The equilibrium shape of a small particle on a substrate where the particle can sink into the substrate is evaluated.

Single-crystal Al2O3 was implanted with cationic impurities in the dose range 1–4 ⊠ 1016/cm2 and subsequently annealed in either an oxidizing or reducing environment. Following annealing at 1200°C or higher, crystalline precipitates or solid solutions are observed, which are consistent with what is expected from the equilibrium phase diagram.

The joining of aluminum nitride by active metal brazing with a Ag–Cu–Ti foil has been investigated in cross section by transmission electron microscopy. The reaction of AIN with the braze alloy results in the formation of continuous TiN and (Ti, Cu, Al)6N (η-phase) layers at the interface. AIN grains at the interface often display arrays of dislocations, presumably arising from thermal expansion mismatch between the AIN and the TiN (the coefficients of thermal expansion are 43 ⊠ 10−7/°C and 80 ⊠ 10−7/°C, respectively). The adjoining TiN contains small Cu precipitates and may also contain numerous defects. Titanium preferentially penetrates the AIN grain boundaries, resulting in finger-like TiN intrusions into the substrate, which sometimes cover entire AIN grains in a TiN shell. On the other side of the TiN, a continuous layer of equiaxed, defect-free η–nitride grains is found. Beyond this η–nitride layer is the remaining mixture of metallic Ag and Cu. High-resolution electron microscopy demonstrates that the AIN–TiN and TiN–η boundaries are abrupt and contain no additional crystalline or amorphous intervening phases. Particular orientation relationships are occasionally observed at the AIN–TiN interface; these are not always the ones that produce the minimum lattice mismatch. The implications of the observed morphology with respect to the reaction sequence, transitional phases, and structural integrity of the joint are discussed.

Since the characteristics of the electrode made from Ag paste greatly affect the dielectric properties of (Nb, Ba) doped TiO2 ceramics, the processing condition, i.e., baking temperature, was investigated. Low melting glass binder contained in Ag paste reacted with TiO2 ceramics to form an interface layer between Ag electrode and TiO2 ceramics during baking. The interface layer was identified as Bi2Ti2O7 by x-ray diffraction (XRD), and the thickness of the Bi2Ti2O7 layer was estimated from line profiles of EPMA and dielectric properties. The interface layers were found to increase with baking temperature. Increased baking temperature lowered the relative dielectric constant and dielectric dissipation factor of TiO2 ceramics, while it raised the resistivity. Controlling the baking condition of the Ag paste electrode on TiO2 ceramics resulted in reasonably good dielectric properties and excellent temperature stability.

The crystal structure of Na2Al2Ti6O16 was refined by the Rietveld method using synchrotron radiation and conventional x-ray powder data, and the agreement factors were Rp = 3.35%, Rwp = 4.30%, and RBragg = 6.39% for synchrotron data. The formula based on the chemical analysis and 16 O atoms is Na1.97Al1.82Ti6.15O16. The crystal data are monoclinic, C2/m, a = 12.1239(3) Å, b = 3.7749(1) Å, c = 6.4180(2) Å, β = 107.59(1)°, V = 280.00(4) Å3, Z = 1, and Dx = 3.82 g cm−3. The site occupancy refinement showed a partial ordering of Al3+ and Ti4+ ions in the two-crystallographically independent octahedral sites.

Densification arising from chemical vapor deposition within a porous preform is modeled as a problem in diffusion and reaction. The effect of reaction rate on the nonuniformity of the final densified composite is explored. The possibility of achieving uniform densification by starting with an initially nonuniform porosity distribution is demonstrated with a simulation model. This model permits one to design a densification process from first principles using a simulation tool that yields results on a timescale far less than that of the processing laboratory.

A simple analytic theory to quantify the extent of “trapping” of cracks by a crystal lattice and its dependence on the range of the interatomic force law is presented. The theory requires a priori knowledge of only one or two lattice-dependent, but force law independent, parameters and can then reproduce nearly all previous (numerically demanding) theoretical results. Moreover, the extent of lattice trapping does not decrease monotonically with increasing range of the force law. For realistic interatomic potentials, however, lattice trapping is quite small. The analysis is then extended to the case of chemical corrosion by direct chemical attack of the crack tip bond. It is shown that similar slow crack growth thresholds can be predicted by the thermodynamic Griffith approach and a fracture criterion based on a local bond instability.

Niobium aluminide-based intermetallic matrix composites are currently being considered as potential high-temperature structural materials. One of the key factors in the selection of a reinforcement material is its chemical stability in the matrix. In this study, chemical interactions between two niobium aluminides, Nb3Al and Nb2Al, and several potential ceramic reinforcement materials, which include carbides, borides, nitrides, and oxides, were analyzed from thermodynamic considerations. Several thermodynamically stable reinforcement materials have been identified for these two matrices.

We have prepared a series of synthetic n-type pyrite (FeS2) crystals by chemical vapor transport and high temperature solution growth. These and natural crystals were characterized with respect to Hall mobility, carrier concentration, and conductivity, and chemically analyzed by mass spectroscopy and atomic emission spectroscopy. The results are compared in detail to previous work on natural crystals. Conductivity and carrier concentration in our measurements varied between 0.05 and 3.5 (Ω cm)−1 and between 6.9·1015 and 5.4·1017 cm−3, respectively. The peak mobilities have values ranging from 100 to 2000 cm2/Vs. We find that sulfur deficiency and a number of impurity elements, such as Si, Cu, and Al, can qualitatively account for the trends in the electronic properties.

We have produced thin photoluminescent films of Eu-doped Y2O3 (Y2O3: Eu) by low-pressure metalorganic chemical vapor deposition (MOCVD). Yttrium and europium β-diketonate precursors are thermally decomposed in a hot-walled MOCVD reactor in the presence of oxygen at a total reactor pressure of 1 Torr. The films are microcrystalline and photoluminescent as-deposited and contain <1 at.% of carbon contamination. Subsequent annealing at 1200°C in air enhances and stabilizes the luminescence. We have measured the photoluminescent lifetimes of the MOCVD films as a function of temperature for the Eu+35D1 ⇉ 7F1 and 5D0 ⇉ 7F2 transitions using excimer laser excitation at 248 nm. We present a kinetic model which can account for the observed emission lifetimes from 20–1000°C.