperature and gas flows were varying in order to study the modified system's behaviour. Experiments with a hot nitrogen showed that there is a limit for maximum obtainable temperature inside the quencher, 1195 – 795 °C, for the given quencher design. A high temperature gradients between measurements points were registered that may be an indication of high heat losses. The temperature gradient increases with increasing gas temperature and decreases with increasing gas flow.Quenching effect (temperature drop) was measured in quenching experiments with propane, and expected temperature drop was calculated for the applied experimental conditions. Low measured temperature drop in quenching experiments indicates poor mixing of hot and cold gases. Quenching effect of propane introduction increases with increasing hot gas temperature because heat consumption by endothermic reactions increases, that is a consequence of increasing propane conversion.A product mixture from quenching experiments with propane was analysed, and conversion of propane, yield and selectivities to products were calculated. Ethylene and propylene are considered to be most valuable products. Ethylene yield increases with conversion and is around 37 % at 100 % conversion. Propylene yield goes through a maximum at 55 % conversion and approaches zero at 100 % conversion. The main identified problems for the proof of concept study are a poor gas mixing, high temperature gradients through the quencher and high heat loss from the quencher part. In addition, problems with connection ceram tube – quencher at high temperature have occurred.

Nonlinear luminescence quenching has been documented in numerous systems such as organic crystals, rare earth insulators, laser materials, semiconductors, and phosphors. In each of these systems nonlinear luminescence quenching occurs under conditions of high excitation density from interactions between excited centers, the result is an additional nonradiative decay pathway that lowers luminescence quantum efficiency. During investigations into the spectra and dynamics of Eu2O3, an apparent saturation dip in the excitation spectra of nano-sized Eu2O3 particles was observed. This thesis describes the investigation into the nature of the saturation effect. The samples studied using luminescent spectroscopy included micron sized Eu2O3 crystals of both cubic and monoclinic phases, nanocrystal monoclinic Eu2O3, and a large fused crystal of monoclinic Eu2O3. It was determined that the saturation effect was due to nonlinear luminescence quenching occurring at the wavelengths of absorption maxima. The mechanism of nonlinear luminescence quenching was concluded to be upconversion by energy transfer.
Master of Science

Heat treatment of different steel products have existed for thousands of years. It has always been an important tool to get the microstructure and resulting properties such as hardness and case hardness and it is even more important today than ever before. This project concentrated on the quenching process and means to decrease the distortion caused by this process. The effect of different oils, temperatures, agitation and if gas quenching could give better results were investigated. The results showed that Miller´s 75 quench oil was better than Park´s 420 at slow agitation and that the viscosity of the oils influenced how much changes in agitation speed and oil temperature affected the distortion. It also shows that gas quenching is an alternative to oil quenching if the microstructure can be improved. Otherwise using Miller´s 75 with low agitation in the Surface combustion furnace will give best results.

GaN is a III-V semiconductor that is a promising material used in production of light emitting devices and high power/high frequency electronics. The electronic and optical properties of GaN are subdued by defects that occur during the growth processes of this material. The emitted photoluminescence (PL) from optically excited GaN gives insight into the origins and effects of point defects within the crystal lattice structure of GaN. In this study, PL spectroscopy is used to examine and analyze the point defects that occur in Zn-doped GaN. The blue luminescence band seen in undoped and Zn-doped GaN have identical fine structure and properties. This band is attributed to a ZnGa acceptor. In Zn-doped, the PL intensity quenches abruptly at certain temperatures, which increase with increasing excitation intensity. This behavior is different from the PL quenching in undoped GaN. The PL behavior was simulated with a phenomenological model based on rate equations. A program created with mathematical modeling software, in conjunction with the basic rate equations, was used to explain the unusual behavior of the abrupt thermal quenching observed in Zn-doped GaN.

Thesis (M.S.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Fire Protection Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

