The effect of gallium oxide on cristobalite formation in Pyrex borosilicate glass has been studied. XRD results show that with 20 vol.% gallium oxide, the precipitation of cristobalite at 800 °C is completely prevented. This result is further evidenced by the linear thermal expansion measurement, in which, in contrast to the system without a sufficient amount of gallium oxide (0–10 vol.%), the thermal expansion coefficient of the composite with 20 vol.% gallium oxide remains unchanged with sintering time at the temperature investigated and, moreover, is very close to the theoretically calculated value.

Recent morphological studies of vapor-deposited diamond indicate (1) that growth on the diamond {100} surface is mediated by steps and (2) that some defects cause rapid initiation of new layers, resulting in characteristic pyramids with edges parallel to surface 〈110〉 directions. If growth on the diamond {100} surface does indeed occur at steps, knowledge of their atomic structure is essential to an understanding of diamond growth. We reexamine results of a recent STM study of homoepitaxial diamond films and suggest that the surfaces on which growth occurs consist of regularly spaced double height steps separated by integral numbers of dimer rows running parallel to the step edges.

A seeding and multistep deposition process has been developed to nucleate and grow diamond films directly on Ni substrates in a hot filament chemical vapor deposition system. High quality diamond films have been deposited without graphite codeposition on both 〈100〉 oriented single-crystal Ni and polycrystalline Ni substrates. Both 〈100〉 and 〈111〉 oriented diamond nuclei have been observed depending upon the underlying substrate orientations. Molten metallic phases were found surrounding the diamond nuclei, and it is speculated that a liquid layer composed of nickel, carbon, and hydrogen also formed on the diamond surface during the growth. The oriented diamond is believed to have been achieved by the reorientation of seeded diamond particles into alignment with the Ni substrate due to interaction between the diamond and Ni lattices.

Sixty−70 μm diameter preceramic fibers, extruded from THF solutions containing 1:2:3 stoichiometric mixtures of yttrium, barium, and copper carboxylates, were pyrolytically transformed into ceramic fibers using controlled heating schedules and reactive atmospheres. The objectives of the work reported here were to identify appropriate processing conditions such that during pyrolysis the preceramic fibers would (1) eliminate the organic ligands without pore or void formation, (2) reach full density with a controlled grain size, and (3) form orthorhombic phase, 123 fibers with reasonable mechanical properties. The mechanisms of organic ligand decomposition and loss were examined using mass spectral fragmentation and TGA. Microstructural and phase evolution were correlated with heating schedules and atmospheres, using XRD, DTA, SEM, and limited flux exclusion studies. The mechanisms of decomposition of the spinnable mixtures suggest intermolecular rather than intramolecular decomposition pathways. Different pyrolysis atmospheres were also examined to explore methods of controlling the degradation process. Slow pyrolysis in air followed by oxygen anneals appears to give superior fibers in terms of controlled grain size and phase. The pyrolyzed fibers exhibit the appropriate orthorhombic phase according to x-ray powder diffractometry. Preliminary flux exclusion measurements demonstrate that the fibers are superconducting although the measured δTc is not exceptional.

To correlate structural and compositional parameters in the bismuth 2212 system, single crystals with the composition (Bi1−yPby)2Sr2(Ca1−xRx)Cu2O8+δ (R = Y, Er, Ho, Tm, and Yb; y = 0, 0.1; 0 ≤ x ≤ 0.5) have been studied at room temperature by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The main results are as follows: (i) The actual content, x, of R (R = Y, Er) in samples is in significant excess over its content x′ in the melt for x′ < 0.5. The opposite effect (but several times smaller) takes place for Ca. At x′ = 0.5, the x value practically coincides with x′. (ii) For all R under examination and x′ = 0.1, the value of x is within the limits of 0.43 ≤ x ≤ 0.51; i.e., x exceeds x′ several times. (iii) The total content of Ca, R (R = Y, Er), and Sr is close to 3 through the whole range 0 ≤ x′ ≤ 0.5. At x′ < 0.5 Ca is partly substituted for Sr, while R occupies only Ca crystallographic positions. Thus the actual formula of the samples is (Bi1-yPby)2+∊Sr2-z(Ca1+z-xRx)Cu2O8+δ. (iv) The evidence was received that the nonlinear dependence c(x) at x < 0.5 is connected with the partial substitution of Sr with Ca. The dependence of c, namely on the R = Y content in the denoted range of x, is close to linear with the slope ∂c/∂x = −0.67(2) Å/at.

Superconducting Ba0.6K0.4BiO3 thin films with transition temperatures up to 25 K have been successfully grown on SrTiO3 substrates using RF magnetron sputtering and postgrowth oxygen annealing. Systematic variation of the sputtering process parameters showed that optimum films can be grown on substrates heated to 400 °C in a 10 mTorr atmosphere containing 96% Ar and 4% O2 and using a target containing Ba, K, and Bi in ratios of 0.6:1.2:1.4. High transition temperature superconductivity was observed only in highly oriented, crystalline films having the ideal stoichiometry, Ba0.6K0.4BiO3.

