No description available.

No description available.

No description available.

No description available.

This thesis is concerned with the structure of shocks occurring in dense regions of molec?ular clouds. These shocks are associated with the outflows from young stars, Herbig-Haro objects, expanding HII regions and the interaction of supernovae remnants with molecu?lar clouds. Momentum, mass and energy are imparted to the cloud. A full understanding of the shock process is thus needed if we are to understand the structure of molecular clouds and the impact on star formation. Emission from the near-infrared transitions of molecular hydrogen is commonly excited in these shocks. A major puzzle is that emis?sion is seen at velocities that would collisionally dissociate molecular hydrogen, and this is a central question that this thesis seeks to answer. This is approached observationally by trying to relate the observed emission to shock models. Fairly accurate semi-analytic derivations of the emission spectrum expected from hydrodynamic and magnetohydrodynamic molecular shocks are used to fully explore the parameter space of the initial conditions, without resort to expensive numerical calculations. The emission spectrum is then related to that observed. Most of this work is based on a spectroscopic multi-line study of the near-infrared H2 emission in two sources, the Orion outflow and the supernova remnant IC443. These observations are then compared with those expected from the models. In both sources it is found that planar hydrodynamic jump-type shocks (J) are consistent with the new observations. Whplanar magnetically moderated continuous shocks (C), which have been invoked to explain the emission from the shock in Orion, are not. Neither shock types can explain the intensities of CO rotational lines and the H2 line ratios simultaneously. The high velocities that are observed still present a problem. In IC443 the conclusion is the same but, in addition, the pressure needed to explain the observations is higher than that observed in the supernova remnant. It is suggested that this discrepancy may naturally occur when radiative shocks are driven through a clumpy medium. This approach of using line ratios as shock discriminators is extended by velocity resolved spectroscopy of three highly excited emission lines from Orion. These observa?tions demonstrate that there are no discernible differences in the line ratios with velocity despite the large change in the energies of the upper energy levels involved. It is discussed how this further constrains the shock type and limits the contribution from non-thermal excitation (such as fluorescence). The possible physical processes that could lead to high velocity, shocked molecular hydrogen are then discussed. Models proposed in the past are, it is argued, inadequate. It is then shown that the line ratios observed can be closely matched with non-planar continuous type shocks which occur in a bow shock. The densities and pressures needed are still high. The general conclusions are that previous plane parallel C-shock models invoked to explain the molecular shocks are inconsistent with the observations. The line ratios imply that either J-type shocks, in which the cooling takes a long time compared to the initial heating, or C-type bow shocks which produce a range of temperatures are responsible for the emission. It is finally suggested that C-shocks in gas with a very high magnetic field can produce the high velocity H2 emission observed without dissociating the molecules.

