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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.

Zirconium press plates have been developed for the production of melamine coated particle board, using shot-peening to achieve the desired plate (and therefore coated particle board) surface texture. Service temperatures of the press plates approach 200��C. This study examined the microstructural effects of extended exposure of shot-peened zirconium to temperatures of 200��C and 300��C. Softening of the surface may reduce wear resistance and possibly the surface morphology of the plate, affecting the usability of the plates. It was discovered that the shot-peened surface of the plates experienced a loss in hardness from approximately 230 VHN (DPH) to about 220 VHN after 560 hrs at 200��C. The same drop in hardness was experienced after only 5.5 hrs at 300��C. This decrease in hardness was determined from hardness profiles before and after heat treating the zirconium to various times from 0.5 hours to 4458 hrs and 2790 hrs at 200��C and 300��C respectively. The decrease in hardness is believed to be a result of static recovery, the annihilation of point and/or line defects and/or alignment of dislocations into relatively low misorientation substantially relatively close to the shot-peened surface (about 35 ��m), and decreased more modestly over the next 100 ��m until virtually no drop was experienced further than approximately 150 ��m from the surface. The shot-peening hardens the surface region which extends about 150 ��m from the surface. Thus, the level of recovery appears to depend on the stored energy associated with cold work, or ambient temperature deformation. This increases from about 2-3% cold work (equivalent percent cold reduction from rolling) in the bulk of the specimens to near 99% at the surface resulting from shot-peening. The dislocation structure of the shot-peened zirconium was examined in the as-peened as well as the annealed conditions using transmission electron microscopy. / Graduation date: 1997

Graduation date: 1952

Graduation date: 1991

In this study, the mechanical properties of nanocrystalline Mo and Zr thin films are examined. The specimens of nanocrystalline Mo and Zr thin films were all fabricated by DC magnetron sputtering at various temperatures. These specimens were annealed in RTA system and then investigated by X-ray diffractormeter as well as TEM. After that, nanocrystalline Mo and Zr thin films were tested by nanoindentation. The average grain sizes in Zr thin films annealed are larger than deposited at high temperature, but the films after annealing are stripped away from the substrates due to the thermal shock. The average grain sizes estimated by XRD patterns are in common with those estimated by TEM images. We suggest that the difference is deviation. Nanocrystalline Mo thin films were first tested by both nanoindentation and tensile tests. Mo thin films were stripped away during tensile tests. We consider that the phenomenon is due to the honeycombed structure of the films. The X-ray diffraction patterns and TEM observations indicated that there is no evident grain growth in the nanocrystalline Zr thin films, deposited at 100 ¢XC, 200 ¢XC, and 300 ¢XC, except at 400 ¢XC. The deposition temperature for apparent grain growth in the Zr thin films is at least above 300 ¢XC. After nanoindentation tests, the hardness (H) and Young¡¦s modulus (E) of specimens deposited at 400 ¢XC are higher than that of other specimens. Compared to coarse-grained Zr metals, we suggest that the slope k in the Hall-Petch relationship is quite small and in the range of nanocrystalline Zr thin films.

The development of fast reactor systems capable of burning recycled transuranic (TRU) isotopes has been underway for decades at various levels of activity. These systems could significantly alleviate nuclear waste storage liabilities by consuming the long-lived isotopes of plutonium (Pu), neptunium (Np), americium (Am), and curium (Cm). The fabrication of metal fuel alloys by melt casting pins containing the volatile elements Am and Np has been a major challenge due to their low vapor pressures; initial trials demonstrated significant losses during the casting process. A low temperature hot extrusion process was explored as a potential method to fabricate uranium-zirconium fuel alloys containing the TRU isotopes. The advantage of extrusion is that metal powders may be mixed and enclosed in process canisters to produce the desired composition and contain volatile components. Uranium powder was produced for the extrusion process by utilizing a hydride-dehydride process that was developed in conjunction with uranium alloy sintering studies. The extrusions occurred at 600 degrees C and utilized a hydraulic press capable of 450,000 N (50 tons) of force. Magnesium (Mg) metal was used as a surrogate metal for Pu and Am because of its low melting point (648 degrees C) and relatively high vapor pressure (0.2 atm at 725 degrees C). Samples containing U, Zr, and Mg powder were prepared in an inert atmosphere glovebox using copper canisters and extruded at 600 degrees C. The successful products of the extrusion method were characterized using thermal analysis with a differential scanning calorimeter as well as image and x-ray analysis utilizing an electron microprobe. The analysis showed that upon fabrication the matrix of the extruded metal alloy is completely heterogeneous with no mixing of the metal particle constituents. Further heat treating upon this alloy allows these different materials to interdiffuse and form mixed uraniumz-irconium phases with varying types of microstructures. Image and x-ray analysis showed that the magnesium surrogate present in a sample was retained with little evidence of losses due to vaporization.

Thèse d'exercice : Chirurgie dentaire : Université de Nantes : 2007. / Bibliogr.