Thesis (M.Sc. (Physics)) -- University of Limpopo, 2015 / While the effect of sulphur on c-ZrO2 is often considered in application of advanced solid oxide fuel cells and biomass gasification cleanup, there has been little study on the effect of sulphur on general structure of c-ZrO2. In this work a study of the structural, energetic, electronic and elastic properties of doped c-ZrO2-xSx, t-ZrO2xSx and m-ZrO2-xSx solid solutions has been carried out using ab-initio total energy calculation of the density functional theory under plane wave pseudopotential method within generalized gradient approximation using the self-consistent virtual crystal approximation (VCA). It has been shown that all the calculated properties obtained after relaxation are in good agreement with available experimental and other calculated values, particularly at x=0. Furthermore, the formation and cohesive energies were calculated to determine the relative stability of all three non-sulphated and sulphated polymorphs of ZrO2. The density of states and band structures have been computed for x = 0.0 - 0.5, and the actual size of the band gap of ZrO2 compounds narrowed with partial replacement of oxygen by sulphur, while peaks above Fermi level move towards the Fermi level. The material changes its insulating properties to semiconductor material as a function of sulphur concentration, which might be useful for potential application. We also investigated and calculated, for the first time, the effect composition variation on mechanical stability, the independent elastic constants and other elastic parameters of the sulphated compounds. The polycrystalline bulk moduli, shear moduli, Young and Poisson’s ratio have been deduced by using Voight-Reuss-Hill (VRH) approximation. In addition we also show the geometric and electronic structure of pure ZrOS and ZrS2 and compare them with the obtained geometric and electronic structures of ZrO2-xSx.

In order to achieve a better understanding of the processes that occur during formation and sintering of zirconia, various chemical and physical techniques were used. Along with Perturbation Angular Correlation spectroscopy, that allowed us to investigate microscopic properties inside the nanometer-size zirconia grains, such techniques as Scanning Electron Microscopy and X-ray diffraction were used for determining the size of particles before and after sintering, and Neutron Activation Analysis was employed for measuring the impurity levels in zirconia powders. By controlling the initial conditions and heat treatment of the powders, we investigated the dependence of formation of the charged defects on the existing molecular structure and morphology of zirconia particles. During the study, it was discovered that at low temperature the PAC frequencies of tetragonal zirconia behave very similarly for all materials that were used in this study. If stabilization is achieved by heavy doping, there are shifts and line-broadening due to the presence of dopants but no obvious differences in the essential physics. One material included in this group is Nb-doped zirconia that has no oxygen vacancies. It was concluded that there are no detectable oxygen vacancies in our pure or lightly doped tetragonal zirconia powders before they are heated into the temperature region where sintering occurs. Vacancies are incorporated as the samples are heated above 1050��C, the temperature at which sintering becomes important. The oxygen vacancies in samples that have been heated to 1200��C remain when cooled. We see no vacancy concentration dependence on the atmosphere for samples not doped with +5 valent elements in order to reduce the vacancy density at 1200��C. In several instances, samples that had been heated to a maximum temperature of 1050��C or 1100��C contained a vacancy density that was small (<100 ppm) but measurable. A reduced oxygen pressure increased the oxygen vacancy density by a measurable amount in these samples. Samples that are tetragonal at 800��C are well-sintered after being heated to 1200��C. Samples that are monoclinic below 1170��C are very poorly sintered at 1200��C and contain few vacancies. Flowing Cl in the system as the samples are sintering retards the densification of the grains. These samples had the smallest density of oxygen vacancies. / Graduation date: 1998

