The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 °C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, kp), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced kp values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi2 in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.

A series of computer experiments has been conducted in order to study the combined combustion synthesis-densification process, in which a mechanical load is applied to a sample as it undergoes a combustion synthesis process. The current work is an extension of a theoretical model of the combustion synthesis process that was developed previously.1,2 In this work, the appropriate constitutive equations for sample deformation have been incorporated, in order to account for the pore-volume change that may take place when the mechanical load is applied, thus densifying the sample. It was shown that the brief appearance of a liquid phase in the combustion wave front provides an important opportunity for densification when the self-propagating combustion synthesis process is conducted in conjunction with an applied mechanical load. That is, the concomitant decrease in the (local) total volume fraction of the solid phases—due to the elementary melting and dissolution processes that also occur (locally)—effectively lowered the (local) apparent yield strength of the sample, thus allowing for the compaction and densification of the sample (i.e., locally). Results indicated that the mechanical load should be applied at the instant at which the sample is ignited, in order to ensure that articles whose density is uniform throughout the sample can be fabricated. This work provided a more detailed and quantitative understanding of this unique process for preparing dense articles by the self-propagating combustion synthesis process, that is, when it is conducted in conjunction with an applied mechanical load.

The rate of liquid-liquid phase separation was experimentally studied in the PbO-B2O3 system. The in situ measurements were made by observing the melts with a videocamera continuously as the melts were cooled down from homogenization temperatures at a rate of 2.5 °C/min. The time interval between the beginning and the completion of the darkening of the visual field was determined as a measure of the separation rate. The phase-separation rate was estimated to be at least 900 times larger than that of the metastable phase separation below the liquidus.

“Effects of boron on the deformation behavior of Ni3Al” [J. Mater. Res. 9, 1742–1754 (1994)]

Thin amorphous films of ceramic capacitor materials were successfully deposited using sol-gel chemistry onto titanium wire using a continuous, computer-controlled process. By repeatedly depositing and calcining very thin layers of material, smooth and even coats can be produced. Surface analyses revealed the layered nature of these thin coats, as well as the amorphous nature of the ceramic. The electrical properties of the better coatings, all composed of niobrium, bismuth, and zinc oxides, were then evaluated.

Interstitial diffusion along twist grain boundaries was studied using the Embedded Atom Method (EAM). Six (100) twist grain boundaries (8.79°–43.6°) in copper were investigated. Interstitial formation energies were found to be much lower (0.26-0.78 eV) than in the bulk, and migration energies were found to be comparable (0.01 eV-0.24 eV) to the bulk values (0.09 eV). The vacancy mechanism is favored for low angle boundaries, and the interstitial mechanism is favored for high angle boundaries. The trends in formation energy versus twist angle were partly explained in terms of the volume expansion of the grain boundary. The total diffusion rate due to both mechanisms was calculated, and agreed reasonably well with experimental data for Cu in polycrystalline Cu. The calculated diffusion rate for specific twist boundaries also agreed well with experimental measurements for Zn/Al.

Surface chemical studies on zinc phosphate glasses were carried out with an ammonia probe using FTIR and XPS. Low softening point zinc phosphate glasses can be co-extruded with high softening point polymers to form polymer filled blends. NH3 reacts with P-OH groups (Br⊘nsted acid sites) to form bound NH4+ and with the zinc ions (Lewis acid sites) to form coordinately bound NH3. Bulk nitridation reactions, forming various P-N bonds to >100 nm, occur concurrently. The glass surfaces were depleted in Zn compared to the batch compositions. Exposure to ambient water vapor removed Lewis acid bound ammonia; aqueous washing removed both types. Di- and tri-methyl amines also reacted with surface Br⊘nsted and Lewis acid sites. These amine reactions have the potential for binding polymer chains to the glass surface.

To enhance a nucleation rate of diamond particles, hydrogenated amorphous carbon (a-C: H) intermediate layers have been formed by radio frequency plasma chemical vapor deposition (CVD) on silicon substrates prior to diamond deposition by hot filament CVD, and the effect of a-C: H intermediate layers on the nucleation and growth rate of diamond particles is studied by varying the thickness of a-C: H films. It is found that diamond particles are well synthesized on thin a-C: H intermediate layers and the nucleation density and growth rate are decreased with increasing the thickness of a-C: H films. Atomic force microscope observations show that a-C: H intermediate layers with rough surface are more effective than the smooth surface for diamond synthesis. Raman spectroscopy shows that the bonding state of carbon atoms in a-C: H films does not change by varying the thickness of a-C: H films. It is proposed that diamond nucleation is affected by the surface morphology rather than the bonding state of carbon atoms in a-C: H films.

