The reaction between SiC powder and H2O has been studied at 400°–800 °C under 10 and 100 MPa. Silicon carbide reacted with H2O to yield amorphous SiO2 and CH4 by the reaction SiC + 2H2O→SiO2 + CH4 above 500 °C. Cristobalite and tridymite crystallized from amorphous silica after the almost complete oxidation of SiC above 700 °C. The oxidation rate, as calculated from the weight gain, increased with temperature and pressure. The Arrhenius plotting of the reaction rate based on a Jander-type model gave apparent activation energies of 167–194 kJ/mol. Contrasted with oxidation in oxidative atmosphere, oxidation in H2O is characterized by the diffusion of H2O and CH4 in an amorphous silica layer where the diffusion seemed to be rate determining. Present results suggest that the oxidation of SiC includes the diffusion process of H2O in silica layers when atmospheres contain water vapor.

Results are reported of (00l) x-ray studies of the α(Ic = 10.65 Å) and β(Ic = 11.49 Å) phases of stage 1 graphite-CsBix ternary intercalation compounds that are air stable. The integrated intensities of the (OOl) x-ray lines are analyzed in terms of a centrosymmetric sandwich model that consists of two laycrs each of Cs, Bi, and C. The results of these analyses indicate that the Bi layers are nearly centered in the cell as Suggested by Lagrange et al. [Synth. Met. 12, 201 (1985)]. Furthermore, stoichiometries of the α and β phases are found to be Cs0.99 Bi0.43C4 and Cs0.83Bi0.81C4, respectively. The β phase is found to contain nearly twice as many Bi atoms'. The sample mass change upon intercalation is in good agreement with the x-ray analyses. The substantial difference in the Bi concentration in the α and β phases would be expected to have strong influence on the superconducting properties of these graphite intercalation compounds (GIC's).

Four types of phase transformation that involve the conversion of crystalline phases into more disordered forms are reviewed: melting, disordering of superlattices, amorphization by diffusion between crystalline phases, and irradation amorphization. In the review emphasis is placed on evidence for the heterogeneous nucleation of the product phases; in this connection, the role of surfaces, antiphase domain boundaries, dislocations, vacancies, and grain boundaries is specifically discussed. All of these features have been either observed, or hypothesized, to play a role as heterogeneous nucleation sites in one or more of the four transformations. An attempt is made to draw parallels between nucleation mechanisms in the various processes.

Ion implantation and ion beam mixing have been investigated as alternative techniques to hightemperature diffusion for introducing dopants into LiNbO3. Heavy ion bombardment at both 77 and 300 K initiated a near-surface decomposition causing Li to diffuse to the surface where it formed a nonuniform agglomerate. The damage and annealing characteristics of this effect were studied by ion scattering/channeling, secondary ion mass spectrometry, and optical microscopy. The origins of the surface decomposition are discussed along with possible solutions, and selected samples were evaluated for waveguide properties.

We present the first known systematic mapping of quantum well and tunnel barrier thicknesses in a resonant tunneling structure by transport measurements. The technique derives a 1 Å averaged resolution for quantum well and barrier thicknesses, independently for the quantum well and adjacent tunnel barriers. Contour maps of the structure reveal an asymmetric shallow ring growth structure for one of the epilayers. Current-voltage characteristics and temperature dependence of the resonant tunneling structures will also be discussed.

Temperature-dependent chemical interactions between Au and nine III-V compound semiconductors (III = Al,Ga,In and V = P,As,Sb) have been calculated using bulk thermodynamic properties. Enthalpic considerations alone are insufficient to predict metal/ compound semiconductor reactivities. The entropy of vaporization of the group V elements is shown to be an extremely important driving force for chemical reactions involving the III-V's, since it enables several endothermic reactions to occur spontaneously under certain temperature and pressure conditions. Plots of either Gibbs' free energies of reaction or equilibrium vapor pressure of the group V element versus temperature are used to predict critical reaction temperatures for each of the systems studied. These plots agree extremely well with previous experimental observations of thin film reactions of Au on GaAs.

A promising method for the preparation of sizeable quantities of structurally well-defined, highpurity MgO powder is reported. The morphology and surface uniformity of the powder is comparable to that of MgO smokes but with narrow size distribution of particles. Sample characterization of these oxide powders is accomplished by combining structural TEM/STEM examination with krypton gas adsorption isotherms. The latter technique is sensitive to the presence of surface hydroxyl groups and of surface roughness on an atomic scale. Highresolution TEM indicates a perfect cubic morphology, intra-crystallite orientation, and dendritic sintering of cubes. In the STEM mode sharp convergent beam electron diffraction patterns are obtained, and in thick specimen regions Kikuchi lines appear, indicating the absence of crystal defects. After prolonged outgassing to remove surface hydroxyl groups, a krypton adsorption isotherm contains a near vertical submonolayer riser and second layer step along with partial wetting features near saturation. These near-ideal dendritic ceramic powders, therefore, provide a research bridge between single crystal surface studies and large-scale powder technology.

