The oxidation behaviour of TiAlN based hard coatings with the addition of Cr and/or Y was investigated using scanning electron microscopy, scanning/transmission electron microscopy, energy dispersive X-ray analysis, thermogravimetry and X-ray diffraction. The coatings were deposited using the combined cathodic arc/unbalanced magnetron deposition technique. The main practical application of these films is dry high speed cutting in difficult to cut materials such as AISI A2 steel. Especially in the case of TiAlCrYN coating with an oxynitride and Cr-enriched overcoat, extensive research on the oxidation behaviour was performed and described here. Heat treatments in air between 600-1000°C for different duration were carried out. The Ti[0.44]Al[0.54]Cr[0.02]N coating was used as the starting point for the investigations. The effect of heat on the composition of the interface region was investigated. This region is of utmost importance for the adhesion of the film. In the case of TiAICrN the interface stability was not guaranteed because of diffusion of the substrate elements Cr and Fe to the coating surface after annealing for 1h at 900°C. In comparison, the diffusion of substrate elements Cr and Fe in a ~2.3 mum thick coating of Ti[0.43]Al[0.52]Cr[0.03]Y[0.02]N and of Ti[0.34]Al[0.62]Cr[0.03]Y[0.01]N with overcoat, reached only a distance of ~600nm into the coating. This was achieved by the diffusion of Y to the grain boundaries. Y probably reacted at the same time with inward diffusing O. The diffusion of Y to the boundaries was observed after heat treatment for 1h at 900°C or 10h at 800°C.Ti[0.26]Al[0.26]N/Cr[0.48]N was the coating with the least oxide layer growth after oxidation for 1h at 900°C. An oxide layer thickness of only ~100nm was measured. For the TiAICrYN coating with overcoat an oxide layer of 230nm and for TiAICrYN of 430nm formed after 1h at 900°C. TiAlCrN in comparison formed an oxide layer of ~800nm after 1 h at 900°C.The oxide layers formed after 1h at 900°C consisted mainly of an Al[2]O[3] and TiO[2] bi-layer in the case of TiAlCrN and TiAlCrYN. The addition of a Cr-rich oxynitride overcoat led to the formation of a mixture out of Al[2]O[3], Cr[2]O[3] and TiO[2] in the oxide layer. In the case of TiAlN/CrN, a solid solution consisting of Cr[2]O[3] and Al[2]O[3] was observed. In general a stress relief after heat treatment was observed. At the same time the formation of voids along the column boundaries was identified. This was explained with the relaxation and diffusion of defects created during the deposition process. The effect of different substrate materials on the oxidation behaviour was also investigated. It was found that the formation of substrate oxides on the coating surface is very much dependent on the onset point of oxidation of the substrate material itself. The oxidation of substrate material occurred mainly through growth defects and pinholes. In cases where cracks formed during heat treatment of the coating, the formation of oxides out of substrate elements were observed in cracks connecting the substrate with the coating surface. Changing the bias voltage altered the formation of cracks. This research emphasises the importance of Y in the oxidation mechanism of TiAlN based hard coatings. Y blocks the diffusion path along the column boundaries and thus stowed down the diffusion and oxidation process. At the same time the addition of Cr can increase the oxidation resistance considerably, which was observed in the TiAlCrYN coating with and without overcoat.

