Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 155-166).

Thesis (PhD.)--University of Technology, Sydney, 2000. / "A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy, University of Technology Sydney, November 2000" Includes bibliographic references.

Thesis (Ph. D.)--Washington State University, December 2006. / Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 143-154).

Processing of Bulk Metastable Oxide Ceramics Oxide ceramic materials are extensively used in various modern application domains which require properties such as high temperature stability or creep resistance, wear resistance, chemical stability, useful electronic, optical and magnetic properties. In the diverse arena of materials technology that interlinks processing, structure, properties and performance, the advancement of new processing techniques to develop oxide ceramics facilitates the extension and refinement of their desirable properties and also mitigate their limitation in terms of application. Progress in processing science/technology offers a real impetus to the development of metastable ceramics with novel, non-equilibrium structures that exhibit scientifically interesting properties and have the potential to cater to the requirements of modern application areas. In the absence of the equilibrium state of a material system, metastable states can be formed with amorphous phases, extended solid solutions, metastable crystal structures and nanocrystallinity. Such metastable states can be retained by imposing kinetic constraints, which means that under the conditions of temperature and pressure, atomic mobilities are inadequate for the transformation. Metastable ceramics that are produced using non-equilibrium processing routes, such as rapid solidification, vapour deposition, spray pyrolysis, sol-gel technique etc., have been known to possess potentially useful properties, such as hard and soft magnetic characteristics, semiconductivity, varistor action, optical transmittivity and superconductivity. Apart from possessing attractive properties, a metastable phase may also serve as a precursor to a desired microstructure; for instance, controlled crystallization of an amorphous phase is a possible way enroute to nanocrystalline structure. It was well recognized that the comprehensive study and use of nanostructured and other metastable ceramics requires consolidation of the fine discontinuous forms (powders or flakes) produced from non-equilibrium processing routes, into bulk shapes with greater control on the fine scale of the structure. Such efforts have not been entirely successful. Non-equilibrium processing techniques do not produce the metastable materials in bulk form. Consolidation of metastable ceramics into dense forms needs high temperatures, mechanical activation (in the form of static pressures or shock waves), or a combination of both. Such activation may trigger the transformation of the desired metastable phase into a more stable phase with concurrent grain growth. Conversely, conditions that allow the desired metastable phase to be retained may not be adequate for complete densification. The subject of the present thesis is two-fold: (i) processing of dense amorphous Al2O3-Y2O3 materials through a novel densification route involving hot pressing of amorphous powders, produced by co-precipitation, at low temperatures and moderately high pressures, (ii) detail study of possible deformation mechanisms of the amorphous phase from mechanical testing at elevated temperatures. Unusual deformation behavior of the bulk amorphous material has been related to the densification process. Development of Pressure Consolidation Technique Amorphous powder of composition Al2O3-15 mol% Y2O3 (A15Y) was synthesized by co-precipitating a hydroxide from metal nitrate precursor’s solution by using ammonium hydroxide. Chemical homogeneity was ensured by a molecular level mixing of precursors of aluminium and yttrium at room temperature. The as-precipitated powder undergoes thermal decomposition (pyrolysis) to an amorphous oxide by ~770-800o C. The crystallization temperature was found from thermal analysis to be ~900o C, with γ-Al2O3 as the initial product of crystallization. The true density of the amorphous A15Y phase was measured to be only 2.69 g cm-3 by pycnometry, which is ~2/3 of the assemblage of equilibrium crystalline phases consisted of Al2O3 and YAG. Uniaxial hot pressing was performed with decomposed, classified powders (large agglomerates with sizes more than 10 µm were removed by sedimentation technique) at low temperature of about 630-640o C and moderately high pressure of 710-750 MPa. Pressure was held constant for 30-45 minutes. Cold compaction at pressures of 50-65 MPa for 8-10 minutes was carried out prior to hot pressing to ensure green strength of the compacts. All hot-pressed compacts revealed significant densification (95-96% relative densities) with uniformly distributed fine porosity. X-ray diffraction, electron microscopy analysis, Raman spectroscopy and differential thermal analysis established the amorphous nature of the dense, hot-pressed pellets. The amorphous phase displays an elastic modulus of ~ 50-60 GPa and a hardness of 4-5 GPa, which are considerably lower than those of the crystalline counterpart. Deformation Behavior of Amorphous Al2O3-Y2O3 The experiments described above clearly demonstrated the feasibility of producing bulk metastable ceramics in the Al2O3-Y2O3 system by a novel consolidation (viz., low temperature-high pressure) route of amorphous powders. This section of the thesis concentrates on studying the deformation mechanisms of the amorphous phase, which are found to be characteristic of the temperature domain of the experiment. Uniaxial compression tests at temperatures of 650-850o C with constant engineering strain rates of ~3-4 X 10-4 s-1 were conducted on dense amorphous samples made from the hot-pressed compacts. At a temperature of 850o C, i.e., close to the crystallization temperature, the amorphous phase was characterized by homogeneous deformation with continuous work hardening after yielding, accompanied by an increase in the true density of this glass by 10-12%. X-ray and electron microscopy analysis confirmed that the density increase was not due to the formation of nano-crystals at this high temperature. Raman spectroscopy and differential thermal analysis further corroborates that the glass was amorphous even after deformation. No shear instabilities were formed at the side surfaces due to the deformation. Significantly large compressive longitudinal strains up to about 28% were observed before unloading. Moreover, an interrupted loading-unloading test established that the bulk density increase was monotonic with the existence of multiple amorphous states enroute to a succession of denser structures. A simultaneous increase in both hardness (H) and modulus (E) of the amorphous phase of up to 100% after deformation bolstered this experimental observation of bulk density increment at constant porosity. The above evidence clearly points towards significant structural changes of the amorphous phase during high temperature deformation process and therefore a phenomenon of molecular densification of the amorphous structure through a hierarchy of dense amorphous phases was hypothesized, analogous to density or entropy driven amorphous-to-amorphous phase transitions (polyamorphism). Note that the densification described here does not refer to the conventional removal of porosity in a ceramics. At an intermediate temperature of 725o C, which is significantly (~200o C) below the crystallization temperature, plastic deformation commences at a stress (yield stress) of 700-780 MPa (considerably higher compared to the yield stress at 850o C) and continued to deform plastically with a slowly decreasing flow stress before reaching a plateau. Thus, the glass exhibited flow softening, in contrast to flow hardening observed at 850oC. Plastic deformation of this glass is largely non-viscous through shear instabilities (akin to the low temperature deformation behavior of metallic glass) and resulted in 8% increment in bulk density after deformation. Once again, the amorphous nature of the glass after deformation was confirmed by X-ray and electron microscopy analysis. Therefore, this intermediate temperature domain was characterized by both densification and shear. Deformation at even lower temperature, viz., at the temperature of hot pressing (650o C), was also characterized by elastic-plastic behavior (similar to flow softening described above), with immediate yield drop after yielding and resulted in a fairly large amount of plasticity of about 17% before unloading. The bulk density was found to be increased only by 2%. Another very interesting experimental finding from the present investigation is the time-dependent deformation (viz., creep densification) exhibited by this glass. It was established from the result of longer term creep experiment at 850o C that the glass revealed large uniaxial compression of about 15% with 5.5% densification to a density of 3.02 g cm-3. Strain rate sensitivity of the A15Y glass was revealed by another stress jump test. To summarize, the present thesis elucidates the discovery of a new class of ceramics with unusual physical properties in an amorphous mixture of Al2O3-Y2O3, which is in contrast to the conventional brittle ceramics. This new class of ceramics deforms plastically without any hydrostatic containment, like ductile metal, at temperatures about 1000o C below those at which their crystalline counterpart would deform. The behavior of this amorphous ceramics under stress that leads to unusually large change in shape, density, hardness and modulus with hierarchies of amorphous structures is demonstrated in detail with experimental evidence.

