Aluminum gallium nitride (AlxGa1−xN) films, grown by metalorganic chemical vapor deposition on sapphire, were characterized by low-temperature cathodoluminescence (CL) and photoluminescence (PL), and room-temperature optical absorbance. The aluminum fractions are estimated to range from x = 0 to x = 0.444. Most films were silicon-doped. The absorption spectra have a Urbach (exponential) form below the bandgap. The width of the Urbach edge, EU, increases with Al fraction, x, as EU = (0.045 +1 0.104x) eV. The luminescence (CL or PL) spectra show a relatively narrow band-edge peak and a broad deep-level peak. The full-widths at half-maximum of the band-edge CL peaks (measured at T = 15 K) are remarkably similar to the Urbach absorption widths, EU (measured at T = 300 K). PL spectra were obtained from the top surfaces and the film-substrate interfaces of several films. The interface PL spectra of some films show an extra peak 0.15 eV to 0.45 eV below the bandgap, which is ascribed to structural defects or impurity phases localized near the interface. The energy of the band-edge luminescence peak shifts with excitation mode (CL, top-surface PL, or interface PL). This effect is attributed to the variation of the excitation depth, between the top surface and film-substrate interface, with excitation mode, together with the depth variation of film properties such as residual stress or aluminum fraction.

The structural behavior of zirconolite (CaZrTi2O7) under reducing conditions at high temperature has been studied, mainly by scanning electron microscopy (SEM) and x-ray diffraction (XRD), but also with x-ray absorption spectroscopy, thermogravimetry, and electron paramagnetic resonance. The partial reduction of Ti4+ to Ti3+, associated with a reducing atmosphere heat treatment, led to the initial formation of perovskite (CaTiO3) as a second phase. As the concentration of Ti3+ in the zirconolite increased, so did the amount of perovskite until the zirconolite was totally transformed into a fluorite structured phase. Analysis of the reduced zirconolites showed them to be consistently deficient in Ca and enriched in Zr, in proportion to the concentration of Ti3+. To determine how electroneutrality was preserved in these reduced zirconolites, a series of zirconolites were prepared in air using In3+ and Ga3+ as models for Ti3+. These samples were then investigated by neutron and x-ray diffraction, SEM, solid state nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR). 71Ga MAS NMR studies of the Ga substituted zirconolite exhibited a narrow resonance at ˜13 ppm which was attributed to six-coordinate Ga incorporated in a trace perovskite phase. Broadline 71Ga NMR and 69/71Ga NQR were required to characterize the Ga incorporated in the zirconolite. The resultant quadrupolar parameters of CQ = 30.0 ± 0.05 MHz and η = 1.0 ± 0.03 indicate that the Ga site is in a highly distorted environment which would suggest that it is located on the five-coordinate Ti site within the zirconolite lattice. These results were complemented by Rietveld refinement of the neutron diffraction data from the In-doped zirconolite sample, which was optimal when all the In was located on the five-coordinate Ti site with the excess Zr located on the Ca site. It would therefore appear that charge compensation for the presence of Ti3+ in zirconolite is effected via the substitution of an appropriate amount of Zr on the Ca site. The Ti3+-stabilized fluorite structure was readily oxidized back to a single phase zirconolite upon heating in air.

A soft process for the intercalation of different ammonium cations into vanadium pentoxide was developed. By refluxing an aqueous solution containing ammonium , tetramethylammonium (CH3)4N+, and tetraethylammonium (C2H5)4N+ with V2O5 powders, intercalation compounds containing corresponding ammonium cations were obtained. The compound with aniline C6H5NH2 was also synthesized by the same process. The compounds with (CH3)4N+, (C2H5)4N+, and also C6H5NH2 had diffraction patterns consisting of very sharp 00l lines. The spacing for sandwiching the intercalates (the identity period), (CH3)4N+, (C2H5)4N+, and C6H5NH2, was 1.29, 1.31, and 1.39 nm, respectively.