This thesis explores the properties of galaxies that reside in high density regions in the local Universe. The driving motivation is to shed light on how galaxies stop forming stars and the role played by the environment in this process. A wide variety of environmentallyrelated quenching mechanisms have been proposed and the smoking gun proving that these are actually transforming galaxies from being star forming to passive is the existence of a population of galaxies in transition with intermediate features. In this Thesis I will mainly focus on the properties of two population of galaxies: those that are forming stars at a reduced level compared to other galaxies at similar mass, and those that have recently and abruptly interrupted their star formation. The data exploited in this Thesis come from surveys which focus on different environments in the local Universe: the WIde-field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. 2006) with its recent extension OmegaWINGS (Gullieuszik et al., 2015a, Moretti et al. submitted) and the Padova Millennium Galaxy and Group catalogue (PM2GC, Calvi et al. 2011). The Thesis starts with a characterization of the WINGS survey and then focuses on the new OmegaWINGS data, illustrating the most important aspects of the photometric and spectroscopic observations. I will describe in detail my personal contribution to the survey, from the data analysis to the redshift and velocity dispersion measurements, to the membership assignments. I will also present the complete galaxy catalog built from the combination of the two cluster surveys considered. While halo mass estimates are quite easily obtained for clusters, at the group regime it is less straightforward to derive robust measurements. I will then present a procedure I developed to compute halo masses from observable quantities for the PM2GC. My approach exploits mock galaxy catalogs extracted from the De Lucia & Blaizot (2007) semi-analytic model, run on the Millennium Simulation (Springel et al. 2005). Subsequently, I will discuss the methods adopted to compute the galaxy stellar properties, exploiting both spectroscopic and photometric information. Using the data from the WINGS+OmegaWINGS surveys, I will present an analysis of the ongoing Star Formation Rate (SFR) and the Specific Star Formation Rate (SSFR) - stellar mass (M∗) relations in field and cluster mass limited samples. I contrast trends in the different environments and, in clusters, at different clustercentric distances. The main result is that in clusters a population of galaxies with a reduced SFR is detected, named transition galaxies, which is much rarer in the field. The spatial location of these galaxies, together with the analysis of the star formation histories, colors and average ages, suggest that transition galaxies have had a reduced SFR for the past 2-5 Gyr. This is compatible with a strangulation scenario, even if other processes like ram pressure stripping can not be excluded. I characterize the properties of post starburst (PSB) galaxies, which are galaxies that abruptly interrupted their star formation sometimes during the past < 1.5 Gyr and present recognizable features in their spectra (no emission and Hδ in absorption). Exploiting an observed magnitude limited sample drawn from WINGS+OmegaWINGS, I will present the first complete characterization of PSBs in clusters and contrast their properties to those of passive (PAS) and emission line (EML) galaxies. The main finding is that the incidence of PSBs increases from the outskirts toward the cluster centers and, more gently, from the least toward the most luminous and massive clusters. PSBs have stellar masses, magnitudes, colors and morphologies intermediate between PAS and EML galaxies, typical of a population that has recently become passive. The phase-space analysis and the velocity dispersion profiles also suggest that PSBs represent a combination of galaxies with different accretion histories. In particular, PSBs with the strongest Hδ are consistent with being recently accreted. This analysis suggests that as a galaxy is accreted onto a cluster and approaches the cluster virialized region, ram pressure stripping (or other interactions) induces either a burst of the star formation with a subsequent fast quenching, or simply a fast quenching and this is more efficient in more massive clusters. I then characterize the PSB population also in smaller systems. Combining the WINGS+OmegaWINGS and PM2GC data, I will show how the fraction of PSB galaxies and the quenching efficiency depend on the halo mass, and increase from single galaxies, to binaries, groups and clusters. In the different environments, different physical mechanisms are probably responsible for the production of PSB galaxies, but all of them produce a truncation of star formation on short time-scales. If ram pressure stripping is the most likely candidate in clusters, gravitational interaction between galaxies could be more efficient in lower density regions. To conclude, the role of environment is undisputed in driving the galaxy evolution. In this Thesis I identified two possible evolutionary pathways related to the end of star formation in clusters: one requires a gradual reduction of star formation (long time-scale quenching t>2Gyr), the other a rapid truncation of the star formation activity (t∼0.1Gyr). Comparing the fraction of PSBs to the fraction of galaxies in transition, it appears that the short timescale star-formation quenching channel contributes two times more than the long timescale one to the growth of the passive population in clusters. The emerging picture is that ram pressure stripping is probably the most successful process affecting star formation in clusters, followed by strangulation. Other mechanisms might play a role, even though they most likely take place in lower density environments.
L’obiettivo del lavoro presentato in questa tesi è la caratterizzazione osservativa delle proprietà delle galassie, per vincolarne gli scenari di formazione ed evoluzione. In particolare, mi sono concentrata nel determinare come l’ambiente in cui evolvono le galassie influisca sul processo di formazione stellare nelle regioni ad alta densità nell’Universo locale. Tra i diversi meccanismi proposti come responsabili del quenching, ovvero la tendenza delle galassie a smettere di produrre stelle, quelli relativi all’ambiente sembrano svolgere un ruolo cruciale. La prova che confermerebbe l’azione primaria di questi meccanismi dovrebbe essere l’esistenza, in ammassi e gruppi di galassie, di una popolazione di galassie in transizione con caratteristiche intermedie. Con lo scopo dunque di identificare questa nuova popolazione, mi concentrerò sullo studio delle proprietà di due particolari classi di galassie: quelle che stanno formando stelle ad un livello ridotto rispetto ad altre galassie di massa simile, e quelle che hanno recentemente, e molto probabilmente bruscamente, interrotto la loro formazione stellare. Utilizzerò i dati provenienti da diverse campagne osservative: la WIde-field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. 2006) con la sua recente estensione OmegaWINGS (Gullieuszik et al., 2015a, Moretti et al. 2017), e il Padova Millennium Galaxy and Group catalogue (PM2GC, Calvi et al. 2011). La prima parte della tesi è riservata alla descrizione delle survey WINGS e OmegaWINGS. Particolare attenzione sarà dedicata alla presentazione degli aspetti più importanti delle osservazioni fotometriche e spettroscopiche di OmegaWINGS, in cui sono stata personalmente coinvolta. Descriverò in dettaglio il mio contributo alla survey, che va dall’analisi dei dati alle misurazioni di redshift e dispersione di velocità, all’individuazione delle galassie appartenenti agli ammassi. Presenterò dunque il catalogo completo costruito dalla combinazione delle due survey considerate. Mentre è relativamente facile ricavare una stima della massa di alone per gli ammassi, e più complicato ottenere misurazioni affidabili per i gruppi. Ho quindi sviluppato una procedura per derivare le masse di alone per i dati PM2GC da quantità osservabili, che descriverò in dettaglio. Tale procedura utilizza cataloghi di galassie estratti dai modelli semi analitici di De Lucia & Blaizot (2007), applicati alla Millennium Simulation (Springel et al. 2005). Successivamente, descriverò i metodi adottati per il calcolo delle propriet`a delle galassie, sfruttando le informazioni provenienti da osservazioni spettroscopiche e fotometriche. Utilizzando i dati WINGS+OmegaWINGS, discuterò l’analisi delle relazioni tra il tasso di formazione stellare (SFR), il tasso specifico di formazione stellare (SSFR) e la massa stellare di una galassia (M∗), nel campo e negli ammassi, in campioni limitati in massa. Confronterò gli andamenti nei diversi ambienti e, negli ammassi, a diverse distanze dal centro. Il risultato principale è l’individuazione di una popolazione di galassie in ammasso con SFR ridotta, denominate galassie in transizione, che è molto più rara nel campo. La distribuzione spaziale di queste galassie, insieme all’analisi della storia di formazione stellare, dei colori ed età medie, suggeriscono che le galassie in transizione abbiano avuto un SFR ridotta per circa 2-5 Gyr. Questo è compatibile con uno scenario di “strangulation”, anche se altri processi come la ram pressure stripping non possono essere esclusi. Successivamente caratterizzerò le proprietà delle galassie cosiddette post starburst (PSB), cioè galassie che hanno bruscamente interrotto la loro formazione stellare all’incirca 1 miliardo di anni fa e che presentano caratteristiche ben riconoscibili nei loro spettri (nessuna emissione e Hδ in assorbimento). Sfruttando un campione limitato in magnitudine apparente estratto dai dati WINGS + OmegaWINGS, presenterò la prima caratterizzazione completa di galassie PSB in ammasso e confronterò le loro proprietà con quelle di galassie passive (PAS) e con righe di emissione (EML). Il principale risultato riguarda il numero relativo di galassie PSB, che aumenta leggermente dalla periferia verso il centro degli ammassi e dall’ammasso meno luminoso/massiccio a quello più luminoso/massiccio. Le galassie PSB hanno proprietà, quali masse stellari, magnitudini, colori e morfologie, intermedie tra le PAS e EML, tipiche di una popolazione che è recentemente diventata passiva. L’analisi dello spazio delle fasi e dei profili di dispersione di velocità indicano anche che le PSB rappresentano una combinazione di galassie con diverse storie di accrescimento. In particolare, PSB con forte Hδ sono consistenti con l’essere state recentemente accresciute. Questa analisi suggerisce che, nel processo di accrescimento di una galassia su un ammasso, all’avvicinarsi alla regione virializzata, per effetto della ram pressure stripping (o di altre interazioni) viene indotto un rapido quenching, preceduto o meno da un forte episodio di formazione stellare; inoltre quest’effetto è più forte in ammassi più massicci. Descriverò successivamente la popolazione di PSB in sistemi più piccoli. Combinando i dati WINGS+OmegaWINGS ai dati PM2GC, mostrerò come la frazione di galassie PSB e l’efficienza del quenching dipendano dalla massa dell’alone e aumentino andando da galassie singole, a sistemi binari, gruppi e ammassi. Nei diversi ambienti, diversi meccanismi fisici sono probabilmente responsabili per la produzione di galassie PSB, ma tutti producono un troncamento della formazione stellare su brevi scale temporali. Mentre negli ammassi la ram presssure stripping sembra essere il candidato più probabile, l’interazione gravitazionale tra galassie potrebbe essere più efficiente nelle regioni a bassa densità. Dal confronto tra la frazione di PSB e di galassie in transizione, si può dedurre che il canale di quenching più rapido constribuisce circa due volte di più alla crescita della popolazione di galassie passive rispetto al canale di quenching più lento. Il quadro che emerge è che la ram pressure stripping è probabilmente il processo che maggiormente incide sul quenching della formazione stellare negli ammasi, seguito dalla strangulation. Altri meccanismi potrebbero influire, anche se probabilmente con effetto maggiore in ambienti a minore densità.