A new interpretation of the reaction mechanism between active metal thin-film filler and ceramic substrate is proposed. The authors predict the possibility of prebonding reactions, prior to melting of the filler, at the interface of the system described above. To prove this, solid-state reactions of Ag–Cu–Ti thin films on sapphire substrates have been studied with Auger electron spectroscopy (AES) and x-ray diffraction (XRD). Reaction process and products have been clarified at the temperature just below the melting point of the filler. It is evident that Cu3Ti3O (diamond structure of Fd3m) is formed by the reaction between Cu3Ti and O which results from the reduction of sapphire. It seems that Cu3Ti3O contributes to bonding between metals and sapphire as an intermediate phase.

Ti−50.8% Al has been cast using unidirectional solidification to produce specimens with a highly directional structure. Each specimen consisted of the intermetallic phases, TiAl and Ti3Al. Both phases existed as single crystal lamellae which were parallel to each other throughout the specimen and aligned along specific crystallographic orientations. Mechanical properties were assessed by deforming specimens such that slip occurred in different directions relative to the lamellae. High-temperature stability was assessed by heating for various times and examining the microstructure. Deformation revealed a highly anisotropic structure since ductility depended upon the principal direction of slip. Poor slip, occurring in a direction through the lamellae, was attributed to the Ti3Al phase where cracking initiated. Heating at 1200 °C resulted in recrystallization of the TiAl phase. The recrystallization started at the ends of the specimens as a result of residual mechanical damage, before spreading along individual lamellae into the bulk.

Mechanically alloyed mixtures of elemental Al and Ti powders have been characterized in the as-milled condition, as well as after hot consolidation. Ball-milling in an argon atmosphere first induces alloying of the elements, followed by the formation of a certain amount of an amorphous-like phase. This amount increases as the equiatomic composition is approached. However, milling in nonsealed mills usually leads to the production of some form of titanium nitride, particularly in Ti-rich mixtures. In the consolidated products obtained from long-milled powders, intermetallic compounds were found to be the predominant phases. The existence of an amorphous phase in the as-milled powders considerably facilitates their hot consolidation.

Carbon-supported Pt catalysts were prepared from H2PtCl6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.

We present evidence indicating that rapid, self-sustained, high-temperature reactions play an important role in the formation of tetragonal MoSi2 during room-temperature high-energy ball milling of elemental powders. Such reactions appear to be ignited by mechanical impact in an intimate, fine-grained, Mo–Si physical mixture formed after an initial milling period. Under certain conditions, limited propagation of self-sustained reactions in these uncompacted powder mixtures renders the compound formation seemingly gradual in bulk-averaged analysis. It is suggested that this type of reaction is an important mechanism in the mechanical alloying of highly exothermic systems. Results are discussed in comparison with similar reactions we observed in ball-milled Al–Ni powders, with self-sustained combustion synthesis previously reported for Mo–Si powders, and with interfacial diffusional reactions in Mo–Si powders or thin-film diffusion couples.

The reliability of integrated-circuit wiring depends strongly on the development and relaxation of stresses that promote void and hillock formation. In this paper an analysis based on existing models of creep is presented that predicts the stresses developed in thin blanket films of copper on Si wafers subjected to thermal cycling. The results are portrayed on deformation-mechanism maps that identify the dominant mechanisms expected to operate during thermal cycling. These predictions are compared with temperature-ramped and isothermal stress measurements for a 1 μm-thick sputtered Cu film in the temperature range 25–450 °C. The models successfully predict both the rate of stress relaxation when the film is held at a constant temperature and the stress-temperature hysteresis generated during thermal cycling. For 1 μm-thick Cu films cycled in the temperature range 25–450 °C, the deformation maps indicate that grain-boundary diffusion controls the stress relief at higher temperatures (>300 °C) when only a low stress can be sustained in the films, power-law creep is important at intermediate temperatures and determines the maximum compressive stress, and that if yield by dislocation glide (low-temperature plasticity) occurs, it will do so only at the lowest temperatures (<100 °C). This last mechanism did not appear to be operating in the film studied for this project.

Based on the results of Shiue and Lee [J. Appl. Phys. 70, 2947 (1991)], the effect of plastic zone and grain boundary on the dislocation emission criterion was investigated. The emission criterion is based on the concept of spontaneous emission. The critical stress intensity factor for dislocation emission increases with the increasing size of dislocation-free zone and the number of piled-up dislocations in the plastic zone, but decreases with increasing grain size. The ductile versus brittle behavior of material was determined by the competition of critical stress intensity factors for dislocation emission and crack propagation. A material with larger grain size is easier to emit dislocation and allows more dislocations to be piled up, so that it behaves more ductile.