Nanocrystalline powder with an average crystallite size of 8–12 nm, which was produced by a combustion synthesis process, was used to prepare dense, nanocrystalline articles. Green compacts of high green density were prepared by dry pressing and densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final grain size and final density. On the other hand, the ranges of heating rates and dwell times that were used had a much less significant effect on the final density and final grain size. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) can be simultaneously achieved under three different firing conditions. The high densification rates are, in part, a result of the minimal coarsening that the particles undergo when the sample is taken rapidly through the temperature regime in which surface diffusion predominates to the temperature regime in which the densification mechanisms of grain boundary and lattice diffusion predominate.

Glass formation from the melt of organic monomers was studied for a variety of different organic molecular structures with Tg near ambient temperature. Crystallization is suppressed by one or more of the molecular properties, hydrogen bonding, interlocking, dipolar, and hydrogen bonding, combined with hindered rotational isomerism. Examples of materials in each category are presented for illustration. The viscosity of undercooled liquids was characterized by the Vogel-Tammon-Fulcher (VTF) equation, η = A cxp[DT0/(T - T0)], where A, D, and T0 are experimentally determined parameters. Our experimental D values are discussed in relation to the molecular structure and glass formation mechanism. The insight provided by our interpretation is intended to assist in the design of new molecular structures with controlled viscosity-temperature characteristics, as well as glass-forming ability by cooling from melts.

Nanocrystalline zirconia doped with 0–10 mol % Y2O3 has been prepared by a combustion synthesis process, followed by a rapid densification process. The concentration of Y2O3 in the as-reacted zirconia appeared to have a significant influence on the reduction of the crystallite size, in the combustion temperature range studied (450 °C–550 °C), as well as on the stabilization of the tetragonal and/or cubic phases. The green compacts were densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final average grain size and the final density of the article. On the other hand, the ranges of heating rates and dwell times that were used in this study were shown to have a much less significant effect on the article's final density and final average grain size. The yttria content had the largest influence on the final density and final average grain size. The densification took place much more rapidly in the 4 mol % Y2O3-ZrO2 samples than in the 10 mol % Y2O3-ZrO2 samples. In particular, the difference in densification rates between the samples with different Y2O3 content was attributed to the influence and magnitude of the associated grain-growth process. It was determined, however, that a high final density (>99% ρth) and a very fine final average grain size (<200 nm) could be simultaneously achieved with each of three different heating rates for the 4 mol% Y2O3-ZrO2 articles.

Four-point bending creep tests were conducted in air on two alumina matrix composites reinforced with 18 and 30 vol% of silicon carbide whiskers, respectively. In both materials, the SiC whiskers tended to form agglomerates. In the temperature range from 1673 to 1823 K, the stress exponents, n, were ∼3.9 and ∼6.3 and the activation energies for creep, Q, were ∼690-740 and ∼930-1010 kJ mol−1 for the composites containing 18 and 30 vol % of SiC, respectively. It is shown that the higher value of n in the composite with 30 vol % of SiC whiskers may be lowered to ∼3 by incorporating a threshold stress. The creep strength of both composites was enhanced by comparison with a similar composite containing 9.3 vol % of SiC whiskers, but there was only a very minor improvement in creep strength when the volume fraction of whiskers was increased from 18 to 30 vol %.

Chemical interactions between polyvinyl alcohol (PVA) and triethanol amine titanate chelate were studied using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The titanate chelate cross coupled the PVA solution and produced a viscous gel. The gel had a three-dimensional network structure containing -CPVA-O-Ti-O-CPVA- organic complexes. A new C(ls) signature at 285.7 eV and an O(ls) signature at 531.25 eV were associated with the formation of these complexes. The water of the PVA solution was physically retained in the gelled structure and was readily available for chemical reactions. The removal of this entrapped water was irreversible and led to a collapsed film of Ti-cross-linked PVA.

The sintering of glass powder compacts was studied by dilatometer in the (25 - x) Na2O–xK2O–75SiO2 glass system. The dilatometrically determined sintering temperature (Tsin) at constant heating rate decreases and the shrinkage at isothermal sintering increases when Na2O is replaced by K2O up to the mole fraction of K2O/(Na2O + K2O) = 0.5. This is due to the decrease in viscosity and means that sintering can be possibly accelereted by introduction of mixed alkali oxides in this system. According to calculation using the VFT-equation, the viscosity value at Tsin is almost independent of glass composition. From these results it may be supposed that the dilatometrically determined sintering begin temperature (Tsin) can be a characteristic point for the sintering of glass.

