Rutherford backscattering spectrometry (RBS) was used to measure the titanium concentration profile for hydrogen-reduced, vacuum-reduced, and as-received, stoichiometric rutile. These profiles give the degree of reduction, specifically, the extent of oxygen deficiency, as a function of depth below the sample surface. Using forward-recoil spectrometry (FRES), the hydrogen-reduced rutile was found to contain more bulk and near-surface hydrogen than the as-received, stoichiometric rutile. This observation provides additional evidence for a hydrogen-diffusion model for the reduction of rutile in a hydrogen environment.

The microstructure of pressureless sintered silicon carbide (SiC) materials with alumina (Al2O3) addition was investigated using analytical electron microscopy and nuclear magnetic resonance. A sintered body with a density of higher than 99% theoretical was obtained with an addition of 5 wt.% Al2O3. The sintered body (SiC–Al2O3) has high strength, high fracture toughness, and high fatigue resistance. Its fracture toughness is approximately 5 MPa-m1/2, which is twice as high as that of pressureless sintered SiC materials with boron and carbon additions (SiC–B–C). The correlation between the microstructure and the mechanical properties is presented here. The starting β–SiC powder is mostly transformed to α–SiC with various polytype distributions during the sintering process. The microstructure has homogeneously distributed, fine, plate-like interlocking gains with a high aspect ratio. Well-developed basal planes form parallel and elongated boundaries, and the crystal structure is mostly the 6H polytype (56%) mixed with thin lamellar 4H.

Beta–SiC specimens possessing 15% stacking fault density were annealed at various temperatures for various time periods under an Ar or a N2 atmosphere, and the mechanisms of stacking fault annihilation and grain growth were investigated. The values of the geometric factor in the Avrami–Erofeev equation indicated that the rate of stacking fault annihilation is controlled by the atomic diffusion process. On the other hand, the rate of grain growth was found to be limited by surface diffusivity. Coincidence in the values of activation energy for stacking fault annihilation and grain growth within experimental errors firmly suggested that the annihilation of stacking faults is an apparent phenomenon resulting from the microstructural development in which the grain growth is controlled by surface diffusivity. Incorporation of nitrogen during heating suppressed the surface diffusivity and, hence, the rate of stacking fault annihilation.

In this work, we explore the use of the chemical vapor composites (CVC) process to increase the rates of silicon carbide (SiC) growth on graphite substrates. Large SiC seed particles are used that are deposited by gravity-driven sedimentation. The results show that addition of large (dp = 28 μm) SiC seed particles to a gas phase containing hydrogen and methyltrichlorosilane increases the deposition rate of SiC by amounts substantially higher than that expected from the addition of the particle volume alone. Insight into the mechanism of this deposition rate enhancement is obtained through analysis of SEM photographs of deposits. Growth rates and deposit structures are consistent with the trends predicted by the previously developed random-sphere model of simultaneous particle-vapor deposition (RASSPVDN), which is used here to interpret the data.

Nanocrystalline α–Fe, Fe3C, and Fe7C3, particles with narrow size distributions were produced by CO2 laser pyrolysis of vapor mixtures of Fe(CO)5 and C2H4. Details of the synthesis procedure are discussed. Mossbauer spectroscopy and x-ray diffraction were used to identify the structural phases and the former was used also to study the magnetism of the nanoparticles. All the nanoparticles were observed to be ferromagnetic in this size range. If excess C2H4 appears in the reactant gas mixture, several monolayers of pyrolytic carbon were observed to form on the particle surface, as deduced from transmission electron microscopy and Raman scattering studies. Results of thermo-gravimetric analysis/mass spectroscopy studies of this carbon coating indicate it is gasified in hydrogen at temperatures T ∼ 250 °C.