The dynamic behavior of a model for the passivation of a metal surface in contact with an aqueous solution is investigated. The model, which is characterized by a three-dimensional state space and five-dimensional parameter space, is obtained by combining elements from passivation models developed by Talbot and Oriani and by Sato. A three-dimensional subspace of parameter space has been studied; the remaining two dimensions are not thought to provide any additional interesting dynamics. The model exhibits remarkably rich dynamics, including the period-doubling, intermittency, and crisis routes to chaos, folds and bubbles in periodic portions of the attractor, and multiple attractors with complex, intertwined basins of attraction.

Vapor-grown carbon fibers (VGCF's) were prepared in a mixture of benzene and Linz–Donawitz converter gas using floating catalytic seeds derived from ferrocene, cobalt acetylacetonate, and thiophene. The diameters of the fibers thus grown were in the range of 2–7 μm. The fibers were heat-treated in argon atmosphere at temperatures between 1700 and 3000 °C. The electrical resistivity at room temperature and magnetoresistance at liquid nitrogen temperature were measured for the as-grown and heat-treated fibers, and morphology of the heat-treated fibers was observed with a scanning electron microscope. The electrical resistivity was nearly similar to that obtained for VGCF's prepared on a substrate in a mixture of benzene and hydrogen. The size effects on the resistivity and magnetoresistance were observed. The magnetoresistance was also found to depend on the heat-treatment time. The magnetoresistance results showed that a transition heat-treatment temperature for the magnetoresistance change from negative to positive was between 2200 and 2300 °C, indicating the graphitizable nature of the present fiber. A characteristic polygonal appearance was observed for the fibers heat-treated above 2500 °C.

Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (χ) from 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the χ data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.

Controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the addition of boron oxide influences the graphite-oxygen reaction. Continuous observations of the process show that at about 450 °C the boron oxide undergoes a strong interaction with both the graphite edge and the basal plane regions, and this results in a spreading of the oxide to form a uniform thin adherent film over the entire substrate. The coated graphite specimens appear to be impervious to attack by oxygen at temperatures below 815 °C. Above this temperature, however, there is a weakening of the additive-graphite interaction with the “armchair” {11$\overline 1$0} faces, and these regions then become vulnerable to attack by oxygen. At the same time very shallow pits are observed to develop in the basal plane, and this action coincides with the appearance of boron carbide species in the electron diffraction pattern. In a complementary series of experiments, it is found that boron carbide is an extremely active catalyst for the graphite-oxygen reaction even at temperatures as low as 100 °C. The impact of these low pressure studies on the behavior of carbon structures used in aerospace applications is discussed.

High-resolution transmission electron microscopy (HRTEM), electron diffraction, and electron energy-loss spectroscopy (EELS) were used to characterize electron-beam-induced structural transformations in crystalline samples of C60 and C70. During these transformations, the electron-diffraction patterns became progressively more diffuse, with the outer diffraction spots disappearing first, followed in succession by the disappearance of the inner spots. We interpret this course of evolution in the diffraction patterns as evidence of degradation of the crystalline structure via the destruction of individual molecules, as opposed to electron-beam-induced motion of intact fullerene molecules. EELS analyses of the data indicate that the final state of these transformations was amorphous carbon.

Copper silicide precipitation in Czochralski silicon influenced by the existence of a GexSi1−x heteroepitaxial layer has been studied. Samples with and without a 100 nm Ge0.016Si0.984 heteroepitaxial layer were annealed together for different periods of time at 1000 °C in air on copper containing iron plate and after copper plating of the backside, respectively. The absorption or emission of silicon self-interstitials (or vacancies) significantly influences the precipitation behavior of copper silicide. Undersaturation of silicon self-interstitials (or vacancy supersaturation) during oxidation of the heteroepitaxial layers is able to prevent the formation of copper silicide precipitate colonies in the surface region.

A comprehensive investigation of antiphase domain boundaries (APB's) in GaAs-on-Si is presented. A comprehensive experimental approach, based on complementary electron microscopy (TEM and SEM) and chemical etch techniques, is developed and used in the study of the structural evolution of APB's on vicinal (001)Si substrates. The question of whether a GaAs selective nucleation or APB annihilation accounts for the absence of APB's in thick GaAs/Si films, grown on substrates misoriented from (001) toward (110), is addressed. APB's are revealed by two different TEM techniques to exist in the first interfacial layers of GaAs/Si even in samples considered to be “APB free”. The APB annihilation mechanism is illustrated in GaAs films grown on substrates misoriented toward (100), either directly, by cross-sectional TEM observations, or indirectly, by combined chemical etch/SEM experiments. In addition, the structural characteristics of APB's and their interaction with other extended crystal defects are clarified by XTEM and TEM observations. Finally, the influence of APB's on GaAs/Si surface morphology and their electrical activity are shown explicitly for the first time.