Epitaxial Nb thin films were grown via molecular beam epitaxy (MBE) at different substrate temperatures on α–Al2O3(0001) substrates. For temperatures of 900 °C to 1100 °C, it was found that Nb grows in the Volmer–Weber growth mode (formation of three-dimensional crystallites). Depending on the growth temperature, different epitaxial orientations of Nb films can be found. At a growth temperature of 900 °C, the Nb{111} planes are parallel to the sapphire basal plane whereas at 1100 °C the Nb grows with the {110} planes on the basal plane of sapphire. These orientations are present even in the initial stages of growth at both temperatures. The formation of two different epitaxial orientations of thick Nb films can be conclusively explained only by considering both the change in the total density of Nb islands with temperature and the influence of island size on the total energy of the islands. The Nb island growth process has been investigated in situ using reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were employed to determine the morphology and structure of Nb islands, Nb films, and Nb/α–Al2O3 interfaces.

The relationships among weight loss, gas evolution, and other changes during sintering were investigated for silicon nitride powders synthesized by silica reduction and imide decomposition. Small weight losses were detected for all powders at low temperatures (below 1400 °C), and large weight losses were found to occur for the silica-reduction powders at high temperatures (1400–1800 °C). Only CO was detected in the sampled gases during sintering, and CO evolution peaks were seen at around 900–1200 (broad), 1400, and 1600 °C. The main deposited material around the samples was SiO. The weight losses at low temperatures were thought to be caused by evaporation of residual binder or adsorbed substances. The main causes of the large weight losses at high temperatures were thought to be CO and SiO evolutions, and CO evolution at around 1600 °C probably originated from the liquid phase accompanied by α–Si3N4 to β–Si3N4 transformation.

An advanced wetting method is used to prepare a composite with a high content of ceramic particles in a matrix of photocured polymethylmethacrylate. The method allows for the preparation at minimum stirring of high quality samples with up to 59% solid in volume. The samples present reasonable flow characteristics prior to curing, with high hardness and fracture toughness in the fully cured state. Practical implications of the preparation method for dental restorative applications are studied. The method is also useful in the preparation of similar composites for other practical applications.

An “Auto Ignition” technique was utilized in synthesizing Al2O3 –ZrO2 powders with nano/nano microstructure. The process used the corresponding nitrates as oxidizers and urea as the fuel. The as-synthesized powders were characterized by x-ray diffraction and transmission electron microscopy. It was observed that the microstructure consisted of crystallites of Al2O3 and ZrO2, both of which were nanocrystalline. As opposed to the other nanocomposite ceramics, this feature of the microstructure classifies the present powders as nano/nano type. This nanocrystallinity of the microstructure (crystallite size less than 100 nm) was maintained even after a soaking at 1200 °C for 2 h. Since the microstructure is stable at high temperatures, it was possible to densify the powders by hot isostatic pressing at 1200 °C. The product was 99% of the theoretical density and maintained nanocrystalline grain size. The average hardness and toughness values, as determined by an indentation technique, were 4.45 GPa and 8.38 MPa · m1/2, respectively. These values represent evidence of ductility in these composites since transformation toughening was ruled out in this case. The potential application of these results is expected to be in net shape deformation forming of ceramics.

The dry sliding wear behavior of 3 mol% tetragonal zirconia polycrystals (3Y-TZP) and a composite containing 20 vol.% SiC whiskers have been examined by transmission electron microscopy. High wear rates for the TZP were associated with dramatic microstructural changes. The extreme outer ∼ 400 nm consisted of an amorphous surface layer containing both alumina and zirconia. Below this, the t-ZrO2 grain size was an order of magnitude smaller than in the starting material. At a depth of 1–2 μm the tetragonal grains had become elongated, with a maximum aspect ratio of 30 : 1. The first monoclinic zirconia was found at a depth of 5 μm. In contrast, the composite exhibited a wear rate 5 orders of magnitude lower, associated with minor microstructural changes.