Ce projet de thèse expérimentale vise à étudier l'interaction d'une détonation se propageant dans une atmosphère gazeuse réactive ensemencée d'un spray d'eau, au sein d'un tube vertical de 4 m de haut ayant une section carré de 52 mm de côté. Le dispositif permet de mesurer les célérités de propagation de la détonation et les niveaux de pression associée, ainsi que d'analyser la structure cellulaire. La caractérisation du spray d'eau par la méthode PDI a permis d'évaluer le diamètre moyen des gouttelettes à 10 μm. Les densités apparentes de spray peuvent atteindre 200 à 250 g/m³.La première étude a consisté à faire varier la dilution Z en argon de 3 à 28, ainsi que la quantité X d'eau injectée (YH2O pouvant atteindre 15%), dans des mélanges de types C2H4-O2-Zar-XH2O(l). Cette étude a permis de faire varier les longueurs caractéristiques de la détonation par rapport à celles du spray. Deux comportements très différents ont été mis en évidence, suivant la taille plus petite (premier comportement) ou plus grande (deuxième comportement) de la longueur d’induction chimique par rapport à celle de l’atomisation secondaire des gouttelettes, dans les conditions de détonation. Ainsi dans le cas idéal où l’épaisseur hydrodynamique – distance moyenne entre le choc et la surface sonique – englobe l’ensemble des interactions diphasiques, la célérité de détonation sera celle de Chapman-Jouguet diphasique, inférieure au cas purement gazeux. De plus dans le cas du premier comportement, la vapeur issue de la phase dispersée ne participera pas l'agrandissement de la structure cellulaire, a contrario du second.Afin de préciser le mécanisme responsable du deuxième comportement, la seconde étude s'est quant à elle attachée à l'analyse de l'influence du spray par rapport à la régularité de la structure cellulaire de la détonation. Deux mélanges sont ainsi considérés, générant une détonation à structure régulière (C2H4-O2-28Ar-XH2O(l)) ou irrégulière (C2H4-O2-11.286N2-XH2O(l)). Cette étude a confirmé que dans cette configuration, la vapeur d’eau issue de la phase dispersée liquide participe alors à l'agrandissement de la structure cellulaire. Elle a également permis de montrer la plus grande résilience des détonations irrégulières par rapport aux détonations régulières vis-à-vis des pertes pariétales. Il a également été constaté que la perte de régularité de la structure cellulaire liée à l'ajout d'eau est associée à l'augmentation de l'énergie d'activation réduite Ea/RTvn et du facteur de stabilité, expliquant par ailleurs l'apparition d'une sous-structure cellulaire, semblable à celle observée dans les détonations initialement irrégulières. La vapeur d’eau ainsi produite par l’évaporation du spray agit alors comme un diluant inerte en aval du choc incident. / The interaction between a gaseous detonation and a water spray was experimentally studied in a 4 m high vertical detonation tube with a 52 mm by 52 mm square section. Detonation pressure signals, average velocity and cellular patterns were recorded.The spray, produced by an ultrasonic generator and injected at the bottom of the tube, was characterized by the Phase Doppler Interferometry (PDI) method. The spray analysis revealed an average droplet diameter of10 μm with Liquid Water Content (LWC) up to 200-250 g/m3. The first study compared the detonation and spray lengths in stoichiometric CzH4-02-Ar-H20(l) mixtures for argon dilution ranging from 3 to 28 and water mass fraction up to Y ttzo 15%. Two distinct behaviors were revealed, driven by the length of the induction zone compared to the secondary breakup length of the spray droplets. It is found that in the ideal case where the hydrodynamic thickness (representing the average length between the shock and the sonic surface) encompass the endothermic multiphase processes, the experimental detonation velocity is equivalent to the Ideal Chapman Jouguet multiphase velocity, which is lower than the ideal detonation velocity in a dry mixture. Moreover, when the induction length is shorter than the secondary breakup length, the water vapor produced by the droplets breakup is not involved in the cellular structure enlargement.The second study highlighted the influence of the water spray on the cellular structure regularity, by using mixtures of CzH.-02-28Ar-H20(l) and C2H.-02-N2-H20(l) with various equivalence ratio. The experiments show that irregular detonations are more resilient compared to regular ones. Moreover the loss of the detonation regularity generated by the water spray addition and the increase in both the reduced activation energy Ea/RTvn and the stability factor are responsible for the sub-structure generation, similar as the one observed in initially irregular detonation. Furthermore, ZND computations indicated that water mainly 5played a thermal role by diluting the reactive gaseous mixture and seemed to have a limited impact on the kinetic nrocesses.

No description available.