The kinetics of changes in void size during annealing of glass-like carbons in the temperature range 1000°−2800 °C for times up to 150 h were studied using small-angle x-ray scattering determinations of the radius of gyration Rg. The results show that Rg ranged from 9 Å at 1000°C to about 24 Å at 2800 °C. A pore coarsening analysis and a superimposition kinetic analysis applied to Rg gave activation energies of 76 ± 4 kcal/mole and 74 ± 9 kcal/mole, respectively, which are associated with migration of vacancies within graphitic layers in the matrix material.

It is demonstrated both theoretically and experimentally that fracture toughness does not directly influence the Weibull modulus of ceramic bending strength for materials that obey the Griffith criterion for crack propagation. Weibull modulus remains unchanged as toughness is increased. However, toughness variations with crack length do affect the Weibull modulus. Thus materials that display R-curve behavior or Dugdale character give an increased Weibull modulus and appear more reliable.

The microstructure of three different types of γ-Fe2O3 particles were examined using transmission electron microscopy. These included pure γ-Fe2O3, γ-Fe2O3 that had been surface modified using Co, and γ-Fe2O3 that had been doped with Co. The major internal microstructural defects found in the particles in all of the samples were pores and antiphase boundaries. Some particles also had a very high density of dislocations and low-angle boundaries. In general, the particles could be described as single crystals with symmetric cross section. The structure is based on a tetragonal unit cell, and each particle is divided into antiphase domains in which the c axis is oriented at 90°with respect to adjoining domains. The particles often exhibited very irregular shapes. No effect of Co modification was seen on the internal or surface structure of the particles. The Co-doped particles were found to be smaller in size and contained a lower density of internal defects. The effect of the microstructural defects and morphological irregularities in these particles on magnetic behavior is discussed.

The Au-Ga-As, Au-Ga-Sb, Au-In-As, and Au-In-Sb ternaries were surveyed using x-ray powder diffraction to determine which metallic phases exist at equilibrium with the III-V compound semiconductors. In closed, small-volume systems (i.e., formation of gas-phase products was prevented), Au does not react with GaAs but does react with the other III-V's investigated to produce Au-group III intermetallic compounds and another solid phase containing the group V element. However, each semiconductor formed pseudobinary systems with at least two different intermetallic compounds. The bulk phase diagrams determined in this study provide frameworks within which much of the experimental data in the literature concerning the products of reactions at Au/III-V interfaces can be understood.

The effects of aging on the upper yield stress τup and serrated flow have been studied in MnO single crystals at 900 °C for oxygen partial pressures ρO2 of 10−11 and 10−7 Pa. Aging initially increases τup as a consequence of segregation of aliovalent impurities to dislocations for both ρO2 values. For long aging times and ρO2 = 10-11 Pa, serrated flow accompanied by solute softening is observed. The data fit predictions of a Portevin-Le Chatelier model for serrations, but with impurity atmospheres causing softening instead of hardening. This is believed to result from changes in local defect equilibria caused by segregation of impurities with valences greater than two to dislocations.

Substrate curvature and submicron indentation measurements have been used recently to study plastic deformation in thin films on substrates. In the present work both of these techniques have been employed to study the strength of aluminum and tungsten thin films on silicon substrates. In the case of aluminum films on silicon substrates, the film strength is found to increase with decreasing thickness. Grain size variations with film thickness do not account for the variations in strength. Wafer curvature measurements give strengths higher than those predicted from hardness measurements suggesting the substrate plays a role in strengthening the film. The observed strengthening effect with decreased thickness may be due to image forces on dislocations in the film due to the elastically stiffer silicon substrate. For sputtered tungsten films, where the substrate is less stiff than the film, the film strength decreases with decreasing film thickness.

Raman scattering from the ∼ 1600 cm−1 graphitic phonons is used to study the stage evolution of graphite-H2SO4 in the first ∼ 1000 Å of the bulk during electrochemical intercalation. The Raman results are compared to staging kinetics in the deep bulk studied previously by 00/ x-ray diffraction. For low cell currents, which establish quasiequilibrium conditions, the c-face surface of the highly oriented pyrolytic graphite (HOPG) in contact with the reactant rapidly changes stage index to n − 1 just as the bulk completes stage n. We conclude that the intercalant must cross the c-face plate boundary of the HOPG, probably entering at either grain boundaries, microcracks, or steps in the plate surface. During stage transitions, the Raman lines are observed to remain Lorentzian in shape, with constant width, indicating that an ordered stage n − 1 compound grows at the expense of an ordered stage n compound. In studies of partially submerged HOPG plates, the surface above the acid level is found to stage last, although the plate surface just below the acid level stages first. Lateral diffusion of the sulfate anions from regions below, to regions above the acid level, is apparently impeded for reasons that are not understood. During portions of the electrochemical reaction requiring only hydrogen rearrangement in the intercalate layers (“overcharging”), Raman spectra taken from above and below the acid level are observed to evolve in concert, indicating the protons are not similarly impeded.