In this work, the pneumatic spray pyrolysis was used to synthesize un-doped and carbon doped zinc oxide (ZnO) nanoparticles. The zinc acetate, tetrabutylammonium bromide and ethanol were used as starting materials for the desired ZnO nanoparticles and the prepared samples were annealed at 400 oC in the furnace. The as synthesized un-doped and carbon doped ZnO NPs were evaluated using X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive x-ray spectroscopy (EDX), High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS) and Ultraviolet-visible spectroscopy (UV-Vis). XRD analysis of the synthesized NPs revealed peaks at 31.90°, 34.50°, 36.34°, 47.73°, 56.88°, 63.04°, 68.20°, and 77.33° belonging to the hexagonal Wurtzite ZnO crystal structure. The incorporation of C species into ZnO lattice was cross examined by monitoring the peak positions of the (100), (002) and (001) planes. These three main peaks of C-ZnO NPs show a peak shift to higher 2θ values which indicates substitutional carbon doping in ZnO NPs. SEM analysis has revealed that the as synthesized NPs have spherical shape and the morphology of the NPs change as the concentration of carbon increases. The EDX spectra of both un-doped and doped ZnO nanoparticles have revealed prominent peaks at 0.51 keV, 1.01 keV, 1.49 keV, 8.87 keV and 9.86 keV. Peaks at, X-ray energies of 0.51 keV and 1.01 keV respectively represent the emissions from the K-shell of oxygen and L-shell of zinc. The L-shell emission at 1.01 keV is considered as convolution of Zn 2p3/2 and Zn 2p1/2 photoelectron energies. The occurrence of these peaks in the EDX endorses the existence of Zn and O atoms in the PSP prepared samples. HRTEM analysis has revealed NPs size modal range from 6.65-14.21 nm for the PSP synthesized samples which is in mutual agreement with the XRD data calculated values. More over the selected area diffraction images displaying the fact that only the diffraction planes of (101), (002) and (100) are responsible for the diffraction pattern belonging to Wurtzite ZnO. RS analysis has revealed that the un-doped ZnO and doped ZnO samples have characteristic Raman vibration modes at 325 cm-1, and 434 cm-1 belonging to Wurtzite ZnO structure. Moreover, the prominent peak at 434 cm-1 which is the characteristic peak of E2(2) (high) mode of the Wurtzite ZnO and the E2(2) (high) has been red shifted by 4 cm-1, as compared to that found in the bulk ZnO. Additionally, the effect of carbon doping through Raman spectroscopy peak shifts of the E2(2) (high) mode, A1(LO) mode and multi-phonon has also been considered and discussed in detail. UV-Vis diffuse reflectance spectroscopy has revealed a red shift of the absorption edge with increase in C doping. Finally, the effect of nano-crystallite size and gradual prominence of C into ZnO lattice due to increase in C doping concentration in the PSP prepared nanoparticles was meticulously elaborated through Raman Spectroscopy analysis.

In this work, zinc oxide nanosheets are synthesised through thermal decomposition of a layered basic zinc acetate precursor and implemented in sensors to investigate the reactions of carbon monoxide, hydrogen and methane. The mean size of nanoparticles within the nanosheets is shown to increase with annealing temperature, and sintering occurs after heating at temperatures of 700°C or higher. X-ray photoelectron and photoluminescence spectroscopy techniques demonstrate that the concentrations of both lattice oxygen species and oxygen-containing surface groups may be enhanced by increasing the annealing temperature. By using an Eley-Rideal-based physical model, the responses of the nanosheets to carbon monoxide are quantitatively related to the reaction parameters of the system. The response characteristics suggest that the carbon monoxide oxidation has activation energy 54 +/- 9 kJ mol-1 while oxygen ionosorption has an energy barrier of 72 +/- 9 kJ mol-1. The sensor recoveries are consistent with corresponding values of 42 +/- 7 kJ mol-1 and 63 +/- 10 kJ mol?¹ for carbon monoxide oxidation and oxygen ionosorption, respectively. In the absence of O- or CO2- surface ions, the energy difference between the Fermi level and the conduction band minimum at the surface is estimated as 590 +/- 90 meV at temperatures close to 400°C. The hydrogen responses of non-functionalised sensors are found to converge at 440°C, despite differing at lower temperatures. This observation is incompatible with the developed model, but it is shown that the phenomenon may be rationalised by considering that the hydrogen concentration close to the sensor surface is decreased due to the rapidity of hydrogen oxidation. Gold nanoparticles significantly enhance the hydrogen response, with gold-decorated nanosheets remaining sensitive to hydrogen below 150°C. Poor sensor recovery is attributed to the formation of long-lived hydroxyl groups formed during hydrogen spillover from the gold surface.

Zinc Oxide has emerged from an unspectacular past in the field of electronics to become one of the most widely researched materials for future devices. Here we investigate the growth and electrical properties of semiconducting ZnO nanowires for future application in the field of Nano-Devices and present a solution to control the behaviour of the electrical contacts. ZnO nanowires (NWs) from initial growth experimentation and optimisation have been thoroughly characterised both structurally and electrically. Structural characterisation revealed the high quality of nanowires from vapour phase and hydrothermal growth that translated to similar measurements of nanowire resistivity. We have confirmed the results of atomic resolution dark field imaging with simulations that no Au catalyst contaminates ZnO nanowires, which makes the material more desirable than Silicon or GaAs. Within the limits of the dark field imaging the interface of the catalyst particle and the nanowire is abrupt, clean and intimate, with no Au diffusion, interfacial layers or roughness. Electron microscopy reveals the Au has an epitaxial relationship with the ZnO and is solid during growth. Using fabrication and contamination free nanoprobe measurements (four-probe scanning tunnelling microscope) in vacuum a transition from rectifying to Ohmic is dependent on contact size and not the materials or structural variations. We have shown this with the application of the nanoprobe on free standing as-grown catalysed ZnO nanowires. Using the most common nanowire growth methods the structure has been thoroughly characterised to allow the interpretation of electrical measurements of resistivity and Au end contacts. A regime of size dependent contacts to ZnO nanowires provides the necessary knowledge and requirements to fabricate ZnO nanowire devices with controlled properties and function. This is a major hurdle for nanodevices overcome without complicated or difficult processing steps. A nanodevice can be fabricated from a substrate, with contacts, in one- step and with tailored interface properties by controlling the catalyst particle size.