<p>Hypersonic flight causes ultra-high surface temperatures which are most intense on sharp leading edges. One way of reducing the surface temperature is to apply a high emittance ceramic (HEC) on the leading edge, increasing the radiation component of heat transfer. An ideal HEC must have a high emittance, while also possessing a strong ablation resistance. From a scientific standpoint, it would be helpful if emittance could be tailored at different wavelengths. For example, materials with tailorable emittance could be used to improve the efficiency of engines, thermo-photo voltaic cells, and other applications. The approach used to create a ceramic with tailorable emittance was to use two different rare-earth elements, adding them to an ultra-high temperature ceramic (UHTC) in small quantities. The samarium element was added to increase the emittance of the UHTC over a large wavelength range (visible to near infrared wavelengths, consistent with the temperature range expected for hypersonic flight), and the erbium element was added to decrease the emittance at specific wavelength ranges. The goal of this study was to create an UHTC with tailorable emittance while maintaining the required ablation resistance. Therefore, ZBS billets with five different Sm to Er ratios and with a nominal total amount of 3 mol.% dopant incorporated were prepared by sintering in vacuum to 2000 °C. The ablation resistance was evaluated by using an oxyacetylene torch and observing at exposure times of 60 s and 300 s, whereas the emittance was evaluated at the Air Force Research Lab facilities via a laser heating testing. The results for the ablation testing showed that ZrB₂-SiC (ZBS) billets co-doped with Sm and Er formed a beneficial <i>c₁</i>-(Sm/Er)_0.2Zr_0.8O_1.9 oxide scale as the majority phase, which is more thermally stable than the <i>m</i>-ZrO₂ oxide scale typically formed in oxidized ZBS systems, resulting in a more adherent oxide scale to the unreacted material. The crystalline oxide scale and the amorphous phase were formed by a convection cell mechanism where the <i>c₁</i>-(Sm/Er)_0.2Zr_0.8O_1.9 crystalline islands precipitate, grow, and coalesce. Moreover, differences in surface temperatures between ZBS samples with different dopant ratios suggest differences in spectral absorptance/emittance between each of the five compositions evaluated. Despite that the emittance profiles with varying Sm:Er molar ratios were similar because <i>m</i>-ZrO₂ was formed as the major oxide phase, the emittance study showed that the erbium oxide influences the emittance profile, as can be noted by the maximum and minimum emittance peaks. Furthermore, results showed that the emittance varies as a function of dopant(s) molar ratios and temperature at shorter wavelength ranges. These changes in the emittance are caused by the different Sm and Er concentration on the surface. Future work should be focused on producing the beneficial <i>c₁</i>-(Sm/Er)_0.2Zr_0.9O_1.8phase directly from the manufacturing process, and therefore, maximize the effect of varying the Sm:Er molar ratios to tailor the emittance. Nonetheless, this study represents the first generation and reported emittance data of UHTC doping ZBS systems with both Sm and Er elements. </p>