The corrosion behavior of aluminized Type 310S stainless steel (SS) in the wet seal of molten carbonate fuel cells was investigated. Coupons of Type 310S SS were aluminized by two different aluminizing methods: thermal spray and slurry-coating. In both types of samples Fe and Cr diffused readily into the Al layer at 650 °C. At first this interdiffusion is limited to the interfacial area. With time, Fe and Cr aluminides precipitate in the Al layer. The slurry-coated layer contains a higher concentration of FeAl and Fe3Al than does the thermal spray layer. Consequently, the slurry-coated layer also displays a greater degree of corrosion than the thermal spray layer.

The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe0.85Gd0.15O3–δ have been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ~0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba0.96Ce0.85Gd0.15O3–δ.

ZrO2 nanopowders derived from zirconium n-propoxide [Zr(OC3H7)4]-acetylacetone-water-isopropanol have been investigated with respect to their tetragonal metastability on heating-cooling processes. The transformation temperature of metastable tetragonal to monoclinic (t′ → m) phase is found to be governed by ultimate firing temperature, time, and atmospheres employed. Crystallite growth is fastened with increase in calcination temperatures over 1000–1400 °C, and the t′ → m transformation temperature is correlated linearly with crystallite size in the studied range of 12–20 nm. Heating in an oxygen environment increases the size of the final crystallites and hence the rate of the t′ → m transformation. It is revealed that the t′ → m transformation temperature depends largely on the heating atmosphere, but only weakly on the cooling one. Based on the findings of this work, surface oxygen deficiencies are attributed to be responsible for low-temperature tetragonal metastability. A crystallite growth model to explain the decline of t′-ZrO2phase is proposed. Kinetic and thermodynamic factors are also discussed in connection with the existing theories of tetragonal metastability.

The fracture toughness of Ni-sapphire interfaces was measured as a function of interfacial embrittlement. Embrittlement was controlled by segregating sulfur to the interface, by limiting the presence of moist air in the test environment, and by altering the distribution of interfacial particulates. Fracture energies scaled with the degree of embrittlement and ranged from 8.5 to 34.2 J/m2. Scanning probe microscopy revealed four distinct plasticity features, the heights of which ranged from 1 μm to 0.5 nm. Plasticity generation processes are determined based on the variation of feature height and position with fracture energy, allowing features associated with the interface decohesion process to be identified.

Nanocrystalline aluminum powders have been prepared by the gas phase condensation method. Samples have been synthesized in a conventional preparation chamber for gas phase condensation and also in the pretreatment chamber of an XPS (x-ray photoelectron spectroscopy) spectrometer so that in situ studies of the passivation process of nanocrystalline aluminum can be performed. For the range of particle sizes (12–41 nm) studied in the present work, we found a universal behavior during passivation with oxygen of the nanocrystalline Al0. An Al2O3 overlayer of 4 nm, which protects the material from further oxidation, was obtained for all samples independently of the route of oxygen dosage. A careful analysis of the photoelectron parameters (binding energy and Auger parameters) for Al and O shows that in the early stages of passivation the alumina overlayer is so thin (<2.5 nm thickness) that the Al2O3 –Al interface induces an increase in the relaxation energy of the photoholes as compared to that of bulk alumina. Conclusions have been drawn about the best way to proceed during passivation of Al ultrafine particles before exposure to an air atmosphere.

X-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectroscopy were used to characterize the microstructure of polycrystalline SiC films grown on as-deposited and annealed polysilicon substrates. For both substrate types, the texture of the SiC films resembles the polysilicon at the onset of SiC growth. During the high temperature deposition process, the as-deposited polysilicon recrystallizes without influencing the crystallinity of the overlying SiC. An investigation of the SiC/polysilicon interface reveals that a heteroepitaxial relationship exists between polysilicon and SiC grains. From this study, a method to control the orientation of highly textured polycrystalline SiC films has been developed.

The growth of epitaxial thin films of Li1–xNb1–xWxO3 from solution precursors on single-crystal LiNbO3 substrates is reported. An all-alkoxide solution readily gave single phase powders after simply mixing the constituent components, whereas an acetate-alkoxide system required additional solution processing stages to give phase pure powders. Heat treatment of films on single-crystal, basal plane LiNbO3 substrates at 600 °C formed a nanocrystalline, porous film which was converted to an epitaxial film after heating to 800 °C. Measurements of second harmonic generation in powders indicate an increase in SHG efficiency with increasing tungsten content. Optical absorption data for films were calculated using reflectance and transmittance spectroscopy data, and indicate a decrease in bandgap with increasing tungsten.

