No description available.

No description available.

Alumina supported manganese oxides were used in the gas phase oxidation of toluene by ozone. Catalyst activity and characterization, the promotional effect of noble metals (Pt and Pd) on the activity of manganese oxides, and the kinetics and mechanism of the reaction were investigated in this PhD thesis. It was shown that MnO2 and Mn2O3 were the active sites of the catalyst capable of oxidizing toluene to CO and CO2 below 100 oC. Catalysts were deactivated at room temperature due to the accumulation of carbonaceous species on their surface. At least 65 oC was required for the stable operation of the catalysts. X-ray absorption spectroscopy was used to study the structure and electronic properties of the mono metallic and bimetallic catalysts. It was found that the catalysts with higher Mn loading resulted in higher oxidation states of Mn which were less favorable for the oxidation of toluene. The addition of Pt to the Mn containing catalyst increased the reaction rate by transferring electrons from Pt to Mn. On the other hand, no promotional effect was observed by the addition of Pd to Mn. The Oxidation state of Mn atoms was one of the most important parameters, controlling the rate of toluene oxidation. Lower oxidation states of Mn were able to easily transfer electrons to ozone, accelerating the rate of toluene oxidation. A reaction mechanism was proposed for the catalytic oxidation of toluene over manganese oxides. In this mechanism, the oxidation of toluene was carried out by the abstraction of hydrogen atoms followed by the oxidation of toluene carbon skeleton. A rate equation was derived based on this mechanism, determining the reaction orders of -1 and 2 for toluene and ozone, respectively. It was concluded that catalytic ozonation is an effective method for the low temperature oxidation of volatile organic compounds (VOCs) in air. The significance of this method is related to energy saving in air purifying systems by reducing the required temperature to oxidize VOCs. Catalytic ozonation can be used in indoor and outdoor applications for removal of VOCs from enclosed environments or polluted industrial streams.

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Two groups of ferroelectric oxides have been studied using a.c. impedance techniques. These were donor-doped BaTiO₃ ceramics showing the positive temperature of resistance, PTCR, effect and single crystal LiTaO₃. Existing theories of the PTCR effect in BaTiO₃ ceramics assume that it is associated with the grain boundary regions. An in-depth analysis of a.c. data, using combined impedance and modulus spectroscopy revealed the presence of at least two components, both of which exhibited PTCR effects. These were attributed to bulk and grain boundary effects because of the different temperature dependence of their associated capacitances: grain boundary effects have temperature independent capacitances whereas bulk effects show a capacitance maximum at the Curie pont and Curie-Weiss behaviour above the Curie point. An explanation for the bulk PTCR effect is proposed. The a.c. data handling techniques used and developed here provide information regarding the inhomogeneous nature of the grain boundary and bulk components which cannot be obtained from d.c. measurements. An equivalent circuit to model the a.c. response of PTCR BaTiO₃ ceramics is presented. The influence of processing conditions on the various bulk and grain boundary PTCR effects, such as the sample cooling rate from the sintering temperature and low temperature anneals, < 400 °C, in various reducing and oxidising atmospheres is discussed. For quickly cooled samples, the PTCR response is dominated by the bulk impedance, whereas for slowly cooled samples, the grain boundary component dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas bulk PTCR effects are relatively insensitive to the atmosphere at low temperatures. Information regarding the conductive core of the grains and the behaviour of component resistances below the Curie point for slowly cooled samples is presented. A general model is proposed which explains the PTCR behaviour of BaTiO₃ ceramics. The effects of both cooling rate and atmosphere are incorporated in this model. A.c. impedance data for single crystal LiTaO₃ with the crystal c axis oriented parallel and perpendicular to the electric field were recorded above and below the Curie point, 590^oC. With the polar c axis parallel to the electric field, the following were measured: the charge polarisation associated with the ferroelectric domains, the intrinsic lattice polarisation, the resistance associated with domain re-orientation and the resistance due to lithium ion migration. The correct choice of equivalent circuit is crucial to the determination of these parameters. With the c axis parallel to the electric field, no ferroelectric behaviour was observed and the crystal was a modest electronic conductor at elevated tempeatures, 500-700^oC. The LiTaO₃ results presented indicate the potential of a.c. impedance techniques for probing the electrical properties of ferroelectric single crystals. Domain re-orientation may be characterised by macroscopic resistance and capacitance values. This allows the temperature dependence of domain re-orientation phenomena to be characterised as the crystal is heated through the Curie point. Such detailed characterisation cannot be obtained from existing fixed-frequency or d.c. measurements.