Sintering of pure SnO2 to high densities is difficult due to its high vapor pressure, and hence, additives are typically used to enhance densification. In this study, the effects of two lithium compounds, LiF and LiNO3, on the densification behavior of SnO2 were evaluated. While LiF resulted in only a modest improvement in densification, LiNO3 additions resulted in densities of ≥ 95% theoretical at 1500 °C in air. Thermal, x-ray, and SEM/TEM microstructural analyses indicated no liquid phase formation. From these studies we attribute the enhanced sintering behavior to the ionic-compensation of Li+ as an acceptor dopant, i.e., 3[Li‴sn] = 2[Voö], which in turn increased the diffusivity of oxygen.

Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of ≍10–15 nm, were prepared under hydrothermal conditions of 10 MPa at 300 °C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1–10 °C/min up to 1350 °C. The undoped CeO2 started to sinter at ≍800–900 °C and reached a maximum density of 0.95 of the theoretical at 1200 °C, after which the density decreased slightly. Isothermal sintering at 1150 °C produced a sample with a relative density of ≍0.98 and an average grain size of ≍100 nm. The samples sintered above 1200 °C exhibited microcracking. The decrease in density and the microcracking above 1200 °C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol% Y2O3 was sintered to almost full density and with a grain size of ≍200 nm at 1400 °C.

The formation of hydroxyapatite by an acid-base reaction between solid calcium phosphates at temperatures from 5 to 60 °C was examined. The basic reactant used is Ca4(PO4)2O, while the acidic reactants include CaHPO4, CaHPO4 · H2O, and Ca(H2PO4)2 · H2O. Rates of heat evolution during reaction were determined by isothermal calorimetry. The variations in the proportions of reactants and hydroxyapatite and the formation of intermediate products were assessed by x-ray diffraction. Development of microstructure was observed. Generally, hydroxyapatite formation occurs by rapid initial reaction followed by a period during which reaction occurs slowly. Apparent activation energies calculated for the reaction when CaHPO4 is the acidic reactant show differing values depending on its surface area. When the acidic reactant is Ca(H2PO4)2 · H2O, intermediate products are formed. At low temperatures the intermediate is CaHPO4 · 2H2O, while at higher temperatures it is CaHPO4. Above 38 °C, the rate during the period of slow reaction decreases with increasing temperature. This appears to be related to the retrograde solubilities of the reactants, CaHPO4 · 2H2O and CaHPO4, and of HAp.

Syntheses of ultrafine glass powders in the MgO–Al2O3–SiO2 ternary oxide system were effected by drying and calcining sols derived from acetylacetonate-tetraethoxysilane ethanol solutions. A mixture of magnesium and aluminum acetylacetonates, dissolved with Si(OC2H5)4 in an ethanol solution in a 2:4:5 Mg: Al: Si mole ratio, was hydrolyzed, spray-dried, and calcined to yield a glass powder of 70 Å primary particle size, which sintered to densities ≥ 2.5 g/cm3 and which formed μ-cordierite as the only crystalline phase below 1000 °C. Incorporation of dopant quantities of boron and phosphorus, via their alkyl esters in the hydrolysis reaction with Si(OC2H5)4 and the acetylacetonates, afforded powders which crystallized in the α-phase directly after sintering to density = 2.66 g/cm3.

Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (β), activation energy (γ), the rate of surface heat loss by convection (ω), the rate of surface heat loss by radiation (δ), and the rate of reaction (λ) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated to be γ > β > δ > ω. The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with δH/Cp > 1.5 × 103 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.

Sol-gel methods were used to prepare chemically modified lead titanate (PT) powders. A PT alkoxide solution was synthesized and doped with Si (2–12 mole%) or with equimolar amounts (2–12 mole%) of Si and Pb through the addition of Si and Pb–Si alkoxide solutions, respectively. PT alkoxide solutions were also prepared with excess Pb and Ti (7 and 10 mole%). Gels were prepared through controlled additions of water. Crystalline phase development of gel-derived powders with heat treatment (400–700 °C) was studied using x-ray diffraction (XRD) and differential thermal analysis (DTA). While PT powders without added Si crystallized directly into a perovskite phase, Si modified materials crystallized first into a pyrochlore phase and at a higher temperature transformed into perovskite. The pyrochlore lattice parameter and the temperature for the transformation to perovskite increased with Si content. In all cases, the crystal structure of the final perovskite phase was not affected by the Si addition. The effect of Si on phase development and mechanisms of transformation is discussed.