This thesis quantitatively investigates gaseous electron-ion recombination in an environmental scanning electron microscope (ESEM) at a transient level by utilizing the dark shadows/streaks seen in gaseous secondary electron detector (GSED) images immediately after a region of enhanced secondary electron (SE) emission is encountered by a scanning electron beam. The investigation firstly derives a theoretical model of gaseous electron-ion recombination that takes into consideration transients caused by the time constant of the GSED electronics and external circuitry used to generate images. Experimental data of pixel intensity versus time of the streaks is then simulated using the model enabling the relative magnitudes of (i) ionization and recombination rates, (ii) recombination coefficients, and (iii) electron drift velocities, as well as absolute values of the total time constant of the detection system, to be determined as a function of microscope operating parameters. Results reveal the exact dependence that the effects of SE-ion recombination on signal formation have on reduced electric field intensity and time in ESEM. Furthermore, the model implicitly demonstrates that signal loss as a consequence of field retardation due to ion space charges, although obviously present, is not the foremost phenomenon causing streaking in images, as previously thought. Following that the generation and detection of gaseous scintillation and electro- luminescence produced via electron-gas molecule excitation reactions in ESEM is investigated. Here a novel gaseous scintillation detection (GSD) system is developed to efficiently detect photons produced. Images acquired using GSD are compared to those obtained using conventional GSED detection, and demonstrate that images rich in SE contrast can be achieved using such systems. A theoretical model is developed that describes the generation of photon signals by cascading SEs, high energy backscattered electrons (BSEs) and primary beam electrons (PEs). Photon amplification, or the total number of photons produced per sample emissive electron, is then investigated, and compared to conventional electronic amplification, over a wide range of microscope operating parameters, imaging gases and photon collection geometries. The main findings of the investigation revealed that detected electroluminescent signals exhibit larger SE signal-to-background levels than that of conventional electronic signals detected via GSED. Also, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases photon collection efficiencies. The attainment of such an improvement being a direct consequence of increasing the `effective' solid angle for photon collection. Finally, in attempt to characterize the scintillating wavelengths arising from sample emissive SEs, PEs, BSEs, and their respective cascaded electrons, such that future photon filtering techniques can be employed to extract nominated GSD imaging signals, the emission spectra of commonly utilized electroluminescent gases in ESEM, such as argon (Ar) and nitrogen (N2), were collected and investigated. Spectra of Ar and N2 reveal several major emission lines that occur in the ultraviolet (UV) to near infrared (NIR) regions of the electromagnetic spectrum. The major photon emissions discovered in Ar are attributed to occur via atomic de-excitation transitions of neutral Ar (Ar I), whilst for N2, major emissions are attributed to be a consequence of second positive band vibrational de-excitation reactions. Major wavelength intensity versus gas pressure data, for both Ar and N2, illustrate that wavelength intensities increase with decreasing pressure. This phenomenon strongly suggesting that quenching effects and reductions in excitation mean free paths increase with imaging gas pressure.