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Transparent conducting oxides (TCOs) have attracted extensive attention for decades due to their remarkable applications in optoelectronic devices. The development of functional nanostructured TCOs with unique properties, and an expansion of their functionalities are therefore research directions of significant current interest. Among TCOs, In2O3 is widely applied because of its high charge carrier concentration and mobility, as well as the ease with which it can be deposited as a thin film. The important role of surfaces in tuning properties in materials shows the importance of studying nanostructured materials with high surface areas. In this thesis I examined the synthesis of phase-controlled In2O3 nanocrystals (NCs) and showed the effect of doping and composition on the materials properties. Owing to the relevance of size, structure, and composition for manipulating properties of nanomaterials, synthesis of well-defined nanocrystals of pure and doped In2O3 has been of considerable interest for fundamental studies as well as for technological applications. Phase controlled synthesis of colloidal In2O3 NCs was achieved via a size-structure correlation. The study of the morphological and phase transformations of In2O3 NCs during their growth in solution implies that corundum (rh-In2O3) is a transient structure in the formation of cubic bixbyite (bcc-In2O3) phase. The formation of NCs smaller than 5 nm leads to the spontaneous stabilization of metastable phases owing to the surface energy and/or surface stress contributions, both of which are dependent on size. The growth beyond the critical size lowers the potential energy barrier height and causes the nanocrystal phase transformation. In addition, phase transformation of colloidal In2O3 NCs in the temperature range of 210-260 ˚C during their synthesis in solution was studied using a combination of structural and spectroscopic methods, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and analyzed data using Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) and interface nucleation models. The phase transformation occurs via nucleation of bcc-In2O3 phase at the interface between contacting rh-In2O3 NCs, and propagates rapidly throughout the NC volume. In situ high temperature XRD patterns collected during nonisothermal treatment of In2O3 NCs reveal that phase transformation of smaller NCs occurs at a faster rate and lower temperature, which is associated with the higher packing density and contact formation probability of smaller nanoparticles. Owing to the fact that NC surfaces and interfaces play a key role in phase transformation, their control through the synthesis conditions and reaction kinetics is an effective route to manipulating NC structure and properties. Although, doping semiconductor NCs is crucial for enhancing and manipulating their functional properties, the doping mechanism and the effects of dopants on the nanocrystal growth and structure are not well understood. We show that dopant adsorption to the surfaces of colloidal In2O3 NCs during incorporation inhibit NC growth and leads to the formation of metastable rh-In2O3 for nanocrystals smaller than ca. 5 nm. Direct comparison between Cr3+ and Mn3+ dopants indicates that the nanocrystal structure directly determines the dopant incorporation limits and the dopant electronic structure, and can be predicted and controlled. These results enable a new approach to designing multifunctional nanostructures and understanding the early stages of crystal growth in the presence of impurities. Nanocrystalline films fabricated from colloidal Cr3+- and Mn3+- doped In2O3 nanocrystals exhibit strong ferromagnetic ordering up to room temperature. The absence of ferromagnetism in the free standing transition metal (TM)-doped In2O3 NCs and appearance of ferromagnetism only in TM:In2O3 films prepared from colloidal NCs, are attributed to the formation of extended structural defects, proposed to be oxygen vacancies at the NC interfaces. In fact, in TM:In2O3 NCs with high surface to volume ratios, more oxygen vacancies are present at the surface of NCs and networking of NCs in the prepared film causes an increase in grain-boundary defects at the interfaces. A comparative study of magnetic circular dichroism (MCD) spectra of Cr3+-doped bcc-In2O3 and Cr3+-doped rh-In2O3 revealed that Cr3+ ions distinctly occupy different symmetry sites in corundum and bixbyite crystal structure of In2O3. In fact, a change in the crystal structure of In2O3 from bixbyite to corundum changes the electronic configuration of Cr3+. By manipulating the NC composition and structure in solution we applied a one-step synthesis of ternary gallium indium oxide (GIO) nanocrystals with variable crystal structures. The structures and sizes of GIO NCs can be simultaneously controlled, owing to the difference in the growth kinetics of In2O3 and Ga2O3 NCs, and the polymorphic nature of both materials. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, were used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor -acceptor pair (DAP) recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In3+ occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga2O3 nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition.