Crack propagation thresholds in brittle materials are explained by consideration of the work done by the applied loading and that needed to create new surfaces as a crack propagates. The threshold mechanical energy release rate is shown to be a measure of the equilibrium surface energy of the material, dependent on the chemical environment. For applied loadings greater than those needed to maintain equilibrium the surface energy term introduces nonlinearities into the crack propagation characteristics. Any surface force or lattice trapping behavior at the crack tip will not influence the observed threshold provided the crack tip remains invariant on crack extension. A simple indentation/strength technique is demonstrated that permits the surface energy in the equilibrium state to be estimated. The technique is applied to the propagation of cracks in sapphire and the surface energy in water estimated as 1.42 J m−2, suggesting that the surfaces in water are stabilized by interactions stronger than van der Waals forces or hydrogen bonding alone.

Optical measurements have been made on glasses quenched from a series of sodium silicate melts as well as germanate, borate, and phosphate liquids containing CoO. The spectral data show that Co+2 is the species stabilized in oxide melts heated in air. The observed bands occur at 2700–3850, 5750–7900, and 15 500–18 600 cm-1. The independence of the optical parameters from melt composition and known melt structures, the high relative intensity of the highfrequency band, and the low calculated value of the Racah B parameter relative to the free ion value indicate that Co+2 forms a distinct tetrahedral complex in the quenched melt.

Pulses of Q-switched Nd-YAG radiation have been used to remove polysilicon target links during the implementation of laser programmable redundancy in the fabrication of silicon memory. The link is encapsulated by transparent dielectric films that give rise to important optical interference effects modifying the laser flux absorbed by the link and the silicon substrate. Estimates of these effects are made on the basis of classical plane-wave procedures. Thermal evolution of the composite structure is described in terms of a finite-difference form of the three-dimensional heat diffusion equation with a heat generation rate having a Gaussian spatial distribution of intensity and temporal shapes characteristic of commercial lasers. Temperature-dependent thermal diffusivity and melting of the polysilicon link are included in the computer modeling. The calculations account for the discontinuous change in the link absorption coefficient at the transition temperature. A threshold temperature and corresponding pressure, sufficiently high to rupture the dielectric above the link and initiate the removal process, are estimated by treating the molten link as a hard-sphere fluid. Numerical results are presented in the form of three-dimensional temperature distributions for 1.06 and 0.53 μm radiation with pulse energies 3.5 and 0.15μJ, respectively. Similarities and differences between heating effects produced by long (190 ns FWHM/740 ns duration) and short (35 ns FWHM/220 ns duration) pulses are pointed out.

Dried gel films of poly (vinyl alcohol) were drawn at high temperature using zone heating devices. Hot air zones 5 and 10 mm wide were used to draw the gels to draw ratios up to λ = 23 × at 200 °C. A 3.2 mm wide heated bar in direct contact with the specimen enabled use of drawing temperatures up to 235°C and resulted in draw ratios of 25-38 ×. Drawn films had a highly fibrous nature, were well oriented, and contained crystals about 10 nm long. Gel permeation chromatography studies of the material before and after drawing showed that drawing at high temperatures caused some changes in molecular weight and dispersity. Storage moduli of the fibers increased with draw ratio, reaching a high of 59 GPa at 31× but, in general, leveled off at high A. Low draw ratio (10-15 ×) fibers drawn in the 5 mm air zone had moduli comparable to high draw ratio (25-38 ×) fibers drawn with the high-temperature contact heater (20-60 GPa). Retraction studies showed that low modulus fibers, even those highly drawn, had low effective molecular extension. Drying the fibers increased both their room temperature modulus and glass transition temperature.

BeF2 glasses are potentially useful materials for light transmission in the near infrared region of the spectrum. While the intrinsic attenuation of BeF2 is thought to be much lower than silica, the optimum wavelength will be further in the infrared, near 2.1 μ. In this spectral region impurities other than transition metals may be important—rare earths, for example. The data required to estimate the contributions to attenuation are normally not available; hence it is the purpose of the present work to provide that information. Using a binary BeF2/ThF4 glass, the optical absorption spectra of a number of rare earths are measured. The experimental procedures, optical spectra, and absorption strength are described.