We report initial results on the synthesis of monolithic aluminum nitride by burning Al–AlN mixtures in high pressure nitrogen. The objective is to synthesize economically large, near-theoretical density AlN parts. In this work, we begin with compacted mixtures of 10 μm Al and 3 μm AlN powder formed into 7.62 cm diameter by 3.81 cm thick disks having densities up to 60% of theoretical. Then, at N2 pressures up to 180 MPa (26 000 psi), we ignite the disk on one face. The fraction of Al converted to AlN, density, and severity of macroscopic cracking vary with N2 pressure and heat transfer from the sample. Presently, products are inhomogeneous, showing regions of relatively high porosity, regions with no porosity but with AlN in a matrix of Al, and regions of nearly theoretical density AlN.

The use of transmission electron microscopy to study the interaction of lattice defects with surfaces of heat-treated ceramic materials is discussed. The approach used throughout the work described has been to prepare a thinned sample in a form suitable for imaging in the electron microscope and then to remove all preparation-induced damage by heat-treating the thinned sample. Applications of the technique to the movement and pinning of individual surface steps in alumina and the grooving of antiphase boundaries in silicon carbide are illustrated.

Acoustic energy has been primarily used in materials research to disaggregate solids and/or disperse them in fluids. It has also been used in a few studies to influence or cause new reactions in organic and organometallic systems. However, the potential of acoustic energy in the area of inorganic reactions and materials science has been virtually unexploited. We report herein new effects of acoustic waves in catalyzing reactions in the systems Al2O3 + H2O and Al2O3 + P2O5 + H2O. In this study the effect of different alumina precursors and their particle size on their reactivity with water and H3PO4 was investigated when exposed to 20 kHz waves.

TiO2 rutile thin films grown on (110) sapphire (α−Al2O3) by the MOCVD technique have been characterized by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The TiO2 rutile thin films grew on the sapphire with two epitaxial orientations. The epitaxial orientation relationships between the rutile films (R) and the sapphire substrate (S) were found to be (1) (101)[010]R ‖ (110)[0001]s. Detailed atomic structures of near-interface regions have been investigated by HREM, providing a clear picture of the initial stage of film growth. HREM images show that about 70% of the nuclei at the interface are the (101) rutile, but most of them are very small, about 5 nm (or 2% of the film thickness) in the growth direction. The film growth was dominated by the (200) orientation. Nucleation and growth of the films will be discussed in terms of the lattice mismatch at the interface and growth rates along the two orientations. Planar defects such as twin boundaries and special grain boundaries are commonly observed in the films, especially in regions close to the substrate. The twin plane and twinning direction are {101} and 〈101〉, respectively. Special grain boundaries are found to be correlated with nucleation and twinning.

Crystallization kinetics and phase transformation characteristics of μ- or α-cordierite seeded gels were compared with those of unseeded monophasic gels. The α-cordierite seeding modified the sequence of phase transformation and lowered the temperature of α-cordierite formation from amorphous gel by ∽150 °C. The μ-seeded transformation was characterized by 3-dimensional isotropic growth with essentially zero activation energy for the nucleation step. Combining the nucleation theory with the transition state theory, we have separately estimated the activation free energy of nucleation and that of growth step in the crystallization of amorphous cordierite gel. The estimated activation free energies showed that the crystallization of unseeded cordierite gel is mainly growth controlled.