The aim of this thesis was to investigate what the relation is between as-quench hardness and final surface hardness for steel beams is, depending on what tempering temperature is used. Also explain how chemistry, dimension and microstructure effects the final mechanical properties of the front axle beam. For this a review of literature concerning the effects was completed.Hardness measurement on the surface was performed on the ends of the beam (bottom and top). This hardness measurement was performed on 6 different front axle articles of the same material (41CrS4) and 2 different front axle articles of another material (40CrMo4). The relation diagram gives an estimation of what type of tempering temperature is needed to achieve the final hardness that is desired. Because the relation was done with some inconsistences it can’t be said to give a perfect answer. The relation diagrams only work for material 41CrS4 and 40CrMo4. For the core hardness test, 2 articles of 41CrS4 and one article of 40CrMo4 was measured on 5 different position on the cross-section, the beams for the respective articles were taken from quenched state and quenched+tempered. The beam dimensions have a significant effect when it comes to cooling down the part and achieve as close to uniform hardness as possible. Even though the Middle point of the I-section sample is one of the closest cores to the surface, it has a softer core compared with the other cores. While there exists hardness difference after quenching between different points in the core they even out after tempering. When comparing the core hardness with the surface hardness it can be said that the surface hardness is not as hard as the core because of decarburization. The microstructure analysis was done on 2 articles of 41CrS4 and one article of 40CrMo4. Samples from the 3 articles is taken from both the as-quenched state and quenched+tempered state. From the optical microscope it could be seen, that the surface of the beams decarbonizes leading to a higher amount of ferrite at the structure and softer surface. Because of this 15 mm into the material is harder than at-surface. Decarburization of the 41CrS4 steels made it so that what should have been a martensite and bainite dominated surface became a ferrite and bainite dominated.To decide the actual amount of retained austenite in the sample an XRD-analysis was performed. The XRD-analysis is done only for one article type of 41CrS4. From the front axle beam three samples of three different locations (bottom, middle, top) was taken for the analysis. For the theoretical calculation of the retained austenite vs the actual amount it can be said that is a very good representation of the total amount of retained austenite in the product. But the theoretical calculation deviates a bit from the actual amount at the top part of the beam.
Syftet med denna avhandling var att undersöka vad relationen är mellan härdat ythårdhet och slut ythårdhet för stålbalkar är, beroende på vilken anlöpnings temperatur som används. Tar också upp hur kemi, dimension och mikrostruktur påverkar de sista mekaniska egenskaperna hos framaxel balken. För detta genomfördes en genomgång av litteraturen om effekterna.Hårdhetsmätning på ytan utfördes på balkens ändar (botten och toppen). Denna hårdhetsmätning utfördes på 6 olika främre axelartiklar av samma material (41CrS4) och 2 olika främre axelartiklar av annat material (40CrMo4). Relationsdiagrammet ger en uppskattning av vilken typ av anlöpningstemperatur som behövs för att uppnå den slutliga hårdheten som önskas. Eftersom förhållandet gjordes med vissa inkonsekvenser kan det inte sägas ge ett perfekt svar. Relationsdiagrammen fungerar endast för material 41CrS4 och 40CrMo4.För kärnhårdhetstestet mättes 2 artiklar av 41CrS4 och en artikel av 40CrMo4 i 5 olika positioner på tvärsnittet, stålen för respektive artiklar togs från härdat tillstånd och härdat + anlöpt. Dimensionerna har en signifikant effekt när det gäller att kyla ner delen och uppnå så nära enhetlig hårdhet som möjligt. Även om mittpunkten i I-sektionsprovet är en av de närmaste kärnorna till ytan, så har det en mjukare kärna jämfört med de andra kärnorna. Det finns hårdhetsskillnad efter härdning mellan de olika punkter men de jämnar ut sig efter anlöpningen. När man jämför kärnhårdheten med ythårdheten kan man säga att ythårdheten inte är så hård på grund av avkolning.Mikrostrukturanalysen gjordes på 2 artiklar av 41CrS4 och en artikel av 40CrMo4. Prover från de 3 artiklarna tas från både härdat tillstånd och härdat + anlöpt tillstånd. Från det optiska mikroskopet kunde man se att stålbalkens yta har blivit utsatt för avkolning vilket leder till en högre mängd ferrit vid strukturen och en mjukare yta. På grund av detta, så är 15 mm in i materialet hårare än vid ytan. Avkolning av 41CrS4 stål gjorde så att det som borde ha varit ett martensit och bainit dominerat yta blev istället ferrit och bainit dominerat.XRD-analysen görs endast för en artikelart av 41CrS4. Från fram axelbalken togs tre prov från tre olika platser (botten, mitten, toppen) för analysen. För att bestämma den verkliga mängden restaustenit i provet utfördes en XRD-analys. För den teoretiska beräkningen av den rest austeniten jämfört med det faktiska beloppet kan man säga det är en mycket bra representation av den totala mängden kvarhållen austenit i produkten. Men den teoretiska beräkningen avviker lite från den faktiska mängden vid stålens övre del.