The cubic fluorite-type solid solution of ZrO₂ containing 15 mole % CaO has been prepared by the hot-pressing process. The effects of annealing on the change.of lattice parameter, electrical properties, and density of the solid solution have been investigated. The lattice parameter of the cubic solid solution was found to depend on the heat treatment of the specimens. The decrease of lattice parameter with annealing temperature and time has been interpreted either in terms of the removal of interstitial oxygen ions from the." lattice or in terms of the inhomogeneous distribution of the CaO in the ZrO₂ lattice. The activation energy for conduction was also found to depend on the heat treatment of the specimens. The variation of activation energy with annealing temperature has been interpreted in terms of pairing and clustering of the oxygen vacancies with the substitutional Ca ions in the solid solution. The minimum activation energy obtained in the present investigation corresponded to the theoretically predicted activation energy for the migration of oxygen vacancies. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The chemical preparation of ceramic materials has been widely studied over the past few decades, and provides the potential for excellent control over the microstructure and properties of the final product. This control is dependent on a comprehensive understanding of the microstructure and physical/chemical processes that occur at each stage. Aqueous routes have much potential for adoption by industry, but in many cases a comprehensive understanding of the microstructure and chemistry is lacking, partly due to the complicated aqueous chemistry of many transition-metals. This investigation has focussed on a specific inorganic, aqueous, sol-gel route for the preparation of pure zirconia (Zr02). Zirconia is a ceramic with a wide range of current and potential applications, such as catalysis, fuel-cells, coatings and biomaterials. The emphasis has been placed on the characterisation of the structure at each stage of the route, leading to an understanding of the various mechanisms that are at work. This project has also provided an opportunity to investigate broader issues concerning the solution-based processing of zirconia, particularly those involving the 'metastable' tetragonal phase. This phase is frequently observed to be formed by non-equilibrium methods, but the mechanisms of formation and de-stabilisation are not properly understood. The studied route consists of a number of stages: the preparation of an aqueous sol of 'zirconium hydroxide' particles by forced hydrolysis of a zirconyl nitrate solution; the conversion of the sol to a gel by removal of the aqueous phase; the conversion of the gel to a crystalline tetragonal zirconia powder by heating; and transformation of the tetragonal phase to the stable monoclinic phase with further treatment. At each stage of processing a number of aspects of the material structure have been investigated, including the short-range order, crystalline lattice parameters, particle packing, porosity, and speciation of the nitrate anion. This has required a wide range of complementary characterisation techniques, including Raman spectroscopy, XRD, TEM, DTA/TGA, SAXS, dynamic light scattering, EXAFS, NMR, and nitrogen sorption. The importance of techniques that allow changes in structure to be characterised in-situ during heating has been emphasised. The particles in the sol and gel are plate-shaped, approximately 0.5 nm thick and 3 - 4 nm across. They are composed of up to several stacked `sheets' of zirconium hydroxide, each of which is composed of zirconium atoms arranged in a regular square lattice, joined by double hydroxy-bridges. Detailed evidence for this structure has not been previously reported. The stages of decomposition of the precursor have been elucidated, including the stages at which oxolation and loss of nitrate occur. The complex crystallisation process at 450°C has been investigated, and a structural mechanism for crystallisation of the 'metastable' tetragonal phase proposed, based on similarities between the tetragonal crystal structure and the disordered sheet structure in the amorphous material just prior to crystallisation. The crystalline material consists of nano-sized crystals, containing unusual intracrystalline mesopores. The lattice parameters of the tetragonal phase change with increasing heat treatment, with the unit-cell tetragonality (c/a) increasing from 1.017 to 1.020. This is a previously-unreported phenomenon which may be associated with the stability of the phase. The tetragonal phase transforms to the monoclinic phase after heating to a 'critical temperature' between 900 and 950°C; this temperature is associated with the loss of residual surface nitrate species and/or a substantial increase in the mass diffusion rate. The crystal size and surface area has little influence on the tetragonal-to-monoclinic transformation, a result which is contrary to much previously-published work and that has significant implications for certain theories explaining the stability of the tetragonal phase. The transformation itself occurs during cooling, over a range between 400 and 100°C, and has been studied in-situ by time-resolved Raman spectroscopy. The conclusions of this investigation contribute not only to the understanding of this particular route for processing zirconia, but also to a broader understanding of aqueous zirconium systems, the chemical processing of zirconia, and the tetragonal-to-monoclinic zirconia transformation mechanisms.

published_or_final_version / Dental Materials Science / Master / Master of Science in Dental Materials Science

published_or_final_version / Dental Materials Science / Master / Master of Science in Dental Materials Science

Zirconia has now been increasingly used in modern dental prosthetic practice due to its high mechanical strength, good esthetics, and excellent biocompatibility. However, the application of zirconia-based dental restorations is still constrained by its chemical inertness and the resultant relatively weak bonding properties, including porcelain to zirconia bonding, and resin to zirconia bonding. Therefore, many investigations have been carried out on the development of an effective method of surface modification on zirconia for improving its bonding ability. The aim of this laboratory study was to evaluate the effects of some new modified surface treatments on the adhesion durability of dental zirconia prosthetic system. In Part I and Part II, the application of laser surface treatment was examined. Laser energy was utilized and applied on zirconia surface before porcelain veneering procedure. Its influence on porcelain zirconia integration interface was mechanically tested and compared with sandblasting treatment. The changes in porcelain zirconia shear bond strength and mechanical strength values of zirconia with the modulation of output energy were recorded. In Part III and Part IV, several types of coating treatment, such as tribochemical approach, silica powder coating, and zirconium silicate coating, were compared with their effects on resin zirconia bonding. The changes in resin zirconia shear bond strength under different aging conditions were observed. The elemental analysis was also performed for clarifying the chemical composition of zirconia surface. The application of laser produced a flake-like micro-retentive structure on zirconia surface. Laser irradiation with the output energy higher than 11.3 W/cm2 was effective in increasing porcelain to zirconia bond strength values. The biaxial flexural strength of zirconia was not significantly affected by laser irradiation with the settings in this study. However, most of the laser treatments still demonstrated slightly lower flexural strength values compared with the control group. No changes in crystalline structure were detected after laser treatment with X-ray Diffraction (XRD) technique. Resin to zirconia shear bond strength was significantly increased after surface coatings. Without surface treatment the bonding between resin and zirconia was susceptible to artificial aging effects. Zirconia and silica-coating groups demonstrated the highest resistance to hydrolytic influence. On the other hand, the chemical changes of zirconia surface were still in need of clarification. It was concluded that porcelain zirconia bonding could be effectively enhanced by applying both sandblasting and a new laser irradiation approach. Laser irradiation might be a potential approach as a surface treatment for improving the quality of porcelain zirconia bonding interface. The strong and reliable resin zirconia integration could not be achieved without appropriate surface pre-treatment. This coating treatment is a promising approach for strengthening resin to zirconia adhesion. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