Aged bricks of AZS and mixed α-β-alumina refractories have been sampled in superstructures of glass making furnaces. α- and β-alumina phases contained in these refractories have been investigated by optical absorption spectroscopy, electron paramagnetic resonance, and electron probe microanalysis. On the side of the brick exposed to the tank atmosphere, β-alumina is the only phase present. The primary corundum grains are transformed into secondary β-alumina under the influence of contaminants from raw materials and oil ashes. The temperature conditions existing in the furnace preclude the formation of β” alumina. The bright blue color of β-alumina originates from the presence of tetrahedral Ni2+ in Al(2) sites, with no evidence for nickel atoms located in the ionic conduction band. By considering the chemical composition of β-alumina, spectroscopic results are consistent with a mutual interaction between divalent and monovalent species during cation diffusion. Indeed, the small divalent cations such as Ni are located in the spinel block and the larger alkali cations play a charge compensation role in the conduction band. As other divalent cations of small ionic radius, nickel hence helps to stabilize β-alumina, which maintains the refractory performance during furnace operation. The spectroscopic evidence of trace amounts of nickel (<100 ppm) in secondary corundum crystals means that this phase formed at the expense of β-alumina inside the high-alumina refractory brick. By considering the diffusion coefficients of Ni2+ in α- and β-alumina, this indicates a fast contamination of the material at an early stage of the furnace history. The formation of a permanent deep layer of primary and secondary corundum has protected the inner part of the refractory brick from further contamination.

Ultrafine, ternary (Si/C/N) ceramic powders have been produced after CO2 laser initiated reactions between silane and mono- or dimethylamine. Various spectroscopic diagnostics and chemical analyses, including ir spectrophotometry, x-ray photoelectron spectroscopy, and x-ray diffraction analysis have been employed to correlate the parameters involved in gas-phase reactions and the powder final composition.

Zinc oxide (ZnO) powders were mixed with Polyacrylic Acid (PAA) to form waterless cements. The optimal ZnO/PAA ratio in terms of mechanical strength was found. Also, the role of the relation ZnO to PAA, the curing time, and the strain rate on the final properties of the composites was studied.

TiB2 conducts the current and forms a liquid phase at the interface with BN. Neighboring crystals of BN and some TiB2 spall due to thermal shock. During pause periods parts of the liquid and fragments are flushed out by the dielectric. Composites rich in TiB2 or with fine TiB2 grains gave high material removal rates. Increasing the amount of the conducting phase by 10% is as effective as decreasing the grain size from 11 to 7 μm. Coarse TiB2 could withstand high pulse durations before wire breaks. Material removal rate increases with pulse duration, frequency, and current. For the same composition and grain size, increasing the pulse duration or current increased the crater depth (the roughness) up to a certain value, beyond which increasing these parameters yielded a smoother surface. The conductivity of the dielectric was effective only for compositions rich in TiB2 content. In such cases, higher water conductivity lowered the energy required for material removal.

The mechanism of fracture arrest in brittle-matrix composites with strong, long fibers is analyzed by using the inclusion method. The maximum stress contribution of the matrix in composites is discussed in this paper. A critical volume fraction of fibers fc is theoretically derived. If the volume fraction f is less than fc, then debonding between fibers and matrix occurs before the crack propagates through the whole section. If f is greater than fc, then no debonding occurs before the crack propagates through the whole section. The value of fc depends on the matrix and fiber properties and the bond character of the interface. To verify the analytical predictions, experiments on fiber reinforced cement composites subjected to uniaxial tension were conducted. The results of the theoretical predictions were also compared satisfactorily with other published experimental data.

The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete containing silica fume has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland cement–silica fume systems. Scanning electron micrographs of air voids are presented for many different ages. The solidification process of portland cement–paste and mortar was frozen at different ages using both a low-temperature scanning electron microscope and freeze drying. During the course of these experiments, it was discovered that air voids can be considered “windows” in the microstructure in which one can observe early hydration features that give insight into the nature of portland-cement systems. A well-developed sheaf of wheat morphology has been observed in concretes incorporating silica fume, and a new interpretation of the microstructure of silica fume concrete is presented.

Berlinite crystals grown in H3PO4, HCl, H3PO4/HCl, H2SO4/HCl, or H3PO4/HCl/H2SO4 solvents are characterized by acoustic microscopy techniques. Surface and subsurface defects can be visualized via acoustical images, whereas elastic parameters of the crystal can be measured on a microscopic scale. They prove to be of great importance in the identification of not only crystal orientations but of preparation methods as well. We show, for example, that a growth in sulfuric and phosphoric mediums improves mechanical behavior of berlinite crystals. Moreover, it seems that anisotropy plays a fundamental role in this characterization technique with an appearance or a disappearance of specific modes.