No description available.

The subject of this thesis is the gas sensing properties of sensors. / Thesis (PhD)--University of South Australia, 2001.

Iron oxide is a crucial hydrophilic mineral in the minerals, metals and materials industries. Colloidal iron oxide dispersions have proven difficult to dewater to high solid-loading during routine hydometallurgical processes. Despite this, there are a limited number of studies integrating interfacial chemistry, flocculation and dewatering behaviour of iron oxide in the literature. / In this study, fundamental investigations of the principles underpinning hematite pulp dewatering behaviour were examined to determine the effectiveness of four polyacrylamide ploymers (home-polymer, PAM N, anionic carboxylate substituted copolymer, PAM A, anionic sulphonate substituted copolymer, PAM S, and cationic trimethyl amino ethyl substituted copolymer, PAM C) as flocculants and four metal salts, Mn(NO), MnSO, and MnC1and KSO) as coagulants. Effects of flocculant structure type and concentration, metal salt type and pH on the surface chemistry, pulp rheology and the dewaterability of dispersions were considered. Experiments were conducted at the isoelectric point (iep-pH 8.5), below (pH 6) and above (pH 11) the iep of the hematite to investigate particle charge effect in the presence and absence of the coagulants and/or flocculants. / Investigations into the polymer adsorption mechanisms conducted using infrared spectroscopy found PAM A to chemically adsorb onto hematite particles via bidentate chelation on either side of the iep, and via monodentate chelation at the iep. These bonding mechanisms were found to influence the pulp particle interactions (rheology) and hence dewatering behaviour. Hydrogen bonding also occurred between PAM A and PAM N and the hematite particles. Spectroscopic evidence was also provided, for the first time, of the partial hydrolysis of non-ionic polyacrylamide at high pH. / Flocculation in general was found to be most effective when the polymer was of similar charge to the hematite particles. Settling rates of over 100 m/h were achieved at and below the iep after flocculation with the charged polymers, whilst consolidation was generally unaffected by flocculation at any pH. Bridging mechanisms between polymers of similar charge to the particles was most effective. Electrostatic and charge patch flocculation mechanisms prevailing between oppositely charged polymer and hematite particles provided less-improved dewatering. / Surface chemistry was greatly influenced by the anionic polymers at and below the iep of hematite. Addition of sulphate ions caused pronounced particle surface charge suppression with significant zeta potential reduction below the iep. The sulphate ion also facilitated PAM A adsorption above the iep which led to a significant improvement in settling rate. Dewatering of flocculated pulps was more efficient at and below the iep of hematite, with the negatively charged dispersion at high pH generally comprising a turbid supernatant. Manganese salts in synergy with PAM A significantly enhanced settling rates at and below the iep, however, the effect was subtle when compared to the addition of the sulphate ion. Application of mechanical shear to the pre-sedimented pulps produced outstanding improvement in particle consolidation, with up to 20 wt% increase in solid loading observed. / Thesis (PhDApSc(MineralsandMaterials))--University of South Australia, 2008.