Nuclear techniques such as perturbed angular correlation (PAC) sample the hyperfine interactions of a large number of probe atoms in specific crystallographic sites. Real crystals contain static defects producing a distribution of electric field gradients (EFGs) that add to the ideal EFG of the crystal at any given probe site. Also, dynamic defects like moving vacancies and interstitial atoms can be present in the crystal and contribute to the distribution of EFGs. The distribution of EFGs leads to line broadening and a change in the observed asymmetry parameter η since the total EFG no longer has the symmetry of the perfect crystal. When both defects are present in a material, obtaining quantitative information from the analysis of PAC spectra is usually very difficult since great care has to be taken to ensure that the source of line broadening is identified correctly. In order to relate the relationship between the static line broadening and changes in the asymmetry parameter η, a uniform random distribution of point charges was used to simulate the static defect EFG. PAC spectra collected on cubic niobium metal, cubic stabilized zirconia and Nb-doped tetragonal zirconia were fitted with this model. Although the quality of the fits is good, more work is needed to clarify the relationship between the new model parameters and the line broadening and asymmetry parameter derived from conventional model fits. The PAC spectra of Nb-doped tetragonal zirconia were fitted with a conventional static model to establish a reliable relationship between line broadening and the asymmetry parameter when only static defects are present in a sample. To account for effects of dynamic defects, a four state stochastic model for vacancy motion was adapted in order to include the line broadening and changes in the asymmetry produced by static defects. As a result, the activation energies corresponding to the rates at which a oxygen vacancy is trapped by, detraps from, and hops among equivalent sites about a PAC probe atom were calculated. The values that were found are physically reasonable, indicating that the dynamics of an oxygen vacancy around a PAC probe atom are satisfactorily described. / Graduation date: 2003

Thesis (Ph.D.)--McMaster University, 2006. / Supervisor: Patrick S. Nicholson. Includes bibliographical references (leaves 146-169).

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 166-174).

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