Thin films of Pb(Mg,Zn)1/3Nb2/3O3 (PMZN) were fabricated by spin casting the partially hydrolyzed Pb-Mg-Zn-Nb-O complex alkoxide sols on (111) Pt-coated MgO(100) planes. Analysis of the isothermal transformation data indicated that the growth of perovskite grains from the pyrochlore matrix is a diffusion-controlled process with an initial fast nucleation. The effect of water concentration in the sol on the preferential orientation of perovskite grains in the corresponding thin film was examined. A strong preferential orientation of (100) perovskite was obtained in the PMZN thin film derived from the sol having a molar ratio of H2O to total metal alkoxides (Rw) of 2. A small-angle x-ray scattering (SAXS) experiment in the Porod region indicated that weakly branched precursor systems led to highly (100)-oriented perovskite grains after thin-film formation. The pyroelectric coefficient and the corresponding figure-of-merit were controlled by suitably varying the degree of preferential orientation during the fabrication of thin films.

We present experimental data for the c-axis electrical resistivity of SbCl5 intercalated graphite between 20 and 300 K. The data are analyzed together with our previous results for these and other samples [O. E. Andersson et al., J. Mater. Res. 7, 2989 (1992)]. Before the analysis, we correct the experimental data to constant volume, as assumed by theorists. We show that the correction factor is much larger for these materials than for normal metals. Although the original data showed significant nonlinearities with T, the corrected data are linear in T to within the experimental accuracy for low-stage compounds below the intercalate crystallization temperature. We compare our results with several models and conclude that both the temperature dependence, the pressure dependence, and the relative changes in the in-plane and c-axis resistivities associated with intercalate crystallization can be best described by a band conduction model.

At the U.S. Department of Energy's Hanford Site, processes are being developed to vitrify waste generated during nuclear materials processing. One of the wastes slated for vitrification is known as neutralized current acid waste (NCAW). The batch chemistry of simulated NCAW was varied with oxidants and reductants. Untreated, formated, nitrated, or sugar-added samples were combined with frit to produce melter feed. Offgas measurements of the formated melter feed showed that formates decomposed at temperatures too low for participation in melt redox reactions. Sugar pyrolyzed and produced CO and H2 at temperatures exceeding 665 °C. For the sugar-added samples, the glass quenched from 1200 °C produced an Fe2+ /ΣFe of 0.79. The measured iron redox ratios from the glasses made from untreated, formated, and nitrated wastes were essentially indistinguishable (0.0024 at 1000 °C and 0.032 at 1200 °C). However, the batch chemistry affected volume expansion and the reaction paths.

Silver has played a critical role for the fabrication of metal/high temperature superconductor composites. Phase equilibrium and microstructure in the ternary PbO-CuO-Ag system have been investigated using differential thermal analysis (DTA), thermogravimetry (TG), scanning electron microscope (SEM), and x-ray diffraction (XRD) techniques. Composition versus temperature diagrams have been established for these systems in air. In the ternary CuO-PbO-Ag system, there is a eutectic reaction CuO + PbO + Ag = L at 750 °C and a composition of 12.04 mol % Ag, 16.35 mol % CuO, and 72.62 mol % PbO. Two immiscible regions near the two binary tie lines PbO-Ag and CuO-Ag were detected. No binary or ternary compound was detected in these systems. SEM and EDS results confirm the presence of two liquid phases and the eutectic point

A SHS process has been established for synthesis of AlN powder under low nitrogen pressures. Al and NaN3 powders were used as the metal and nitrogen sources, respectively. The compact of the mixture of the reactants was wrapped up with aluminum foil and then wrapped up with an igniting agent (i.e., Ti + C). The synthesis reaction was triggered by the combustion of the igniting agent. The wrappings were found necessary in achieving high product conversions under low nitrogen pressures (<1 MPa). The product conversion was also affected by the reactant composition and the nitrogen pressure. High conversions were obtained when the mass ratio of Al to NaN3 was 1/2 or lower and the nitrogen pressure was 0.5 MPa or higher. The AlN powders as synthesized were observed to have two major types of morphology, i.e., granular particles with 0.5–3 μm in diameter and fibers with aspect ratios of 10–800.

The synthesis of tungsten silicides by self-propagating combustion has been successfully accomplished under the influence of an electric field. Materials with starting composition ranging from 6 to 30 wt. % Si were investigated by the method of field-activated combustion synthesis (FACS). A threshold field value was required to initiate a self-sustaining wave; the threshold value depended on composition. It was shown that the level of the applied field can influence the mechanism of silicide formation. The silicide W5Si3 could be formed only at relatively high field values while WSi2 can be formed at any field. The effect of the field on the silicide formation is discussed in terms of its role in liquid phase formation.