The effects of chloromethane on diamond nucleation and growth were studied by employing laser reflective interferometry. Chloromethane enhances the film-growth rate only slightly compared to methane. However, chloromethane greatly enhances the nucleation density and shortens the film-forming stage, more significantly at a lower temperature. Thus, chloromethane facilitates the low temperature growth mainly through the enhancement of nucleation. Nucleation density is strongly dependent on the compositions of H atoms and carbon species prior to diamond growth. The residual diamond seeds by diamond-grit scratching are suggested to be the major nucleation sites. Chloromethane can enhance diamond nucleation by protecting the residual seeds from being etched by H atoms.

Composite preform fiber architectures range from the very simple to the complex, and the extremes are typified by parallel continuous fibers and complicated three-dimensional woven structures. Subsequent processing of these preforms to produce dense composites may depend critically on the geometry of the interfiber porosity. The goal of this study is to fully characterize the structure of a 0°/90° cloth layup preform using x-ray tomographic microscopy (XTM). This characterization includes the measurement of intercloth channel widths and their variability, the transverse distribution of through-cloth holes, and the distribution of preform porosity. The structure of the intercloth porosity depends critically on the magnitude and direction of the offset between adjacent cloth layers. The structures observed include two-dimensional networks of open pipes linking adjacent holes, arrays of parallel one-dimensional pipes linking holes, and relatively closed channels exhibiting little structure, and these different structures would appear to offer very different resistances to gas flow through the preform. These measurements, and future measurements for different fiber architectures, will yield improved understanding of the role of preform structure on processing.

The synthesis of polycarbodiimide and polyimide in a cyanate resin precursor was achieved. A unique procedure for achieving a high molecular weight of the molecular composite reinforcement molecules was demonstrated. In spite of phase separation being present during the processing, the final cured composites were transparent. The enhanced mechanical properties and the presence of a single Tg, which increases with rigid rod content, were indications that a molecular composite was achieved. The agreement between measured mechanical properties and those predicted using molecular mechanics simulations CERIUS2 software was encouraging.

Based on observations of plasticity on fracture surfaces, we propose two fracture mechanism for Ni-sapphire interfaces. A brittle-type mechanism is proposed for the decohesion of Ni from sapphire by which cracks advance in increments of 20 nm. When particulates that increase the interface strength are present, debonding occurs at the leading edge of the particulate, and unsteady crack advance occurs. Additionally, toughening mechanisms are proposed for each type of plasticity feature observed, and the fracture energy of each mechanism is quantified. Comparison of energy dissipated by these mechanisms with measured fracture energies indicates how the fracture energy of the interface varies with sulfur segregation and environmental embrittlement.

Aluminosilicate precursors were prepared by mechanochemical treatment of gibbsitesilica gel mixtures. The effect of grinding on their structure and thermal behavior has been examined by 27Al and 29Si MAS NMR, x-ray diffraction (XRD), differential thermal analysis-thermogravimetry (DTA-TG), and Fourier transform infrared (FTIR). After 8 h grinding, the hydrated alumina was completely changed to an amorphous phase which showed a new exothermic DTA peak at about 980 °C due to the formation of γ–Al2O3 or spinel phase. This behavior was related to changes in the Al and Si environments, as deduced from the MAS NMR spectra. With increased grinding time, some 4-coordinated Al appears, together with an Al resonance at about 30 ppm. Simultaneously, a new Si resonance appears at about −90 ppm, indicating a greater degree of homogeneity in the ground samples. Mullite crystallizes at 1200 °C from samples ground for 8–20 h, its XRD intensity increasing with increased milling times, in agreement with the NMR, DTA, and FTIR data. Changes in the Al and Si environments during heat treatment, as reflected by the NMR spectra, are also reported.