The recently proposed bioassay procedure that is based on the substrate induced quenching (SIQ) of an indicator fluorescence for the measurement of analyte concentrations is evaluated. In this type of assay a enzynatic reaction and a fluorescence quenching interaction are coupled together. Typically, an appropriate dehydrogenase enzyme reduces or oxidises the nicotinamide adenine dinucleotide cofactor. The change in the concentration of NADH results in variations in the excited fluorophore population as observed through fluorescence intensity. This latter aspect is used to monitor substrate (analyte) concentrations. Results on the investigation of the substrate induced quenching bioassay method and possibilities of using it as the basis of (i) a novel enzyme bioassay technique and (ii) a novel bioprobe format are presented. Ethanol was chosen as the model analyte, and a new assay procedure for its measurement was developed. A generic theoretical relation is discussed for the observed assay kinetics of substrate induced quenching (SIQ) and a model is described that includes the effects due to dynamic/static quenching of the fluorophore by either the enzyme substrate or product. The validity of the derived model is shown by comparison with experimental results for a SIQ based ethanol assay. The option of running the dehydrogenase reaction so as to consume NADH rather than generate it is also investigated. In order to demonstrate this approach acetaldehyde was chosen as the model analyte, and a assay procedure for its measurement was developed. The potential of the SIQ technique for incorporation into biosensor based upon a 'reservoir' format was demonstrated through the development of custom optical instrumentation and resevoir flowcell. Applicability of the SIQ technique to other biosensor formats such as flow-injection analysis and 'dry reagent' technology is discussed. The overall applicability of the SIQ technique is assessed through the generation of a number of SIQ assays on the following substrates: ethanol, glucose, glucose-6- phosphate, L-glutamic acid, isocitric acid, acetaldehyde, pyruvic acid, ot-ketoglutaric acid, and oxalacetic acid.

The quenching characteristics of a range of concentrations of sodium polyacrylate, a commercially available polymer quenchant, have been studied. These solutions showed a stable film boiling stage the duration of which increased with increasing concentration. The maximum surface heat transfer coefficients were significantly below those recorded in water or polyalkylene glycol solutions and decreased with increasing concentration. Just after the passage of this maximum the surface heat transfer coefficient declined rapidly to reach values, at a surface temperature of about 300 C, equivalent to those recorded in the film boiling stage. Photography showed that this was associated with a decline in the mobility of the vapour bubbles formed in this stage. The surface heat transfer coefficients were used to calculate the stress and strain generated during quenching using a visco-elasticplastic model of an infinite plate of a low alloy steel. Comparisons of the predicted residual stresses in the case of the sodium polyacrylate solutions with residual stresses predicted in the case of other quenchants indicated that sodium polyacrylate solutions were capable of producing residual stress distributions similar to that produced by a medium speed quenching oil and greatly below those produced in the case of polyalkylene glycol solutions. This was achieved by a decline in the temperature gradient in the specimen before transformation to martensite began associated with the rapid reduction in surface heat transfer coefficient caused by the loss of mobility of the vapour at these surface temperatures. The predicted residual stresses and strains were also compared to experimentally measured residual stresses and strains to validate the model used. Three boundary layer theory models of film boiling were evaluated in the case of quenching in both water and a sodium polyacrylate solution and the predicted surface heat transfer coefficients compared to experimentally obtained values. None of the models produced a close agreement therefore a modification has been proposed to allow the inclusion of a turbulent interface in the models.

Investigation of the thermal quenching of photoluminescence (PL) in semiconductors provides valuable information on identity and characteristics of point defects in these materials, which helps to better understand and improve the properties of semiconductor materials and devices. Abrupt and tunable thermal quenching (ATQ) of PL is a relatively new phenomenon with an unusual behavior of PL. This mechanism was able to explain what a traditional model failed to explain. Usually, in traditional model used to explain “normal” quenching, the slope of PL quenching in the Arrhenius plot determines the ionization energy of the defect causing the PL band. However, in abrupt quenching when the intensity of PL decreases by several orders of magnitude within a small range of temperature, the slope in the Arrhenius plot has no relation to the ionization energy of any defect. It is not known a priori if the thermal quenching of a particular PL band is normal or abrupt and tunable. Studying new cases of unusual thermal quenching, classifying and explaining them helps to predict new cases and understand deeper the ATQ mechanism of PL thermal quenching. Very few examples of abrupt and tunable quenching of PL in semiconductors can be found in literature. The abrupt and tunable thermal quenching, reported here for the first time for high-resistivity ZnO, provides an evidence to settle the dispute concerning the energy position of the LiZn acceptor. In high-resistivity GaN samples, the common PL bands related to defects are the yellow luminescence (YL) band and a broad band in the blue spectral region (BL2). In this work, we report for the first time the observation of abrupt and tunable thermal quenching of the YL band in GaN. The activation energies for the YL and BL2 bands calculated through the new mechanism show agreement with the reported values. From this study we predict that the ATQ phenomenon is quite common for high-resistivity semiconductors.