Microbial fuel cells (MFCs) are emerging energy production technology which converts the chemical energy stored in biologically degradable compounds to electricity at high efficiencies. Microbial fuel cells have some advantages such as use of an inexpensive catalyst, operate under mild reaction conditions (i.e. ambient temperature, normal pressure and neutral pH), and generate power from a wide range and cheap raw materials. These make microbial fuel cell as an attractive alternative over other electricity generating devices. However, so far the major problem posses by this technology is the low power outputs of the microbial fuel cells that hinder its commercialization. Restriction in the electron transfer from bacteria to the anode electrode of a MFC is thought to be one cause for the low power output. Most recent MFC research is focused on using contaminants present in industrial, agricultural, and municipal wastewater as the energy source, with very few studies utilising gaseous substrates. Mediators can be added to MFCs to enhance the electron transfer from the microbe to the anode, but have limited practical applicability in wastewater applications because of the difficulty in recovering the expensive and potentially toxic compound. This thesis describes an investigation of electricity generation in a microbial fuel cell by combining a gaseous substrate with a mediator in the anode compartment. The emphasis being placed on the selection of a mediator to improve the electron transfer process for electricity production in an MFC. Subsequently, methods to improve the performance of a mediator MFC in respect of power and current density were discussed. This type of MFC is purposely aimed to be applied for treating gaseous contaminants in an anaerobic biofilter while simultaneously produce electricity. In this study, ethanol was the first gaseous substrate tested for the possibility to generate electricity in the MFC. Various mediators were previously compared in their reversibility of redox reactions and in the current production, and three best mediators were then selected for the power production. The highest electrical current production i.e. 12 μA/cm2 was obtained and sustained for 24 hrs with N,N,N',N'-tetramethyl-1,4- phenylendiamine TMPD (N-TMPD) as the mediator using glassy carbon (GC) electrode. The maximum power density reached 0.16 mW/cm2 by using carbon cloth (CC) anode. The absorption of these mediators by the bacterial cells was shown to correlate with the obtained energy production, with no N-TMPD was absorbed by the bacterial cells. The 24 hr current production was shown to be accompanied by the decrease in the ethanol concentration (i.e. 1.82 g/L), however ethanol crossover through the proton exchange membrane and ethanol evaporation around the electrodes were most likely to be the major cause of the decrease in the ethanol concentration. A theoretical coulombic efficiency of 0.005% was calculated for this system. The electrokinetics of microbial reduced mediator in the ethanol-mediator MFCs was also examined. Two methods i.e. linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were used to obtained the kinetic parameters. CV method gave a better estimation of the kinetic parameters than LSV method due to the low concentration of the mediators used, affecting the Tafel behaviors. All CVs showed quasi-reversible behaviors compared to the CVs in the absence of the bacteria, which is thought due to the bacteria decreased the amount of the reduced and the oxidised mediator available at the surface of GC electrode. The highest exchange current density (i o ) was obtained by using N-TMPD as the mediator with the same concentration of the mediator used i.e. 0.13±0.01 mA/cm 2. The power output achieved also the highest (0.008 mW/cm 2) with N-TMPD as the mediator. The power density was improved to 0.03 mW/cm2 by using CC electrode. Another main objective of this thesis is to prove anoxic methane oxidation which was believed to occur only in marine sediments, and applies this for power generation in microbial fuel cells. Ferricyanide looked promising when it was used as the electron acceptor (thus as the mediator for the MFC). It was shown that ferricyanide was fully reduced by methanotrophs bacteria with methane as the substrate (versus abiotic and nitrogen control). The highest reduction rate achieved was 3 x10-3 mM/min.g. This finding was supported by ferricyanide peak heights disappearance (spectrophotometry at 420 nm), CO 2 production (sensor readings), ferrocyanide formation (cyclic voltammetry), and no other alternate electron acceptor was present. The total CO 2 produced was equal to 0.015 mmoles of CO 2 from starting concentration ferricyanide of 0.2 mmoles (after substraction with an offset value). CV results show 2.4 mM of ferrocyanide was produced after a total addition of 3 mM ferricyanide into the anoxic methanotrophic suspension. The current and voltage generation in microbial fuel cell reactor from the reduced ferricyanide confirmed that ferricyanide received electrons from the bacterial metabolism. The maximum power density of 0.02 mW/cm2 and OCV of 0.6 V were obtained with 3 mM ferricyanide using LSV method.

We present an equatorial (± 30◦ Decl.) sample of all known single (within 4′′) mid M-dwarfs (M2.5V-M8.0V) extending out to 10 pc. For this sample of 58 stars, we provide photometry, low dispersion optical (6000−9000 ̊A) spectra from which spectral types are determined, Hα equivalent widths, and gravity sensitive NaI indices. For 45 of these 58 stars, strict limits are placed on the presence of companions, based on precise infrared radial velocities. Our spectroscopic results indicate that on average, we rule out the existence of companions with masses of 1.5 MJUP or greater in 10 day orbital periods around slowly rotating (vsini < 6.5 km s−1) M-dwarfs. Similarly, strict limits are placed on the presence of companions to 53 out of the 58 stars with astrometry. Our astrometric results show that, on average, we rule out the presence of companions with masses greater than 9 MJUP with an orbital period of 8 years. These results establish these stars as the nearest set of single mid M-dwarfs. Two additional stars, GJ 867B and LHS 1610, were initially included in this program, but later discovered to be spectroscopic binaries (SB). The binary GJ 867BD is a wide (24.5') companion to the M2 dwarf GJ 867AC. With this discovery, the GJ 867 system (d =8.82 ± 0.08 pc) becomes one of only four quadruple systems with in 10 pc of the Sun and the only among these with all M-dwarf (or cooler components). To measure how the rotational velocities vary with spectral type, we assembled a list of all known single (within 3′′) mid M-dwarfs that have trigonometric parallaxes within 25 pc and reside between −30◦ and +65◦ Decl from the RECONS sample. From this list of 402 stars, only 169 stars have previously reported vsini values. We obtained spectroscopic measurements for an additional 75 stars. Of those, 17 have vsini values above our detection threshold of 3 km s−1. Our data are consistent with the trend of more low mass M-dwarfs having high projected rotational velocity values than high mass M-dwarfs.