No description available.

The chemical preparation of ceramic materials has been widely studied over the past few decades, and provides the potential for excellent control over the microstructure and properties of the final product. This control is dependent on a comprehensive understanding of the microstructure and physical/chemical processes that occur at each stage. Aqueous routes have much potential for adoption by industry, but in many cases a comprehensive understanding of the microstructure and chemistry is lacking, partly due to the complicated aqueous chemistry of many transition-metals. This investigation has focussed on a specific inorganic, aqueous, sol-gel route for the preparation of pure zirconia (Zr02). Zirconia is a ceramic with a wide range of current and potential applications, such as catalysis, fuel-cells, coatings and biomaterials. The emphasis has been placed on the characterisation of the structure at each stage of the route, leading to an understanding of the various mechanisms that are at work. This project has also provided an opportunity to investigate broader issues concerning the solution-based processing of zirconia, particularly those involving the 'metastable' tetragonal phase. This phase is frequently observed to be formed by non-equilibrium methods, but the mechanisms of formation and de-stabilisation are not properly understood. The studied route consists of a number of stages: the preparation of an aqueous sol of 'zirconium hydroxide' particles by forced hydrolysis of a zirconyl nitrate solution; the conversion of the sol to a gel by removal of the aqueous phase; the conversion of the gel to a crystalline tetragonal zirconia powder by heating; and transformation of the tetragonal phase to the stable monoclinic phase with further treatment. At each stage of processing a number of aspects of the material structure have been investigated, including the short-range order, crystalline lattice parameters, particle packing, porosity, and speciation of the nitrate anion. This has required a wide range of complementary characterisation techniques, including Raman spectroscopy, XRD, TEM, DTA/TGA, SAXS, dynamic light scattering, EXAFS, NMR, and nitrogen sorption. The importance of techniques that allow changes in structure to be characterised in-situ during heating has been emphasised. The particles in the sol and gel are plate-shaped, approximately 0.5 nm thick and 3 - 4 nm across. They are composed of up to several stacked `sheets' of zirconium hydroxide, each of which is composed of zirconium atoms arranged in a regular square lattice, joined by double hydroxy-bridges. Detailed evidence for this structure has not been previously reported. The stages of decomposition of the precursor have been elucidated, including the stages at which oxolation and loss of nitrate occur. The complex crystallisation process at 450°C has been investigated, and a structural mechanism for crystallisation of the 'metastable' tetragonal phase proposed, based on similarities between the tetragonal crystal structure and the disordered sheet structure in the amorphous material just prior to crystallisation. The crystalline material consists of nano-sized crystals, containing unusual intracrystalline mesopores. The lattice parameters of the tetragonal phase change with increasing heat treatment, with the unit-cell tetragonality (c/a) increasing from 1.017 to 1.020. This is a previously-unreported phenomenon which may be associated with the stability of the phase. The tetragonal phase transforms to the monoclinic phase after heating to a 'critical temperature' between 900 and 950°C; this temperature is associated with the loss of residual surface nitrate species and/or a substantial increase in the mass diffusion rate. The crystal size and surface area has little influence on the tetragonal-to-monoclinic transformation, a result which is contrary to much previously-published work and that has significant implications for certain theories explaining the stability of the tetragonal phase. The transformation itself occurs during cooling, over a range between 400 and 100°C, and has been studied in-situ by time-resolved Raman spectroscopy. The conclusions of this investigation contribute not only to the understanding of this particular route for processing zirconia, but also to a broader understanding of aqueous zirconium systems, the chemical processing of zirconia, and the tetragonal-to-monoclinic zirconia transformation mechanisms.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available in print.