Concentration quenching effect in ytterbium (Yb) and neodymium (Nd) doped phosphate glasses, thulium (Tm) doped germanate glass, and praseodymium (Pr) doped tellurite glass were studied. The fluorescence and lifetime of these rare-earth doped glasses with different concentrations were measured. Ion pair and clustering are included in the model to explain the quenching effect occurring in highly doped glasses. This study will help us in designing and fabricating high unit gain optical fibers.

Tryptophan fluorescence quantum yield is widely used to follow protein folding for the villin headpiece subdomain (HP-35) and a synthetic peptide Ac-W-(A) ₃ -H + -NH ₂ (WH5). These biopolymers have a histidine residue, which is a potent quencher of tryptophan fluorescence, positioned four amino acids away from tryptophan. Experiments assumed that when folding occurs the fluorescence of tryptophan will be quenched by histidine due to the formation of an alpha helix. The reliability of folding and unfolding rate constants determined by tryptophan fluorescence has been called into question by several computational studies. A method to calculate the electron transfer matrix element was developed for different donor/acceptor systems. This method shows that the electron transfer matrix element is sensitive to orientation at close distances and that it does not follow a simple exponential decay with distance. This thesis improved the methods developed by Callis and coworker by conducting 100 ns long simulations for single tryptophan proteins and by modifying the calculation of the fluorescence quantum yield to account for heterogeneity in the calculated electron transfer rates. In addition the method was extended to calculate electron transfer rate constants for histidine quenching by conducting 1microsecond long simulations of HP-35 and WH5. Calculated tryptophan fluorescence quantum yields for the single tryptophan proteins show better agreement with experiment than was previously reported. Simulations for HP-35 and WH5 indicate that the ability of histidine to quench the fluorescence of tryptophan is surprisingly controlled by the energy gap dependence on the distance that separates them. The energy gap dependence on this distance arises from water solvation around histidine. At large distances this solvation decreases the ability of histidine to accept an electron from tryptophan. Different tryptophan/histidine rotamers control this distance. Even when HP-35 is completely folded much of the time histidine does not quench tryptophan fluorescence contrary to the idea that histidine is only close when HP-35 is folded. The calculated fluorescence quantum yield is sensitive to the distribution of close and far conformations and the rate of exchange between these two conformations. This sensitivity gives credibility to the folding/unfolding rates derived from tryptophan fluorescence quantum yields.

In this thesis the finite element computer code QUEANA simulating the quenching of axisymetric parts and determining the residual stress state was improved by adding pre- and post-processors. The code was further verified by additional numerical experiments and comparison of the results with commercial software &ldquo
MARC&rdquo
. The possible applications of this code are optimization of industrial quenching processes by controlling the evolution of internal stresses and dimensional changes.

The recently proposed and novel steel heat treatment process known as quenching and partitioning (Q&P) is studied, in particular, the partitioning stage. Characterization of the partially quenched microstructure prior to the partitioning stage is enabled by using a base composition containing a higher Mn concentration than for conventional Q&P steels, such that quenching to room temperature produces only partial decomposition of austenite to martensite. Increasing the carbon concentration of this base composition also enabled a study of the effect of steel carbon content, although at the higher carbon contents refrigeration was required to achieve marten site because of the reduced Ms temperatures. Increased carbon also allowed the effect of martensite morphology to be examined. Intercritical annealing of a conventional TRIP steel composition provided an alternative route towards increasing the carbon concentration of austenite prior to partitioning. Light optical microscopy, scanning electron microscopy, X-ray and neutron diffraction were used to characterise the evolution of microstructure. In particular, in-situ examination of the Q&P heat-treatment process, enabled by introducing a furnace into the neutron beam-line, gave real -time observations and analyses of partitioning. Lattice parameter measurements enabled calculation of the carbon content of retained austenite, providing evidence of carbon partitioning from martensite to untransformed austenite during the partitioning stage. Thus, the partitioning process was shown to be effective in thermal and mechanical stabilization of retained austenite. In addition, evidence was found for substantial carbon enrichment of the austenite phase, which might be expected to provide opportunity for new Q&P steel grades with enhanced properties

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.
Vita.
Includes bibliographical references.
This dissertation examines the synthesis of fullerene-based fluorescence quenchers for numerous biosensor applications. The Introduction describes the need for biosensors in our society, what they are and various biosensing schemes that are currently being worked on in our group. Chapter One describes the synthesis of a number of fullerene derivatives. In order to incorporate fullerene derivatives into biosensors, they need to posses a functional group that can be easily reacted with biomolecules. Two of the functional groups by which molecules are conjugated to biomolecules such as amino acids and proteins are amines and carboxylic acids. For this reason, we synthesized amine- and carboxy-containing C₆₀ that could then be conjugated to biomolecules. Chapter Two describes the steps taken towards the incorporation of these fullerene derivatives into biosensors. First, Stern-Volmer experiments were conducted to determine whether or not the fullerene derivatives would be good quenchers for our polymers. Second, a polymer with pendant fullerenes was made to determine whether or not there was an enhancement in the quenching as compared with the Stern-Volmer data. Third, the use of the biotin-streptavidin system to determine how well the fullerene derivatives would perform in a biosensor system is discussed.
by Vanessa Virginia Perez.
S.M.