High-pressure phase transformation of beryllium gallium oxide (BeGa2O4) has been studied. Applying high pressure at elevated temperatures to the original hexagonal BeGa2O4 (β–Si3N4-type structure), a high-pressure modification with orthorhombic structure (olivine-type structure) was obtained, i.e., o-BeGa2O4. Lattice parameters of the new phase were determined to be a = 0.5698, b = 0.9759, and c = 0.4551 nm. The pressure and temperature ranges where the high-pressure phase was observed are 3.5 to 7.5 GPa and 800 to 1600 °C, respectively. A tentative pressure-temperature phase diagram for BeGa2O4 was proposed. Transformation is not straightforward; decomposition of the original phase into single oxides and their recombination to form o-BeGa2O4 are necessary. This process seems to apply in both ways, formation and decomposition of the high-pressure phase. The stability of the high-pressure phase is explained in terms of the total molar volume for the phase, the result of summing up molar volumes of constituent compounds. This is the first known report on transformation of β–Si3N4-type structure into a denser structure.

GeSe2 can exist in both amorphous and crystalline phases. Although most semiconductor devices are constructed from crystalline materials, the use of amorphous materials in devices has high potential. The study of GeSe2 is especially interesting since it has been established that an amorphous-to-crystalline transition can be induced by laser irradiation. In order to better understand this phenomenon, it is necessary to study the microstructure of GeSe2 glass. Therefore, transmission electron microscopy studies were undertaken to investigate the degree of crystallinity of GeSe2 glass. It was found that small microcrystallites with diameters in the range of 100–300 Å were embedded in a glass matrix. These microcrystallites formed larger clusters in some areas.

We have measured the real (dielectric constant) and imaginary (loss factor) components of the complex relative permittivity at 298 K using microwave frequencies (2, 10, and 18–40 GHz) for bulk SiO2-aerogels and for two types of organic aerogels, resorcinol-formaldehyde (RF) and melamine-formaldehyde (MF). Measured dielectric constants are found to vary linearly between values of 1.0 and 2.0 for aerogel densities from 10 to 500 kg/m3. For the same range of densities, the measured loss tangents vary linearly between values of 2 × 10−4 and 7 × 10−2. The observed linearity of the dielectric properties with density in aerogels at microwave frequencies shows that their dielectric behavior is more gas-like than solid-like. The dielectric properties of aerogels are shown to be significantly affected by the adsorbed water internal to the bulk material. For example, water accounts for 70% of the dielectric constant and 70% of the loss at microwave frequencies for silica aerogels. Because of their very high porosity, even with the water content, the aerogels are among the few materials exhibiting such low dielectric properties. Our measurements show that aerogels with greater than 99% porosity have dielectric constants less than 1.03; these are the lowest values ever reported for a bulk solid material.

Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 μm for a film containing 45 wt.% PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite materials.

The kinetics of oil absorption into cotton-phenolic ball-bearing retainer material was determined. The results fit a model of two-step absorption in which the first step can be described by capillary rise and the second step by diffusion of oil from the capillaries into the bulk phenolic resin. The capillaries are associated with the cotton threads and are on the order of several micrometers or less in radius. The model successfully describes data for two cotton-phenolics with different cloth weaves, and five nonpolar liquids (four lubricating oils and heptane) with viscosities that vary over 3 orders of magnitude. The total amount of oil absorbed at the completion of the first step (1.5 to 3.4% v/v in this work) is proportional to the volume of the sample, not its surface area, suggesting that the entire volume of the material stores oil and not just the surface region. If the material is not completely dry when submerged in oil, the capillaries are blocked and only surface absorption of oil can take place. The saturation amount of oil in the resin after diffusion is complete at about 2 to 3% v/v (not including the amount of oil stored in the capillaries). The diffusion coefficient calculated using our model is 3 × 10−12 cm2/s, which is reasonable when compared with available literature data on large and small molecules diffusing into polymers.