Carbon-doped aluminium oxide, α-Al₂O₃ : C, is a highly sensitive luminescence dosimeter. The high sensitivity of α-Al₂O₃ : C has been attributed to large concentrations of oxygen vacancies, F and F⁺ centres, induced in the material during its preparation. The material is prepared in a highly reducing atmosphere in the presence of carbon. In the luminescence process, electrons are trapped in F-centre defects as a result of irradiation of the material. Thermal or optical release of trapped electrons leads to emission of light, thermoluminescence (TL) or optically stimulated light (OSL) respectively. The thermoluminescence technique is used to study point defects involved in luminescence of α-Al₂O₃ : C. A glow curve of α-Al₂O₃ : C, generally, shows three peaks; the main dosimetric peak of high intensity (peak II) and two other peaks of lower intensity called secondary glow peaks (peaks I and III). The overall aim of our work was to study the TL mechanisms responsible for secondary glow peaks in α-Al₂O₃ : C. The dynamics of charge movement between centres during the TL process was studied. The phototransferred thermoluminescence (PTTL) from secondary glow peaks was also studied. The kinetic analysis of TL from secondary peaks has shown that the activation energy of peak I is 0.7 eV and that of peak III, 1.2 eV. The frequency factor, the frequency at which an electron attempts to escape a trap, was found near the range of the Debye vibration frequency. Values of the activation energy are consistent within a variety of methods used. The two peaks follow first order kinetics as confirmed by the TM-Tstop method. A linear dependence of TL from peak I on dose is observed at various doses from 0.5 to 2.5 Gy. The peak position for peak I was also independent on dose, further confirmation that peak I is of first order kinetics. Peak I suffers from thermal fading with storage with a half-life of about 120 s. The dependence of TL intensity for peak I increased as a function of heating rate from 0.2 to 6ºCs⁻¹. In contrast to the TL intensity for peak I, the intensity of TL for peak III decreases with an increase of heating rate from 0.2 to 6ºCs⁻¹. This is evidence of thermal quenching for peak III. Parameters W = 1.48 ± 0:10 eV and C = 4 x 10¹³ of thermal quenching were calculated from peak III intensities at different heating rates. Thermal cleaning of peak III and the glow curve deconvolution methods confirmed that the main peak is actually overlapped by a small peak (labeled peak IIA). The kinetic analysis of peak IIA showed that it is of first order kinetics and that its activation energy is 1:0 eV. In addition, the peak IIA is affected by thermal quenching. Another secondary peak appears at 422ºC (peak IV). However, the kinetic analysis of TL from peak IV was not studied because its intensity is not well defined. A heating rate of 0.4ºCs⁻¹ was used after a dose of 3 Gy in kinetic analysis of peaks IIA and III. The study of the PTTL showed that peaks I and II were regenerated under PTTL but peak III was not. Various effects of the PTTL for peaks I and II for different preheating temperatures in different samples were observed. The effect of annealing at 900ºC for 15 minutes between measurements following each illumination time was studied. The effect of dose on secondary peaks was also studied in this work. The kinetic analysis of the PTTL intensity for peak I showed that its activation energy is 0.7 eV, consistent with the activation energy of the normal TL for peak I. The PTTL intensity from peak I fades rapidly with storage compared with the thermal fading from peak I of the normal TL. The PTTL intensity for peak I decreases as a function of heating rate. This decrease was attributed to thermal quenching. Thermal quenching was not observed in the case of the normal TL intensity. The cause of this contrast requires further study.

No description available.