A viscoelastic-plastic mathematical model was used to calculate the thermal stress and strain generated during the quenching of an infinite plate of high hardenability steel (835M30) in water, oil and Polymer. In the present work the mathematical model was modified to include the effect of initial stress on the rate of stress relaxation, which has been found to be significant. The data required to incorporate this effect into the calculations, were obtained experimentally during the-.present investigation. The effect of an applied stress during transformation (transformation plasticity) was also introduced in the mathematical model. The new model produced a marked improvement in the degree of agreement between the calculated and experimental residual stress, although the corresponding level of agreement in the case of residual strain was less good. In particular, strains after water quenching agreed less well with experiment as a consequence of the change in the model, although this drawback was not found after oil and polymer quenching. The new mathematical model was used to investigate the effect of martempering, section size and transformation temperature range on the generation of thermal stress and strain. A salt bath treatment above the Ms temperature followed by air cooling prevented residual stress development, but an oil quench after the salt bath treatment generated a level of residual stress at the end of cooling that was similar to that obtained after a direct oil quench from 850°C. Neither martempering process was successful in reducing residual strain. With an increase in section size a reduction in the residual stress and an increase in the distortions was obtained after a water quench. However, after oil quenching the overall effect of section size on residual stress and strain was small. The effect of variation in the transformation temperature range was found to be small in the case of residual stress but an increase in Ms temperature produced a significant increase in the level of residual strain.

The critical heat flux (CHF) point for a reactor core system is one of the most important factors to discuss in regards to reactor safety. If this point is reached, standard coolant systems are not enough to handle the temperature increase in the cladding, and the likelihood of meltdown greatly increases. While the nucleate boiling and film boiling regimes have been well-investigated, the transition boiling regime between the point of departure from nucleate boiling (DNB) and the minimum film boiling temperature (Tmin) remains difficult to study. This is due to both the complexity of the phenomena, as well as limitations in measurement, where experiments typically utilize thermocouples for temperature data acquisition. As a result of technological advancement in the field of fiber optics, it is possible to measure the quenching temperature to a much higher degree of precision. Optical fiber sensors are capable of taking many more measurements along a fuel simulator length than thermocouples, which are restricted to discrete points. In this way, optical fibers can act as an almost continuous sensor, calculating data at a resolution of less than one millimeter where a thermocouple would only be able to measure at one point. In this thesis, the results of a series of quenching experiments performed on stainless steel, Monel k500, and Inconel 600 rods at atmospheric pressure, with different subcooling levels and surface roughnesses, will be discussed. The rewetting temperature measurement is performed to compare results between thermocouples and optical fiber sensors in a 30 cm rod. These results are further discussed with regard to future application in two-phase flow experiments.
Master of Science
There are multiple types of boiling that can occur depending on the heat transfer capabilities of the system and the power applied to the coolant. The most common is nucleate boiling, where vapor produced at the surface forms bubbles and move away from the surface due to buoyancy. At a high enough power, the bubbles can coalesce into a film and lead to a point at which the liquid coolant can no longer contact the surface. Since vapor is not as effective at transferring heat from the surface, the temperature will increase drastically. In nuclear reactors, this situation (known as departure from nucleate boiling), can quickly lead to a meltdown of the fuel rods. Another important safety parameter in nuclear reactors is the minimum temperature at which this vapor film can be maintained, Tmin. This parameter is a source of significant concern with regard to accident scenarios such as LOCA (loss of coolant accident), where reintroducing coolant to the rods efficiently is of top priority. While much research has been done on nucleate and film boiling, it has been difficult to study the transition period between the two regimes due to both its transient nature and the lack of continuous measurement capabilities. Typically, temperature is measured using thermocouples, which are point-source sensors that do not allow for high spatial resolution over a large area. This thesis deals with the utilization of optical fibers for temperature measurement, which are capable of calculating data at every millimeter, potentially a much more precise measurement system than with the thermocouples. The experiments performed in this paper are quenching experiments, where a rod embedded with thermocouples and an optical fiber is heated to well above Tmin and quickly plunged into a volume of water, in order to view the transition from film to nucleate boiling.

Zirconium alloys are widely used by the nuclear industry as fuel cladding and structural materials. Many physical and metallurgical properties of zirconium alloys, that are important for their performance in nuclear reactors, are affected by crystallographic texture due to the strong anisotropy of individual crystals. Irradiation assisted growth is one example. Zirconium crystals deform anisotropically under irradiation, which in the presence of strong textures (like the ones observed in cold-rolled sheet) causes undesirable deformation of components during service. For this reason, the nuclear industry is interested in developing thermomechanical processes that produce random textures, taking advantage of the allotropic phase transformation undergone by zirconium, from the low temperature hcp alpha-phase to the high temperature bcc beta-phase. One of these processes is beta-quenching, which has showed certain success in weakening strong rolling textures. However, there is no consensus about the fundamental mechanisms involved.The aim of this work is to study the evolution of the texture of the zirconium alloy Zircaloy-2 during beta-quenching, in order to gain understanding on the mechanisms involved on texture development and evolution during the alpha-to-beta and beta-to-alpha phase transformations. Firstly industrially beta-quenched samples were characterised using well known techniques such as laboratory X-ray diffraction (LXRD) and electron backscatter diffraction (EBSD), which revealed a relationship between peak temperature and the inherited alpha texture. An in situ synchrotron X-ray diffraction (SXRD) experiment provided, for the first time, information of texture evolution of zirconium during rapid changes and at non-ambient conditions. Different peak temperatures and stress/strain conditions were tested. Detailed post mortem EBSD characterisation of samples studied in situ provided insight on the relationship between the microstructure and the texture. Finally, laboratory furnaces were used to beta-quench samples at very high temperature. It was found that there is selection of orientation variants during beta-quenching of zirconium, but while the selection during the alpha-to-beta transformation is almost negligible, depending on the texture evolution of the beta-phase (affected by grain growth and/or plastic deformation), diverse mechanisms of variant selection act during the beta-to-alpha phase transformation. The inherited textures observed result from the combination of these mechanisms. Some of the results of this work can be transferred to other systems such as titanium and the alpha-gamma-alpha phase transformation in steel.

