High purity, ultrafine SiC powders have been produced from gas phase reactants (SiH4, C2H2) in a CO2 laser induced process. The flow reactor designed to operate with a medium power (10–50 W) continuous wave CO2 laser source is described. The mechanism of gas phase reactions involved has been investigated by means of on-line optical diagnostics. Powders produced have been characterized by means of conventional chemical and spectroscopic methods. The x-ray results point out a growth mechanism by coalescence; i.e., whole islands move in the flame to take part in binary collisions, analogously to that observed for particles produced by inert gas evaporation.

Dense polycrystalline thin films of ZrO2 (3 and 8 mol % Y2O3) were produced by the pyrolysis of zirconium acetate precursor films, which were deposited on single crystal Al2O3 substrates by spin-coating aqueous solutions of zirconium acetate and yttrium nitrate. Dense films were heat treated to encourage grain growth. With grain growth, these films broke into islands of ZrO2 grains. Identical areas were examined after each heat treatment to determine the mechanism that causes the polycrystalline film to uncover the substrate. Two mechanisms were detailed: (a) for a composition which inhibited grain growth and produced a polycrystalline film with very small grains, the smallest grains would disappear to uncover the substrate, and (b) for a composition which did not inhibit grain boundary motion, larger grains grew by enveloping a smaller grain and then developed more spherical surface morphologies, uncovering the substrate at three grain junctions. In both cases, the breakup phenomenon occurred when the average grain size was larger than the film thickness. Thermodynamic calculations show that this breakup lowers the free energy of the system when the grain-size-to-film-thickness ratio exceeds a critical value. These calculations also predict the conditions needed for polycrystalline thin film stability.

Solutions of zirconyl salts can be treated in a variety of ways to precipitate hydrous amorphous ZrO2. The phases formed upon heating these precipitates are described and rationalized based upon a theory of hydrolytic polymerization which had been proposed at an earlier stage.

Compressive behavior of an open cell, porous ceramic has been examined and compared to a prior theoretical model. The study involved (i) microstructural characterization, (ii) crushing strength and Young's modulus measurements, and (iii) construction of a deformation-mode map. Initially, the crushing behavior was found to be different than predicted theoretically. Weaker struts throughout the material fractured during the loading and this damage was accumulated until a macroscopic crack or cracks propagated through the material at the crushing stress. Further work showed the discrepancy was related to the uniformity of loading in these porous materials. The use of compliant faces on the loading rams improved the loading uniformity, leading to a substantial reduction in the experimental scatter and increasing the likelihood of unstable crack propagation events rather than damage accumulation. Both crushing strength and Young's modulus were found to be dependent on cell size, but this was considered to be a result of strut cracking at the smallest cell size. A deformation-mode map was constructed using the average stress/strain values at critical points such as the onset of crushing, the minimum crushing stress, and the densification stress. Although some of the details of the deformation map were different from that expected theoretically, the map did appear to be a useful guide to the compressive behavior.

Results of crack growth observations on mica in water-containing environments are described. The study focuses on equilibrium crack states for reversed loading cycles, i.e., for initial propagation through virgin solid and subsequent retraction-repropagation through healed or misoriented-healed interfaces. Departures from these equilibrium states are manifest as steady-state forward or backward crack velocities at specific applied loads. The equilibria are thereby interpreted as quiescent, threshold configurations G = WE, with G the Griffith mechanical-energy-release rate and WE the Dupré work of adhesion, on crack velocity (v-G) diagrams. Generally, WE is found to decrease with concentration of water, in accordance with a Gibbs formalism. Hysteresis is observed in the forward-backward-forward crack propagation cycle, signifying a reduction in the adhesion energy on exposure of the open interface to environmental species prior to healing. This hysteresis is especially marked for those interfaces that are misoriented before healing, indicating that the structure of the underlying solid substrate as well as of the intervening fluid is an important consideration in the interface energetics. The equilibrium states for different environments can be represented on a simple energy-level diagram, as differences between thermodynamic end-point states: initial, closed-interface states refer to crystallographic bonding configurations ahead of the crack-tip adhesion zone; final, open interface states refer to configurations behind the crack-tip zone. The significance of this diagram in relation to the fundamental atomic structure of interfaces in fracture and other adhesion geometries, including implications concerning kinetics, is discussed.

Dilatometry, high-temperature x-ray diffraction, differential thermal analysis, and differential scanning calorirmetry have been performed on LaGaO3, NdGaO3, PrGaO3, SmAlO3, and LaAlO3 single crystals grown by the Czochralski technique. First order phase transitions have been located at 145°C for LaGaO3 and 785°C for SmAlO3, and ΔH has been measured for the LaGaO3 transition. Second order transitions have been identified for LaGaO3, PrGaO3, NdGaO3, and LaAlO3. The usefulness of these compounds as substrates for high temperature superconducting films is discussed in terms of thermal expansion matching.