Orthostatic intolerance following exposure to simulated or actual microgravity is observed following spaceflight and extended periods of bed rest, and is not always associated with simultaneous hypotension. Differential adaptation of cephalic and caudal arterial vasculatures (as a result of removal of the normal hydrostatic gradient) is proposed as a potential mechanism underlying this phenomenon. A potential role for changes to the L-arginine/nitric oxide pathway in such adaptations has been suggested, predominantly from previous in vitro studies; using an established model of simulated microgravity (head-down tilt; HDT). This thesis investigates whether findings in isolated vessels are reflected by in vivo measurements of cephalic and caudal vascular function. Using carotid or iliac artery flow normalized to mean arterial pressure as an index of cerebral or hind limb vascular conductance, autoregulatory cerebral vasodilatation in response to lower body negative pressure was found to be impaired following HDT. In addition, α¬1-adrenoceptor agonist-mediated vasoconstriction was decreased in the cerebral vasculature and increased in the peripheral and hind limb vasculature. Administration of acetylcholine or the non-selective nitric oxide synthase (NOS) inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) demonstrated a decreased contribution of NOS to cerebrovascular tone, but an increased contribution of NOS to peripheral vascular resistance and tone of the hind limb vasculature. Together with a lack of difference in the response to the selective inducible NOS (iNOS) inhibitor 1400W, these results suggest that differential adaptation of eNOS may account for the observed differences between control and HDT animals. Further investigation of the changes to the L-arginine/nitric oxide pathway suggest that these changes are not associated with changes in eNOS expression, but may be related to altered activity of eNOS. Furthermore, the bioavailability (as measured by pharmacokinetic half life) or the vascular effector mechanisms (as measured by the haemodynamic response to exogenously administered nitric oxide) responsible for the effects of nitric oxide were also shown to be unaffected by HDT. These findings suggest that differential adaptation of the L-arginine/nitric oxide pathway may contribute to the inability to raise total peripheral resistance and impaired cerebral autoregulation following HDT, thereby representing a mechanism of orthostatic intolerance following exposure to microgravity. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate

In this work, the optical properties of ZnO doped with arsenic and nitrogen were studied. The ZnO samples were grown by Metalorganic Chemical Vapor Deposition (MOCVD). The solubility of nitrogen in the ZnO films, as well as its activation upon annealing, was also investigated. Hydrogen is known as a major source for passivation of the acceptors in ZnO:N. Therefore, it is crucial to dissociate the complex(es) formed by nitrogen and hydrogen and diffuse out the hydrogen in order to prevent the reformation of such complexes. High temperatures (≥ 600 C) are required for these purposes. In order to effectively remove the hydrogen impurities from the sample, it is important to know the optical fingerprints of hydrogen and its thermal stability. Therefore, the effects of annealing and hydrogen plasma treatment on bulk ZnO (hydrothermally grown) were first studied. The use of bulk material for this purpose was motivated by the well-resolved photoluminescence (PL) lines observed for bulk ZnO, which allow the identification of the different lines related to hydrogen after plasma treatment. Annealing at 850 C was effective for the removal of most of the hydrogen related transitions in the near-band-edge emission. Also, additional transitions at ~3.364 eV and ~3.361 eV were observed after hydrogen plasma treatment, which were ascribed to hydrogen-Zn vacancy complexes. In this work, a comparative study of the annealing ambient and temperature on ZnO films grown on GaAs substrate, using diethyl zinc (DEZn) and tertiary butanol (TBOH), showed that arsenic diffuses in the ZnO films and gives a shallow level in the band gap, which is involved in an acceptor-bound exciton line at 3.35 eV. This shallow level is visible when annealing is performed in oxygen, but not when annealing is performed in nitrogen, and indeed only for annealing temperatures around 550 C. However, annealing in either ambient also causes zinc to diffuse from the ZnO films into the GaAs substrate, rendering the electrical properties deduced from Hall measurements ambiguous. For ZnO:N, NO was used as both oxygen and nitrogen sources. Monitoring the concentration of nitrogen, carbon and hydrogen in the ZnO films, the formation of different complexes from these impurities were deduced. Furthermore, an investigation of the effect of annealing on the concentrations of impurities showed that their out- diffusion was strongly dependent on the crystalline quality of the ZnO films. For porous ZnO films, obtained at low growth temperatures (≤310 C), the out-diffusion of impurities was efficient, whereas for films grown at higher temperatures, which have improved crystalline quality, the out-diffusion was practically nonexistent. The out-diffusion of unwanted impurities may activate the nitrogen dopant in the ZnO films, as was confirmed by the PL measurements on the different samples grown at different temperatures. PL transitions at ~3.24 eV and ~3.17 eV were related to substitutional NO. These transitions were more dominant in the spectra of samples grown at low temperatures. An additional transition at ~3.1 eV was assigned to a donor-acceptor pair transition involving VZn, instead of NO, as previously reported.

No description available.

No description available.