Thesis (M.S.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept of Fire Protection Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Steel continues to be one of the primary materials in use today, however, even after 200 years of published research, innovative methods are being discovered and exploited to produce steel with enhanced properties. One such recent discovery has been termed Quenching and Partitioning (Q&P). The Q&P heat treatment process is reported to produce a multiphase microstructure consisting of retained austenite stabilised to room temperature, and a harder martensitic phase. This combination is prospected to form a new generation of high strength, formable sheet steel well suited to the demands of the automotive industry. Previous research has reported on the application of Q&P to commercial grades of steel, however, the temperatures required have hampered study of the process. In this investigation a model alloy has enabled separation of Q&P into its individual stages for closer inspection. Standard metallographic techniques were used to examine the microstructure of the material, followed by X-ray diffraction measurements to determine the stability of the austenite phase fraction before and after partitioning, confirming the effectiveness of the partitioning process in stabilising retained austenite. Estimations of carbon concentration via lattice parameter measurements have provided evidence of austenite carbon enrichment, but also suggest that some carbon remains unaccounted for, possibly as an equilibrium distribution between epsilon carbides and strained interstitial sites. In-situ partititioning in a neutron diffractometer has been utilised to obtain real-time measurements of the partitioning process. These measurements suggest that redistribution of carbon within martensite occurs before partitioning to austenite becomes kinetically significant. The trapping of carbon within energetically favourable locations is postulated to be a major factor in retarding carbon migration from martensite to austenite when compared to mathematical models of the partitioning process. Neutron diffraction measurements of carbon interstitial occupancy in austenite have also provided further evidence of austenite carbon enrichment during the partitioning process.

Investigating the mechanisms that regulate star formation (SF) across cosmic time is a complex issue. Most of our knowledge of the cosmic SF history at z > 3 relies on UV surveys, which could miss SF obscured by dust. IR/mm-selections of galaxies are thus a crucial tool for unveiling the total amount of SF. In addition, active galactic nuclei (AGN) are considered to play a crucial role in galaxy evolution. According to models, AGN could be decisive in suppressing SF in massive galaxies. Despite this, some theories predict that AGN could also trigger SF inside outflows. These two mechanisms have been named as “negative” and “positive” AGN feedback and their study is crucial to understand what regulates SF inside galaxies. The aim of this thesis is to investigate the processes that fuel and suppress SF across cosmic time. On one side, we focused on the study of cold gas in the early Universe. We derived the first [C II] luminosity function at z~5 from a mm-selection of galaxies. By using [C II] as a SF rate indicator, we measured the cosmic SF rate density at z~5, which shows a possible excess compared to previous estimates. On the other side, we studied the impact of negative and positive AGN feedback on SF. First, we investigated negative feedback in a massive galaxy at z~2, which will possibly evolve into a passive galaxy. Then, we studied positive feedback in a sample of 70 nearby star-forming galaxies by using UV spectroscopy. We detected in several objects blueshifted stellar absorption, which could be indicative of young stars formed inside outflows. Despite the uncertainties affecting our analysis, studying cold gas and AGN feedback represents a powerful approach to constrain the processes that feed and quench SF across cosmic time.

We investigate the relationship between the black hole accretion rate (BHAR) and star formation rate (SFR) for Milky Way (MW) and Andromeda (M31)-mass progenitors from z = 0.2 to 2.5. We source galaxies from the K-s-band-selected ZFOURGE survey, which includes multiwavelength data spanning 0.3-160 mu m. We use decomposition software to split the observed spectral energy distributions (SEDs) of our galaxies into their active galactic nuclei (AGNs) and star-forming components, which allows us to estimate BHARs and SFRs from the infrared (IR). We perform tests to check the robustness of these estimates, including a comparison with BHARs and SFRs derived from X-ray stacking and far-IR analysis, respectively. We find that, as the progenitors evolve their relative black hole-galaxy growth (i.e. their BHAR/SFR ratio) increases from low to high redshift. The MW-mass progenitors exhibit a log-log slope of 0.64 +/- 0.11, while the M31-mass progenitors are 0.39 +/- 0.08. This result contrasts with previous studies that find an almost flat slope when adopting X-ray-/AGN-selected or mass-limited samples and is likely due to their use of a broad mixture of galaxies with different evolutionary histories. Our use of progenitor-matched samples highlights the potential importance of carefully selecting progenitors when searching for evolutionary relationships between BHAR/SFRs. Additionally, our finding that BHAR/SFR ratios do not track the rate at which progenitors quench casts doubts over the idea that the suppression of star formation is predominantly driven by luminous AGN feedback (i.e. high BHARs).

"Quenching is the rapid cooling process from an elevated temperature. Compared to water and oil quench medium, high pressure and velocity gas is preferred to quench medium and high hardenability steel, with the potential to reduce distortion, stress and cracks. Currently, no standard test exists to characterize the gas quench steel hardenability and measure the performance of industrial gas quench furnaces. In this thesis, the fundamental difference between the liquid and gas quenching, heat transfer coefficient, was emphasized. It has been proven that gas quenching with constant HTC cannot generate the similar cooling curves compared to liquid quenching. Limitations on current gas quench steel hardenability tests were reviewed. Critical HTC, a concept like critical diameter, was successfully proved to describe the gas quench hardenability of steel. An attempt to use critical HTC test bar and measure the HTC distribution of gas quench furnace was made. Gas quenching, usually with slow cooling rate, may reduce hardness and Charpy impact toughness, compared to water and oil quenching. Lattice parameter and c/a ratio of as-quenched martensite in steel was measured using high resolution X-ray diffraction and Rietveld refinement. For AISI 4140, Charpy impact toughness decreases when the cooling rate decreases after quenching and tempering. Austenite percentage and carbon content in austenite is proposed as the dominated mechanism."