The effect of 647 nm Kr-ion laser irradiation on small, 1 μm diameter regions of (In0.43Sb0.57)0.87Ge0.13 and In0.43Sb0.57 amorphous as-deposited thin alloy films was determined. Laser irradiation times of 50 ns-20 μs produced three distinct reflectivity states which differed from the original as-deposited values for both alloys. Both alloys exhibited an amorphous state of increased reflectivity after short pulse (∼100 ns to ∼500 ns) laser irradiation which has not been previously observed. Slightly longer pulses caused surface corrugations to form in these high reflectivity regions. The formation of corrugations was consistent with stress relaxation. Further laser annealing caused crystallization and decreased reflectivity. The addition of Ge increased the minimum time to crystallize by one order of magnitude, inhibited the precipitation of pure Sb crystals, and caused crystallites to nucleate at the spot edge rather than from the center of the spot. Optical, microstructure, and electrical changes were correlated. SEM and TEM showed that regions of similar reflectivity could result from significantly different microstructures.

The refractive index of the surface of a zirconium fluoride glass was increased by exchanging fluorine in the glass with gaseous chlorine. The chlorine profile in the glass surface, as measured by Rutherford backscattering, was that expected for diffusion. The refractive index change was measured with ellipsometry, and compared to models of the concentration profile of chlorine.

Recently acoustic emission (AE) techniques have been used to study crack propagation in materials. The application of these techniques to heterogeneous, quasi-brittle materials such as concrete requires a better understanding of how the signal generated from a microfracture is transformed due to wave propagation and due to the transducer response. In this study, piezoelectric transducers were calibrated using displacement transducers. The validity of an elastodynamic Green's function approach was examined for cement-based materials. The acoustic emission source was characterized using moment tensor analysis. Acoustic emission measurements were analyzed for center-cracked-plate specimens of mortar and concrete. It was observed that, as expected, the dominant mode of cracking was mode I (tensile). However, mode II (shear) and mixed mode cracks also occurred, perhaps due to grain boundary sliding and interface debonding. Microfractures appear to localize prior to critical crack propagation. Mode I cracks generally required more energy release than mode II and a smaller inclusion provided a stronger interface bond than the larger ones.

The chemical composition and structure of MoS2 solid lubricant films are intimately related to their friction and wear characteristics. We have conducted an x-ray photoelectron spectroscopy (XPS) study of 1-μm thick radio frequency (rf)-sputter-deposited MoS2 films to determine the chemical state of the films, focusing on the role of oxygen impurities. Concentrations of chemisorbed and bulk species were determined from the Mo 3d, S 2p, and O 1s peak shapes and intensities after annealing the films to temperatures from 425 to 975 K. Films deposited on substrates that were at ∼345 K [ambient temperature (AT) films] and on substrates heated to ∼525 K [high temperature (HT) films] both had ∼10% oxygen within the bulk of the films. The relative areas and shapes of the XPS peaks for the HT films at all annealing temperatures were consistent with the formation of a MoS2−xOx solid solution, where O atoms were probably substituted into S sites in the 2H–MoS2 crystal lattice. In AT films, this phase composition was stable only for annealing temperatures ≥725 K, in agreement with previous studies of the changes in crystal structure of AT films with annealing. The results are discussed in terms of previous studies of the structure and composition of sputter-deposited MoS2 films.

The question of stability of a planar solid-liquid interface in undercooled pure and alloy melts has been reconsidered without the restrictive assumption of no heat flow in the solid made in earlier works. The modified analysis indicates that provided the thermal gradient on the solid side of the interface, Gs, is positive, stability can be achieved in an undercooled alloy melt for growth rates R>Ra, whereas a recent analysis by Trivedi and Kurz, which assumes Gs = 0, suggests that stability is possible only if R>Ra + Rat. Here Ra is the familiar absolute stability limit of Mullins and Sekerka and Rat, is the absolute stability limit in an undercooled pure melt, as identified by Trivedi and Kurz. The absolute stability criterion for steady-state planar growth in an undercooled alloy melt is thus the same as derived earlier by Mullins and Sekerka for directional solidification. Relaxing the restrictive assumption of Gs = 0 also reveals that there is a regime of stability for low growth rates and low supercoolings. Stability is possible under these conditions if Gs>0, and the bath undercooling ΔTb < ΔTO + ΔTh/2, where ΔTO is the freezing range of the alloy and ΔTh is the hypercooling limit for the pure melt. For large supercoolings, Gs < 0, and the interface will be unstable with respect to large wavelength perturbations, even if R > Ra + Rat.