The finite difference method was used to calculate the variable heat transfer coefficient required to maximize mechanical properties of heat treated wrought 7075 aluminum alloy without causing residual stress. Quench simulation enabled determination of maximum surface heat flux bordering on inducing plastic flow in the work piece. Quench Factor Analysis was used to correlate cylinder diameter to yield strength in the T73 condition. It was found that the maximum bar diameter capable of being quenched without residual stress while meeting military mechanical design minimums is 2". It was also found that the cooling rate must increase exponentially and that the maximum cooling rate needed to achieve minimum mechanical properties is well within the capability of metals heat treatment industry.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.
Includes bibliographical references (p. 83-86).
One fundamental challenge in designing organic light-emitting diodes is luminescence quenching near an electrode. In this work, we investigate the underlying mechanism behind luminescence quenching by measuring the reduction in Alq3 photoluminescence due to SnO02. Using an analytical model and a Monte Carlo simulation for exciton dynamics in amorphous organic solids, we find that the exciton diffusion length in bulk Alq3 is in the range of 70--80 A. We also find that for SnO2 films deposited without oxygen in the sputtering ambient, resonant energy transfer from Alq3 to SnO2 is the dominant quenching mechanism. By varying the oxygen content in the Ar/C)2 sputtering gas mixture, we find that the energy transfer distance decreases from 10--25 A for 0% 02 to less than 2 A for 10% 02. Our experimental results suggest that because excess oxygen reduces oxygen vacancies and defect electronic states in SnO2, it leads to a smaller spectral overlap between the emission of Alq3 and the absorption of SnO2, thereby shortening the energy transfer distance and reducing the quenching capability of SnO2.
by Jun Mei.
S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.
Includes bibliographical references (p. 85-88).
In this thesis we perform analyses on simulated data that allow us to demonstrate the sensitivity of the CMS experiment to certain jet quenching observables. In particular, two theoretical scenarios which mimic RHIC data at low PT and which show either no quenching or BDMPS-based quenching at high PT are formulated. The difference between these two scenarios is analyzed for RAA, RCP at different centralities and jet-specific observables such as jet energy spectra, fragmentation functions and jet profiles. We show how these analyses indicate that the large acceptance of the CMS detector, combined with the high granularity in the energy resolution of the calorimeter will be essential tools in studying the phenomenon of jet quenching. Finally, we propose extensions to this work in preparation to analyzing the data from Pb-Pb runs at the LHC. Disclaimer: The work on this thesis does not model the CMS detector geometry with the accuracy required for official analyses, which are fully representative of the CMS detector capabilities. Such analyses require of the full CMS simulation machinery and are left to the CMS Heavy Ion group as a whole.
by David López Mateos.
S.B.

Thesis (Ph. D.)--University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Chemistry and Biochemistry. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

An experimental approach was taken in determining the quantum yield of Ruthenium(II) with Oxygen using two different Ru complexes. This reaction results in Ruthenium(III) and O2-. The Coulombic interactions caused by a carboxylate functional was found to increase the yield of charge separation. This was done using a diode to measure the intensity of the completed reaction over a certain time frame. The intensities were turned into concentrations. The concentration over time was used to determine the quantum yield. This information is useful in creating more efficient light emitting diodes.

There are numerous numerical and experimental studies to find correlations of octane rating with fuel properties. This thesis was based on the hypothesis that quenching characteristics at ignition locations impact the flame development. Conversely, determination of quenching characteristics might serve as an effective measure to determine the fuel mixture octane number. This hypothesis was tested with premixed flame experiments using primary reference fuels (iso-octane and n-heptane) and commercial grade gasoline. Premixed flame experiments were conducted on a flat flame burner. Primary reference fuels of different ratios were taken and correlated to their respective thermal quenching condition by introducing co-flowing inert gasses at room temperature with fuel-air mixture. The inert gasses that were used in the experiment are nitrogen and helium and the results are analyzed using a camera and an imaging spectrometer. The experimental results support the hypothesis that flame quenching can be correlated to fuel mixture octane number, and holds potential as an alternative method to determine the octane number.

Durante os anos desde 1910, diversos trabalhos científicos foram desenvolvidos, tornando o processo de têmpera bem estabelecido nas plantas industriais. Atualmente, existem diferentes tipos de têmpera e mais técnicas estão sendo desenvolvidas para aumentar as propriedades de componentes de aço. Um deles é o de têmpera intensiva, que pode ser considerado como um processo relativamente novo. O método de têmpera intensiva visa otimizar o processo produtivo, ao mesmo tempo em que diminui consideravelmente o custo da etapa de tratamento térmico. Além disso, atualmente o uso de soluções ambientalmente amigáveis torna este processo bem menos agressivo ao planeta. Neste trabalho realizou-se a têmpera intensiva e a têmpera convencional em corpos de prova preparados para teste de tenacidade à fratura. Trata-se, porém, de um método alternativo de teste, relativamente recente, no qual são considerados parâmetros de fratura dúctil e fratura frágil para o cálculo de K1C. Estes ensaios de tenacidade foram feitos a partir de ensaios de tração em corpos de prova com pré-trinca. Embora as tensões residuais compressivas tenham atingido os valores mais altos na têmpera intensiva, os resultados de tenacidade à fratura foram mais positivos para as amostras com têmpera convencional. A junção e aplicação desses métodos pode trazer um novo parâmetro de fabricação e análise de materiais metálicos, em especial aços-mola, que foi o objeto de estudo deste trabalho.
Since the beginning of 20th Century, several scientific works were developed and conventional quenching process became well established into the industrial area. Nowadays new quenching process were introduced increasing mechanical properties of the heat treated components. Intensive quenching is one of these process which optimize the heat treatment process using also quenchants considered nontoxic to the environment. In this work it was made comparative studies in the SAE 5160 samples which were submitted to conventional quenching and intensive quenching. Toughness fracture were evaluated using an alternative test where are analyzed ductile fracture and brittle fracture parameters to calculate KIC. In this method tensile test are performed in pre-cracked samples. Residual stresses were also measured and although intensive quenching promoted highest compressive stresses in the surface, KIC obtained in such samples presented low values compared with conventional quenching. The presented method of analysis will bring a new parameter for production and analysis for metallic materials, particularly spring steel, where compressive stress and toughness are important as properties for suspension components.